JP2017200294A - Power supply device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve operation efficiency when the load on a power supply is light.SOLUTION: A switching power supply device comprises feedback means that feeds back a signal according to an output voltage output from a power supply to switching control means; in a light load state where the output voltage is reduced, the switching control means controls to maintain a turn-on state of switching means for a predetermined time on the basis of a detection value detected by current detection means and the signal from the feedback means.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a DCDC converter as a power supply device that supplies power to the device.

  A half-bridge current resonance power supply is known as an example of a switching power supply apparatus. This current resonance power supply is known to have high operating efficiency in a normal load (when the load current is relatively large) in which the apparatus is operating, and to operate with low noise. For this reason, the current resonance power supply is adopted as a power supply apparatus for many apparatuses. However, it is known that the efficiency of the current resonance power supply at light load is lowered. Therefore, in order to save power in the standby state where the device is not in operation, conventionally, a current resonance power source and a flyback power source are provided separately, and in the case of a normal load, two power sources (two converter configuration) are used. In the case of a light load, a method of stopping the current resonance power supply is adopted.

JP 2013-143877 A

  However, in the configuration of two converters, two electromagnetic transformers (coils) are required, so that the scale of the power supply circuit is increased. Therefore, it is desired to reduce the circuit scale of the power supply apparatus by improving the operation efficiency of the current resonance power supply at a light load to configure the power supply apparatus with one current resonance power supply.

  A power supply apparatus of the present invention for solving the above-described problems includes a rectifying / smoothing means for rectifying and smoothing an AC voltage from a commercial AC power supply, a first resonance circuit connected to the rectifying / smoothing means, and the first A second resonance circuit connected to the resonance circuit; a switching means for switching a current to the first resonance circuit and the second resonance circuit; a switching control means for controlling the switching means; and the first Current detection means for detecting a current flowing in the resonance circuit of the first or second resonance circuit, and feedback means for feeding back a signal corresponding to the output voltage output from the power supply device to the switching control means, and the switching control The means is based on the detection value of the current detection means and the signal from the feedback means at a light load when the output voltage is reduced. , And controlling the on-state of the switching means so as to maintain a predetermined time.

According to another aspect of the present invention, there is provided an image forming apparatus having a fixing device that fixes an image formed on a recording material to the recording material. The fixing device includes an exciting coil and drives the fixing device. A power supply device for rectifying and smoothing an AC voltage from a commercial AC power source, the excitation coil connected to the rectification smoothing device, and the excitation coil connected to the excitation coil. A second resonance circuit; switching means for switching current to the excitation coil and the second resonance circuit; switching control means for controlling the switching means; and current flowing through the excitation coil or the second resonance circuit. Current detection means for detecting
Feedback means for feeding back a signal corresponding to the output voltage output from the power supply device to the switching control means, wherein the switching control means detects the current detection means at a light load when the output voltage is reduced. Based on the value and the signal from the feedback means, the switching means is controlled to be kept on for a predetermined time.

  As described above, according to the present invention, the operation efficiency at the time of light load of the power source can be improved with a simple configuration.

The figure which shows the power supply circuit of Example 1. The figure which shows the operation | movement waveform of the power supply circuit of Example 1. The figure which shows Example 2 a power supply circuit The figure which shows the operation | movement waveform of the power supply circuit of Example 2. The figure showing the fixing device of Example 3. FIG. 10 is a diagram illustrating a temperature distribution of the fixing device according to the third exemplary embodiment. The figure which shows the power supply circuit of Example 3 The figure which shows the operation | movement waveform of the power supply circuit of Example 3. The figure which shows the temperature distribution of the fixing measure of Example 3.

[Example 1]
FIG. 1 shows a circuit of a current resonance power supply according to the first embodiment. In the present embodiment, a half-bridge type current resonance circuit will be described as an example. The power supply circuit of FIG. 1 is characterized in that at the time of a light load, driving of the switching elements 104 and 105 described later is not stopped, but one of the switching elements 104 and 105 is always turned on except for the dead time period. is there. The configuration of the switching means is not a half bridge type as in the present embodiment, but may be, for example, one switching element. That is, the same method can be applied to any power supply device that performs soft switching using resonance.

