JPS6137748B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an induction heating cooker. Induction heating cookers rectify commercial AC power and convert it into DC voltage such as pulsating current, and use this DC power to oscillate an inverter at a high frequency of approximately 20 to 40 KHz, thereby inducing the high frequency alternating current generated. In addition to the heating coil, a magnetic field is generated, and this magnetic field is applied to a cooking pot made of iron metal fittings placed close to the induction heating coil to induction heat the cooking pot. Fig. 1 shows the main circuit including an inverter of this type of cooker, 1 is an AC power supply, 2 is a power switch, 3 is a rectifier circuit, 4 and 5 are a high frequency filter circuit composed of a chiyoke coil and a smoothing capacitor. are doing. Reference numeral 6 denotes an inverter comprising an induction heating coil 7, a resonant capacitor 8, a damper diode 9, and a switching element 10. As the switching element 10, a semiconductor switching element such as a transistor or a GTO (gate turn-off) thyristor is used. (CT) is a current transformer that detects input current, and 11 is a cooking pot. FIG. 2 is a circuit block diagram for driving and controlling the main circuit 12, in which 13 is a starting signal generation circuit that provides an inverter oscillation recording signal, and 14 is a load that discriminates between no load, small object load, and appropriate load. Detection circuit, 15
is an on-signal generation circuit when performing self-excited oscillation,
An on signal is applied to the switching element 10 which is in the off state to make it conductive. 16 is an input power setting circuit that detects the input current to the main circuit 12 using a current transformer CT and compares it with a preset value to control the input power; 17 is an output from the input power setting circuit 16; In response to this, a frequency control circuit that determines the off timing of the switching element 10 generates an off signal for the switching element 10 so that the oscillation frequency becomes a frequency corresponding to the set power and load. 18 is activated when the load detection circuit 14 detects no load, an inappropriate load, or a small load;
This latch circuit 18 inhibits the next stage NAND gate 19, and is released by inputting a start signal. 20 is a waveform shaping circuit, and 21 is a gate drive circuit for the switching element 10. However, when the activation signal is now generated, this activation signal passes through the waveform shaping circuit 20 and the gate drive circuit 21 and is applied to the gate of the switching element 10 in the main circuit 12, making it conductive and causing the inverter 6 to start oscillating. When the switching element 10 becomes conductive and power begins to be supplied to the load, its input current becomes current.
The input power is applied to the input power setting circuit 16 via the transformer CT, and is compared with a predetermined voltage level corresponding to the frequency value, and the frequency control circuit 1 is adjusted to match the predetermined voltage level.
The frequency is controlled by 7. This frequency control is performed by changing the conduction period of the switching element 10. If the load is an accessory load, the load sensing circuit 14 is activated and sets the latch circuit 18. Therefore, Nandgate 19 is banned,
The on signal generated by the on signal generation circuit 15 is not transmitted to the waveform shaping circuit 20, and oscillation is stopped. In a cooker having such a configuration, an excessive current may flow through the switching element 10 depending on the material of the cooking pot 11. For example, a GTO thyristor is used as the switching element 10, and the pot material is 18
-8 stainless steel (18% chromium, 8% nickel)
), the GTO thyristor has approximately
A current of 60 A or more flows through the device, and although it does not destroy the device, it causes abnormal heat generation that accelerates device deterioration. Incidentally, at an appropriate load, the element current is approximately
50A, and the rated current of the GTO thyristor is
It is about 70A. The present invention aims to prevent the generation of such excessive current and protect the switching element 10. That is, in the present invention, a protection circuit 22 is newly added as shown in FIG. Protection circuit 2 below
The specific structure and operation of 2 will be explained based on FIGS. 4 and 5. A resonance voltage Vp is applied to the base of the transistor 23 via a diode 24. Resistors 25, 26, 27, and 28 are divided resistors, and set the base potential to turn on the transistor 23 when Vp is positive. 29 is a noise absorption capacitor, and 30 is a speed up capacitor. The transistor 23 outputs a positive voltage signal to the collector during a period when Vp is negative, that is, during a period when the diode 9 is conductive. This signal passes through an integrating circuit 33 consisting of a resistor 31 and a capacitor 32, and is input to the ○+ input terminal of a comparator circuit 34. A constant voltage is applied to the ○− input terminal of the comparator circuit 34.
A reference potential signal obtained by dividing Vcc by resistors 35 and 36 is provided. The output of the comparison circuit 34 is
The signal is applied via resistors 37 and 43 to the + reference input terminal of a comparator circuit 38 that constitutes the frequency control circuit 17. The frequency control circuit 17 controls the oscillation frequency according to the input set by the input power setting circuit 16. After that, it is converted into a potential level signal,
Divided by dividing resistors 43 and 44 to comparator circuit 38
Input to the ○+ input terminal. Here, the dividing resistor 43,
Reference numeral 44 maintains the reference level of the comparator circuit 38 so that the oscillation frequency does not fall below approximately 20 KHz when no output is generated from the input power setting circuit 16, thereby preventing the generation of audible noise. 45 is about 20~ from the waveform shaping circuit 20
A 40KHz oscillation pulse is applied to its base,
The transistor operates on and off, and its collector potential is input to the - input terminal of the comparator circuit 38. Reference numeral 46 denotes a capacitor connected in parallel with the transistor 45. Since the transistor 45 is charged during the OFF period and discharged during the ON period, the collector potential of the transistor 45 has a sawtooth waveform. A switching element 10 is connected to the output of the comparison circuit 38.
OFF signal is obtained. When heating an appropriate load, the conduction period of the diode 9 during one resonance period is relatively short as shown by the solid line waveform in FIG.
The charging period of the capacitor 32 is short and the discharging period is long. Its output is therefore kept at a low level. This integrated signal level is determined by the comparator circuit 34.
Since it is lower than the ○+ reference potential V+, the comparator circuit 3
The four outputs remain at the "H" level, and the frequency control circuit 17 performs the heating operation with the set input without being restricted by the protection circuit 22. On the other hand, when a pot made of 18-8 stainless steel (containing 18% chromium and 8% nickel) is heated, the conduction period of the diode 9 during one resonance period becomes longer, as shown by the broken line waveform in FIG. Therefore, the charging period of the capacitor 32 of the integrating circuit 33 becomes longer, the discharging period becomes shorter, its output level increases, and the capacitor 32 of the integrating circuit 33
exceeds the reference level V+. Therefore, the output of the comparison circuit 34 changes to the "L" level, and the resistor 37 is connected in parallel to the capacitor 42. As a result, the reference level of the comparator circuit 38 changes from V ₀ to V ₁
As a result, the output signal period of the comparator circuit 38 becomes shorter. The table below shows the measurement results for the iron enamel pot (appropriate load) and the 18-8 stainless steel pot.

