JP3977666B2 - Inverter cooker - Google Patents

Inverter cooker Download PDF

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Publication number
JP3977666B2
JP3977666B2 JP2002054419A JP2002054419A JP3977666B2 JP 3977666 B2 JP3977666 B2 JP 3977666B2 JP 2002054419 A JP2002054419 A JP 2002054419A JP 2002054419 A JP2002054419 A JP 2002054419A JP 3977666 B2 JP3977666 B2 JP 3977666B2
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Prior art keywords
input
pulse width
control means
drive
drive pulse
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JP2003257604A (en
Inventor
秀竹 林
等 滝本
照也 田中
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株式会社東芝
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Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inverter cooker provided with a bridge type inverter circuit.
[0002]
[Problems to be solved by the invention]
FIG. 25 shows an electromagnetic cooker that is an inverter cooker. In the figure, a DC power supply circuit 1 generates a DC power supply by smoothing and rectifying a commercial AC power supply 2 of, for example, 100 V, a diode bridge 3 for rectification, a reactor 4 and a capacitor 5 for smoothing. And is configured. The reactor 4 also has a function of suppressing noise.
[0003]
The inverter circuit 6 includes a heating coil 7, a resonance capacitor 8, and switching elements 9 and 10, and free wheel diodes 11 and 12 are connected in reverse parallel to the switching elements 9 and 10. The switching elements 9 and 10 are turned on and off by the drive unit 13. For example, as shown in FIG. 26, by alternately turning on and off with a dead time, a high-frequency current is passed through the heating coil 7, Induction heating.
[0004]
In the above-described electromagnetic cooker, normally, input adjustment is continuously performed between input powers 400 W to 3 kW by changing the drive frequency (on / off frequency) of the switching elements 9 and 10 of the inverter circuit 6. is there. Note that when the input power is less than 400 w, the inverter circuit 6 is operated intermittently, but there is a problem that the heating becomes intermittent, and the input is continuously adjusted from a high input to a low input. There is a request to do.
[0005]
Here, in order to adjust the input power to 400 W or less by continuous adjustment by changing the drive frequency, the drive frequency of the switching elements 9 and 10 becomes too high, which increases the switching loss and enlarges the cooling device. Not practical.
[0006]
Therefore, when the on-duty ratio is uniquely reduced (the drive pulse width is relatively reduced) and the driving frequency of the switching elements 9 and 10 is changed from high input to low input, even if the input power is about 50 W, The drive frequency need not be as high as 100 kHz.
[0007]
However, in this case, on the contrary, in the high input region, when the switching element 10 of the lower arm is turned on, a vertical short circuit occurs during the reverse recovery time of the free wheel diode 11 of the upper arm (while the free wheel diode is reverse conducting). When the upper arm switching element 9 is turned on, a vertical short circuit occurs during the reverse recovery time of the free wheel diode 12 of the lower arm. For this reason, inverter loss increases and noise is generated due to a short-circuit current.
[0008]
FIG. 27 shows a configuration in which a snubber circuit 15 including a snubber capacitor 15a is provided to prevent the noise. In this case as well, in the continuous adjustment of the driving frequency at a constant duty ratio, a short-circuit current is generated as described above. appear. In FIG. 28, the state of switching of the switching elements 9 and 10 and the state of the current flowing through the inverter circuit 6 are divided into modes (a) to (j). FIG. 29 shows the relationship between the on / off states of the switching elements 9 and 10 (base voltages VGE1 and VGE2), the current IL and the current Ic1, and the collector-emitter voltage VCE2 of the switching element 10. . Note that the timings a to j in FIG. 29 coincide with the timings of the modes (a) to (j).
[0009]
When the input (input current IL) becomes large, when switching from mode (i) to mode (j), the charging current flows through the snubber capacitor 15a, and the switching element 9 is turned on in the middle of the voltage drop. There is a possibility that a short-circuit current flows and destroys the switching element 9.
[0010]
Further, when the mode (d) is changed to the mode (e), a charging current flows through the snubber capacitor 15a, and a large short-circuit current flows because the switching element 10 is turned on while the voltage is rising. 10 may be destroyed. There is also a method of changing only the driving pulse width without changing the driving frequency, but in this case, it is not possible to cover from high input to low input.