  Hereinafter, it demonstrates in detail based on FIG. In FIG. 1, reference numeral 160 denotes a power supply device, which converts an AC voltage input from the commercial AC power supply 100 into a DC voltage and supplies the DC voltage to the load 121. Reference numeral 101 denotes noise filter means. The circuit 150 is an example of a rectifying / smoothing means. The AC voltage is full-wave rectified by the diode bridge 102 and the capacitor 103. The circuit 151 is an example of feedback means, and the voltage supplied to the load 121 is stabilized by transmitting the output voltage of the power supply device 160 to the circuit 152 which is an example of switching control means. The circuit 151 as feedback means is a circuit that divides the voltage of the capacitor 114 by the resistors 119 and 120 and compares the divided voltage with the reference voltage in the shunt regulator 117 and adjusts the current flowing through the resistor 115 and the LED 116 of the photocoupler. When the current of the photocoupler LED 116 changes, the current of the photocoupler phototransistor 118 also changes, and the change in the output voltage is transmitted to the circuit 152 as a signal (voltage). A capacitor 108 is shown as an example of the second resonance circuit 155. Reference numerals 106, 114, 123, 125, and 129 denote capacitors and half-bridge circuits as an example of the switching means 157. Reference numeral 104 denotes a switching element provided on the high side, and reference numeral 105 denotes a switching element provided on the low side. As an example of the first resonance circuit 154, a primary winding 109 wound around a transformer 107 is shown. 110 and 111 are secondary windings wound around the transformer 107, 112, 113, 122, and 124 are diodes, and 115, 127, 128, and 130 are resistors.

  Resonant current flows through the switching element 104, the switching element 105, the capacitor 108, and the primary winding 109 in accordance with the switching operation of the switching element 104 and the switching element 105 (operation for alternately turning on and off). In this embodiment, as an example of the current detection unit 156, a circuit in which the current of the capacitor 108 is divided by the capacitor 129 and detected as the voltage across the resistor 128 is shown. Instead of detecting the current as in this circuit, for example, a current transformer (coil) may be used, or a circuit that detects the voltage at both ends by connecting a low resistance in series with the capacitor 108 may be employed. When the load 121 in this embodiment is driven by a normal load (also referred to as a rated load), the on-duty of the switching element 104 and the switching element 105 is fixed to 50%, and the output voltage is controlled by changing the frequency. It is carried out.

A switching control IC 126 is a main part of the switching control means 152. Hereinafter, it is abbreviated as IC126. 1 to 10 of the IC 126 are terminal numbers. The function of each terminal of the IC 126 is shown below.
Terminal 1: A high voltage input terminal that is a starting terminal for starting the IC 126 in response to the voltage from the AC input line or the voltage of the capacitor 103. In this embodiment, the capacitor 103 is connected.
Terminal 2: The power supply terminal of the IC 126.
Terminal 3: Current detection terminal (current detection value is input).
• Terminal 4: A terminal for feedback voltage input.
Terminal 5: GND terminal.
Terminal 6: A bootstrap power supply terminal for operating the driver of the switching element on the high side.
Terminal 7: The gate output terminal of the switching element on the high side.
Terminal 8: A terminal connected to an intermediate potential serving as a bootstrap reference.
Terminal 9: The gate output terminal of the switching element on the low side.
Terminal 10: A terminal connected to the source terminal S of the switching element on the low side.

  FIG. 2 shows operation waveforms of the circuit of FIG. 1 in this embodiment. The operation will be described below with reference to FIG. Before describing each operation waveform, the direction of voltage and current is defined. The drain voltage of the switching elements 104 and 105 is positive when the voltage at the drain terminal is higher than the voltage at the source terminal, and the drain current of the switching elements 104 and 105 is positive when the drain current flows from the drain terminal to the source terminal. The gate voltage of the switching elements 104 and 105 is positive in the direction in which the voltage at the gate terminal is higher than the voltage at the source terminal. The direction in which the current of the capacitor 108 flows from the terminal connected to the primary winding 109 of the transformer 107 to the terminal connected to the source of the switching element 105 is positive, and the reverse direction is negative. The voltage of the secondary winding voltages 110 and 111 is positive in the direction in which the voltage is higher when the voltage is connected to the anode terminals of the diodes 112 and 113.

