KR20220037304A - Soft-Start Control Algorithm for Induction Heating System - Google Patents

Abstract

The present invention relates to control of an induction heating device which uses a voltage resonance method using a commercial AC power source. According to the fundamental principle of the induction heating device, since a high-frequency AC suitable for a material of a cooking vessel has to flow through a working coil to induce eddy current in the cooking vessel, a structure in which a DC power generated by rectifying the commercial power supplied to the working coil is switched to a power control semiconductor device at high speed is provided. When the induction heating device starts a switching operation by a pulse wave modulation (PWM) output of a micro control unit (MCU) in a standby mode in which a switching element is turned off while the DC power generated by rectifying the commercial power is supplied to the working coil, an initial high DC voltage is suddenly applied, noise is generated by a high current flowing in a working coil resonator and a semiconductor switching element, and the switching element can be burnt due to overshoot of a resonant voltage induced in the working coil resonator. To prevent generation of annoying noise and destruction of the switching element due to the initial excessive resonance voltage, the present invention provides a soft start control method of initial operation to safely move the induction heating device in a standby state to normal operation.

Description

Safe initial operation control of induction heating device {Soft-Start Control Algorithm for Induction Heating System}

The present invention is an induction heating inverter device that generates heat by inducing an eddy current in a cooking vessel by flowing a high-frequency current to a working coil by high-speed switching of a Power MOSFET, including an IGBT, and a SiC power device using an MCU. It relates to a power control driving method of an induction heating device that controls the duty ratio of the PWM function built into the MCU (Micro-Controller Unit) for safe initial operation of the induction heating device by minimizing the start-up transient.

There are several power control methods in the power control system using DC voltage, but one of the most commonly used methods is to adjust the temporal ratio of the amount of current flowing in the load. It is a method of controlling the average current by the ratio of the time when it does not flow.

By using the ON/OFF ratio of the PWM function of the control unit used in the induction heating device, the average current flowing through the working coil, which is the load, can be finely adjusted. A stable initial operation can be realized by starting by lowering the pulse width of the switching control signal and increasing it gradually.

In the induction heating device, the pulsating voltage generated by full-wave rectification of the AC voltage is made into DC voltage through the LC smoothing circuit. is in a state of being hung.

Therefore, when a switching control signal with a wide pulse width is suddenly applied to the switching element in the standby state at the time when the AC voltage is high, a large current flows in the working coil resonance part, and excessive voltage generated in the working coil resonance part composed of the working coil and parallel capacitor. .The switching element may be destroyed (burned out) by the current, or noise may be generated by the induced current. The present invention is an induction heating control method that solves the problem of the conventional transient phenomenon occurring in such initial operation.

As mentioned in the background art, the cause of the problem is a transient that may occur in the initial operation transitioning from the standby mode to the normal heating control mode. will do

The pulse width of the first switching control signal applied to the switching element in the standby mode starts from a short value at which the resonance voltage generated across the working coil resonator does not occur, and periodically increases the pulse width, which is the switching control signal, to a smaller value. If you increase it, there will be no transients.

In addition, when the first switching control signal is synchronized at the zero-cross point of the AC voltage and applied to the switching device, the voltage energy itself is low, so the power calculated by 'power = voltage (zero) x current (pulse width)' is the lowest. It can be seen that the probability of occurrence of transient phenomena is the lowest at this point.

By using the PWM function of the controller MCU, which is conventionally used without hardware components added to the present invention, the switching element due to a transient that may occur in the initial operation of the induction heating device may be burned out or generated by a high current. It can solve the noise problem.

1 to 4 are views showing an embodiment for realizing the present invention,
1 is a general functional block diagram representing the entirety of the present invention.
2 shows commercial power AC voltage waveforms, pulsating waveforms, standby mode, the first section/second section of the soft start control section, and AC voltage zero-cross detection pulse waveforms.
3 shows the switching pulse width change in each of the standby mode, the soft start control period, and the heating mode, the generation of the resonance voltage, and the resonance voltage low point-cross detection waveform.
4 is a PWM block diagram of an MCU that changes the pulse width of a switching control signal.

The present invention will be described in detail with reference to the accompanying drawings 1 to 4 . 1 to 4 are shown in order to explain in detail a preferred embodiment of the present invention, the invention may be practiced individually or in combination.