[0011]
In the case of drive frequency change control, the oscillation frequency of the inverter circuit 6 may exceed the set value due to the characteristics of the circuit, and the resonance circuit in the inverter circuit 6 changes from inductive to capacitive. There was an increase in losses.
[0012]
The present invention has been made in view of the above circumstances, and an object thereof is to provide an inverter cooker that can be continuously varied from a high input to a low input without excessively increasing the driving frequency or causing a short-circuit current to flow. It is in.
[0013]
[Means for Solving the Problems]
  The invention of claim 1 includes a bridge type inverter circuit having a heating coil or a high frequency transformer, a resonance capacitor, and a plurality of switching elements, and generating high frequency power based on switching driving of the switching elements alternately.
  A snubber circuit provided in at least one of the switching elements;
  Drive frequency control means for controlling the drive frequency of the switching element;
  Drive pulse width control means for controlling the drive pulse width of the switching element;
  Input detection means for detecting input power;
With
  The drive frequency control means performs input adjustment between the high input region and the low input region, andThe drive pulse width control means operates only in a low input region,
  In the low input region, as the input power detected by the input detection means is lowered, the drive frequency is sequentially increased while reducing the drive pulse width.However, it has characteristics.
[0014]
  The input power is changed depending on the input setting or is changed depending on the load. In this case, if it is attempted to change the input only by adjusting the drive frequency at a constant duty ratio, the frequency becomes too high or a short-circuit current flows. However, in the invention of claim 1, the dynamic frequency control means performs input adjustment between the high input region and the low input region, andThe drive pulse width control means operates only in the low input region. In the low input region, the drive frequency is sequentially increased while the drive pulse width is reduced as the input power detected by the input detection means decreases.Therefore, the input can be continuously changed from the low region to the high region without causing the frequency to be excessively high and without generating a short-circuit current.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a first embodiment in which the present invention is applied to, for example, a two-mouth cooking heater will be described with reference to FIGS. In FIG. 2, the external appearance of the built-in type cooking heater 20 is shown. In FIG. 2, on the top surface of the top plate 21, pan mounting portions 22a, 22b, and 22c are displayed by printing at three locations. Of these, heating coils 23 and 23 (see FIG. 1) are respectively provided below the left and right pot placing portions 22a and 22b, and a nichrome wire (not shown) is provided below the central pot placing portion 22c. A heater is provided.
[0029]
A roaster 24 is disposed on the left side of the cooking heater 20 and an operation panel 25 is disposed on the right side. The operation panel 25 is provided with an operation unit 26 such as various switches and dials. An input setting device 26a serving as an input setting unit for heating power control of the heating coils 23 and 23 is provided in the operation unit 26. Is provided. In addition, the pans 27 and 27 which are loads are mounted on the pan mounting portions 22a and 22b.
[0030]
Next, the electrical configuration will be described with reference to FIG. In FIG. 1, only the configuration for driving one heating coil 23 is shown, but actually, a circuit for driving two heating coils 23 and 23 and a nichrome wire heater is configured. It is. The DC power supply circuit 28 has a configuration in which the AC input terminal of the full-wave rectifier circuit 29 is connected to the commercial AC power supply 30, and the DC connection terminal is connected between both terminals of the smoothing capacitor 32 via the reactor 31.
[0031]
An inverter circuit 34 is connected between both terminals of the smoothing capacitor 32 via DC buses 33a and 33b. The inverter circuit 34 is configured by connecting IGBTs 35a and 35b, which are switching elements, a resonant capacitor 36, the heating coil 23, and free wheel diodes 37a and 37b as shown in the figure. That is, between the DC buses 33a and 33b, arms including IGBTs 35a and 35b as positive and negative switching elements are connected, and free wheel diodes 37a and 37b are connected in parallel to the IGBTs 35a and 35b, respectively. Has been. One end of the heating coil 23 is connected to the output terminal of the inverter circuit 34, and the other end of the heating coil 23 is connected to the DC bus 33 b via the resonance capacitor 36. The resonance circuit 38 is configured. Each IGBT 35a, 35b of the inverter circuit 34 is supplied with a drive signal from the drive unit 39 to the gate.