  FIG. 2A shows an example of operation waveforms in the rated load state (including the vicinity of the rated load). When a voltage is applied between the gate and source of the switching element 104 and the switching element 104 is turned on, a current flows through a path such as the capacitor 103, the switching element 104, the primary winding 109 of the transformer 107, and the capacitor 108. The secondary winding 110 of the transformer 107 is wound in the same direction as the primary winding 109. When the voltage of the secondary winding 110 becomes higher than the forward voltage of the diode 112, a current flows from the secondary winding 110 of the transformer 107 to charge the capacitor 114. Hereinafter, the operation will be described for the periods 1 to 7 in FIG.

<Period 1>
When the switching element 104 is kept on, the drain current of the switching element 104 becomes a sinusoidal current that flows in the series resonance circuit of the leakage inductance of the transformer 107 and the capacitor 108 as shown by waveform 4. The voltage between the gate and source of the switching element 104 is lowered and turned off in a time shorter than the leakage period of the transformer 107 and the resonance period of the series resonance circuit of the capacitor 108.

<Period 2>
Even when the switching element 104 is turned off, the current flowing through the primary winding 109 of the transformer 107 is preserved. The energy stored in the transformer 107 causes a current to flow through the path of the primary winding 109 -capacitor 108 -capacitor 106 to charge the capacitor 106, and the drain voltage of the switching element 105 decreases as indicated by waveform 3.

<Period 3>
When the drain voltage of the switching element 105 becomes lower than the source voltage, the body diode of the switching element 105 becomes conductive and current flows in the negative direction (waveform 5).

<Period 4>
Zero voltage switching (also referred to as soft switching) is performed by applying a voltage between the gate and source of the switching element 105 as indicated by waveform 2 during the period in which the body diode of the switching element 105 is flowing. After the switching element 105 is turned on, the drain current of the switching element 105 flows in the negative direction until the energy of the primary winding 109 of the transformer 107 is exhausted.

<Period 5>
When the energy of the primary winding 109 is exhausted, the voltage stored in the capacitor 108 is maximized, and current begins to flow in the negative direction through the capacitor 108 using the energy stored in the capacitor 108 as a power source. The current flowing through the capacitor 108 is shunted by the capacitor 129 and appears as a voltage across the resistor 128. The voltage of resistor 128 is shown in waveform 6. A current flows from the capacitor 108 to a path passing through the primary winding 109 and the switching element 105. The voltage appearing at the secondary winding 110 of the transformer 107 is in the negative direction, and the voltage appearing at the secondary winding 111 is in the positive direction. The capacitor 114 is charged from the secondary winding 111 through the diode 113.

<Period 6>
When the switching element 105 is turned off as shown by the waveform 2, the current flowing in the primary winding 109 charges the capacitor 106 by the energy of the primary winding 109 and the remaining energy of the capacitor 108, and the drain voltage of the switching element 105 increases. (Waveform 3).

<Period 7>
When the voltage of the capacitor 106 rises and becomes higher than the voltage of the capacitor 103, current starts to be supplied to the 103 through the body diode of the switching element 104 (waveform 4). By turning on the switching element 104 during this period, the switching element 104 also performs a soft switching operation. At the rated load, such a series of operations is alternately turned on and off with an on-duty of about 50%.

  Next, the operation at light load is shown in FIG. When the load 121 becomes small (the current flowing through the load becomes small), the voltage of the capacitor 114 increases. Then, the voltage of the resistor 120 becomes higher than the reference voltage of the shunt regulator 117, and a current flows between the cathode (K) and the anode (A) of the shunt regulator 117. This current is transmitted to the IC 126 by the LED 116 and the phototransistor 118 of the photocoupler, and the voltage of the feedback terminal 4 of the IC 126 is lowered. The IC 126 increases the frequency when the voltage decreases. As the frequency increases, the output from the current resonant power supply decreases and the voltage of the capacitor 114 decreases. As described above, the IC 126 keeps the output voltage constant by increasing the switching frequency while maintaining the duty of 50%.

  Here, comparing FIG. 2A and FIG. 2B, the period of FIG. 2B is shorter than that of FIG. 2A. Further, the waveforms 4 to 6 in FIG. 2 (b) are not a sinusoidal waveform as in FIG. 2 (a) but a triangular waveform in order to move away from the resonance frequency.