1, the induction heating device is a line filter unit 10, a rectifier unit 20, an LC smoothing unit 30, a working coil resonance unit 40, a resonance voltage low-cross detection unit 50, a switching element 60 ), a current detection unit 70 , a switching element driving unit 80 , an MCU control unit 90 , and an AC voltage zero-cross detection unit 100 .

The line filter 10 is to attenuate the conductive EMI emitted to the power line. Commercial power AC220V (or AC110V) of 60Hz (or 50Hz) is the line filter 10 and the bridge diode (Bridge Diode; BD) rectifying unit ( 20), it is changed to a pulsating waveform, and since the negative part of the AC waveform is raised to the positive region, when the commercial AC power is 60Hz, the pulsating waveform has a period as shown in FIG. to 120 Hz.

The pulsating waveform is made into a DC voltage by the L-C smoothing unit 30 and supplied to the working coil resonator 40 in which a working coil LR and a capacitor CR are configured in parallel. In the standby mode, since the switching element 60 is turned off, the DC voltage smoothed by the inductor Lf3 and the capacitor C3 constituting the L-C smoothing part 30 is maintained. However, when the switching element 60 starts a switching operation with high-speed Turn-On/Turn-Off, starting from the anode of the bridge diode BD, the LC smoothing unit 30, the working coil resonator 40, and the switching Since the current flowing in the closed loop constituted by being connected to the element 60 and the cathode of the bridge diode BD is as large as several tens of amperes, the DC voltage is not covered by the amount of charge charged in the smoothing capacitor C3. It may not be maintained and may have a shape close to the pulsating waveform.

The working coil resonance unit 40 constitutes a parallel resonance circuit with a working coil LR and a capacitor CR, and when the switching element 60 is turned on, the working coil resonance unit 40 is hung current starts to flow by the DC voltage;

The switching element 60 continues to increase during the turn-on time according to the pulse width of the switching control signal output from the PWM block of the MCU controller 90 . When the switching element 60 is turned off (Turn-off), the closed circuit is opened (Open Loop), so the working coil (LR) and the capacitor (CR) have parallel resonance in which the electrical energy accumulated during the turn-on time is repeatedly charged and discharged. this can start

As the switching device 60, a power control semiconductor device that switches the working coil resonator 40 at high speed is used, and functionally, high-speed response, high withstand voltage, and high current characteristics are required. IGBTs, power MOSFETs, and SiC can be used, but IGBTs with relatively low prices can be mainly used for induction heating devices for home appliances.

The current detector 70 mainly uses a shunt resistor or a current transformer, and the current flowing through the switching element 60 is converted into a voltage (voltage = resistance x current);

The converted minute voltage is amplified by the OP-Amp built into the MCU control unit 90 and converted into a digital value by the built-in AD converter to calculate the current (Is) value, and the pulsating current applied to the front end of the working coil resonator 40 It is necessary to calculate the current power of the induction heating device by calculating with the voltage (Va) (power: P = Va X Is ).

The switching element driver 80 is a driving circuit for making a switching control signal capable of sufficiently driving the switching element 60 because the PWM output built into the MCU controller 90 is a weak signal to directly drive the switching element 60 . can be configured

Resonance voltage low-cross detection unit 50 is below the potential (lower point of resonance voltage) of the pulsating voltage which becomes the reference voltage by decreasing the resonance voltage VR across both ends of the working coil resonance unit 40 from the highest value (Peak value). It is a circuit that detects the point of cross (crossing). When the switching element 60 is turned off, when viewed from the ground potential, the pulsating voltage applied to the left side of the working coil resonator 40 becomes the reference voltage, and the pulsating voltage + the resonance voltage VR is applied to the right side. The two voltages at both ends are distributed at an appropriate resistance ratio and applied to a voltage comparator built in the MCU control unit 90. The '+' input of the voltage comparator is connected to the pulsating voltage side, which is the reference voltage, and a '-' input. is connected to the 'pulse voltage + resonance voltage' side. Therefore, when the resonance voltage decreases from the peak value and crosses to the pulsating voltage potential, the output of the voltage comparator changes from Low to High, so the low-crossing point of the resonance voltage can be detected.

