JP2006351371A - Induction heating cooker - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely control heating power in an induction heating cooker. <P>SOLUTION: This cooker is constituted so that a high frequency current generated at an inverter circuit (3) is supplied to a serial resonance circuit (4) formed by a heating coil (27) and capacitors (28, 29) for inductively heating a heated object (31), and the heated object is heated. Firstly, a coarse adjustment of input power is carried out by varying a switching frequency of the inverter circuit, and next, the input power is made to coincide with a target power value by a fine adjustment of a DC voltage supplied to the inverter circuit. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an induction heating cooker that performs cooking by induction heating of a cooking device load such as a pan, and more particularly, induction heating suitable for heating a load made of a material having a low magnetic permeability and high resistance such as aluminum and copper. It relates to a cooker.

  Induction cooking devices are safe because they do not use fire, and are easy to adjust temperature because they are electric heating. Therefore, they have recently been rapidly incorporated into system kitchens. By the way, since the cooking pot etc. which are heating objects are made with various materials, it is calculated | required that an induction heating cooking appliance can heat all those materials efficiently.

Generally, when a high frequency current is applied by placing a coil near the surface of a conductive material, an induced current flows in the material by electromagnetic induction, and when the induced current flows, Joule heat is generated due to the resistance of the conductive material and the temperature of the material rises. . This is the heating principle of induction heating. When a uniform high-frequency magnetic field having an amplitude H0 is generated in the vicinity of the surface of a conductive material having a flat surface, the amplitude i of the current density of the induced current within the distance x from the surface of the material is expressed by the following equation.
i = (ωκμ) ^1/2 · H0 · exp (− (ωκμ / 2) ^1/2 · x) (1) When the frequency is f, ω = 2πf, κ is the conductivity of the conductive material, and μ is The magnetic permeability of the conductive material.

According to equation (1), the induced current density i decreases exponentially as it enters the interior from the surface. A material with a large value of (ωκμ) ^1/2 has a large reduction rate of current density, and in such a material, the induced current flows only near the surface layer of the material. Further, since the current density i on the surface (x = 0) is (ωκμ) ^1/2 · H0, the material having a larger value of (ωκμ) ^{1/2 has} a higher current density i on the surface.

Comparing the values of (ωκμ) ^1/2 for aluminum and iron, which are conductive materials, when ω is the same, the value of aluminum is 1/5 to 1/6 of the value of iron. Therefore, when the high frequency magnetic field in the immediate vicinity of the surface is the same, the surface current density of aluminum is as small as 1/5 to 1/6 that of iron, and the current penetration depth is 5 to 6 times.

Joule heat generated per unit volume in the conductive material is calculated by (i ² / κ). Considering the integral value of i ² for the effective volume portion through which the induced current flows, the effective volume of aluminum is about 5 times that of iron, but the current density is about 1/5 ² times smaller. For this reason, the integral value of i ² is reduced to about 1/5 times. Furthermore, when comparing the value of conductivity κ, aluminum is about six times that of iron. Therefore, the total Joule heat generated in the conductive material is such that the value of aluminum is a small value of about 1/30 of the value of iron when the high frequency magnetic field in the immediate vicinity of the surface is the same. That is, aluminum can be said to be a material that is difficult to induce heating.

According to equation (1), even if the value of κμ is different, the distribution of induced currents in the conductive material is the same if the value of (ωκμ) ^1/2 is the same. Therefore, even in the case of aluminum, if the frequency f is increased so that the value of (ωκμ) ^1/2 is the same as that of iron, the distribution of induced current can be made substantially the same as that of iron, and the total Joule heat generated Can be increased. However, there still remains a difference due to the difference in conductivity κ. The remaining means to fill the difference is to make the value of the magnetic field H0 closest to the surface in the equation (1) larger than that of iron. In order to increase the value of the magnetic field H0, it is necessary to increase the current flowing through the coil or increase the number of coil turns.

  However, increasing the frequency f of the high frequency magnetic field is not so easy. When the frequency f is increased, the switching loss of the inverter circuit that supplies current to the coil increases. In order to solve these problems, development of circuit technology for efficiently heating an object to be heated such as aluminum or copper having a small value of (κμ) has been continued.

  For example, Patent Document 1 discloses a technique for increasing the number of turns of a heating coil and lowering the capacity of a resonant capacitor when heating an aluminum material. This is considered to be intended to increase the strength of the generated magnetic field, but it is difficult to sufficiently increase the heating efficiency because high frequency switching is required.

  Patent Documents 2 and 3 disclose circuit techniques for generating a high-frequency magnetic field having a frequency corresponding to twice the switching frequency of the inverter circuit without changing the number of turns of the heating coil. This is intended to increase the frequency of the high-frequency current flowing through the heating coil without increasing the switching frequency of the inverter circuit. However, when heating an iron pot or the like, high heating power heating is difficult because the number of turns of the heating coil is large and the coil current hardly flows.

  The high-frequency current flowing in the heating coil is generated by the switching operation of the switching elements in the inverter circuit. Recently, the gate signal for determining the switching timing of the switching elements is generated by digital calculation in the microcomputer. . For this reason, if the switching frequency is increased, a microcomputer having a high calculation speed is required, resulting in a problem of an increase in cost.

  Furthermore, there is a problem when adjusting the heating power. The heating coil is connected in series with a capacitor to form a resonance circuit, and high frequency current is supplied to the resonance circuit from the inverter circuit. As the current flowing through the resonance circuit increases, a stronger high-frequency magnetic field is generated. The current flowing through the resonance circuit is maximized when the frequency of the high-frequency current supplied from the inverter circuit matches the resonance frequency of the resonance circuit. When a difference occurs between the two frequencies, the resonance current decreases. For this reason, the heating power is adjusted by varying the switching frequency of the inverter circuit.

  The sharpness of the change in the resonance current when the frequency of the high-frequency current supplied to the resonance circuit is changed is represented by a value of Q. The Q value of the heating coil is calculated by the equation (ω · L / r). ω is the angular frequency, L is the inductance of the coil when a pan or the like is placed on the heating coil, and r is the high-frequency resistance in that state.

  Aluminum has a higher conductivity than iron. Therefore, the Q value when heating an aluminum pan or the like is higher than when heating an iron material. When the value of Q is high, the resonance current changes greatly due to a slight change in the switching frequency of the inverter circuit, and the heating power also changes greatly. FIG. 8 shows an example of a change in input power (heating power is approximately proportional to this value) to the inverter circuit when the frequency of the high-frequency current passed through the resonance circuit is changed. In the case of an aluminum pan, the input power changes greatly with a slight frequency change.

This means that in order to adjust the heating power to the aluminum pan, it is necessary to accurately control the switching frequency of the inverter circuit at fine frequency intervals. In order to increase the frequency accuracy, it is necessary to increase the calculation accuracy of the microcomputer, and for this purpose, it is necessary to increase the bit length of the operation numerical value. For this reason, when heating an aluminum pan, there is a problem that an expensive microcomputer having a long bit length and a high calculation speed is required.
JP-A 61-16491 JP 2001-160484 A Japanese Patent Application No. 2004-330401

  The present invention has been made to solve such problems, and the problem is that the heating power for heating an object to be heated made of a material having a low magnetic permeability and high conductivity such as aluminum and copper is highly accurate. It is to provide an induction heating cooker that can be controlled by heating and that can be heated with high efficiency.