[0032]
A microcomputer 40 as a main body for driving and controlling the inverter circuit 34 is configured to include a ROM, a RAM, and the like, and drives and controls the IGBTs 35a and 35b in accordance with input power. 41, a function as drive pulse width control means 42 is provided. The microcomputer 40 also has a function as the switching means 43. The drive frequency control means 41 is for changing the drive frequency of the IGBTs 35a and 35b, and the drive pulse width control means 42 is for changing the drive pulse width of the IGBTs 35a and 35b.
[0033]
The switching circuit 44 switches between the output of the drive frequency control means 41 and the output of the drive pulse width control means 42 and supplies it to the drive unit 39.
The input setting unit 26a is for setting the input power as a thermal power, and for example, as shown in FIG. 3, the input power is arbitrarily set between 3 kW and 50 W. The input set value by the input setting device 26a is supplied to the switching means 43, the drive frequency control means 41 and the drive pulse width control means 42.
[0034]
Now, operations of the drive frequency control means 41, the drive pulse width control means 42 and the switching means 43 in the microcomputer 40 will be described with reference to FIGS.
When the input set value by the input setter 26a is 800 W or more and 3 kW or less, that is, in the high input region, the drive frequency control means 41 sets the drive frequency while keeping the on-duty ratio constant as the input set value becomes higher. Control is performed so as to increase (shorten the cycle) (see FIGS. 4A and 4B).
[0035]
When the input set value by the input setter 26a is less than 800 W and 3 kW or more, that is, in the low input region, the drive pulse width control means 42 keeps the drive frequency constant as the input set value decreases. Is controlled so as to decrease (increase the dead time) (see FIGS. 4C and 4D).
[0036]
The switching means 43 outputs a selection signal Sa when the input set value by the input setting device 26a is 800 W or more (high input region), and the switching circuit 44 receives the control signal a from the drive frequency control means 41. Operate to accept. The drive frequency control means 41 controls the drive frequency according to an input set value of 800 W or more and outputs a control signal a.
Further, when the switching means 43 is less than 800 W, the switching means 43 outputs a selection signal Sb and causes the switching circuit 44 to operate so as to receive the control signal b from the drive pulse width control means 42. The drive pulse width control means 42 controls the drive pulse width according to the input set value less than 800 W and outputs the control signal b.
[0037]
According to such an embodiment, the drive frequency control means 41 and the drive pulse width control means 42 are switched according to the input power, in this case, the input power set by the input setting device 26a, that is, the drive frequency control means 41 and Since the input adjustment is performed by the drive pulse width control means 42, the input can be continuously changed from the low region to the high region without causing the frequency to become excessively high and generating a short-circuit current.
[0038]
Further, according to this embodiment, input adjustment in the high input region is performed by drive frequency control by the drive frequency control unit 41, and input adjustment in the low input region is performed by drive pulse width control by the drive pulse width control unit 42. I have to. In this case, since the drive frequency control is responsible for the high input region, unlike the case where it is responsible for the high input region to the low input region, the input in the high input region can be satisfactorily continuous without generating a short-circuit current. Can be changed. In addition, since the low input area is handled by the drive pulse width control, the input can be lowered continuously by gradually decreasing the drive pulse width from the appropriate one. In this case, the drive frequency is not increased. That's it.
[0039]
FIG. 5 shows a second embodiment. In this embodiment, a snubber circuit 45 having a snubber capacitor 45a is connected to the IGBT 35a, and a snubber circuit 46 having a snubber capacitor 46a to the IGBT 35b. Is different from the first embodiment.
According to this embodiment, the snubber circuits 45 and 46 can alleviate the rise of the voltages of the IGBTs 35a and 35b to suppress the generation of noise and switching loss, thereby contributing to the improvement of efficiency. Even in the configuration provided with such snubber circuits 45 and 46, the high input area is handled by the drive frequency control and the low input area is handled by the drive pulse width control, so that it is continuously from the high input area to the low input area. Thus, good input adjustment can be achieved. The snubber circuit may be composed of a snubber capacitor and a resistor. Furthermore, one snubber circuit may be used for a plurality (two) of IGBTs.
[0040]
FIG. 6 shows a third embodiment. This embodiment is different in that an input detection means 51 is provided in place of the switching means 43 in the second embodiment. In addition to the setting input from the input setting unit 26a, the input detection means includes an input current detection signal by an electric current detection means (not shown), an inverter current detection signal, or a voltage detection of the resonance capacitor 36 by an electric voltage detection means (not shown). In response to a signal or the like, the input power is detected. When the detected input power becomes the same value as shown in FIG. 3 in the first embodiment, the same control as in the first embodiment is performed.