The AC voltage zero-cross detection unit 100 crosses (crosses) the zero potential when the AC voltage changes from '+' to '-' or from '-' to '+' (Zero-Crossing). point) is a circuit where a pulse signal is made. It is necessary to inform the MCU control unit 90 of the lowest point of the AC voltage. (See Figure 2)

The MCU control unit 90 receives the detection signal and information from the resonance voltage low-cross detection unit 50, the current detection unit 70, and the AC voltage zero-cross detection unit 100, and converges the output of the induction heating device to a target value. to correct the difference between the measured output (power) value and the target value; A negative feedback control is performed in which the calculated pulse width of the switching control signal is applied to the switching element 60 through the switching element driver 80 using the built-in PWM function. Other secondary functions of induction heating devices; For example, overall control such as detecting and blocking high voltage, determining whether or not a cooking vessel is present, and a user interface is performed, but the description is omitted because it is not directly related to the present invention.

On the other hand, since the current flowing through the switching element 60 is controlled by the pulse width of the switching control signal controlled by the MCU control unit 90, the transient problem caused by immediately shifting from the standby mode to the normal heating mode is eliminated by the switching control signal. It can be solved by softly starting (starting) in standby mode by finely adjusting the pulse width and moving to normal heating control mode. That is, it is solved by executing the 'soft start control' section in the middle of the induction heating device transitioning from the standby state to the high-speed switching operation state.

Referring to FIG. 2 , the soft start control period T2 is synchronized with the zero-cross time point ZP1 of the first alternating voltage encountered in the standby mode T1 and the switching control signal is applied to the switching element 60 . It starts and ends in synchronization with the second AC voltage zero-cross time point (ZP2), and at the same time, it can transition to the normal heating control mode (T3).

The reason for synchronizing the soft start control section (T2) to the zero-cross time point (ZP1) of the first AC voltage encountered in the standby mode (T1) is that the lowest potential of the pulsating voltage is the zero-cross point of the AC voltage and the switching element ( This is because the lower the voltage when 60) is turned on, the less transient phenomenon occurs.

In addition, the soft start control section (T2) ends in synchronization with the second AC voltage zero-cross time point (ZP2) and at the same time can transition to the normal heating control mode (T3) step is the switching element in the soft start control section (T2) The pulse width of the switching control signal applied to (60) is gradually increased after the first pulse, and the pulse width is sufficiently increased during the half cycle of the AC voltage (8.333 ms in the case of 60 Hz) to enter the normal heating control mode (T3) step. Because implementation is possible.

Therefore, the soft start control application period is fixed because it is limited to a half cycle (one cycle of pulsating voltage) of the AC voltage from the zero-cross time point (ZP1) of the first AC voltage to the next zero-cross time point (ZP2) in the standby mode. It is possible to predict the change width of the switching control signal pulse width, the execution time, and the number of pulses in the soft start period T2 having the specified time.

3 is a diagram showing a soft start control section (T2) added between the standby mode (T1) and the normal heating control mode (T3), the first section (A) and the second section included in the soft start control section (T2) Resonance voltage generation according to switching pulse period and pulse width change in section 2 (B), and the resonance voltage low point-cross detection signal relationship are expressed.

Referring to FIG. 3 , the soft start control period T2 includes a first period A and a first period A in which a pulse of a switching control signal is applied to the switching element 60 at an arbitrarily set period in the range of 40us to 128us. It is divided into a second section (B) in which a pulse of a switching control signal is generated in synchronization with the low point-cross point of the resonance voltage, that is, a pulse period is determined by the resonance voltage, and the first section (A) and the pulse generation period of the switching control signal of the second section B may be different.

In the induction heating device described in the present invention, the voltage resonance waveform generated in the working coil resonator 40 by turning on/off of the switching element 60 is switched again at the lowest potential (lowest point of resonance voltage-cross detection time). Since the soft switching method of turning on the element 60 is used, the pulse of the switching element control signal is not automatically generated unless a low-cross of the resonance voltage is detected.

Since it should start in standby mode (T1) with a pulse width of the switching control signal that is sufficiently small that the transient problem (noise and burning of the switching element) of the initial operation of the induction heating device does not occur, the soft in standby mode (T1) The pulse width of the first switching control signal of the first section A in which the start T2 starts may be set to a small value at which the low-point-cross signal of the resonance voltage is not detected (a preferred embodiment of the present invention is 1us starting with a pulse width of ).