  The invention according to claim 1 for solving the above-mentioned problem is a series resonance circuit of a heating coil and a capacitor, an inverter circuit for supplying a high-frequency current to the series resonance circuit, and a DC voltage for the inverter circuit. In an induction heating cooker that includes a DC power supply circuit and induction-heats an object to be heated by flowing a high-frequency current through the series resonance circuit, the frequency of the high-frequency current supplied from the inverter circuit is first varied. An induction heating cooker characterized in that rough adjustment of input power is performed, and then the DC voltage supplied to the inverter circuit is finely adjusted so that the input power matches the target power value.

  In order to efficiently heat an object to be heated made of a material having a low magnetic permeability and high conductivity such as aluminum and copper, it is necessary to use a high-frequency high-frequency current. However, fine adjustment of the frequency in the high frequency region is difficult, and fine adjustment of the heating power by frequency adjustment is difficult. On the other hand, the fine adjustment of the supply voltage to the inverter circuit is easy, and the heating power can also be adjusted by this. According to the configuration of the present invention, it is not necessary to finely adjust the frequency of the high-frequency current with high accuracy, and the heating power of the object to be heated can be accurately matched with the target value.

  Further, since the final fine adjustment is performed by the DC voltage, it is not necessary to adjust the frequency of the high-frequency current at such a fine frequency interval. Accordingly, the inverter circuit is not required to adjust the switching frequency at such a fine frequency interval. Therefore, there is an advantage that a relatively low-cost product that does not have a high calculation speed can be used for a microcomputer or DSP for generating timing pulses for driving the inverter circuit by software.

  The invention according to claim 2 is a series resonance circuit (4) of a heating coil (27) and a capacitor (28, 29), an inverter circuit (3) for supplying a high frequency current to the series resonance circuit, A DC power supply circuit (2) for supplying a DC voltage to the inverter circuit, a gate control circuit (5) for adjusting the ON / OFF timing of a switching element of the inverter circuit to vary the frequency of the high-frequency current, and the gate An induction heating cooker comprising: a power control means (6) for supplying a target value of the frequency of the high-frequency current to the control circuit and a target value of the DC voltage to the DC power supply circuit, wherein the power control The means includes a predetermined high-frequency current of which the input power of the DC power supply circuit substantially matches the indicated target power value when the DC voltage supplied to the inverter circuit is a standard value. The wave number is given as a target value to the gate control circuit, and the target value of the DC voltage applied to the DC power supply circuit is adjusted so that the input power of the DC power supply circuit matches the input power target value in that state. It is an induction heating cooking device characterized by being made.

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. Moreover, according to this structure, even if the to-be-heated object heated with a heating coil differs from the assumed standard to-be-heated object, the power consumption of a heating coil can be made to correspond with the instruct | indicated target electric power value. .

  The invention according to claim 3 is the induction heating cooker according to claim 2, wherein the power control means is in a state where the DC voltage supplied to the inverter circuit is equal to a predetermined standard value. A relationship table between the frequency of the high-frequency current and the input power of the DC power supply circuit is held, and when the target power value is instructed, the input power of the DC power supply circuit is a frequency that substantially matches the target power value. Then, a frequency higher than the resonance frequency of the series resonance circuit is read from the relation table and given to the gate control circuit as a frequency target value so that the input power of the DC power supply circuit matches the input power target value in that state. When the DC voltage target value applied to the DC power supply circuit is controlled, and the DC voltage of the DC power supply circuit becomes a predetermined value or more during the control of the DC voltage target value, The frequency target value is decreased by a predetermined value, and conversely, when the DC voltage of the DC power supply circuit falls below a predetermined value, the frequency target value is increased by a predetermined value, and in that state, the DC target is increased again. It is configured to repeatedly perform control for adjusting the target value of the DC voltage applied to the DC power supply circuit so that the input power of the power circuit matches the input power target value.

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. In a region where the frequency of the high-frequency current flowing through the series resonance circuit is higher than the resonance frequency, the power consumption of the heating coil decreases as the frequency of the high-frequency current increases. In this configuration, when the input power is significantly different from the input power target value, coarse adjustment of the frequency of the high-frequency current is performed again. Therefore, even when the object to be heated by the heating coil is greatly different from the object to be heated that is assumed as a standard, the power consumption of the heating coil can be matched with the instructed target power value.

  The invention according to claim 4 is the induction heating cooker according to claim 2, wherein the power control means is in a state in which the DC voltage supplied to the inverter circuit is equal to a predetermined standard value. A relationship table between the frequency of the high-frequency current and the input power of the DC power supply circuit is held, and when the target power value is instructed, the input power of the DC power supply circuit is a frequency that substantially matches the target power value. A frequency lower than the resonance frequency of the series resonance circuit is read from the relation table and given to the gate control circuit as a frequency target value so that the input power of the DC power supply circuit matches the input power target value in that state. When the DC voltage target value applied to the DC power supply circuit is controlled, and the DC voltage of the DC power supply circuit becomes a predetermined value or more during the control of the DC voltage target value, The frequency target value is increased by a predetermined value, and conversely, when the DC voltage of the DC power supply circuit falls below a predetermined value, the frequency target value is decreased by a predetermined value, and in that state, the DC target value is again increased. It is configured to repeatedly perform control for adjusting the target value of the DC voltage applied to the DC power supply circuit so that the input power of the power circuit matches the input power target value.

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. In a region where the frequency of the high-frequency current flowing through the series resonance circuit is lower than the resonance frequency, the power consumption of the heating coil increases as the frequency of the high-frequency current increases. In this configuration, when the input power is significantly different from the input power target value, coarse adjustment of the frequency of the high-frequency current is performed again. Therefore, even when the object to be heated by the heating coil is greatly different from the object to be heated that is assumed as a standard, the power consumption of the heating coil can be matched with the instructed target power value.

  The invention according to claim 5 is the induction heating cooker according to any one of claims 2 to 4, wherein the series resonance circuit has a resonance frequency by switching a capacity of the capacitors (28, 29). The power control means includes material detection means for detecting the type of material of the object to be heated to be heated by the heating coil, and determines the frequency target value of the high-frequency current. The capacitor is switched to a predetermined capacity according to the type of material of the object to be heated detected by the material detection means, taking into account the fact that the switching has been performed and the type of material of the object to be heated. The frequency target value is determined.

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. The optimum frequency of the high-frequency current differs greatly depending on whether the object to be heated is an iron-based material or an aluminum-based material. Efficient heating can be performed by detecting the material of the object to be heated and changing the resonance frequency of the resonance circuit according to the material to determine the frequency target value as in this configuration.

  The invention according to claim 6 is the induction heating cooker according to any one of claims 1 to 5, wherein the inverter circuit includes first and second switching elements (20) connected with antiparallel diodes. , 21) and a series connection circuit of third and fourth switching elements (22, 23) connected to anti-parallel diodes, and the gate control circuit When one of the first and second switching elements is in the ON state, the other is in an OFF state and is operated alternately. When one of the third and fourth switching elements is in the ON state, the other is in the OFF state. The first switching element is in an ON state for 3/4 of the switching period, and the third switching element is in an ON state for 1/4. And the switching of the third switching element to the ON state is controlled at a timing delayed by 1/4 of the switching period from the switching of the first switching element to the ON state. And

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. In addition, if the four switching elements of the inverter circuit are operated as in this configuration, the ratio of the frequency of the high-frequency current flowing through the heating coil to the switching frequency can be set to 2: 1, and the inverter circuit has a low switching frequency. Can be operated.

  The invention according to claim 7 is the induction heating cooker according to claim 6, wherein the first switching element has an ON state of 5/8 of the switching period, and the third switching element has 3 / 8 is operated so as to be in the ON state, and the switching of the third switching element to the ON state is delayed by 1/8 of the switching cycle from the switching of the first switching element to the ON state It is controlled by.

