CN1472990A

CN1472990A - Apparatus and method for shortening heating time before starting magnetron

Info

Publication number: CN1472990A
Application number: CNA021278261A
Authority: CN
Inventors: 应建平; 郭兴宽; 曾剑鸿
Original assignee: Delta Electronics Inc
Current assignee: Delta Electronics Inc
Priority date: 2002-08-01
Filing date: 2002-08-01
Publication date: 2004-02-04

Abstract

A high-frequency heating apparatus for shortening a magnetron start-up heating time, comprising: a transformer connected with the DC power supply; the first resonant circuit is connected with the primary side winding of the transformer and comprises at least one switching device and at least one capacitor, and the first resonant circuit is formed by switching of the switching device and the primary side winding of the transformer; the rectifying device is connected with a secondary side coil of the transformer; a noise filter connected with the secondary side winding of the transformer; a resonant capacitor connected to the noise filter; and a magnetron connected to the noise filter, the resonant capacitor and the magnetron to form a second resonant circuit. With the invention, before the magnetron is started, the power supply is operated in a resonance mode, thereby increasing the output of the heating power of the magnetron and shortening the heating time before the magnetron is started, and after the magnetron is started, the operation mode of the power supply shifts the resonance point, thereby reducing the output of the heating power and maintaining the cathode temperature of the magnetron not lower than 2100K.

Description

Apparatus and method for shortening heating time before starting magnetron

(1) Field of the invention

The present invention relates to an apparatus and method for shortening the heating time before starting a magnetron.

(2) Background of the invention

Fig. 1 is a schematic circuit diagram of a conventional magnetron (magnetron). As shown in fig. 1, a magnetron is a vacuum tube for generating microwave, and its normal operation conditions are: when the cathode temperature exceeds 2100K (absolute temperature), a negative high voltage (thousands of volts) is applied between the cathode and the anode. However, different magnetrons have different operating voltages but substantially similar voltage-current characteristics, as shown in FIG. 2. When the voltage between the cathode and the anode reaches a working voltage, the magnetron generates a microwave, the voltage between the cathode and the anode is clamped near the working voltage, and the characteristic of the magnetron is equivalent to a voltage regulator tube.

After the magnetron is started, a microwave with a frequency of 2.45GHz is generated, but other radio frequency waves are generated at the same time, and a noise filter is needed for eliminating the radio frequency waves. As shown in fig. 3, the noise filter includes two inductors L1 and L2 and a capacitor C1 to eliminate other rf waves.

The conventional apparatus for shortening the heating time before starting the magnetron is as follows: the cathode of the magnetron is a negative high voltage end, and in practical application, a winding is directly added on a main transformer to be used for heating the magnetron, so that other high voltage isolation transformers are not needed to be used for providing a heating power supply for the cathode of the magnetron. As shown in fig. 4, n1, n2, and n3 represent the primary winding and the secondary winding of the main transformer, respectively. The capacitor C1 is used to filter out radio frequency ripples, and its value is small and can be ignored in the following analysis. Assume that the voltage of the heating winding n2 shown in fig. 4 is:

u_n2＝Vsinωt (1)

wherein V is a voltage peak on the winding n2, and ω is a voltage angular frequency of the primary side of the main transformer.

The current flowing through the cathode is:

wherein, R is the heating equivalent resistance of the cathode, and L is the sum of the inductors L1 and L2.

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The heating power is as follows:

from the above reasoning, the method for shortening the heating time before starting the magnetron is: (1) increasing the voltage peak value V; or (2) reduce the angular frequency ω of the input power.

However, in the first method, i.e. increasing the input voltage, the terminal voltage of the other secondary winding n3 of the main transformer is increased. At this time, the magnetron is not yet started, resulting in an increase in voltage stress of the rectifying diode on the secondary side. In the second method, for the magnetron power supply, there still exists the problem of the voltage stress of the secondary side rectifying diode increasing. The concrete description is as follows: the transformer applied to a microwave oven power supply is a high voltage transformer, the primary side and the secondary side of the transformer are generally separated from each other by a winding, so that a large leakage inductance is generated, and the magnetron power supply structure shown in fig. 4 can be equivalent to that shown in fig. 5. An inductor L in the figure_kIs the leakage inductance of the high voltage transformer u_n2、u_n3High frequency ac voltages of the secondary windings n2 and n3, respectively.