  Next, an operation at a further light load will be described with reference to FIG. If the frequency is increased too much, the operating efficiency decreases, so an upper limit frequency is set in the IC 126 in advance. When the load 121 becomes smaller, the IC 126 performs control so that the output voltage becomes constant by increasing the frequency as described above. However, when the load 121 is further reduced (light load state), the voltage of the capacitor 114 increases even at the upper limit frequency. Then, the photocoupler LED 116 is kept emitting light by the shunt regulator 117, and the phototransistor 118 connected to the feedback terminal 4 continues to pass current. For this reason, the voltage of the feedback terminal 4 does not increase.

  If the voltage at the terminal 4 does not rise even at this upper limit frequency, the IC 126 waits for the voltage at the terminal 4 to rise by extending the ON time of the low-side switching element 105. A waveform 2 shows an operation waveform when the on-time of the low-side switching element 105 is extended. In this state, no energy is supplied to the capacitor 114, so that when the current continues to flow through the load 121, the voltage of the capacitor 114 eventually decreases. Then, the shunt regulator 117 does not flow current between the cathode (K) and the anode (A), and the phototransistor 118 also stops flowing current due to the LED 116 of the photocoupler, so that the feedback terminal voltage increases. On the other hand, the IC 126 detects a current value obtained by dividing the current of the capacitor 108 by the resistor 128 and the capacitor 129 by the terminal 3. The IC 126 detects the enable state while the capacitor C108 is discharged, that is, when the voltage of the resistor 128 becomes equal to or lower than a predetermined threshold value for detecting the enable state as shown by the waveform 6. The IC 126 is in an enabled state, and turns off the gate of the switching element 105 when the voltage of the FB terminal (terminal 4) increases to a predetermined threshold value or more. Then, C106 is charged by the energy stored in the primary winding 109 of the transformer 107 and the residual energy of the capacitor 108, and the drain terminal voltage of the switching element 105 rises. When the source terminal voltage of the switching element 104 becomes higher than the drain terminal voltage of the switching element 104, the body diode of the switching element 104 becomes conductive. The IC 126 turns on the switching element 104 after the dead time. Thus, by turning off the switching element 105 in the enabled state, a current flows in the positive direction to the switching element 105 at the time of turn-off. Thereafter, the drain voltage of the switching element 105 rises, and the switching element 104 is turned on during a period in which a current flows through the body diode of the switching element 104.

  Thus, according to the present embodiment, when the load 121 becomes small (light load) and the on-time of the switching element 104 becomes the minimum on-time, the switching element 104 is turned off after the switching element 104 is turned off. The on-time is extended as shown by waveform 2 in FIG. The on-time of the switching element 105 is terminated when the voltage at the terminal 4 of the IC 126 rises and exceeds the threshold value, and the voltage of the resistor 128 is equal to or lower than the threshold value for detecting the enable state. That is, the ON state of the switching element 105 is maintained for a predetermined time. In this way, the resonance current is intentionally passed through the capacitor 108, the primary winding 109 of the transformer, and the switching element 105, whereby the next turn-off of the switching element 105 and the switching element 104 are turned on. That is, by operating so as to wait for the timing for switching the switching state, it is possible to perform soft switching even with a light load. As a result, the current resonance power supply can efficiently operate from a rated load to a light load. That is, a power supply device can be configured with one current resonance power supply.

[Example 2]
The feature of the second embodiment is that a capacitor capacity switching means 153 for switching the capacity of the capacitor 108 is added to the second resonance circuit 155. In this way, the period of the resonance current can be changed by switching the capacitance of the capacitor of the second resonance circuit 155. As a result, the frequency of turning on the switching element 104 is increased and the output voltage ripple is suppressed.

  The power supply circuit of Example 2 is shown in FIG. The same parts as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted. Note that the capacitor 108 in the first embodiment will be described by distinguishing the reference numeral 201 in this embodiment. 202 is a capacitor, 203 is a switching element. The terminal 11 of the IC 126 is a terminal for driving the switching element 203. Reference numeral 153 denotes capacitor capacity switching means, which includes a capacitor 201, a capacitor 202, and a switching element 203 as a plurality of capacitors. Then, the combined capacity of the capacitor 201 and the capacitor 202 is switched by turning on and off the switching element 203 to change the capacitor capacity of the second resonance circuit 155 and select (switch) the resonance frequency.