  The induction cooking device configured as described above has the same effect as that of the first aspect of the present invention. In addition, if the four switching elements of the inverter circuit are operated as in this configuration, the ratio of the frequency of the high-frequency current flowing through the heating coil to the switching frequency can be set to 4: 1, and the inverter circuit can be switched low. Can be operated at frequency.

  The invention according to claim 8 is the induction heating cooker according to any one of claims 2 to 7, wherein the DC power supply circuit rectifies an AC voltage supplied from an external power supply (11). And a step-up chopper type DC / DC converter (7) for converting the direct-current voltage output from the rectifier circuit and outputting the boosted direct-current voltage instructed by the power control circuit. It is characterized by.

  In the case of this configuration, the same effect as that of the first aspect of the invention can be obtained. In order to heat an object to be heated with high output, it is necessary to supply a high DC voltage to the inverter circuit. According to this configuration, after the commercial AC voltage is rectified, the required high DC voltage can be easily generated by boosting the commercial AC voltage with the boost chopper type DC / DC converter.

  The invention according to claim 9 is the induction heating cooker according to claim 8, wherein the step-up chopper type DC / DC converter is connected between both ends of the output of the rectifier circuit (12), A series connection circuit of a diode (14) and an output smoothing capacitor (15), a switching element (13) for opening and closing between both ends of the series-connected diode and the output smoothing capacitor, and an open / close drive signal to the switching element A drive circuit (7a), and an output voltage control circuit (7b) that provides an opening / closing timing signal to the drive circuit so that the voltage across the output smoothing capacitor matches the value instructed by the power control circuit. In addition, an input signal isolation circuit is provided at the input portion of the drive circuit so that the opening / closing timing signal is transmitted as an electrically isolated signal. Characterized in that the sea urchin configuration.

  In the case of this configuration, the same effect as in the eighth aspect of the invention can be obtained. By providing the input signal isolation circuit, there is an effect that the possibility that the output voltage control circuit malfunctions due to a large current flowing in the negative power supply line of the rectifier circuit is reduced.

  Hereinafter, an embodiment of an induction heating cooker according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing the circuit configuration of the induction heating cooker. The induction cooking device 1 includes a DC power supply circuit 2, an inverter circuit 3, a series resonance circuit 4, a gate control circuit 5, and a power control circuit (corresponding to power control means) 6.

  The DC power supply circuit 2 is a circuit that supplies a boosted DC voltage Vdc to the inverter circuit 3 in the next stage, and is constituted by a full-wave rectifier circuit 11 and a DC / DC converter 7. The full-wave rectifier circuit 11 converts an AC voltage supplied from the external power supply 10 into a DC voltage and supplies the DC voltage to the DC / DC converter 7. The DC / DC converter 7 is a boost chopper type DC / DC converter that boosts the DC voltage supplied from the rectifier circuit 11 and is also called a boost chopper circuit. The DC / DC converter 7 includes a reactor 12, a switching element 13, a diode 14, a smoothing capacitor 15, a drive circuit 7a, and an output voltage control circuit 7b.

  The reactor 12, the diode 14, and the smoothing capacitor 15 are connected in series between the output terminals of the rectifier circuit 11 in this order. A ground line 18 is connected to a connection point between the rectifier circuit 11 and the smoothing capacitor 15, and the smoothing capacitor 15 is charged to a positive voltage by a current passing through the diode 14. Switching element 13 is connected between the interconnection point of reactor 12 and diode 14 and ground line 18. The switching element 13 is a self-extinguishing type switching element, such as a bipolar transistor or IGBT.

  The switching element 13 is controlled to open and close (ON / OFF) by the drive circuit 7a. When the switching element 13 is turned on, current flows from the rectifier circuit 11 to the ground line 18 through the reactor 12 and the switching element 13, and electromagnetic energy is accumulated in the reactor 12. Thereafter, when the switching element 13 is turned off, the electromagnetic energy accumulated in the reactor 12 charges the smoothing capacitor 15 via the diode 14.

  When the switching element 13 is turned off, a high voltage is generated across the reactor 12, so that a voltage obtained by boosting the output voltage of the rectifier circuit 11 can be obtained across the smoothing capacitor 15. The voltage Vdc across the smoothing capacitor 15 is the output voltage Vdc of the DC power supply circuit 2.

  The value of the output voltage Vdc is controlled by the output voltage control circuit 7b. FIG. 2 is a configuration example of the output voltage control circuit 7b for adjusting the output voltage Vdc. The output voltage control circuit 7 b includes an error amplifier 71, a comparator 72, and a sawtooth wave generation circuit 73. The error amplifier 71 receives the target value Vdcs of the output voltage Vdc output from the power control circuit 6 and the output voltage Vdc as feedback. The error voltage ΔVdc is output from the output terminal. The error voltage ΔVdc is input to the comparator 72 and compared with the sawtooth wave having a constant frequency generated by the sawtooth wave generation circuit 73. As a result, a PWM-modulated pulse signal having a pulse width proportional to the error voltage ΔVdc is generated at the output of the comparator 72. The PWM-modulated pulse signal is applied to the drive circuit 7a, where it is subjected to voltage conversion, power conversion, etc., and is applied to the switching element 13. The output voltage Vdc is controlled so as to coincide with the target value Vdcs by such proportional control by feedback.

  In order to prevent the output voltage control circuit 7b from malfunctioning due to the influence of a large load current flowing in the ground line 18, a signal insulation circuit using a photocoupler is provided at the input portion of the drive circuit 7a to perform PWM. The modulated pulse signal may be transmitted in an electrically insulated state.

  The output voltage Vdc of the DC power supply circuit 2 is supplied to the inverter circuit 3 through the positive power supply line 17 and the ground line 18. The inverter circuit 3 generates a high-frequency voltage from the DC output voltage Vdc by the switching operation and supplies the high-frequency voltage to the series resonance circuit 4. The inverter circuit 3 includes a first half bridge circuit in which first and second switching elements 20 and 21 are connected in series between a power supply line 17 and a ground line 18, and third and fourth switching elements 22, This is a full bridge type inverter circuit composed of a second half bridge circuit in which 23 are connected in series.

  Antiparallel diodes (free wheel diodes) 20a, 21a, 22a, and 23a are connected to the first to fourth switching elements 20 to 23, respectively. The switching element is, for example, an IGBT or a bipolar transistor. A series resonance circuit 4 serving as a load between the interconnection point N1 of the first and second switching elements 20 and 21 and the interconnection point N2 of the third and fourth switching elements 22 and 23 is an output terminal. Is connected. Snubber capacitors C24 and C25 for reducing switching loss are connected between the two output terminals N1 and N2 and the ground line 18.

  The first and second switching elements 20 and 21 are alternately operated so that when one is in an ON state, the other is in an OFF state. The third and fourth switching elements 22 and 23 are also operated alternately so that when one is ON, the other is OFF.

  The gate control circuit 5 controls the ON / OFF operation of each switching element of the inverter circuit 3. The gate control circuit 5 includes a gate signal generation circuit 5a and a gate drive circuit 5b. The gate signal generation circuit 5a is a circuit that receives a command of a frequency target value from the power control circuit 6 and generates a gate drive pulse signal for turning on / off each switching element. The gate drive pulse signal is given to the gate drive circuit 5b, where the level is converted and given to the switching elements 20-23. The pulse signal generation operation of the gate signal generation circuit 5a will be described in detail later.

