JP2000113973A - Induction heater device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized and low-cost induction heater device capable of reducing the loss in a switching element with a simple circuit structure and having a reliable protective function. SOLUTION: This induction heating device having a small loss and a reliable protective function is provided with a stop means having a current resonance inverter structure, cutting off a switching element 13 with a zero current, detecting the inverse conducting current of the switching element 13, and stopping it at the prescribed value or above and a clamping means self-clamping the switching element 13 at the prescribed value or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating device such as an induction heating cooker used in general households and restaurants, and more particularly, to a switching element of high-frequency power conversion means (hereinafter referred to as an inverter). And a protection operation.

[0002]

2. Description of the Related Art A conventional inverter circuit configuration and control method of an induction heating device will be described with reference to FIGS.

FIG. 8 shows a basic circuit of an inverter of an induction heating apparatus. Reference numeral 1 denotes a DC power supply, which is obtained from a commercial AC power supply via a rectifier. Reference numeral 2 denotes a heating coil, on which an object to be heated is placed and heated by a high frequency magnetic field from the heating coil 2. 3
Is a resonance capacitor connected in parallel with the heating coil 2, and 4 is a switching element, and a high-frequency current is supplied to the heating coil by turning on / off the element. The switching element 4 uses an IGBT which requires much less driving power than a bipolar transistor or the like, and has a withstand voltage of 90.
0V and the current rating is 60A. Reference numeral 5 denotes a reverse conducting diode connected in parallel to the switching element 4, and reference numeral 6 denotes a control circuit that detects a collector-emitter voltage of the switching element 4 and controls on / off of the switching element 4.

FIG. 9 is a diagram showing waveforms at various points during the operation of the inverter of FIG. (A) is a drive signal of the switching element 4 output from the control circuit 6, and the switching element 4 is turned on when HIGH. (A) shows the current flowing through the switching element 4 and the reverse conducting diode 5. (C) is a voltage generated between the collector and the emitter of the switching element 4.

FIG. 10 is an enlarged view of a collector current and a collector-emitter voltage during a period when the switching element 4 transitions from on to off (ie, at the time of current interruption = turn-off) in the operation waveform of FIG. In the figure, the tail current is I
This phenomenon is peculiar to the GBT, and the slower the switching speed of the element, the longer its occurrence period. The temperature characteristic of the tail current is positive, and the higher the temperature of the switching element, the longer the generation period and the greater the loss.

As described above, the loss of the switching element 4 caused by the operation of the present inverter can be divided into the conduction loss generated during the HIGH period of the drive signal in FIG. Are classified into two types.

[0007] The conduction loss is determined by the product of the collector current of the switching element 4 and the on-voltage correlated with the collector current. In general, the loss temperature characteristic of the conduction loss is almost flat or negative depending on the performance of the switching element.

In the present inverter, the switching element 4
Is turned on and off at a frequency of about 20 kHz to 30 kHz, and the generated loss depends on the element performance.
It is about 40W. Further, the ratio of the turn-off loss to the generated loss depends on the operating frequency, but is approximately 30 to 50.
%. Since the generated loss is large, the switching element 4 is attached to a heat sink and is forcibly cooled by a cooling fan.

Although not particularly shown in the figure, a current transformer for detecting an input current is provided on a power supply side for controlling input power supplied to a load.

[0010]

However, in the above-described conventional induction heating apparatus, since the cooling fan is driven during operation, the noise is loud and the user is uncomfortable. This noise is a problem particularly when the user surrounds and uses a cooking device such as a pan. In the case of application to a rice cooker, when the timer rice cooker or the like is operated in the early morning or late at night, the noise similarly gives a user discomfort. This problem is not a general electric heater type or a cooking device such as a gas stove, and is therefore a serious problem unique to an induction heating device. As for the temperature characteristic of the entire loss, the temperature characteristic of the turn-off loss is dominant, and when the element temperature of the switching element 4 rises,
Since the loss also increases, the design of forced air cooling with a fan needs to be sufficiently studied, and there is a problem in the development man-hour.

Another problem is that the power consumption during operation is large because the loss of the switching element 4 is large, and the control of the element may cause some abnormality (power failure such as instantaneous power failure or lightning surge or power supply abnormality). If the timing deviates from the predetermined timing due to external noise or the like, there is a possibility that a voltage higher than the withstand voltage of the switching element 4 may be generated. Generally, the switching element 4 is immediately destroyed when a voltage higher than the withstand voltage is applied. Therefore, although not particularly shown in the drawing, the voltage across the switching element 4 and the input voltage are detected, and oscillation is immediately performed in the event of an abnormality. A protection circuit for stopping is required. This protection circuit is one of the factors that increase the cost.