[Table] (A): When the protection circuit operates
(b): When no protection circuit is provided As is clear from this table, due to the operation of the protection circuit 22, both the frequency and the current peak value become almost the same values as the appropriate load. In the above example, the output of the integral circuit 33 is, in the case of an iron enamel pot,
The result was 1.2V, and 1.87V for the 18-8 stainless steel pot. Therefore, the reference potential V of the comparison circuit 34
+ is within the range of both voltage values above, for example, in the middle,
It should be set to about 1.7V. As described above, the induction heating cooker of the present invention prevents the generation of excessive current flowing through the inverter when heating a load made of a special material, thereby preventing thermal damage to the switching elements and extending the life of the inverter. be able to. Furthermore, whereas conventionally, overcurrent measurement of switching elements was carried out by inserting a current transformer on the inverter side, the present invention
Since there is no need for a current transformer that complicates the circuit configuration and has problems with insulation, the circuit configuration can be simplified and reliability can be improved.

[Brief explanation of the drawing]

Fig. 1 is a general circuit diagram of a main circuit including an inverter in an induction heating cooker, Fig. 2 is a block diagram including a control section, Fig. 3 is a block diagram of an embodiment of the present invention, and Fig. 4 is a main part. The circuit diagram and FIG. 5 are signal waveform diagrams. 6... Inverter, 12... Main circuit, 17... Frequency control circuit, 22... Protection circuit.

Claims (1)

[Claims] 1. A high-frequency inverter including an induction heating coil, a switching element and a diode connected in antiparallel, an oscillation frequency control circuit that turns on and off the switching element, and a signal according to the input to the oscillation frequency control circuit to vary its frequency. an input power setting circuit that detects the proportion of the conduction period of the diode during one oscillation period of the inverter to obtain a voltage signal, and sets the signal to a predetermined voltage level determined based on the appropriate current of the switching element; When the above occurs, the oscillation frequency control circuit is provided with a protection circuit that reduces the input to the inverter, and the oscillation frequency control circuit is configured to set the minimum oscillation frequency to a value higher than the audible frequency when the input power setting circuit is not operating. An induction heating cooker comprising adjustment means.