[0041]
That is, when the detected input power is 800 W or more, the selection signal Sa is output, and the switching circuit 44 is operated to receive the control signal a of the drive frequency control means 41. At this time, as the detected input power increases, the drive frequency control means 41 performs control so as to increase the drive frequency (shorten the cycle) while keeping the on-duty ratio constant (see FIGS. 4A and 4B). . Further, when the detected input power is less than 800 W, the selection signal Sb is output, and the switching circuit 44 is operated to receive the control signal b of the drive pulse width control means 42. At this time, the drive pulse width control means 42 performs control to reduce the drive pulse width while keeping the drive frequency constant as the input set value decreases (see FIGS. 4C and 4D).
[0042]
7 to 9 show a fourth embodiment, which differs from the third embodiment in the following points. That is, a drive frequency control / drive pulse width control means 52 configured as one unit from a drive frequency control means and a drive pulse width control means is provided as a control means for controlling the low input region.
[0043]
  When the detected input power is less than 800 W, the selection signal Sb is output, and the switching circuit 44 is operated to receive the control signal b of the drive frequency control / drive pulse width control means 52. At this time, the drive frequency control / drive pulse width control means 52 decreases the drive pulse width as the detected input power decreases.ShiHowever, the drive frequency is controlled so as to increase sequentially (see FIGS. 9C and 9D).
[0044]
According to this embodiment, the input adjustment in the low input region is performed not only in the drive pulse width control but also in the drive frequency control. Therefore, if the drive frequency is set in accordance with the drive pulse width, the frequency is not increased. Detailed control is possible.
In place of one unit of the drive frequency control / drive pulse width control means 52 having the drive frequency control means and the drive pulse width control means, the drive frequency control means 41 and the drive pulse width control means 42 are used. Drive frequency control and drive pulse width control may be performed simultaneously. In short, the functions of the drive frequency control means and the drive pulse width control means may be provided as one unit or as a single unit.
[0045]
10 to 12 show a fifth embodiment, which differs from the third embodiment in the following points. That is, a drive frequency control / drive pulse width control means 53 configured as one unit from a drive frequency control means and a drive pulse width control means is provided as a control means for controlling the high input area.
[0046]
When the detected input power is 800 W or more, the control signal a from the drive frequency control / drive pulse width control means 53 is received by the switching circuit 44. At this time, the drive frequency control / drive pulse width control means 53 increases the drive frequency as the detected input power increases, and further reduces the drive pulse width as compared with the case of the same on-duty ratio. (See FIGS. 12A and 12B).
[0047]
According to this embodiment, since input adjustment in the high input region is performed not only by drive frequency control but also by drive pulse width control, finer input adjustment control can be performed without generating a short-circuit current in the high input region. Can be done. Note that the above-described drive pulse width may be increased as compared with the case of the same on-duty ratio.
[0048]
13 to 15 show a sixth embodiment. In this embodiment, the input adjustment in the high input region is handled by the drive frequency control / drive pulse width control means 54, and the input adjustment in the low input region is handled by the drive frequency control means 55. This is different from the embodiment. That is, when the detection input power is 800 W or more, the drive frequency is changed to be smaller while the drive pulse width is increased as the detection input is increased. (See FIGS. 15A and 15B). When the detected input power is less than 800 W, a constant duty ratio is set, and the drive frequency is controlled to increase sequentially as the detected input power decreases (see FIGS. 15C and 15D).
[0049]
According to this embodiment, the input adjustment in the high input region is handled by the drive pulse width control, and in addition to this, the drive frequency control is also performed. Therefore, a finer input can be performed without generating a short-circuit current in the high input region. Adjustment control can be performed. And since the input adjustment in the low input region is handled by the drive frequency control, the input is continuously lowered without increasing the frequency by increasing the drive frequency by setting the drive pulse width to a duty ratio that does not increase. I can go.
[0050]
FIGS. 16 to 18 show a seventh embodiment, which is different from the sixth embodiment in that the input adjustment in the high input region is handled by the drive pulse width control means 56. That is, when the detection input power is 800 W or more, the detection input power is changed to increase the drive pulse width at a constant frequency as the detection input increases. (See FIGS. 18A and 18B).