Before the magnetron is started, the magnetron is equivalent to an open circuit, and the voltage stress u of the rectifying diodes D1 and D2 is expressed by the following equation:

wherein f is_sIs the power supply u_n3Frequency of (f)₀Is a resonant frequency of the leakage inductance and the two capacitors C3 or C4:

if C is equal to C3 or C4, the result is that

A voltage gain M ═ u can be obtained_c3/u_n3And f ═ f_s/f₀Fig. 6 shows a characteristic graph of (a). Unless the frequency of the power supply drops to a very low value, for example 0.2 times the resonant frequency, the voltage across the load capacitor is still very large. The reduction in the power supply frequency may saturate the transformer with an increased excitation current.

As can be seen from the above description, although the heating time is shortened, the voltage stress of the rectifier diode is also increased by the first and second methods.

(3) Summary of the invention

The present invention provides a device and method for shortening the heating time before starting a magnetron, which can shorten the heating time before starting the magnetron without increasing the voltage stress of the secondary side rectifier diode of a transformer.

According to an aspect of the present invention, there is provided an apparatus for shortening a heating time before starting a magnetron, connected to a dc power supply, comprising: a transformer connected to the DC power supply; a first resonant circuit connected to the primary winding of the transformer, including at least one switching device and at least one capacitor, and forming the first resonant circuit with the primary winding of the transformer by switching of the switching device; a rectifier connected to a secondary coil of the transformer; a noise filter connected to the secondary winding of the transformer; a resonant capacitor connected to the noise filter; and a magnetron connected to the noise filter, wherein the noise filter, the resonant capacitor and the magnetron form a second resonant circuit to shorten the heating time of magnetron start-up.

Preferably, the noise filter comprises: a first inductor having one end connected in series with one end of the magnetron; a second inductor, one end of which is connected in series with the other end of the magnetron; and a first capacitor connected in parallel with the first inductor and the second inductor.

Preferably, the resonant capacitor is connected in series with any one of the first inductor and the second inductor.

Preferably, one end of the resonant capacitor is connected to a connection point of the first inductor and the first capacitor.

Preferably, one end of the resonant capacitor is connected to a connection point of the second inductor and the first capacitor.

Preferably, one end of the resonant capacitor is connected in series with the second inductor, and the other end is connected to one end of the first capacitor.

Preferably, one end of the resonant capacitor is connected in series between the first inductor and the first capacitor.

Preferably, the resonant capacitor is connected in series between the first inductor and the magnetron.

Preferably, the resonant capacitor is connected in series between the second inductor and the magnetron.

Preferably, the noise filter further includes a resonant inductor connected in series to the input terminal of the noise filter.

Preferably, the rectifying device is one of the following devices: a full wave voltage doubler rectification device (full wave voltage doubler rectification); half-wave voltage doubler rectification; a full wave rectification device (full wave rectification); full bridge rectification (full bridge rectification).

Preferably, the transformer is a transformer with leakage inductance.

According to another aspect of the present invention, there is provided a method for shortening a heating time for starting a magnetron in a magnetron high-frequency heating apparatus including a noise filter connected to the magnetron, the method comprising the steps of: adding a resonant capacitor to the noise filter; a series resonant circuit is formed by the resonant capacitor, the noise filter and the magnetron, and before the magnetron is started, the device is operated in a resonant mode, so that the heating power output of the magnetron is increased, and the starting heating time is shortened.

For further explanation of the objects, structural features and effects of the present invention, the present invention will be described in detail with reference to the accompanying drawings.

(4) Description of the drawings

FIG. 1 is a schematic circuit diagram of a conventional magnetron (magnetron);

FIG. 2 is a schematic diagram of voltage-current characteristics of a conventional magnetron;

FIG. 3 is a circuit diagram of a noise filter applied to a magnetron heating circuit in the prior art;

FIG. 4 is a schematic diagram of a conventional heating circuit applied to a magnetron;

FIG. 5 is a schematic diagram of the equivalent circuit of FIG. 4;

FIG. 6 is a graph showing the voltage gain versus frequency characteristics of a magnetron;

FIG. 7 is a circuit diagram of a noise filter of a magnetron heating circuit according to a first preferred embodiment of the present invention;

FIG. 8 is a schematic diagram of the equivalent circuit of FIG. 7;

fig. 9 is a graph showing the heating calibration power versus frequency characteristic of the magnetron, in which fig. 9A is a graph showing the heating calibration power P1 with the quality factor Q being 10 versus the calibration frequency, and fig. 9B is a graph showing the heating calibration power P1 with the quality factor Q being 20 versus the calibration frequency;