Therefore, in the first section (A), since the switching control signal that is the PWM output of the MCU control unit 90 that is automatically triggered in synchronization with the low point-cross detection signal of the resonance voltage is not generated, the MCU control unit 90 sends the first pulse The switching operation can be smoothly performed until the resonance voltage is detected by outputting the output and gradually increasing the pulse width of the switching control signal periodically using the internal timer function.

A preferred embodiment of the present invention uses the Over-flow (OVF) interrupt cycle of the 10-bit PWM counter built in the MCU control unit 90 to set the switching control signal pulse width without using a separate timer every 128us cycle. The first section (A) can be performed by increasing the PWM duty value by 0.125us in the generated OVF interrupt processing routine. In case of 10-bit PWM, when 8Mhz (period: 0.125us) clock is input to PWM Up-Counter and 1024 counts are counted, overflow occurs and OVF interrupt occurs. That is, the pulse width of the switching control signal can be gradually changed by increasing the PWM duty value in the OVF interrupt processing routine that occurs every 0.125 x 1024 = 128us time.

On the other hand, if the pulse width of the switching control signal is gradually increased at a constant period in the first section (A), a resonance voltage will be generated at some point by the increased pulse width, and a resonance voltage low point-cross signal will be detected. In order to avoid an unstable detection signal that may be caused by fluctuations in the pulsating voltage that is the detection reference comparison voltage of the resonance voltage low-cross detection unit 50 due to noise, the first detection signal does not immediately move to the second section (B). , after the detection signal is generated three or more times is determined as a stable time point, and the second section (B) is shifted.

That is, the first section (A) becomes the first section (A) until the point at which the low point-cross signal of the stable resonance voltage is detected, and in the first section (A), even if the low point-cross detection signal of the resonance voltage occurs, the stable number of times (3 or more) If it is not reached, the pulse of the switching control signal is not generated in synchronization with the detection signal and is generated in the period (128us) of the first section (A). This can be selected by setting whether to output the PWM block built into the MCU control unit 90 or not.

The second section (B) is directly connected to the first section (A) and starts from the point when the stable resonance voltage low-cross detection is determined, is synchronized with the resonance voltage low-cross detection signal, and is built into the MCU control section 90 Since the operation of the PWM block is triggered and outputted automatically, the pulse period of the switching control signal can be determined by the sum of the time width of the resonance voltage and the pulse width of the switching control signal.

The difference between the first section A and the second section B included in the soft start control section T2 is only different from the timing and period at which the switching control signal pulse is output, and the switching applied to the switching element 40 is The pulse width of the control signal may be gradually increased from the first pulse of the first section A to the zero-cross point ZP2 of the AC voltage at which the second section B ends.

However, the gradual increase of the pulse width in the second section stops the increase when the increased pulse width reaches the pulse width value corresponding to the target power of the normal heating control mode (T3), and the second section (B) ends. The same pulse width may be maintained until the zero-cross time point ZP2.

In a preferred embodiment of the present invention, when AC220V, 60Hz is used, the pulse width of the first switching control signal of the first section (A) is 1us, and the pulse width is gradually increased at intervals of 0.125us for each repetition period to target soft I was able to implement the start effect.

The second section (B) included in the soft start control section (T2) is synchronized with the zero-cross time point (ZP2) of the subsequent AC voltage, and is terminated and transferred to the normal heating control mode (T3).

In the normal heating control mode (T3) step, the first switching control signal having a pulse width corresponding to the target power set to be controlled by the induction heating device is synchronized with the zero-cross detection (ZP2) of the AC voltage to the switching element 60 It starts with accreditation to The pulse width of the switching control signal at the end of the soft start control period (T2) including the second period (B) is close to the pulse width of the first applied switching control signal at which the normal heating control mode (T3) stage starts Therefore, it can be smoothly transitioned to the normal heating control mode (T3).

Even if there is a difference between the last pulse width at the end of the second section (B) and the pulse width corresponding to the target power of the normal heating control mode (T3), the pulsating voltage is the lowest at the zero-cross time point (ZP2) of the AC voltage. Since the changed pulse width is applied in synchronization, transient problems (noise and burning of the switching element) may not occur.

For example, in an electric rice cooker, which is one of induction heating applications, when the normal heating control mode (T3) is 'warm', the target power to be controlled is low, so the pulse width value corresponding to the target power of the keep warm function may also be very small. . In this case, the pulse width gradually increased in the second section B may reach the pulse width of the target power (warming) long before the end of the second section B. Therefore, the final pulse width of the second section (B) is limited to the pulse width corresponding to the target power of the warming function and is the same as the first pulse width of the normal heating control mode (T3), so it is normal in the soft start control section (T2) It can be smoothly transitioned to the heating control mode T3.