[0051]
According to this embodiment, the input adjustment in the high input region is handled by the drive pulse width control. Therefore, unlike the case of performing the drive pulse width control from the high input region to the low input region, the high input with the reasonable pulse width is performed. Input adjustments can be made in the area.
[0052]
FIGS. 19 to 21 show an eighth embodiment. This embodiment is different from the seventh embodiment in that the low input region is handled by the drive frequency / drive pulse width control means 57. That is, when the detected input power is less than 800 W, the drive pulse width is changed to be smaller as the detected input becomes lower, in addition to increasing the drive frequency. (See FIGS. 21 (c) and 21 (d)).
[0053]
FIG. 22 shows a ninth embodiment. This embodiment differs from the third embodiment in that an inverter stop means 58 is provided. That is, the inverter stop means 58 detects where the detected input power rises or falls past the threshold value 800 W, and temporarily outputs an inverter stop signal to the drive unit 39 to stop the inverter operation. It has become. Therefore, switching between the drive frequency control of the drive frequency control means 41 and the drive pulse width control of the drive pulse width control means 42 is performed after the operation of the inverter circuit is stopped.
According to this embodiment, since the drive frequency control and the drive pulse width control are switched and performed after the operation of the inverter circuit 34 is stopped, the operation of the inverter circuit 34 is not suddenly changed, and the IGBTs 35a and 35b may be damaged. Disappears.
[0054]
A short-circuit current detection means for detecting a short-circuit current of the inverter circuit 34 is provided. When the short-circuit current is detected by the short-circuit current detection means, the drive frequency control of the drive frequency control means 41 and the drive of the drive pulse width control means 42 are performed. Switching to pulse width control may be performed. If it does in this way, since it switches from the control form in which the short circuit current generate | occur | produced to another control form, generation | occurrence | production of a short circuit current can be prevented. Further, the operation of the inverter circuit 34 may be stopped before this switching.
[0055]
FIG. 23 shows a tenth embodiment, which is different from the first embodiment in that a microwave oven is shown as an inverter cooker. That is, a high-frequency transformer 60 for driving the magnetron 59 is provided instead of the heating coil. In this embodiment, the same effect as that of the first embodiment can be obtained.
[0056]
FIG. 24 shows an eleventh embodiment, which differs from the third embodiment (FIG. 6) in the following points. That is, the snubber circuit 46 is provided for the IGBT 35b, and a resonance capacitor voltage phase detection means 61 for detecting the voltage phase of the resonance capacitor 36 is provided. The microcomputer 40 has functions as phase difference setting means 62, phase difference detection means 63, comparison calculation means 64, drive frequency control means 65, and drive pulse width control means 66.
[0057]
The phase difference setting unit 62 sets the phase difference based on the detection input from the input detection unit 51. The phase difference detection means 63 is supplied with a first signal S1 in which the output voltage of the inverter circuit 34 is correlated, and the phase is correlated with the output current of the inverter circuit 34 output from the resonance capacitor voltage phase detection means 61. A second signal S2 is provided to detect the phase difference between them. The phase difference detection value of the phase difference detection means 63 and the phase difference setting value of the phase difference setting means 62 are given to the comparison calculation means 64, and the comparison calculation means 64 compares the two values so that they are equal. A command is issued to the drive frequency control means 65.
[0058]
The drive frequency control means 65 outputs a drive frequency control signal corresponding to the command from the comparison calculation means 64 and gives it to the drive pulse width control means 66. At this time, the drive pulse width control means 66 adjusts the drive pulse width according to the detection input of the input detection means 51.
[0059]
According to the eleventh embodiment, the frequency is directly changed to an appropriate frequency from the comparison result by the comparison calculation means 64, so that it is possible to prevent the resonance circuit 38 from being driven under a frequency condition in which the impedance is capacitive, and the resonance is always performed. It becomes possible to drive at a frequency or a frequency that becomes an inductive impedance, and the derivation of loss can be suppressed as much as possible. Since the pulse width is adjusted according to the input while maintaining the frequency, a wide input adjustment from the high input region to the low input region can be performed.