  The series resonance circuit 4 that is a load of the inverter circuit 3 includes a heating coil 27, capacitors 28 and 29, and a switch 30. The capacitor 29 and the switch 30 are connected in series to the capacitor 28 in series. This capacitor circuit and the heating coil 27 constitute a series resonance circuit. The switch 30 is for switching the resonance frequency by opening / closing the connection of the capacitor 29, and the opening / closing thereof is controlled by the power control circuit 6. When a high-frequency current flows through the heating coil 27, the object to be heated 31 is heated by induction heating.

  When the switch 30 is in the OFF state, the heating coil 27 and the capacitor 28 form a series resonance circuit. The capacity of the capacitor 28 is determined so that the resonance frequency when heating the aluminum-based object to be heated in this state is about 80 kHz. On the other hand, when the switch 30 is in the ON state, the capacitors 28 and 29 are arranged in parallel, and the capacitance and the heating coil 27 constitute a series resonance circuit. The parallel capacitances of the capacitors 28 and 29 are determined so that the resonance frequency when heating the iron-based object to be heated is about 20 kHz. The capacity of the capacitor 28 is determined first, and the capacitor 29 is determined later. A resonance frequency of about 20 kHz is selected when heating an object to be heated of an iron-based material, and a resonance frequency of about 80 kHz is selected when heating an object to be heated such as aluminum or copper.

  The power consumed by the heating coil 27 is controlled by the power control circuit 6. The power control circuit 6 is a circuit that performs control so that the power consumed by the heating coil 27 matches the target power value instructed from the outside. Since the current flowing through the heating coil 27 is a high-frequency current, it is not easy to detect the power consumption of the heating coil 27. Therefore, in the present embodiment, instead of directly controlling the power consumed by the heating coil 27, control is performed to match the input power of the DC power supply circuit 2 with the following input power target value calculated based on the target power value. As a result of the control, the power consumption of the heating coil 27 is made to coincide with the target power value.

  The input power of the DC power supply circuit 2 is the sum of power consumption in the heating coil 27 and circuit loss in other circuit portions. The circuit loss includes a loss that is proportional to the power consumption of the heating coil 27 and a loss that is not proportional to the power consumption, but the value can be grasped in advance through experiments and calculations. Therefore, if the input power of the DC power supply circuit 2 is controlled so as to coincide with the value obtained by adding the target power value of the heating coil 27 indicated by the circuit loss (hereinafter referred to as the input power target value), the heating coil 27 is controlled. The power consumption becomes equal to the instructed target power value.

  The power control circuit 6 performs control to make the input power of the DC power supply circuit 2 coincide with the input power target value by feedback control. For this purpose, the power control circuit 6 includes a load power control circuit 6a and an input power detection circuit 6b. The input power detection circuit 6b is supplied with the value of the AC voltage supplied to the DC power supply circuit 2 and the value of the input current. The input current is detected by the current transformer 16. The input power detection circuit 6b detects the input power from the input voltage and current values and inputs the detected input power to the load power control circuit 6a. An instruction value for the target power value from the outside is also input to the load power control circuit 6a. Further, in order to perform feedback control, the output voltage Vdc of the DC power supply circuit 2, that is, the supply voltage Vdc to the inverter circuit 3 is also input.

  The power control circuit 6 follows the control calculation flow described later, and the output voltage control circuit 7b in the DC power supply circuit 2 receives the target value of the output voltage Vdc (supply voltage Vdc to the inverter circuit 3) and the gate in the gate control circuit 5. The signal generation circuit 5a outputs a frequency target value of the high-frequency current that flows through the heating coil 27.

  In addition, the power control circuit 6 includes a coil current detection circuit 6c. The coil current detection circuit 6c detects the current flowing through the heating coil 27 and inputs it to the load power control circuit 6a. The input current value is referred to when determining the material of the object to be heated. The current in the heating coil 27 is detected by a current transformer 32.

  The control calculation in the load power control circuit 6a and the gate signal generation circuit 5a is performed by digital calculation using a common set of microcomputer or DSP (Digital Signal Processor).

  Next, a control flow for matching the power consumed by the heating coil 27 with the target power value instructed from the outside under the above configuration will be described with reference to FIG. The flow of FIG. 3 is a flow executed by the load power control circuit 6a.

  This control flow is started when a heating start signal (not shown) is input from the outside. In the first step S1, the target power value of the heating coil 27 input from the outside is read. In step S2, an input power target value Ps obtained by adding the aforementioned circuit loss to the target power value is calculated.

  In subsequent step S3, the material of the object to be heated is determined. This is because the optimum heating frequency (frequency of the high-frequency current flowing through the heating coil 27) differs greatly between the iron-based and aluminum-based objects to be heated as described in "Background Art". There are various methods for determining the material. One of them is a judgment method that utilizes the fact that the magnetic permeability differs greatly between iron-based and aluminum-based materials. In this case, for example, 25 kHz is instructed to the inverter circuit 3 as the target value of the heating frequency to the gate signal generation circuit 5a with the switch 30 turned on. On the other hand, the output voltage control circuit 7b of the DC power supply circuit 2 is given a relatively low voltage that is predetermined as a target value of the output voltage Vdc (supply voltage Vdc to the inverter circuit 3). Then, the current value of the heating coil 27 when operating under such conditions is read from the coil current detection circuit 6c.

  When the object to be heated is an aluminum system, the inductance of the heating coil 27 becomes a small value because the magnetic permeability is low. Therefore, the resonance frequency of the series resonance circuit 4 is greatly deviated from 25 kHz, for example, 50 kHz. In this case, only a small current flows through the heating coil 27. On the contrary, when the object to be heated is iron-based, the resonance frequency is about 20 kHz. Since the heating frequency 25 kHz is considerably close to that value, a larger current flows in the heating coil 27 than in the case of an aluminum material. Accordingly, it can be determined from the magnitude of the current value of the heating coil 27 whether the material is iron or aluminum. It can be determined from the magnitude of the input power detected by the input power detection circuit 6b instead of the magnitude of the current flowing through the heating coil 27. In this case, the value of the input power is small for aluminum-based materials and larger for iron-based materials.

  When the material of the object to be heated is determined in this way, the switch 30 of the series resonance circuit 4 is switched according to the material (step S4). If the material is aluminum, the switch 30 is turned off to set the resonance frequency to about 80 kHz (step S5). If it is iron-based, the switch 30 is turned on to set the resonance frequency to about 20 kHz (step S6).

  If the configuration of the series resonant circuit 4 is determined in this manner, the value of the supply voltage Vdc supplied to the inverter circuit 3 at the start of heating is determined in the next step S7. This value is determined in advance as an intermediate value in the allowable output voltage range of the DC power supply circuit 2. This value will be referred to as a standard supply voltage to the inverter circuit 3. The load power control circuit 6a gives this standard supply voltage to the output voltage control circuit 7b as the target value Vdcs of the supply voltage Vdc.

  In subsequent step S8, an initial target value of the heating frequency f to be given to the gate control circuit 5 is determined. In order to determine the target value of the heating frequency f to be given first, the load power control circuit 6a previously stores the relationship data between the heating frequency f and the input power Pin when the supply voltage to the inverter circuit 3 is equal to the standard supply voltage. Remember. FIG. 4 is an example in which the relational data is represented by a graph. The curve (1) in the figure is an operation curve connecting the operating points when an aluminum-based material is heated at a standard supply voltage of 360 V (hereinafter referred to as the curve of (1)). Is referred to as a standard operating curve).