  For example, when the capacity of the capacitor 201 is ¼ of the capacity of the capacitor 108 of the first embodiment and the capacity of the capacitor 202 is ¾ of the capacity of the 201 capacitor 108, the combined capacity is the capacity of the capacitor 108 when the switching element 203 is turned on. Is almost the same. When the switching element 203 is turned off, the combined capacity becomes 1/4 of the capacitor 108. The switching element 203 is in an ON state until the load 121 reaches a light load at which the IC 126 reaches the upper limit frequency from the rated load. Accordingly, the explanation of the operation at the rated load and at the light load is the same as that of the first embodiment, and the explanation is omitted.

  FIG. 4 shows the waveform of each part at a further light load. The resonance frequency is determined by the combined capacitance of the capacitors 201 and 202 and the inductance of the primary winding 109. Therefore, the resonance frequency when operating at the rated load is the same as when the capacitor 108 of the first embodiment is connected. When the load is light, the IC 126 extends the on-time of the switching element 105 and turns off the switching element 203 if the voltage at the terminal 4 does not rise even when the upper limit frequency is reached. During the period when the switching element 105 is on, the inductance of the primary winding 109 of the transformer 107 and the capacitor 201 resonate. Since the switching element 203 is off, the combined capacitance that determines the resonance frequency is ¼, so the resonance period is ½. As a result, the enable timing is ½ that of the first embodiment. The solid line waveform in FIG. 4 is a waveform when the switching element 203 is off, and the dotted line waveform is a waveform when the switching element 203 is on.

  The on-time of the switching element 105 is not a continuous value because it can be turned off only in the enabled state. In other words, even if the output voltage decreases and the feedback terminal voltage of the IC 126 increases, a waiting time occurs in which switching cannot be resumed until the enable state is reached. Since the time that cannot be resumed is as short as several μsec, the ripple of the output voltage slightly increases. When it is desired to further suppress the ripple of the output voltage, switching may be performed so as to reduce the combined capacitance for determining the resonance frequency as in this embodiment. As a result, the maximum delay time that cannot be reacted until the enable appears after the voltage at the feedback terminal 4 rises can be shortened. As a result, it is possible to reduce the fluctuation of the output voltage.

  In this embodiment, the capacitor capacity switching means 153 has been described in the form of connecting only one circuit in series with the switching element 203 and the capacitor 202 in parallel to the capacitor 201. However, for example, one series circuit may be added, and the capacitor may be selectively switched according to the time until the voltage at the feedback terminal 4 of the IC 126 rises to the threshold value. By controlling in this way, the enable period can be controlled more finely, and the ripple of the output voltage can be further reduced.

  As described above, according to this embodiment, the current resonance power supply can efficiently operate from a rated load to a light load. That is, a power supply device can be configured with one current resonance power supply. Furthermore, the ripple of the output voltage can be further reduced.

[Example 3]
In the third embodiment, an example in which the power supply device 160 described in the first and second embodiments is applied to an electromagnetic induction heating type fixing device provided in the image forming apparatus 170 will be described. FIG. 5 shows the converter 516 applied with the primary circuit 171 of the first embodiment (FIG. 1) and the second embodiment (FIG. 3). In this fixing device, the heating width depends on the frequency of the current flowing in the exciting coil, and it is necessary to drive with the switching frequency fixed. In addition to extending the above-mentioned on-time and waiting for the soft switching timing in order to improve the efficiency at light load, the period (frequency) of the resonance current that flows during the on-time extension is also heated. To the required period (frequency). As a result, control is performed so as not to change the frequency of the current flowing in the coil even at light loads. For example, an electrophotographic image forming apparatus can be applied to the image forming apparatus 170 in this embodiment. Description of well-known components not directly related to the present embodiment will be omitted. Also, the fixing device is described only for coil unit A (described later) corresponding to the load of converter 516.

  The converter 516 is obtained by replacing the transformer 107 of the power supply device 160 shown in the first and second embodiments with the exciting coil 503, the magnetic core 502, and the fixing sleeve 501 of the coil unit A. The fixing sleeve is a cylindrical heating member, and generates heat when a current flows through the exciting coil 503 and an electric field is generated. Further, the object of the feedback means shown in the first and second embodiments is replaced with the temperature of the sleeve 501 from the voltage. In the first and second embodiments, the switching elements 104 and 105 are turned on and off at a fixed 50% duty at the rated load, and the output voltage is controlled by the frequency. In contrast, the present embodiment is different in that duty control with a fixed frequency, so-called PWM control, is performed for the reason described later.