[0060]
In addition, this invention is not limited to each above-mentioned Example, You may change as follows. The input adjustment by the drive frequency control of the drive frequency control means may be performed at a frequency that is equal to or higher than the resonance frequency of the inverter circuit. In this way, the inverter circuit can be operated in an inductive situation, and no short-circuit current is generated due to the reverse recovery characteristics of the freewheeling diode. The snubber circuit may be composed of a capacitor and a resistor. Further, the threshold value 800W between the high input region and the low input region may be changed as appropriate. Furthermore, a plurality of threshold values may be provided, and control by drive frequency control, drive pulse width control, or both may be appropriately performed in each input region.
[0061]
【The invention's effect】
As is apparent from the above description, the present invention can be continuously varied from a high input to a low input without excessively increasing the driving frequency or causing a short-circuit current to flow.
[Brief description of the drawings]
FIG. 1 is an electrical configuration diagram showing a first embodiment of the present invention.
FIG. 2 is an external perspective view.
FIG. 3 is a diagram showing the relationship between input and drive control mode
FIG. 4 is a diagram showing an on / off state of an IGBT according to LA, LB, LC, and LD at each input in FIG. 3;
FIG. 5 is a view corresponding to FIG. 1 showing a second embodiment of the present invention.
FIG. 6 is a view corresponding to FIG. 1 showing a third embodiment of the present invention.
FIG. 7 is a view corresponding to FIG. 1 showing a fourth embodiment of the present invention.
FIG. 8 is equivalent to FIG.
FIG. 9 is a view corresponding to FIG.
FIG. 10 is a view corresponding to FIG. 1 showing a fifth embodiment of the present invention.
11 is a view corresponding to FIG.
FIG. 12 is equivalent to FIG.
FIG. 13 is a view corresponding to FIG. 1, showing a sixth embodiment of the present invention.
14 is a view corresponding to FIG.
15 is equivalent to FIG.
FIG. 16 is a view corresponding to FIG. 1 showing a seventh embodiment of the present invention.
FIG. 17 is a view corresponding to FIG.
18 is equivalent to FIG.
FIG. 19 is a view corresponding to FIG. 1 showing an eighth embodiment of the present invention.
20 is equivalent to FIG.
FIG. 21 is a view corresponding to FIG.
FIG. 22 is a view corresponding to FIG. 1, showing a ninth embodiment of the present invention.
FIG. 23 is a view corresponding to FIG. 1, showing a tenth embodiment of the present invention.
FIG. 24 is a view corresponding to FIG. 6 showing an eleventh embodiment of the present invention.
25 is a view corresponding to FIG. 1 showing a conventional example.
FIG. 26 is a diagram showing how switching elements are turned on / off.
FIG. 27 is a circuit diagram for showing the current and voltage of each part in the inverter circuit;
FIG. 28 is a diagram showing the current state of the inverter circuit during switching.
FIG. 29 is a waveform diagram showing changes in switching, current, and voltage.
[Explanation of symbols]
23 is a heating coil, 26a is an input setting device, 28 is a DC power supply circuit, 34 is an inverter circuit, 35a and 35b are IGBTs (switching elements), 36 is a resonance capacitor, 37a and 37b are free wheel diodes, 40 is a microcomputer, 41 is a drive frequency control means, 42 is a drive pulse width control means, 43 is a switching means, 45a and 46a are snubber capacitors, 45 and 46 are snubber circuits, 51 is an input detection means, 52, 53, 54 and 57 are drive frequencies. Control / drive pulse width control means (drive frequency control means, drive pulse width control means), 55 a drive frequency control means, 56 a drive pulse width control means, 58 an inverter stop means, 61 a resonance capacitor voltage phase detection means, 62 is a phase difference setting means, 63 is a phase difference detection means, and 64 is a comparison operation means.

Claims (1)

  1. A bridge-type inverter circuit that has a heating coil or a high-frequency transformer, a resonance capacitor, and a plurality of switching elements, and generates high-frequency power based on alternately switching driving the switching elements;
    A snubber circuit provided in at least one of the switching elements;
    Drive frequency control means for controlling the drive frequency of the switching element;
    Drive pulse width control means for controlling the drive pulse width of the switching element;
    Input detection means for detecting input power ; and
    The drive frequency control means performs input adjustment between the high input area and the low input area, and the drive pulse width control means operates only in the low input area,
    In the low input region, as the input power detected by the input detection means becomes low, the drive frequency is sequentially increased while reducing the drive pulse width .
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