In the normal heating control mode (T3), a negative feedback control that corrects the difference between the target power and the currently measured power according to the conventional automatic control method is performed, and the changed pulse width is switched to compensate for the power difference Since the control signal is always synchronized with any one of the zero-cross detection signals of the AC voltage and applied to the switching element 60, stable switching control can be performed.

4 is a block diagram of a PWM function for changing the pulse width of a switching control signal embedded in the MCU control unit 90 used in a preferred embodiment of the present invention. Determination of the first pulse width at which the soft start control period T2 starts on the basis of the MCU controller 90 having the PWM function of FIG. 4 and the pulse period and execution time of the first period A, the second The pulse period and execution time of the section (B), and the last pulse width of the second section (B) can be calculated.

The PWM block in Fig. 4 has 10-bit Resolution, and there are 4 types of clocks applied to the 10-bit counter, but 8MHz, the clock with the highest frequency, is selected. The 10-bit counter of the PWM block is incremented every 8MHz clock, and if an overflow occurs, an interrupt is generated in the CPU. Also, when the resonance voltage low-cross signal is detected, the PWM 10-bit counter is automatically cleared to '0' and an interrupt is generated in the CPU.

Since the built-in Interval Timer block is not sufficient in the MCU control unit 90 and a separate Interval Timer cannot be used, the period of the pulse output of the first section (A) is determined by using the 10-bit Counter Overflow (OVF) interrupt of the PWM block. did with PWM 10-bit Counter OVF interrupt period can be calculated as 0.125us (8MHz Clock period) x 1024 (10-bit Counter) = 128us.

The pulse width of the switching control signal to the extent that the first start pulse of the first section A does not generate a resonance voltage by the working coil resonator 40 may be experimentally obtained as 1us. 10-bit Counter of PWM block Increases the PWM duty value by '1' (1/8MHz=0.125us) which is the minimum value in the OVF interrupt processing routine every 128us period when the OVF interrupt occurs and controls the switching of the pulse width increased by 20 times in total Assuming that the stable resonance voltage low-cross signal is detected three or more times when the signal is applied to the switching element 40,

The execution time of the first section (A) becomes 128 [us] x 20 [times] = 2.56 [ms] , and the pulse width at the end of the first section (A)) is 1us + (0.125 [us] x 20) [times]) = 3.5 [us] .

In the second section (B), when the PWM block is set so that the pulse of the switching control signal is automatically output in synchronization with the low point-cross timing of the resonance voltage, it is triggered by the detection signal of the resonance voltage low point-cross detection unit 50 and PWM An output signal (switching control signal pulse) is applied to the switching element 60 and a CPU interrupt is generated at the same time. In the interrupt processing routine, the pulse width of the switching control signal is gradually increased by increasing the PWM duty value by '1' (1/8MHz=0.125us).

In the case of home induction heating device, since the average switching frequency of about 25KHz suitable for the cooking vessel is used, the average switching period is about 40us, and the second section (B) ends at the zero-cross point (ZP2) of the AC voltage, so the 60Hz When using commercial AC power, the soft start control section (T2) is about 8.333ms. Therefore, the time for which the second section B is performed may be calculated as 8.333 [ms] - 2.56 [ms] = 5.773 [ms] .

Also, the pulse width at the end of the second section B may be increased to 3.5 [us] + {0.125 [us] x (5.773 [ms] / 40 [us])} = 21.54 [us] . That is, it can be increased up to the pulse width of the average power stage of the normal heating control mode (T3) (the stage in which the sum of the pulse width and the resonance voltage time width becomes the average switching period of 40us, and the pulse width corresponds to 20us), so soft start control The control mode can be smoothly changed by matching the final pulse width of the mode T2 with the first pulse width of the normal heating control mode T3.

If you want to increase the final pulse width at the end of the second section (B), reduce the initial start pulse width of the first section (A) to 1us or less, or reduce the pulse generation period to 128us or less to reduce the first section (A) Instead of reducing the execution time of , it may be possible if the execution time of the second section (B) is increased.