  The electromagnetic induction heating type fixing device causes the conductive layer of the rotating body (hereinafter referred to as a fixing sleeve) 501 to generate electromagnetic induction heat by the magnetic flux generated by the exciting coil 503, and the image formed on the recording material by the heat of the fixing sleeve 501. Is fixed to a recording material (hereinafter referred to as paper). In FIG. 5, A is a perspective view of a coil unit provided in the fixing device. In general, a recording material on which an image is formed often uses paper of a size from letter size to A5 size. When fixing a recording material having a small size, the temperature tends to increase because the recording material does not pass through the end of the fixing sleeve 501. Therefore, when fixing a recording material having a small size, it is necessary to take measures such as suppressing heat generation at the end, radiating heat at the end, or waiting until the temperature decreases by decreasing the throughput.

  Next, a mechanism for generating an induced current in the conductive layer of the fixing device will be described in detail. The coil unit A is disposed inside the fixing sleeve 501, has a spiral-shaped portion whose spiral axis is substantially parallel to the generatrix direction of the fixing sleeve 501, and an alternating magnetic field for causing the conductive layer of the fixing sleeve 501 to generate heat by electromagnetic induction. Excitation coil 503 is formed. Furthermore, the magnetic core 502 for arrange | positioning in a spiral shape part and guide | inducing a magnetic flux is provided. A magnetic core 502 serving as a magnetic core member is disposed through a hollow portion of the fixing sleeve 501 by a fixing means (not shown), and has magnetic poles NP and SP. The magnetic core 502 has an end shape, and the magnetic flux generated by the exciting coil 503 forms an open magnetic path. The material of the magnetic core 502 is preferably a material having a high relative magnetic permeability and a high saturation magnetic flux density. In this embodiment, sintered ferrite having a relative magnetic permeability of about 1800 is used. The coil 503 is formed by spirally winding a single conducting wire around the magnetic core 502 in the hollow portion of the fixing sleeve 501. At that time, the magnetic core 502 is wound so that the gap is closer to the end than the center. The material of the exciting coil 503 is formed by insulating a metal material having a low resistivity such as copper, and may be a litz wire in consideration of eddy current loss at high frequencies. The coil 503 is wound in a direction intersecting the axis X of the magnetic core 502.

  Coil unit A is connected to converter 516 via terminals 503a and 503b, and is supplied with a high-frequency current by converter 516. The temperature detection elements 509, 510, and 511 detect the temperature of the fixing sleeve 501, and the fixing control unit 521 of the engine control unit 530, which is the control unit of the image forming apparatus 170, detects the paper width and edge temperature detection elements 510 and 511. The drive frequency is determined based on the temperature information. Then, the determined drive frequency is transmitted to the frequency setting unit 520. In addition, the fixing control unit 521 calculates the excess or deficiency of power from the temperature of the temperature detection element 509 in the center, and transmits it to the power control unit 519 to control the converter 516.

  In the coil unit A that generates an induced current flowing in the circumferential direction of the fixing sleeve 501 as in the present embodiment, the heat generation distribution in the busbar direction (also referred to as the longitudinal direction) of the fixing sleeve 501 changes when the drive frequency of the converter 516 is changed. I know you will. The longitudinal direction is the same as the axis X. FIG. 6 shows a temperature distribution of the fixing sleeve 501 when the driving frequency of the converter 516 is adjusted in the range of 20 KHz to 50 KHz so that the center of the fixing sleeve 501 in the generatrix direction (longitudinal direction) is maintained at 200 ° C. It can be seen that the amount of heat generated at both ends of the fixing sleeve 501 decreases as the driving frequency is lowered. Therefore, for example, by adjusting the frequency according to the paper size of the recording material, it is possible to obtain a heat generation distribution optimum for the paper size. Further, by providing the temperature control elements 510 and 511 at the ends, finer control is possible.

  FIG. 7 shows an example of the circuit of converter 516. In the first and second embodiments, the feedback unit 151 detects and feeds back the output voltage. This embodiment is different in that the feedback means feeds back the temperature. The rectifying / smoothing means 150 including the diode bridge 102 and the capacitor 103 reduces the capacitance of the capacitor to a pulsating direct current so that the power factor is approximately 1, and the line loss due to the reactive current is suppressed.