FIG. 10 is a schematic diagram of a magnetron heating circuit according to a first preferred embodiment of the invention;

FIG. 11 is a partial schematic diagram of the equivalent circuit of FIG. 10;

FIG. 12 is a partial schematic view of a magnetron heating circuit according to a second and third preferred embodiment of the present invention;

FIG. 13 is a partial schematic view of a magnetron heating circuit according to fourth to fifth preferred embodiments of the present invention;

FIG. 14 is a partial schematic view of a magnetron heating circuit according to seventh to eighth preferred embodiments of the present invention; and

FIG. 15 is a partial schematic view of a magnetron heating circuit according to a tenth preferred embodiment of the invention.

(5) Detailed description of the preferred embodiments

In order to solve the problems of the prior art, the invention utilizes the noise absorption inductor of the magnetron and combines the working principle of series resonance to shorten the heating time of the magnetron. As shown in fig. 7.

Since the capacitance of the capacitor C1 is small and can be ignored, a resonant circuit is formed by the resonant capacitor C2 and the two inductors L1 and L2. The equivalent circuit diagram is shown in FIG. 8, wherein L is the sum of two noise absorption inductors L1 and L2, and a resistor R is the equivalent resistance of the magnetron cathode heating.

u_n2＝Vsinωt (7)

Where V is the peak voltage value of a winding n2 and ω is an angular frequency of the primary-side voltage of the transformer.

And the current flowing through the cathode is:

wherein,

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the heating power is as follows:

therefore, as long as it is reduced at the time of starting

Of (2) can

To shorten the heating time before the magnetron is started.

Suppose that

P 1 = \frac{P}{V^{2} / 2 R};

From this, a relationship of a heating calibration power P1, a calibration frequency f' and a Quality factor Q can be obtained, as shown in FIG. 9. Fig. 9A is a characteristic curve of the heating calibration power P1 and the calibration frequency f' with the quality factor Q being 10. Fig. 9B is a graph showing the heating calibration power P1 versus the calibration frequency f' for the Q-factor Q-20. Before starting the magnetron, if the primary side power supply operating frequency is at the resonant frequency f₀₁Nearby, the heating power will be greatly increased, and its maximum value is V²/2R (full resonance state).

Therefore, according to the analysis, the invention not only greatly shortens the heating time, but also the voltage stress of the rectifier diode is basically kept unchanged. To further explain the problem, please refer to fig. 10, which is a schematic diagram of a magnetron heating circuit according to a first preferred embodiment of the present invention. The magnetThe control tube heating circuit is connected with a DC power supply V_dcThe method comprises the following steps: a transformer T, a first resonant circuit 1, a rectifying device 2, a noise filter 3, a resonant capacitor C2 and a magnetron 4. The transformer T is connected with the DC power supply V_dc. The first resonant circuit 1 is connected to the primary winding of the transformer T, and includes at least one switching device and at least one capacitor, and forms a first resonant circuit with the primary winding of the transformer T by switching of the switching device. The rectifying device 2 is connected to a secondary side coil of the transformer T. The noise filter 3 is connected to the secondary winding of the transformer T. The resonant capacitor C2 is connected to the noise filter 3. And the magnetron 4 is connected with the resonant capacitor C2, wherein the noise filter 3, the resonant capacitor C2 and the magnetron 4 form a second resonant circuit to shorten the starting heating time.

In addition, the noise filter 3 includes: a first inductor L1, one end of which is connected in series with one end of the magnetron; a second inductor L2, one end of which is connected in series with the other end of the magnetron 4; and a capacitor C1 connected in parallel with the first inductor L1 and the second inductor L2.

In the circuit shown in fig. 10, since the transformer is a high voltage transformer, the primary winding and the secondary winding are generally separated according to the insulation requirement, so that a large leakage inductance is generated. For convenience of analysis, please refer to fig. 11, which is a partial schematic diagram of the equivalent circuit of fig. 10. u. of_n2、u_n3Is a high frequency AC voltage, an inductor L_kThe leakage inductance is obtained.

Regarding the second part, the relationship between the voltage gain and the frequency before starting is still shown in FIG. 6. The relationship between the heating target power P1, the target frequency f' and the quality factor Q (Qualityfactor) for the first portion is shown in FIG. 9. Before the magnetron is started, the power supply of the magnetron is set to f₀₁Frequency operation, in which the heating power is high, by appropriate selection of f₀₁And f₀May be such that the voltage stress of the rectifier diode does not change. After the magnetron is started, the power supply of the magnetron is recoveredAnd (4) working at a normal frequency, and reducing the heating power to only maintain the cathode not less than 2100K.

