JPH06231877A - Induction heating device - Google Patents

Abstract

PURPOSE:To eliminate a voltage dividing circuit of high withstand voltage and high accuracy from a synchronous detecting circuit, and lower cost by constituting the synchronous detecting circuit in such a way as capable of generating a synchronous signal based upon voltage generated in a search coil wound around a magnetic core forming a part of a magnetic circuit laid in the vicinity of a heating coil. CONSTITUTION:When a switching element 6 is actuated with a pulse signal from a PWM modulator 12 via a driver 13 and an inverter 5 is thereby caused to start operation, rectified and smoothing current supplied to the inverter 5 is cyclically switched on and off, and converted into a high frequency of several tens of hundreds Hz. This high frequency is applied to a heating coil 4 for the induction heating of a pan. In this case, voltage waveforms of several volts easy to be processed with a comparator 16 are generated by the search coil 14, depending upon a switching waveform applied to the coil 4. The generated voltage waveforms are compared in the comparator 16. Then, the rising edge of the waveform is detected with a differentiating circuit 22 on the basis of output from the comparator 16, thereby generating a synchronous signal. This synchronous signal is used to keep synchronous state with the pulse signal from the modulator 12 for driving the element 6. Thus, a switching loss is restrained to low level, and good switching operation is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an electromagnetic cooker, I
The present invention relates to an induction heating device such as an H cooker, and more particularly to the structure of a synchronization detection circuit thereof.

[0002]

2. Description of the Related Art An induction heating device such as an electromagnetic cooker is composed of a heating coil for induction heating a heated object such as a pan by applying a high frequency wave, and a switching element composed of a transistor for rectifying and smoothing voltage using the heating coil as a resonance inductor. A resonance type inverter or the like for periodically switching and converting to the high frequency is provided, and the switching element is on-driven with the zero point or the like of the collector resonance voltage as a drive timing in order to suppress a switching loss to a small level. . As a synchronization detection circuit which takes on-driving timing of a switching element with respect to this collector resonance voltage, for example, Japanese Patent Publication No.
As disclosed in Japanese Unexamined Patent Publication No. 8-36473, there is one in which a collector resonance voltage is compared with a rectified and smoothed voltage to obtain a synchronization signal.

FIG. 8 shows an induction heating apparatus provided with such a conventional synchronous detection circuit. In the figure, reference numeral 1 is an AC power supply, and an AC voltage from the AC power supply 1 is rectified by a rectification bridge 2 and then made into a rectification smoothed voltage via a choke coil 3. This rectification smoothed voltage constitutes the heating coil 4 as a resonance inductor. It is inputted to the inverter 5 of the voltage resonance type (Ichiishi type quasi E class). 6 in the inverter 5 is a switching element composed of a transistor, 7
Is a freewheeling diode, 8 is a resonance capacitor, and 9 is a bypass capacitor. Reference numeral 25 denotes a synchronization detection circuit, which divides the rectified and smoothed voltage and the collector resonance voltage of the switching element 6 into appropriate voltage values by voltage dividing circuits 26 and 27 each of which is composed of a resistor, and then compares the voltage values with a comparator 28 to obtain a synchronization signal. Is obtained. Since the rectified and smoothed voltage or the like becomes a high voltage, the voltage dividing circuits 26 and 27 are required in the preceding stage of the comparator 28 as described above. 11 is a power control circuit, 12 is a PWM modulator,
Reference numeral 13 is a driver, and a pulse signal having a duty cycle corresponding to the control signal from the power control circuit 11 is P
The switching element 6 is output from the WM modulator 12, and the driver 13 drives the switching element 6 based on the pulse signal to control the electric power.

[0004]

In the conventional synchronous detection circuit in the induction heating device, the rectified smoothed voltage is compared with the collector resonance voltage of the switching element to obtain the synchronous signal. Since it becomes a high voltage (about 600 Vpeak), two voltage dividing circuits are required before the comparator. However, since it is necessary for the voltage dividing circuit to divide the switching waveform of several tens of kHz, it is difficult to use a high resistance, and thus the loss of the voltage dividing circuit becomes large. Further, in order to reliably operate even in a portion where the switching waveform becomes small, such as near the zero-cross of the commercial AC power supply, it is necessary to accurately match the voltage division ratio of the two voltage dividing circuits, which requires high accuracy.

Therefore, it is an object of the present invention to provide an induction heating apparatus which does not require a high withstand voltage and highly accurate voltage dividing circuit in a synchronization detection circuit and can reduce the cost.