  In FIG. 7, 107 corresponds to the coil unit A, the temperature of the fixing sleeve 501 in 107 is detected by the temperature detection element 509, and switching control is performed as temperature control information by the engine control unit 530. . Reference numerals 510 and 511 in FIG. 5 denote temperature detection elements at the end, which are provided for determining the frequency based on the paper size and further changing the frequency based on the temperature at the end when heating is performed.

  In this embodiment, since the drive frequency of the converter 516 is changed according to the heating width with respect to the paper size, if the paper is the same size, the frequency is fixed and the temperature control is controlled by the on-duty by the PWM method. The PWM method is a control method in which the power is changed by changing the duty without driving at 50% duty unlike the rated loads of the first and second embodiments. An example of such a waveform is shown in FIGS. 8 (a) and 8 (b). In FIG. 8A, for example, the cycle is 20 μs (50 kHz) corresponding to the case where the paper is letter size, and it can be seen that PWM control is performed in which the ON times of the switching element 104 and the switching element 105 are different. In the PWM method, the on-duty of the switching element 104 is controlled to control power. In order to fix the frequency, the switching element 105 is turned on for a time obtained by subtracting the dead time after the switching element 104 is turned off.

  FIG. 8B shows a period of 50 μs (20 kHz) corresponding to the A5 size. For example, the engine control unit 530 determines the frequency based on the size of the recording material and instructs the IC 126 on the frequency using a transmission unit (not shown). When the on-time of the switching element 105 exceeds the capacitance determined by the capacitor capacitance switching means 153 and 1/2 of the resonance period of the series resonance circuit formed of the coil 109, resonance loss occurs. For this reason, the resonance period of the series resonance circuit is configured to be at least twice the maximum period to be driven. For example, the maximum period is when the A5 size paper width is supported and 20 kHz (50 μs) is supported. In that case, the connection of the capacitors 701, 702, and 704 is controlled by turning on and off the switching elements 703 and 705 so that the IC 126 has a period of 10 kHz (100 μs) or more.

  In the case of a fixing device, a wide control range is required for the power control range, for example, at startup (about 700 W to 1200 W), continuous printing (about 400 W to 700 W), and idle rotation such as between papers (about 60 W to 400 W). It becomes. For example, when the output power is about 400 W or more, the duty control is performed based on the PWM control. However, when the output power is 400 W or less, the on-duty of the switching element 104 becomes too small and the control becomes difficult. The IC 126 is provided with a lower limit of the ON width in advance, and is configured to extend the ON time of the switching element 105 when the voltage at the terminal 4 does not increase even when the lower limit ON width is reached. At this time, the switching elements 703 and 705 provided in the capacitor switching unit 170 are switched to determine the resonance period during which the switching element 105 is on.

  For example, an example of power control of 400 W or less will be described based on the waveform of FIG. In order to correspond to the letter size paper width, the resonance period of the capacitor 701 and the primary winding 109 when the switching elements 703 and 705 are turned off is set to 20 μs (corresponding to 50 kHz). When the switching elements 703 and 705 are turned off, the resonance current continues to resonate at 20 μs as shown by the solid line in FIG. 8C even if the ON time of the switching element 105 is increased. For this reason, even if the power is reduced and the PWM cycle of the switching elements 104 and 105 is lengthened, the driving can be continued without impairing the frequency characteristics. The timing when the switching element 703 and the switching element 705 are turned off or on is preferably the period when the body diode of each switching element is conductive, that is, the timing when the current flows in the direction in which the capacitor 701 is discharged.

  As described in the first embodiment, the switching element 105 is turned off by detecting a current obtained by dividing the resonance current by the capacitor 129 and the resistor 128 and enabling the timing when the capacitor 701 is discharged. When the feedback terminal 4 is controlled by temperature control and the voltage of the feedback terminal 4 rises and exceeds a threshold value, the switching element 105 is turned off. Further, after the dead time, the switching element 104 is turned on. By controlling as described above, soft switching can be performed without greatly changing the frequency component of the current flowing in the coil even with a light load.

  When the paper width is small (corresponding to A5 size), the switching element 703 is turned on and the resonance period is set to 50 μs (corresponding to 20 kHz). The waveform is as shown by the dotted line in FIG.