  The load power control circuit 6a reads the heating frequency f at which the input power Pin becomes equal to the aforementioned input power target value Ps from the standard operation curve (1). As an example, when the input power target value Ps is 1100 W, the heating frequency f is 87.4 kHz corresponding to the point A in the figure. In this case, the frequency of the high-frequency current flowing through the heating coil 27 can be changed only in units of 0.1 kHz, and in the case where the heating frequency f corresponding to the point A is 87.37 kHz, it was obtained from the figure. The adjustable frequency closest to the value may be determined. The relationship between the heating frequency f and the input power Pin may be held in a table instead of having a graph as shown in FIG. 4 (actually an approximate expression).

  The load power control circuit 6a gives the first target value of the heating frequency f determined in this way to the gate signal generation circuit 5a, and then instructs to start the inverter circuit 3. Upon receiving the instruction, the gate signal generation circuit 5 a starts controlling the ON / OFF timing of the four switching elements 20 to 23 so that the high-frequency current having the instructed heating frequency f is supplied to the heating coil 27. The method of controlling the ON / OFF timing will be described in detail later.

  If heating is thus started, the process proceeds to step S9. Steps S9 to S17 are a control flow for accurately matching the value of the input power Pin with the input power target value Ps. That is, an instruction is given to the DC power supply circuit 2 so that the standard supply voltage is supplied to the inverter circuit 3 in step S7. In step S8, the input power Pin matches the input power target value Ps under the standard supply voltage. The heating frequency f was instructed to the inverter circuit 3. As a result, the input power Pin matches the input power target value Ps, and it seems that the heating coil 27 consumes power equal to the target power value instructed from the outside.

  However, in practice, the input power Pin cannot be matched with the input power target value Ps only by giving such an instruction. There are two main reasons. The first reason is that the frequency of the high-frequency current flowing through the heating coil 27 cannot be changed continuously as described above, but can be changed only in units of 0.1 kHz, for example. This is because the load power control circuit 6a and the gate signal generation circuit 5a are controlled by digital calculation using a microcomputer (or DSP).

  Since the microcomputer operates with a clock having a constant frequency, the heating frequency can be changed only in units of 0.1 kHz, for example. Of course, if the clock frequency is increased, the adjustment unit can be reduced. However, this requires the use of an expensive microcomputer. Heating of the aluminum-based material needs to be performed at a high frequency (for example, 90 kHz), and since the Q value of the heating coil 27 is increased, the input power is changed as shown in FIG. Pin varies greatly. For this reason, the input power Pin is different from the input power target value Ps.

  The second reason is that the condition of the object to be heated assumed by the standard operation curve (1) in FIG. 4 is different from the condition of the actual object to be heated. The resonance frequency of the series resonant circuit 4 varies with the same aluminum pan if the bottom thickness is different or the pan contents are different. As a result, the input power Pin and the input power target value Ps do not coincide with each other under the operation condition obtained from the standard operation curve (1) in FIG.

  If the input power Pin does not match the input power target value Ps, the power consumption in the heating coil 27 does not match the instructed target power value. If this happens, cooking will be hindered. Therefore, in the induction heating cooker 1 of the present embodiment, in order to make the input power Pin coincide with the input power target value Ps in spite of the cause of the above first and second reasons, the following method is used. adopt.

  That is, since the heating frequency f is difficult to finely adjust, only the power is roughly adjusted at the heating frequency f. If the rough adjustment is performed by first adjusting the heating frequency f, the fine adjustment of the supply voltage Vdc to the inverter circuit 3 is performed while the value of the heating frequency f is maintained at that value. Then, the error power remaining in the coarse adjustment is compensated. Fine adjustment of the supply voltage Vdc is easy. If the value of the supply voltage Vdc is likely to deviate from the allowable output voltage range of the DC power supply circuit 2 during fine adjustment of the supply voltage Vdc, the heating frequency is slightly changed and coarse adjustment is performed again to make fine adjustment with the supply voltage Vdc. Make adjustments. By repeating such rough adjustment and fine adjustment, the input power Pin is finally matched with the input power target value Ps.

  How to proceed with the rough adjustment and fine adjustment will be described with reference to the example shown in FIG. If the input power target value Ps is 1100 W, the operating point on the standard operating curve (1) in FIG. 4 is point A, and the heating frequency f is 87.4 kHz. Initially, the target value Vdcs of the supply voltage Vdc to the inverter circuit 3 is set to a standard value of 360 V, and the heating frequency f is set to 87.4 kHz.

 However, if the actual operating curve when the supply voltage Vdc is 360 V is the curve (2) in FIG. 4 due to the reasons described above for the first and second reasons, the operating point is point B. Point B is the operating point for coarse adjustment. Since the input power Pin at point B is about 950 W, which is smaller than the input power target value Ps of 1100 W, it is increased to 1100 W by fine adjustment.

  Therefore, the target value Vdcs is increased without changing the heating frequency f, and the supply voltage Vdc to the inverter circuit 3 is increased. When the supply voltage Vdc is increased, the operating curve (2) moves upward toward the curve (3), and the operating point B approaches the A point. If the operating point B coincides with the A point before the supply voltage Vdc reaches the maximum allowable output voltage (for example, 400 V) of the DC power supply circuit 2, the input power Pin coincides with the input power target value Ps, and the purpose is It will be reached.

  If the value of the supply voltage Vdc reaches the allowable maximum output voltage of the DC power supply circuit 2 before the point B coincides with the point A, the supply voltage Vdc cannot be increased further. In that case, the coarse adjustment is performed again. In this example, the heating frequency f is decreased from 87.4 kHz by an adjustable Δf (for example, 0.1 kHz) to 87.4 kHz. At the same time, the supply voltage Vdc is returned to the initial standard value 360V.

  When the heating frequency f is decreased, the input power Pin is increased, and the operating point moves to point C on the operating curve (2). Assume that the input power Pin at point C is 1150 W, which is slightly higher than the input power target value Ps (1100 W). Therefore, this time, as a fine adjustment, the supply voltage Vdc is decreased. When the supply voltage Vdc is decreased, the operation curve (2) moves downward toward the curve (4), and the operation point C approaches the point D where the input power Pin becomes equal to the input power target value Ps (1100 W). If the operating point C coincides with the D point before the value of the supply voltage Vdc reaches the allowable minimum output voltage (for example, 320 V) of the DC power supply circuit 2, the input electric power Pin coincides with the input electric power target value Ps. It will be reached.

  If the fine adjustment cannot be performed within the allowable output voltage range of the DC power supply circuit 2, the coarse adjustment is performed again and the fine adjustment is performed. If such rough adjustment and fine adjustment are repeated, the input power Pin can finally be matched with the input power target value Ps within the allowable output voltage range of the DC power supply circuit 2. Since the value of the supply voltage Vdc can be varied almost continuously unlike the heating frequency f, the input power Pin can be matched with the input power target value Ps with high accuracy.

Steps S9 to S17 are a control flow for repeatedly executing coarse adjustment and fine adjustment based on the above-described idea. In step S9, the input power Pin is read from the input power detection circuit 6b. In the next step S10, a power deviation ΔP between the input power target value Ps and the read input power Pin is calculated. In the subsequent step S11, the target value Vdcs of the supply voltage Vdc supplied to the inverter circuit 3 is calculated by performing a proportional-integral calculation on the power deviation ΔP according to the following equation.
Vdcs = Ai · ∫ΔP · dt + Ap · Δp (2) This is for making the deviation ΔP zero by the feedback control of the proportional integral control. Ai is an integral constant, and Ap is a proportionality constant.