The present invention solves the above-mentioned conventional problems, and can sufficiently reduce the loss of the switching element, thereby reducing the cooling fan noise, the power consumption, and the cost reduction by a simple and reliable protection circuit. It is an object of the present invention to realize a comfortable and energy-saving and inexpensive induction heating device.

[0013]

In order to solve the above-mentioned problems, the present invention provides a heating coil for heating an object to be heated by a high-frequency magnetic field and a reverse conduction type switching for high-frequency switching of a current supplied to the heating coil. And a high-frequency power converter that has a configuration in which the high-frequency current flowing through the switching element is cut off when the current flowing through the switching element is zero as a resonance current, and a reverse conduction current of the switching element is detected. Stopping means for stopping high-frequency oscillation when the detection current is equal to or more than a predetermined value,
Clamping means for self-clamping the switching element when the voltage across the switching element is equal to or higher than a predetermined value.

[0014]

The invention according to claim 1 has a heating coil for heating an object to be heated by a high-frequency magnetic field, and a reverse conduction type switching element for switching a current supplied to the heating coil at a high frequency, The high-frequency current flowing through the switching element is a resonance current, a high-frequency power conversion unit configured to cut off when the current flowing through the switching element is zero, and a reverse conduction current of the switching element is detected. An induction heating apparatus comprising: a stop unit that stops high-frequency oscillation when the voltage is equal to or more than a predetermined value; and a clamp unit that self-clamps the switching element when the voltage across the switching element is equal to or more than a predetermined value. Things.

According to this configuration, the current flowing through the switching element has a resonance waveform and turns off while the current is zero or flows through the reverse conducting diode. Therefore, no turn-off loss occurs as a loss of the switching element. Only the loss results, and the loss of the switching element can be greatly reduced. Furthermore, since there is no turn-off loss, the loss temperature characteristic becomes almost flat or negative, and an inverter circuit which is extremely stable thermally and can be easily designed for cooling can be obtained.

Further, by providing the stopping means and the clamping means, a no-load start which may occur in a normal use state is stopped by the stopping means, and the withstand voltage of the switching element due to a power failure or a control circuit malfunction is reduced. In the instantaneous mode, the clamp operation can be performed first, and then the operation can be stopped by the stopping means, so that a simple and reliable protection function can be realized.

The invention according to a second aspect of the present invention has a detecting means for detecting a current flowing through the switching element, the input current is controlled to be constant using the detected current, and the reverse conducting current is detected by the detecting means. As a stopping means for detecting the

According to this configuration, as compared with the configuration of the first aspect, the input current detecting means and the current detecting section of the stopping means can be used in common, and the circuit can be reduced in size and cost can be reduced.

According to a third aspect of the present invention, in particular, there is provided a second stop means for detecting a voltage between both ends of the switching element and stopping high-frequency oscillation when the voltage between both ends is a predetermined value or more. It is.

With this configuration, it is possible to realize an induction heating device that can reliably protect the device even during an abnormal operation in which a reverse conduction current does not flow.

[0021]

(Embodiment 1) Hereinafter, a first embodiment of the present invention will be described. FIG. 1 is a diagram showing a first embodiment of the present invention. In FIG. 1, reference numeral 11 denotes a DC power supply, specifically, a commercial AC power supply is obtained via a rectifier. 12 is
The heating coil is connected in series to the DC power supply 11, and although not particularly shown in the drawing, an object to be heated such as a pot is placed on the coil. Reference numeral 16 denotes a resonance capacitor connected in parallel with the heating coil 12. A choke coil 15 is connected in series with the heating coil 12. Reference numeral 13 denotes a switching element, and in this embodiment, an IGBT having a withstand voltage of 900 V.
You are using A reverse conducting diode 14 is connected in parallel with the switching element 13. The high-frequency switching of the switching element 13 causes the heating coil 1
2 is supplied to the object to be heated through
Numeral 0 is a high-frequency power conversion means. With this circuit configuration, when a normal oscillation signal is applied to the switching element 13 during normal operation, the current flowing through the switching element 13 becomes a resonance current, and when the current is zero (the current flows through the reverse conducting diode). (Including when it is flowing).