  The case of dealing with an intermediate size paper width in the meantime can be dealt with by changing the ratio of the driving time at the two resonance periods of 20 μs and 50 μs. For example, in order to obtain a temperature distribution equivalent to 30 kHz, the time ratio between the resonance periods of 20 μs and 50 μs may be set to 3: 7. In order to obtain a temperature distribution equivalent to 40 kHz, the time ratio between the resonance periods of 20 μs and 50 μs may be set to 7: 3. FIG. 9 shows the result of changing the temperature distribution at the time ratio in this way. By changing the driving cycle according to the time ratio, it is possible to perform heating with a temperature distribution optimum for a desired paper size.

In this embodiment, as the second resonance circuit 155, the capacitor 701 is connected to the capacitor capacity switching means 153 comprising two series circuits of a capacitor and a switching element, and the frequency is switched in three stages. This is because there are the following three driving methods as described in the present embodiment.
1. When the switching element is PWM controlled with the minimum period (rated power, paper size is A5 width).
2. When the control is performed to extend the ON time of the switching element 105 with the letter size paper width.
3. When control is performed to extend the ON time of the switching element 105 with the A5 size paper width.

  Due to the difference in the paper width of the recording material, the series circuit of the capacitors and switching elements constituting the capacitor capacity switching means 153 may be increased or decreased depending on the corresponding paper type. For example, if one series circuit of a capacitor and a switching element is added so as to correspond to 30 μs (33 kHz), it is possible to drive without switching the capacitor switching means when driving at 33 kHz. Further, even when driving with a period between 20 μs and 30 μs, or with a period between 30 μs and 50 μs, the temperature distribution in the paper width direction can be reduced, so that temperature control with less temperature ripple can be performed. Become.

  As described above, according to the present embodiment, the power supply devices according to the first and second embodiments can be applied as a driving power source for the fixing device of the image forming apparatus, and the temperature distribution is optimum for the size in the width direction of the recording material. Control with less temperature ripple can be executed.

150 rectifying / smoothing means 151 feedback means 152 switching control means 154 first resonance circuit 155 second resonance circuit 156 current detection means 157 switching means

Claims (7)

Rectifying and smoothing means for rectifying and smoothing an AC voltage from a commercial AC power supply; a first resonance circuit connected to the rectifying and smoothing means; a second resonance circuit connected to the first resonance circuit; Switching means for switching current to the first resonance circuit and the second resonance circuit, switching control means for controlling the switching means, and current flowing in the first resonance circuit or the second resonance circuit are detected. A current detecting means for
Feedback means for feeding back a signal corresponding to the output voltage output from the power supply device to the switching control means;
The switching control means performs control so as to maintain the ON state of the switching means for a predetermined time based on a detection value of the current detection means and the signal from the feedback means at a light load when the output voltage is reduced. A power supply device characterized by that.   2. The power supply device according to claim 1, wherein the first resonance circuit is a coil on a primary side of the transformer, and the second resonance circuit is a capacitor.   The second resonance circuit includes a plurality of the capacitors, and switches a resonance cycle in a state where the switching unit maintains an ON state by switching capacitances of the plurality of capacitors. 2. The power supply device according to 2.   The switching means includes a first switching element and a second switching element, and maintains the ON state of the second switching element for a predetermined time after the first switching element is switched from ON to OFF. The power supply device according to claim 1, wherein the power supply device is a power supply device. In an image forming apparatus having a fixing device for fixing an image formed on a recording material to the recording material,
The fixing device has an exciting coil,
A power supply device for driving the fixing device;
The power supply device
Rectifying and smoothing means for rectifying and smoothing an AC voltage from a commercial AC power source;
The exciting coil connected to the rectifying and smoothing means;
A second resonant circuit connected to the excitation coil;
Switching means for switching current to the excitation coil and the second resonant circuit;
Switching control means for controlling the switching means;
Current detection means for detecting a current flowing in the excitation coil or the second resonance circuit;
Feedback means for feeding back a signal corresponding to the output voltage output from the power supply device to the switching control means;
The switching control means performs control so as to maintain the ON state of the switching means for a predetermined time based on a detection value of the current detection means and the signal from the feedback means at a light load when the output voltage is reduced. An image forming apparatus. A temperature detecting element for detecting the temperature of the fixing device;
The image forming apparatus according to claim 5, wherein the feedback unit feeds back a signal corresponding to a temperature detected by the temperature detection element to the switching control unit.   The image forming apparatus according to claim 5, wherein the fixing device includes a cylindrical heating member, and the excitation coil is provided inside the heating member.

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