Although preferred embodiments of the present invention have been shown and described in detail, the present invention is not limited to the above-described embodiments, and those of ordinary skill in the art to which the present invention pertains without departing from the gist of the present invention as claimed in the claims It is to be understood that various modifications may be made by any person skilled in the art, and such modifications are intended to be within the scope of the appended claims.

Claims (16)

In an induction heating device that uses the inverter principle to rectify commercial AC power into DC and convert it into high-frequency AC, switching in standby mode in which the switching element is turned off while DC voltage is supplied to the working coil When a PWM (Pulse Width Modulation) switching control signal of the controller is applied, the switching element is turned on and a switching control operation is started;
Induction heating device with soft-start control to prevent noise that can be caused by high current flowing through a high initial voltage suddenly entering the resonance part of the working coil and damage to the switching element due to excessive resonance voltage. The method according to claim 1, wherein the soft start control period is synchronized at the zero-cross time point of the first alternating voltage encountered in the standby mode, and starts when the switching control signal is applied to the switching element, and is synchronized at the second zero-cross time point An induction heating device that transitions to the normal heating control stage at the same time as it is finished. The induction heating apparatus according to claim 2, wherein the soft start control application period is limited to a half cycle of the AC voltage (one cycle of the pulsating voltage) from the zero-crossing time of the AC voltage to the next zero-crossing time. The induction heating apparatus according to claim 2, wherein a pulse width of the switching control signal applied to the switching element is gradually increased in the soft start control period. The method according to claim 2, wherein the soft start control period comprises: a first period in which the switching control signal having an arbitrary set period is applied to the switching element;
An induction heating device divided into a second section in which the detection signal is synchronized at the low point-cross point of the stable resonance voltage, the switching control signal is generated, and the generation period is determined. The induction of claim 5, wherein the pulse width of the first switching control signal of the first section applied to the switching element is the low point of the resonance voltage generated at both ends of the working coil resonant part - a pulse width in a range in which a cross is not detected. heating device The induction heating device according to claim 6, wherein, when AC220V 60Hz is used, the pulse width of the first switching control signal of the first section is 1us, and the pulse width is gradually increased at any time interval ranging from 0.1us to 0.15us per repetition period . The method according to claim 7, The pulse width repetition period of the switching control signal in the first section is set to an arbitrary value in the range of 40us to 128us, the low-point of the resonance voltage - the pulse width of the switching control signal until the cross signal is stably detected This progressively increased induction heating device. The method as set forth in claim 8, wherein the criterion for determining the timing at which the low point-cross signal of the resonance voltage is stably detected is the resonance voltage low point-cross point detected after the low point-cross detection signal of the resonance voltage is generated three or more times. Induction heating device with hysteresis characteristics. The induction heating apparatus according to claim 5, wherein the second section starts at the point when the low-point-cross signal of the resonance voltage is stably detected and ends at the zero-cross point of the first encountered AC voltage. The method according to claim 10, wherein the pulse of the switching control signal applied to the switching element in the second period is generated in synchronization with the low-point-cross detection signal of the resonance voltage, the pulse width is any time interval in the range of 0.1us to 0.15us An induction heating device that is gradually increased to The method according to claim 11, When the pulse width of the switching control signal generated in synchronization with the low point-cross detection signal of the resonance voltage in the second section reaches a pulse width corresponding to the target power of the normal heating control mode, the pulse width is increased Induction heating device in which the pulse width is maintained until the zero-cross point of the AC voltage at which the second section ends. The method according to claim 12, wherein the target power is determined according to an operating function (eg, keeping warm or cooking, heating, etc.) of the induction heating device to be controlled in the normal heating control mode following the end of the soft start control section, corresponding to the target power An induction heating device that calculates and applies the pulse width experimentally. The method according to claim 13, wherein the normal heating control mode starts at the zero-cross point of the AC voltage at the end of the second section, and the pulse width of the first switching control signal of the normal heating control mode is a pulse width corresponding to the target power Induction heating device determined by The induction heating apparatus according to claim 14, wherein the normal heating control mode conforms to the conventional automatic control method and performs negative feedback control to correct the difference between the target power and the currently measured power. The induction heating apparatus according to claim 15, wherein the switching control signal of the pulse width changed to correct the power difference in the normal heating control mode is always synchronized with any one of the zero-cross detection signals of the AC voltage and applied to the switching element.

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