Reference numeral 17 denotes a control circuit including an oscillation circuit, which controls the switching element 13. Reference numeral 18 denotes a self-clamping means, which has a predetermined value at both ends of the switching element 13 (specifically, a value higher than the voltage generated during normal operation and lower than the withstand voltage of the switching element 13; in this embodiment, When the voltage during normal operation is about 400 V and the withstand voltage of the switching element 13 is 900 V, the voltage is set to 600 V.) When the voltage becomes equal to or more than that, the switching element 13 is self-clamped. In the case of this embodiment, a withstand voltage of 600 V is applied between the collector side of the switching element 13 and the gate.
This is realized by inserting a zener diode and a reverse blocking diode connected in series.

Reference numeral 19 denotes stop means for detecting a reverse conduction current flowing through the switching element 13d and stopping the oscillation of the switching element 13 via the control circuit when the detected current is equal to or more than a predetermined value. In this embodiment, a current transformer is used as the current detecting means to detect only the reverse conduction current.

Although not particularly shown in the figure, power control in the present embodiment is performed by detecting an input current input from a commercial power supply to a rectifier by a current transformer.

FIG. 2 shows a drive signal of the switching element 13 and a current (Ic) voltage (Vce) waveform during normal operation.

In the figure, the switching element has a driving signal of HIG.
Turns on when H and turns off when LOW. The oscillation frequency in this embodiment is about 45 kHz. Fig. 2 (a)
As shown in the figure, the current waveform becomes a resonance waveform by adopting the present inverter circuit configuration, and the current is turned off while the current flowing through the switching element 13 is zero or the current is flowing through the reverse conducting diode 14. No loss occurs, and significant loss reduction is possible.

FIG. 3 shows a switching element at the time of no-load start-up (for example, when a load (a pan or the like) that is normally placed on the heating coil 12 is not placed due to a user forgetting to place it). 3 shows the waveforms of each part. In this embodiment, the oscillation is started when the input voltage of the commercial power supply is detected and the voltage is sufficiently small (ie, 50 Hz / 6).
It is set to a valley of the power supply voltage envelope which can be set to a frequency of 0 Hz. In the case of no load, in the inverter constant (the inductance value of the heating coil 12 and the like) of the present embodiment, as shown in FIG.
An extremely large current flows during the normal operation. When a large current flows through the reverse conducting diode 14, the recovery current flowing through the reverse conducting diode 14 also increases, and a surge voltage is generated in the voltage between both ends of the switching element as shown in (d).

As shown in (a), immediately after the start of the start, the surge voltage is small, but as the power supply voltage increases, the surge voltage also increases, and the current flowing through the reverse conducting diode 14 reaches a predetermined value. At this time, the oscillation is stopped by the stopping means 19. If the oscillation is not stopped (and the clamp means 18 is not present), a surge voltage exceeding the withstand voltage of the switching element 13 is generated near the power supply voltage peak, and the switching element 13 is destroyed. Further, depending on the allowable current of the reverse conducting diode 14, the excessive conducting current may cause the reverse conducting diode 14 to be thermally destroyed.

FIG. 4 shows waveforms of various parts of the switching element 13 when the oscillation starts erroneously near the peak of the power supply voltage and there is no load due to abnormal operation of the control circuit 17 due to external noise or the like. In this case, the surge voltage generated at the voltage between both ends of the switching element 13 becomes extremely large (exceeds the breakdown voltage without the clamping means 18) from the first time. No element destruction occurs.
In addition, since the stopping means 19 detects the reverse conducting diode current at this time and stops the subsequent oscillation, there is no possibility of continuous self-clamping. In general, the self-clamp of the switching element 13 causes an extremely large loss at the time of clamping, and the element generates a large amount of heat. Therefore, when continuous self-clamping is performed, there is a possibility that the switching element 13 may be thermally destroyed. Further, the destruction due to the first surge voltage cannot be avoided only by the stopping means 19.

As is apparent from the above description, according to the first embodiment, since the loss of the switching element 13 can be reduced with a simple structure, a small-sized and low-cost induction heating device requiring less cooling can be obtained. Can be. Stopping means 1
9, the load abnormality can be detected, and the stopping means 12
In the event of an abnormality that cannot be made in time, self-clamping means 1
8 operates to prevent the switching element 13 from being destroyed. Further, there is no need for a means for detecting the input voltage in the protection circuit as in the conventional induction heating device, and the cost can be reduced.