[0006]

In order to solve the above-mentioned problems, the present invention firstly proposes a heating coil for inductively heating an object to be heated by applying a high frequency, and a DC voltage for use as a resonance inductor for the heating coil. A resonant inverter that periodically switches by a switching element to convert to the high frequency,
In an induction heating device having a synchronization detection circuit that generates a synchronization signal that takes the drive timing of the switching element, the synchronization detection circuit is wound around a magnetic core that constitutes a part of a magnetic circuit provided near the heating coil. The gist is that the synchronizing signal is generated based on the voltage generated in the turned search coil.

Secondly, in the first configuration, the synchronization detection circuit is configured to generate a synchronization signal based on a voltage obtained by advancing the voltage generated in the search coil by a required phase in a phase advancing circuit. What is done is the gist.

Thirdly, in the first or second configuration, the search coil is constructed by using a center conductor of a shield wire, and the outer conductor of the shield wire is grounded at either one of both ends. The point is to open the other side.

[0009]

In the above construction, firstly, the voltage generated in the search coil can be changed in voltage value depending on the number of windings on the magnetic core, and the number of windings can be easily handled by an electronic circuit such as a comparator. It becomes possible to obtain the voltage of the volt. A synchronizing signal is generated by processing such as comparing this voltage with a reference voltage of an appropriate value by a comparator. As a result, the synchronization detection circuit does not need a high-voltage and high-precision voltage divider circuit.

Second, by synchronizing the voltage generated in the search coil with the required phase by the phase advancing circuit, the delay of the voltage generated due to the constituent circuits and the like is corrected and an appropriate drive timing can be obtained. It becomes possible to reliably obtain a signal.

Thirdly, noise on the AC power supply line may occur in the voltage generated in the search coil due to the heating coil being directly connected to the AC power supply line. Configure the search coil with the center conductor of the shield wire,
By grounding one end of the outer conductor of the shielded wire, the generation of the noise in the voltage generated in the search coil is prevented, and the switching element can be driven well.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 are views showing a first embodiment of the present invention. In FIG. 1, components that are the same as or equivalent to the devices, elements, etc. in FIG. 8 are designated by the same reference numerals as those used above, and redundant description will be omitted. In the present embodiment, the synchronization detection circuit 10 has a search coil 14 wound around a magnetic core forming a part of a magnetic circuit provided in the vicinity of the heating coil 4.
It is configured to generate a synchronization signal based on the voltage generated at the. FIG. 2 shows the configuration of the search coil 14 portion. Reference numeral 17 is a pot as a heated object, 18 is a top plate, and ferrite 19 serving as a magnetic core is arranged in an appropriate manner on the bottom surface of the heating coil 4. The ferrite 19 serves as a part of the magnetic circuit of the magnetic flux generated in the heating coil 4 and is provided for efficiently inductively heating the pot 17. The search coil 14 is constructed by winding an insulating wire around the ferrite 19 once. Search coil 14
Can change the generated voltage to a desired value by changing the number of turns. In addition, in FIG.
The heating coil support plate, the ferrite support plate, the heat insulating material, etc. are not shown. The sync detection circuit 10 is configured to compare the voltage generated in the search coil 14 with a reference voltage in a comparator 16 to generate a sync signal. The comparator 16 has a small Schmitt width by giving a positive feedback from its output terminal to the non-inverting input terminal with a resistor, thereby giving a margin to the generation of the synchronization signal.

The induction heating apparatus of this embodiment is configured as described above, and when the pulse signal from the PWM modulator 12 drives the switching element 6 via the driver 13 to start the inverter 5, the inverter 5 is input to the inverter 5. The rectified and smoothed voltage is periodically switched and converted into a high frequency of several tens of kHz, and this high frequency is applied to the heating coil 4
And the pan 17 is induction-heated. At this time, according to the switching waveform supplied to the heating coil 4, the search coil 14 generates a voltage waveform of several V which is easy to handle in the comparator 16, and this voltage waveform is zero V in the comparator 16.
The output of the comparator 16 is compared with the reference voltage of
A synchronizing signal is generated by detecting the rising edge by the differentiating circuit 22 composed of CR. Further, the diode 23 is a grumper for suppressing the differential output with respect to the rising edge of the output of the comparator 16. By this synchronizing signal, the collector resonance voltage of the switching element 6 and the pulse signal output from the PWM modulator 12 are synchronized, the switching element 6 is ON-driven at an appropriate timing, and the switching loss is suppressed to a small level, thereby achieving good switching. The action is taken.

FIG. 3 shows a second embodiment of the present invention. In this embodiment, the comparator 16 in the synchronization detection circuit 20
A phase advancing circuit 21 including a capacitor C and a resistor R for advancing the voltage generated in the search coil 14 by a required phase is connected to the preceding stage.