Referring to fig. 12, a partial schematic diagram of a magnetron heating circuit according to a second and a third preferred embodiments of the invention is shown.

Please refer to fig. 13, which is a partial schematic diagram of a magnetron heating circuit according to the fourth to fifth preferred embodiments of the present invention. The difference from fig. 12 is that the capacitor C1 is ignored because its capacitance value is small.

Please refer to fig. 14, which is a schematic diagram of a magnetron heating circuit according to seventh to eighth preferred embodiments of the present invention. The biggest difference from the foregoing preferred embodiment is that the resonant inductor L3 is further included in the embodiment.

Referring to FIG. 15, a schematic diagram of a magnetron heating circuit according to a tenth preferred embodiment of the invention is shown. The difference from fig. 14 is that the capacitance C1 is ignored because its capacitance value is small.

In summary, the circuit structures shown in fig. 7, 12 to 15 disclosed in the present invention can be applied to any power circuit to shorten the starting time of the magnetron, and all of them belong to the scope of the present patent. Therefore, the present invention can provide a magnetron high-frequency heating apparatus to shorten the heating time before the start.

Of course, those skilled in the art will recognize that the above-described embodiments are illustrative only, and not intended to be limiting, and that changes and modifications of the above-described embodiments are intended to be within the scope of the appended claims, as they fall within the true spirit and scope of the present invention.

Claims

1. A high-frequency heating apparatus for shortening the starting heating time of a magnetron connected to a DC power supply, comprising:

a transformer connected to the DC power supply;

a first resonant circuit connected to the primary winding of the transformer, including at least one switching device and at least one capacitor, and forming the first resonant circuit with the primary winding of the transformer by switching of the switching device;

a rectifier connected to a secondary side coil of the transformer;

a noise filter connected to the secondary winding of the transformer;

a resonant capacitor connected to the noise filter; and

a magnetron connected to the noise filter, wherein the noise filter, the resonant capacitor and the magnetron form a second resonant circuit to shorten the start-up heating time.

2. The high-frequency heating apparatus according to claim 1, wherein the noise filter comprises:

a first inductor having one end connected in series with one end of the magnetron;

a second inductor, one end of which is connected in series with the other end of the magnetron; and

a first capacitor connected in parallel with the first inductor and the second inductor.

3. The high-frequency heating apparatus according to claim 2, wherein the resonance capacitor is connected in series with one of the first inductance and the second inductance.

4. The high-frequency heating apparatus according to claim 3, wherein one end of the resonance capacitor is connected to a connection point of the first inductor and the first capacitor.

5. The high-frequency heating apparatus according to claim 3, wherein one end of the resonance capacitor is connected to a connection point of the second inductor and the first capacitor.

6. The high-frequency heating apparatus according to claim 3, wherein one end of the resonance capacitor is connected in series to the second inductor, and the other end is connected to one end of the first capacitor.

7. The high-frequency heating apparatus according to claim 3, wherein one end of the resonance capacitor is connected in series between the first inductor and the first capacitor.

8. The high-frequency heating apparatus according to claim 3, wherein the resonance capacitor is connected in series between the first inductance and the magnetron.

9. The high-frequency heating apparatus according to claim 3, wherein the resonance capacitor is connected in series between the second inductance and the magnetron.

10. The high-frequency heating apparatus according to any one of claims 4 to 9, wherein the noise filter further comprises a resonance inductor connected in series to an input terminal of the noise filter.

11. The high-frequency heating apparatus according to claim 1, wherein the rectifying means is one of:

(1) a full-wave voltage-doubler rectifying device;

(2) a half-wave voltage-multiplying rectifying device;

(3) a full-wave rectifying device;

(4) a full bridge rectifier device.

12. The high-frequency heating apparatus according to claim 1, wherein the transformer is a transformer having a leakage inductance.

13. A method for shortening the starting heating time of a magnetron applied to a magnetron high-frequency heating apparatus including a noise filter connected to the magnetron, the method comprising the steps of:

adding a resonant capacitor to the noise filter;

a series resonant circuit is formed by the resonant capacitor, the noise filter and the magnetron, and before the magnetron is started, the device is operated in a resonant mode, so that the heating power output of the magnetron is increased, and the starting heating time is shortened.