(Embodiment 2) Hereinafter, a second embodiment of the present invention will be described. FIG. 5 is a view showing a second embodiment of the present invention. In FIG. 5, the detecting means of the stopping means 19 detects the current flowing through the reverse conducting diode 14 in the same manner as in the configuration of claim 1, and further detects the current flowing through the switching element 13 in the same manner. The configuration is such that power control is performed within 17. With this configuration, as compared with the configuration of the first aspect, a current transformer for detecting an input current is not required, and a low-cost induction heating device can be realized.

(Embodiment 3) Hereinafter, a third embodiment of the present invention will be described. FIG. 6 is a diagram showing a third embodiment of the present invention. In FIG. 6, reference numeral 21 denotes the switching element 13.
Of the control circuit 17
This is the second stopping means for stopping the oscillation via.

FIG. 7 shows operation waveforms when the off period of the switching element 13 is longer than that during normal operation for some reason. As shown in the figure, the current flowing through the switching element 13 is in a mode in which no current flows through the reverse conducting diode 14, and when the current is forcibly turned off, a large surge voltage is generated across the switching element 13.

According to this embodiment, even in such a mode, the switching element 13 can be protected because the second stopping means is provided.

[0035]

As described above, according to the first aspect of the present invention, the current flowing through the switching element has a resonance waveform,
Turn off at zero current, no turn-off loss occurs, extremely low loss and thermally stable inverter can be realized, and a small and low-cost induction heating device with little required cooling can be obtained with a simple configuration. Can be. Further, by providing the stopping means and the clamping means, a more reliable induction heating device can be obtained with a simple configuration.

According to the second aspect of the present invention, in particular, since the current detection of the stop means is also used as the input current detection means, it is possible to provide an induction heating apparatus which is less expensive and has a small number of parts. .

According to the third aspect of the present invention, the protection operation can be performed even in an abnormal state in which the current does not flow through the reverse conducting diode, and a highly reliable induction heating device can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing operation waveforms during a normal operation;

FIG. 3 is a diagram showing an operation waveform at the time of an abnormal load.

FIG. 4 is a diagram showing operation waveforms when the control circuit malfunctions;

FIG. 5 is a circuit diagram showing a circuit configuration of an induction heating apparatus according to a second embodiment of the present invention.

FIG. 6 is a circuit diagram showing a circuit configuration of an induction heating apparatus according to a third embodiment of the present invention.

FIG. 7 is a diagram showing an operation waveform at the time of an abnormal operation.

FIG. 8 is a circuit diagram showing a circuit configuration of a conventional induction heating device.

FIG. 9 is a diagram showing operation waveforms of the same.

FIG. 10 is a diagram showing operation waveforms when the switching element is turned off.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 12 Heating coil 13 Switching element 18 Clamping means 19 Stopping means 20 High-frequency power conversion means 21 Second stopping means

Continued on the front page (72) Inventor Naoaki Ishimaru 1006 Kazuma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. Person Hideki Omori 1006 Kazuma Kadoma, Kadoma City, Osaka Pref. Matsushita Electric Industrial Co., Ltd. (72) Inventor Toshiyuki Kosaka 1006 Kadoma Kadoma, Kadoma City, Osaka Pref. AD07 AD23 AD25 AD28 AD30 AD35 AD37 BD07 CD10 3K059 AA03 AA08 AB04 AC03 AC07 AD07 AD23 AD25 AD28 AD30 AD35 AD37 BD07 CD10 CD48

Claims (3)

[Claims] 1. A heating coil for heating an object to be heated by a high-frequency magnetic field, and a reverse conduction type switching element for high-frequency switching of a current supplied to the heating coil, wherein a high-frequency current flowing through the switching element is: A high-frequency power conversion unit configured to cut off when a current flowing through the switching element is zero as a resonance current; and to detect a reverse conduction current of the switching element and stop high-frequency oscillation when the detected current is equal to or more than a predetermined value. Means for stopping,
An induction heating apparatus comprising: a clamping means for self-clamping the switching element when a voltage across the switching element is equal to or higher than a predetermined value. 2. A stopping means for detecting a current flowing through a switching element, controlling an input current to be constant using the detected current, and detecting a reverse conduction current by the detecting means. Item 7. An induction heating device according to Item 1. 3. Detecting a voltage across the switching element,
3. The induction heating apparatus according to claim 1, further comprising a second stopping means for stopping high-frequency oscillation when the voltage between both ends is equal to or more than a predetermined value.