  If a new target value Vdcs is obtained from the equation (2), it is checked in the next step S12 whether the target value Vdcs exceeds the allowable maximum output voltage (for example, 400 V) of the DC power supply circuit 2. If not, the process moves to step S14, and this time, it is checked whether it is less than the allowable minimum output voltage (for example, 320V). If not, the process proceeds to step S17, and the target value Vdcs calculated by equation (2) calculated in step S11 is output to the output voltage control circuit 7b. Then, the process returns to step S9. In the example of FIG. 4, the operation of bringing the operating point closer to the A point is continued.

  If the allowable maximum output voltage is exceeded in step S12, further fine adjustment cannot be performed only with the output voltage Vdc, so coarse adjustment is performed by readjustment of the heating frequency f. In step S13, the value of the heating frequency f is decreased by an adjustable Δf (for example, 0.1 kHz). Then, the process proceeds to step S16. In step S16, the target value Vdcs is returned to 360 V, which is the standard value determined in step S8. At the same time, the integral value in the above equation (2) in step S11 is reset. Then, the process proceeds to step S17, where the new heating frequency f calculated in step S13 is output to the gate signal generation circuit 5a, and the target value Vdcs returned to 360V is output to the output voltage control circuit 7b. Then, the process returns to step S9. In the example of FIG. 4, the operating point is moved to the C point.

  If it is lower than the allowable minimum output voltage in step S14, the value of the heating frequency f is increased by Δf for rough adjustment. Then, the processes of steps S16 and S17 are performed, the process returns to step S9, and fine adjustment is resumed. If the coarse adjustment and the fine adjustment as described above are repeated, the input power Pin finally matches the input power target value Ps with high accuracy.

  In the description so far, the operation curve indicating the relationship between the heating frequency f and the input power Pin is lowered to the right as shown in FIG. 4, that is, the case where the input power Pin is decreased as the heating frequency f is increased has been described. . In general, when the series resonant circuit 4 is excited at a frequency equal to the resonant frequency, the input power Pin is maximized. In the region where the excitation frequency is higher than the resonance frequency, the operation curve goes down to the right, and in the low region, goes up to the right. Therefore, when operating in a region that rises to the right, it is necessary to increase the excitation frequency in order to increase the input power Pin, and to decrease the excitation frequency in order to decrease it. In that case, Δf in step S13 in the control flow of FIG. 3 may be changed from subtraction to addition, and Δf in step S15 may be changed from addition to subtraction. Then, similar results can be obtained by the control flow shown in FIG.

  Next, the control operation of the gate signal generation circuit 5a will be described. The gate signal generation circuit 5a receives an instruction of the target value of the heating frequency f from the load power control circuit 6a, and the first to fourth four switching circuits so that a high-frequency current matching the indicated frequency flows through the heating coil 27. This is a circuit that generates a pulse signal for ON / OFF operation of the elements 20-23.

  FIG. 5 shows a typical timing at which the four switching elements 20 to 23 are turned on and off, and a waveform of the high-frequency current flowing through the heating coil 27 in that case. 5 (2) shows the ON / OFF timing of the first switching element 20, FIG. 5 (3) shows the ON / OFF timing of the third switching element 22, and (1) of FIG. The waveform of the high frequency current which flows through is shown. The second switching element 21 performs an OFF operation when the first switching element 20 performs an ON operation, an ON operation when the first switching element 20 performs an OFF operation, and the reverse ON / OFF operation. Since the waveform is clear, it is omitted. Similarly, the fourth switching element 23 performs an ON / OFF operation opposite to that of the third switching element 22. Since the waveform is clear, it is omitted.

  The first switching element 20 is turned ON / OFF with a 50% duty. The third switching element 22 performs an ON / OFF operation opposite to that of the first switching element 20. At the timing when the first switching element 20 is turned on, the charging voltage of the capacitor 15 passes a current that returns to the capacitor 15 again through the first switching element 20, the series resonance circuit 4, and the fourth switching element 23. Try to. Meanwhile, a voltage substantially equal to the supply voltage Vdc, which is the voltage across the capacitor 15, is applied to both ends of the series resonant circuit 4.

  On the other hand, at the timing when the first switching element 20 is turned off, the charging voltage of the capacitor 15 is such a current that returns to the capacitor 15 again through the third switching element 22, the series resonance circuit 4, and the second switching element 21. Try to flow. Meanwhile, a voltage substantially equal to the supply voltage Vdc that is the voltage across the capacitor 15 is applied to both ends of the series resonant circuit 4 in the opposite direction. That is, the series resonant circuit 4 is applied with a rectangular wave-like alternating voltage whose direction changes every half cycle as shown in FIG.

  The voltage applied to the series resonance circuit 4 is a rectangular wave-like alternating voltage, but the series resonance circuit 4 is a series circuit of a coil and a capacitor and prevents a sudden current change. A substantially sinusoidal current flows as shown in FIG. The value of the current flowing through the series resonance circuit 4 (hereinafter referred to as resonance current) becomes the largest when the switching frequency of the ON / OFF operation matches the resonance frequency of the series resonance circuit 4.

  When the excitation is performed at a frequency higher than the resonance frequency as in the example shown in FIG. 4, the impedance of the series resonance circuit 4 becomes inductive, and the phase of the resonance current is larger than the phase of the voltage applied to both ends of the series resonance circuit 4. Be late. Considering immediately after the first switching element 20 is turned on in FIG. 5, at this time, the second switching element 21 is immediately after the OFF operation. At this moment, the resonance current flows from the interconnection point N2 toward the interconnection point N1 due to the phase delay of the resonance current. The current flows into the capacitor 15 through the antiparallel diode 20a connected to the first switching element 20 because the second switching element 21 is OFF.

  Similarly, at this moment, the current flowing out from the negative side of the capacitor 15 flows into the series resonance circuit 4 through the antiparallel diode 23a connected to the fourth switching element 24. That is, while the delay current immediately after the first switching element 20 is turned on, the capacitor 15 is not discharged but charged by the resonance current. The same applies to the period during which the delayed current flows immediately after the first switching element 20 is turned off. In this case, the delayed current charges the capacitor 15 through the antiparallel diodes 21a and 22a.

  In the description so far, the first and second switching elements 20 and 21 and the third and fourth switching elements 22 and 23 perform the opposite ON / OFF operations. However, in practice, a dead time is provided in which both of them are turned off at the moment when the ON / OFF operation is switched. In the meantime, the resonance current continues to flow. During the dead time, current flows through antiparallel diodes connected to each switching element.

  The snubber capacitors C24 and C25 serve to reduce the current flowing through the antiparallel diode and reduce the switching loss of the inverter circuit 3. Its capacity is a small value. If it is too large, the switching loss increases conversely.

  When the four switching elements 20 to 23 are turned on / off at the timing shown in FIG. 5 by such an operation, a resonance current flows through the heating coil 27 and the object to be heated 31 is inductively heated. However, in the operation at the timing shown in FIG. 5, the switching frequency needs to be the same as the frequency of the resonance current. Therefore, for example, when heating an aluminum material that requires heating at a high frequency, the switching frequency must be as high as about 90 kHz.

  Increasing the switching frequency increases switching loss. Further, as described above, in order to generate an accurate ON / OFF timing signal in a high frequency region, an expensive microcomputer (or DSP) having a high calculation speed is required.

  For these reasons, in the induction heating cooker 1 of this embodiment, when heating an object to be heated that can be heated with sufficiently high efficiency even at a low frequency of about 20 kHz, such as an iron-based material, heating is performed at the timing shown in FIG. Do. On the other hand, in the case of an object to be heated that requires heating at a high frequency, such as aluminum, an inverter drive that can reduce the switching frequency to a frequency lower than the resonance current frequency as described below. Adopt the method.