FIG. 4 shows the operation waveforms of the search coil output voltage a, the phase advance waveform b, the comparator output c and the collector resonance voltage Vce in the normal operation of this embodiment. The same operation waveforms of each part when starting are shown. A and b shown here are Vce
However, this is due to the convenience of the P, W, and M modules, and is not limited to the opposite phase, but may be the same phase. From FIG. 4, the phase-advancing waveform b obtained by advancing the search coil output voltage a by the required phase by the phase-advancing circuit 21 ensures that the comparator output c (synchronization signal) at an appropriate timing with respect to the collector resonance voltage Vce is obtained. I understand. Further, it can be seen from FIG. 5 that synchronous detection is reliably performed on the collector resonance voltage Vce even when the inverter 5 is started.

6 and 7 show modified examples of the search coil in the first and second embodiments. In this embodiment, the search coil 15 is formed by using the central conductor of the shield wire, and one of both ends of the outer conductor of the shield wire is grounded and the other is open. FIG. 7 shows a circuit configuration when the search coil 15 is applied to the synchronization detection circuit of FIG. As shown in Figure 1,
Noise on the AC power supply line may occur in the voltage generated in the search coil due to the heating coil 4 being directly connected to the AC power supply line. In contrast,
By configuring the search coil 15 with a shield wire as in this modification, the above noise is prevented from being generated in the voltage generated in the search coil 15, and reliable synchronization detection can be performed.

[0017]

As described above, according to the present invention,
First, the synchronization detection circuit is configured to generate the synchronization signal based on the voltage generated in the search coil wound around the magnetic core forming a part of the magnetic circuit provided in the vicinity of the heating coil. The voltage generated in the coil can change its voltage value depending on the number of times it is wound around the magnetic core, and it is possible to obtain a voltage of several volts that can be easily handled by the electronic circuit that constitutes the synchronization detection circuit, such as a comparator, by the number of turns. Therefore, a high withstand voltage and high precision voltage dividing circuit is not required in the synchronization detection circuit, and the cost can be reduced.

Secondly, since the synchronization detection circuit is constructed so as to generate a synchronization signal based on a voltage obtained by advancing the voltage generated in the search coil by a required phase in the phase advancing circuit, the generation caused by the configuration circuit or the like. It is possible to reliably generate a synchronization signal that corrects the delay of the voltage and can obtain an appropriate drive timing.

Thirdly, since the search coil is composed of the central conductor of the shielded wire and the outer conductor of the shielded wire is grounded at one of both ends and the other is open, the heating coil is connected to the AC power supply line. Noise on the AC power supply line is prevented from being carried in the voltage generated in the search coil due to the direct connection, and the switching element can be driven well.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an induction heating device according to the present invention.

FIG. 2 is an exploded perspective view showing a configuration of a search coil portion in the first embodiment.

FIG. 3 is a circuit diagram showing a synchronization detection circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram showing operation waveforms of various parts during normal operation in the second embodiment.

FIG. 5 is a diagram showing operation waveforms of various parts when the inverter is started in the second embodiment.

FIG. 6 is a perspective view showing a modified example of the search coil in the first and second embodiments.

FIG. 7 is a circuit diagram when the search coil is applied to the synchronization detection circuit of the first embodiment.

FIG. 8 is a circuit diagram showing an induction heating device including a conventional synchronization detection circuit.

[Explanation of symbols]

 4 Heating Coil 5 Voltage Resonance Inverter 6 Switching Element 10, 20 Synchronous Detection Circuit 14, 15 Search Coil 16 Comparator 17 Pot (Heated Object) 19 Ferrite (Magnetic Core) 21 Phase Advance Circuit

Claims (3)

[Claims] 1. A heating coil that induction-heats an object to be heated by applying a high frequency, a resonance type inverter that uses the heating coil as a resonance inductor, and periodically switches a DC voltage by a switching element to convert the DC voltage into the high frequency. In an induction heating device having a synchronization detection circuit that generates a synchronization signal that takes a drive timing of a switching element, the synchronization detection circuit is wound around a magnetic core that constitutes a part of a magnetic circuit provided near the heating coil. The induction heating device is configured to generate the synchronizing signal based on a voltage generated in the search coil. 2. The synchronization detection circuit is configured to generate a synchronization signal based on a voltage obtained by advancing the voltage generated in the search coil by a required phase in a phase advancing circuit. Item 1. The induction heating device according to item 1. 3. The search coil is configured by using a center conductor of a shield wire, and the outer conductor of the shield wire is configured such that one of both ends is grounded and the other is open. The induction heating device according to claim 1 or 2.