  FIG. 6 shows one of the inverter drive systems, and shows the timing waveform of the system in which the switching frequency is half the resonant current frequency. (2) and (3) in the figure indicate the timing of the ON / OFF operation of the first and third switching elements 20 and 22. Since the second and fourth switching elements 21 and 23 always perform the opposite operations to the ON / OFF operations of the first and third switching elements 20 and 22, respectively, their waveforms are omitted.

  As shown in (2) in the figure, the first switching element 20 is in the ON state during the 3/4 period of the switching period, and is in the OFF state during the remaining 1/4 period. As shown by (3) in the figure, the third switching element 22 is in an ON state for a quarter period and in an OFF state for the remaining 3/4 period. The timing at which the third switching element 22 is turned on is delayed by a quarter period from the timing at which the first switching element 20 is turned on.

  When the switching operation is performed at such timing, a voltage having a waveform indicated by (4) in the figure (hereinafter referred to as an excitation voltage) is applied to both ends of the series resonance circuit 4. That is, the excitation voltage is applied twice every 1/4 period during one switching period. The value of the excitation voltage is almost equal to the charging voltage Vdc of the capacitor 15. The direction of the voltage is a direction in which a current flows from the interconnection point N1 toward the interconnection point N2. This direction is defined as a positive direction (resonant current also flows in this direction as a positive direction). In this case, the non-excitation period between the two excitation periods is a quarter cycle.

  In such a state that the excitation voltage is periodically applied, it is assumed that the resonance frequency cycle of the series resonance circuit 4 is ½ of the switching frequency cycle. Then, the series resonant circuit 4 receives the excitation voltage for ½ period during one period of the resonance frequency. The direction of the excitation voltage is always the same positive direction, and there is a non-excitation period of ½ period in which the excitation voltage is zero between excitation and excitation.

  In the series resonance circuit 4, a resonance current having a frequency twice as high as the switching frequency as shown in FIG. When 1/2 of the switching frequency is higher than the resonance frequency, the impedance of the series resonance circuit 4 becomes inductive, and the phase of the resonance current lags behind the resonance voltage. Even in this case, the ratio between the frequency of the resonance current and the switching frequency is 2: 1.

  If the switching operation is performed at the timings (2) and (3) in FIG. 6 in this way, the series resonance circuit 4 can resonate at a switching frequency that is ½ of the resonance frequency. Therefore, for example, when it is desired to heat an object to be heated made of aluminum at 90 kHz, the switching frequency can be halved to 45 kHz.

  FIG. 7 shows the timing waveform of the driving method in which the switching frequency is ¼ times the frequency of the resonance current. The first and third switching elements 20 and 22 are switched at the timings (2) and (3) in the drawing. Since the second and fourth switching elements 21 and 23 always perform the opposite operation to the ON / OFF operation of the first and second switching elements 20 and 22, respectively, their waveforms are omitted.

  As shown in (2) in the figure, the first switching element 20 is in the ON state during the 5/8 cycle and the OFF state during the remaining 3/8 cycle. As shown by (3) in the figure, the third switching element 22 is in an ON state for 3/8 cycles and in an OFF state for the remaining 5/8 cycles. The timing at which the third switching element 22 is turned on is delayed by 1/8 cycle from the timing at which the first switching element 20 is turned on.

  When the switching operation is performed at such timing, the excitation voltage applied to both ends of the series resonance circuit 4 has a waveform indicated by (4) in the figure. As in the case of (4) in FIG. 6 described above, two positive excitation voltages are applied every 1/8 period during one switching period. The non-excitation period between the two excitation periods is 3/8 period, which is an odd multiple of 1/8 period.

  If the period of the resonance frequency of the series resonance circuit 4 is ¼ of the period of the switching frequency, １ ／ period of the switching frequency corresponds to ½ period of the resonance frequency. Then, for the same reason as described in the case of FIG. 6, a resonance current having a frequency four times the switching frequency as shown in (1) of FIG. When 1/4 of the switching frequency is higher than the resonance frequency, the resonance current becomes a lagging current.

  Thus, if the switching operation is performed at the timings (2) and (3) in FIG. 7, the series resonant circuit 4 can be resonated at a switching frequency that is 1/4 of the resonant frequency. Therefore, for example, when it is desired to heat an object to be heated made of aluminum at 90 kHz, the switching frequency can be reduced to 1/4 of 22.5 kHz.

  Next, how to generate timing pulses as shown in FIGS. 5 to 7 will be described. The timing pulse is generated by the gate signal generation circuit 5a. The target value of the frequency of the high-frequency current flowing through the heating coil 27 is given from the load power control circuit 6a. The timing at which the pulse is generated in FIGS. 5 to 7 is determined according to the material of the object to be heated.

  In the case of an iron-based material, the resonance frequency in a state where the switch 30 is turned on is adjusted to about 20 kHz, for example, and a high frequency current of 25 to 35 kHz is passed. Since the frequency is low, in this case, heating is performed with the timing pulse of FIG. In the case of an aluminum-based material, the resonance frequency in a state where the switch 30 is turned off is adjusted to, for example, about 85 kHz, and a high frequency current of 88 to 92 kHz is passed. In this case, since the frequency is high, heating is performed with the timing pulses of (2) and (3) in FIG. The switching frequency is 1/4 to 22 to 23 kHz, which is close to the switching frequency when heating the iron-based material.

  Such a timing pulse can be generated by an analog circuit, but in order to reduce the frequency fluctuation with respect to the target frequency, a method of generating by software calculation based on the clock pulse is suitable.

  The logic of timing pulse generation by software calculation is relatively simple. For example, in the case of generating the timing pulses of (2) and (3) in FIG. 7, an addition-type repetition counter that counts from 0 to N and then returns to 0 is provided by software. The value of N is determined so that the time required to count N reference clock pulses is equal to the target one switching cycle time. One switching cycle time is four times as long as one cycle time of the target high-frequency voltage. Then, when the count value of the counter is between 0 and (5/8) · N, a pulse for turning on the first switching element 20 is counted, and the count value is (1/8) · N to (4 / 8) A pulse for turning ON the third switching element 22 is generated between N. In this way, the timing pulses shown in (2) and (3) of FIG. 7 can be generated. Other timing pulses can be generated in the same way.

  As is clear from the above description, in the induction heating cooker of this embodiment, the input power is first roughly adjusted by the switching frequency of the inverter circuit, and then the DC voltage supplied to the inverter circuit with the switching frequency fixed. Is finely adjusted by feedback control to match the input power with the target value. A high-frequency high-frequency current must be used to heat the aluminum object to be heated, but fine adjustment of the input power by adjusting the switching frequency in such a high-frequency region is difficult. In this embodiment, since the fine adjustment part is performed by adjusting the DC voltage, which can be finely adjusted, the input power can be matched with the target value with high accuracy.

  Further, since the fine adjustment by the DC voltage is performed as described above, it is not necessary to adjust the frequency of the high-frequency current at such a fine frequency interval. Therefore, it is not necessary to adjust the switching frequency of the inverter circuit at such a fine frequency interval. Therefore, there is an advantage that the timing pulse for driving the inverter circuit can be generated without using an expensive microcomputer or DSP having a high calculation speed.

It is a block diagram of one circuit composition of induction heating cooking appliance 1 concerning the present invention. It is a structural example of the output voltage control circuit 7b. It is a control calculation flow of the load power control circuit 6a. It is a figure explaining the adjustment principle of the input power in this invention. It is a wave form diagram in case the inverter circuit 3 is driven with the switching frequency equal to the resonant frequency of the direct current resonant circuit 4. FIG. FIG. 6 is a waveform diagram when the inverter circuit 3 is driven at a switching frequency that is ½ of the resonance frequency of the DC resonance circuit 4. FIG. 6 is a waveform diagram when the inverter circuit 3 is driven at a switching frequency that is ¼ of the resonance frequency of the DC resonance circuit 4. It is an example of the relationship between the frequency of the heating current by the material of to-be-heated material, and input electric power.

Explanation of symbols

  In the drawings, 1 is an induction heating cooker, 2 is a DC power supply circuit, 3 is an inverter circuit, 4 is a series resonance circuit, 5 is a gate control circuit, 6 is a power control circuit, 7 is a step-up chopper DC / DC converter, 7a Is a drive circuit, 7b is an output voltage control circuit, 11 is a rectifier circuit, 12 is a reactor, 13 is a switching element, 14 is a diode, 15 is an output smoothing capacitor, 20 is a first switching element, and 21 is a second switching element. , 22 is a third switching element, 23 is a fourth switching element, 20a, 21a, 22a and 23a are antiparallel diodes, 24 and 25 are snubber capacitors, 27 is a heating coil, 27 and 28 are capacitors, and 30 is switching. , 31 indicates an object to be heated.

Claims (9)

A series resonance circuit including a heating coil and a capacitor, an inverter circuit that supplies a high-frequency current to the series resonance circuit, and a DC power supply circuit that supplies a DC voltage to the inverter circuit, the high-frequency current flowing through the series resonance circuit In an induction heating cooker that induction-heats an object to be heated,
First, the frequency of the high frequency current supplied from the inverter circuit is varied to roughly adjust the input power of the DC power supply circuit, and then the DC voltage supplied to the inverter circuit is finely adjusted to set the input power to the target power value. An induction heating cooker characterized in that it matches with the above. A series resonant circuit (4) of a heating coil (27) and capacitors (28, 29);
An inverter circuit (3) for supplying a high-frequency current to the series resonant circuit;
A DC power supply circuit (2) for supplying a DC voltage to the inverter circuit;
A gate control circuit (5) for adjusting the ON / OFF timing of the switching element of the inverter circuit to vary the frequency of the high-frequency current;
A power control means (6) for providing a target value of the frequency of the high-frequency current in the gate control circuit and a target value of the DC voltage in the DC power supply circuit;
The power control means gate-controls a predetermined high-frequency current frequency at which the input power of the DC power supply circuit substantially coincides with the indicated target power value when the DC voltage supplied to the inverter circuit is a standard value. The circuit is configured to adjust the target value of the DC voltage applied to the DC power supply circuit so that the input power of the DC power supply circuit matches the input power target value in that state. Induction heating cooker featuring. 3. The induction heating cooker according to claim 2, wherein the power control means includes a frequency of the high-frequency current in a state where a DC voltage supplied to the inverter circuit is equal to a predetermined standard value, and the DC power supply circuit. Maintain a relationship table with input power,
When the target power value is instructed, the gate is obtained by reading from the relation table a frequency at which the input power of the DC power supply circuit substantially matches the target power value and higher than the resonance frequency of the series resonance circuit. Give the control circuit as a frequency target value, and in that state, control the target value of the DC voltage applied to the DC power supply circuit so that the input power of the DC power supply circuit matches the input power target value,
When the DC voltage of the DC power supply circuit becomes equal to or higher than a predetermined value during the control of the target value of the DC voltage, the frequency target value is decreased by a predetermined value, and conversely, the DC voltage of the DC power supply circuit is When the frequency becomes less than a predetermined value, the frequency target value is increased by a predetermined value, and in this state, the input power of the DC power supply circuit is again applied to the DC power supply circuit so as to match the input power target value. An induction heating cooker configured to repeatedly perform control for adjusting a target value of a DC voltage. 3. The induction heating cooker according to claim 2, wherein the power control means includes a frequency of the high-frequency current in a state where a DC voltage supplied to the inverter circuit is equal to a predetermined standard value, and the DC power supply circuit. Maintain a relationship table with input power,
When the target power value is instructed, the gate is obtained by reading from the relation table a frequency at which the input power of the DC power supply circuit is substantially equal to the target power value and lower than the resonance frequency of the series resonance circuit. Give the control circuit as a frequency target value, and in that state, control the target value of the DC voltage applied to the DC power supply circuit so that the input power of the DC power supply circuit matches the input power target value,
When the DC voltage of the DC power supply circuit becomes equal to or higher than a predetermined value during the control of the target value of the DC voltage, the frequency target value is increased by a predetermined value, and conversely, the DC voltage of the DC power supply circuit is increased. When the frequency becomes lower than a predetermined value, the frequency target value is decreased by a predetermined value, and in this state, the input power of the DC power supply circuit is again applied to the DC power supply circuit so as to match the input power target value. An induction heating cooker configured to repeatedly perform control for adjusting a target value of a DC voltage. 5. The induction heating cooker according to claim 2, wherein the series resonance circuit is configured such that a resonance frequency thereof is switched by switching a capacity of the capacitor (28, 29),
The power control means includes a material detection means for detecting a material type of an object to be heated to be heated by the heating coil, and when the frequency target value of the high-frequency current is determined, the to-be-heated object detected by the material detection means The capacitor is switched to a predetermined capacity according to the type of material of the object, and the frequency target value is determined in consideration of the switching and the type of material of the object to be heated. Induction heating cooker. It is an induction heating cooking appliance in any one of Claims 1 thru | or 5, Comprising: The said inverter circuit is the same as the series connection circuit of the 1st, 2nd switching element (20, 21) which connected the anti-parallel diode. It is configured as a full bridge circuit composed of a series connection circuit of third and fourth switching elements (22, 23) connected with antiparallel diodes,
The gate control circuit operates alternately so that when one of the first and second switching elements is ON, the other is OFF, and one of the third and fourth switching elements is ON. In this case, the other is operated alternately so that the other is in the OFF state,
The first switching element is operated so that ¾ of the switching cycle is ON, the third switching element is operated so that ¼ is ON, and the third switching element is switched to the ON state. An induction heating cooker characterized in that switching is controlled at a timing delayed by 1/4 of the switching period from switching of the first switching element to the ON state.   The induction heating cooker according to claim 6, wherein the first switching element is operated so that 5/8 of a switching cycle is ON, and the third switching element is ON of 3/8. In addition, the induction heating cooking is characterized in that the switching of the third switching element to the ON state is controlled at a timing delayed by 1/8 of the switching period from the switching of the first switching element to the ON state. vessel.   The induction heating cooker according to any one of claims 2 to 7, wherein the DC power supply circuit includes a rectifier circuit (11) for rectifying an AC voltage supplied from an external power supply, and a DC output from the rectifier circuit. A step-up chopper type DC / DC converter (7) for converting a voltage and outputting a stepped-up DC voltage of a value instructed from the power control circuit. vessel. The induction heating cooker according to claim 8, wherein the step-up chopper type DC / DC converter includes a reactor (12), a diode (14), an output smoothing capacitor (15) connected between both ends of the output of the rectifier circuit. ), A switching element (13) that opens and closes both ends of the series-connected diode and the output smoothing capacitor, a drive circuit (7a) that supplies an opening / closing drive signal to the switching element, and the output smoothing An output voltage control circuit (7b) that provides an opening / closing timing signal to the drive circuit so that the voltage across the capacitor matches the value instructed by the power control circuit, and an input section of the drive circuit An input signal isolation circuit is provided on the switch so that the opening / closing timing signal is transmitted as an electrically isolated signal. Induction heating cooker.

Images