JPH038077B2 - - 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 40kHz, 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 ferrous metal placed close to the induction heating coil to induction heat the cooking pot.

Figure 1 shows the main circuit including the inverter of this type of cooker, 1 is an AC power supply, 2 is a power switch,
3 constitutes a rectifier circuit, and 4 and 5 constitute a high frequency filter circuit by a chiyoke coil and a smoothing capacitor. Reference numeral 6 denotes an inverter, which includes an induction heating coil 7, a resonant capacitor 8, a damper diode 9, and a switching element 10. Switching element 1
0, a semiconductor switching element such as a transistor or a CTO (gate turn-off) thyristor is used. (CT) is a current transformer that detects input current, and 11 is a cooking pot. Figure 2 shows
This is a circuit block diagram for driving and controlling the main circuit 12, in which reference numeral 13 is a start signal generation circuit that provides an inverter oscillation start signal, 14 is a load detection circuit that discriminates between no load, small load, and appropriate load, and 15 is a self-excited oscillation circuit. The on-signal generating circuit used when performing this applies an on-signal 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), compares it with a preset value, and controls the input power; Upon receiving the output, a frequency control circuit that determines the off timing of the switching element 10 so that the oscillation frequency becomes a frequency corresponding to the set power generates an off signal for the switching element 10. Reference numeral 18 denotes a latch circuit which is activated when the load detection circuit 14 detects no load or a small load, and inhibits the next stage NAND gate 19. This latch circuit 18 is released by inputting a start signal. 20 is a waveform shaping circuit, 2
1 is a gate drive circuit for a switching element.
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, the input current is applied to the input power setting circuit 16 via a current transformer (CT) and compared with a predetermined voltage level corresponding to the frequency value. The frequency is controlled by the frequency control circuit 17 to match this. 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, the NAND gate 19 is prohibited, the ON signal generated by the ON signal generation circuit 15 is not transmitted to the waveform shaping circuit 20, and oscillation is stopped.

For a cooker with such a configuration, the input power range is, for example, from 200W to 1200W.
It is set within a predetermined range. In this case, the upper limit is often controlled by frequency, usually 20kHz.
At this time, it is designed to obtain an input of approximately 1200W. On the other hand, the lower limit is often limited by power, and is set at about 200W.

However, even if the input range is set to about 200 to 1200W using standard loads such as iron pots, loads of other materials, such as 18-8 stainless steel (contains 18% chromium and 8% nickel) - iron -18-8 stainless steel laminated pot actually has about 225 or
The input range will be 1200W, and with an 18 stainless steel pot that also contains 18% chromium, the input range will be approximately 150W.
Or 1100W. Especially in the former case, if the input is set to the minimum input, the output of the comparator circuit included in the input power setting circuit will be fixed at the "L" level.
A problem arises in that the operation of the frequency control circuit at the next stage that receives this is stopped, and the oscillation of the inverter is stopped.

The present invention has been made in response to such circumstances, and when the input power setting circuit is set to the minimum value, the minimum input is fixed to, for example, 200W regardless of the material of the load, and the minimum input is maintained at a constant minimum input. This is an inverter oscillation.

An embodiment of the present invention will be described below with reference to FIGS. 3 to 5. FIG. 3 shows a specific example of the input power setting circuit 16 and the frequency control circuit 17. The input current detected from the current transformer (CT) is adjusted in value by a resistor 22 and a variable resistor 23, and then sent to a rectifier circuit 24. input. The rectifier circuit 24 has a fourth
Pulsating flow signal (frequency 100Hz or
120Hz), and inputs it to the input terminal of the comparator circuit 27 through a filter circuit consisting of a resistor 25 and a capacitor 26. The input terminal of the comparator circuit 27 receives a potential signal V+, which is set by dividing the power supply voltage +Vcc by the resistors 28, 29, 30 and the variable resistor 31.
is given. By adjusting the variable resistor 31, the input power is set within the range of 200W to 1200W.
As shown in waveform B in FIG. “H”
The level period becomes shorter if the set level V+ is lowered (input decrease), and becomes longer if V+ is increased (input increase). The output of the comparator circuit 27 whose “H” level period is set according to the input is the divided resistor 3
2, 33, resistor 34 and capacitor 35
The voltage is applied to an integrating circuit consisting of a voltage level signal and converted into a voltage level signal. Furthermore, this voltage level signal is divided by resistors 36 and 37 to become a signal C (V+) and is input to the input terminal of a comparator circuit 38. On the other hand, the input terminal of the comparator circuit 38 is given the output of a time constant circuit made up of a resistor 39 and a capacitor 40. This time constant circuit repeats charging and discharging in synchronization with the on/off of the transistor 41 connected in parallel to the capacitor 40, and generates a sawtooth waveform shown in waveform D (V-) in FIG. Get a signal. The transistor 41 is connected to the waveform shaping circuit 20 by
It receives an inverter oscillation frequency signal of approximately 20 to 40kHz and is driven on and off. When the signal D (V-) reaches the level signal C (V+), the comparison circuit 38 outputs an "L" level pulse signal E, which is as follows.
It is applied to the waveform shaping circuit 20 as an off signal for the switching element 10. In this way, by advancing or delaying the off signal output timing, the frequency can be changed, the peak of the high frequency current flowing through the inverter 6 can be controlled, and the input can be varied by controlling the on time of the switching element. .

Waveforms A', B', C' shown by points A and B in Figure 4,
D' and E' indicate cases in which the signal level V- of the comparison circuit 27 does not fall below the set level V+, and this phenomenon occurs when the input power is set to the minimum. In other words, 18-8 stainless steel-iron-18 with high equivalent resistance
-8 Switching element 1 for stainless steel loads
Even if the 0 ON time is the same as the standard load, the heating input increases and the pulsating current signal A' (V-) detected by the current transformer becomes at a higher level than the standard load. Therefore, if the level V+ of the variable resistor 31 is set to the minimum (actually below the input level of 225 W), the pulsating current signal A' will not fall below the set level V+. At this time, the output of the comparison circuit 27
B′ is fixed at “L” level, but
The input terminal of the comparator circuit 38 receives a voltage signal C' (V
+), the comparator circuit 38 operates and outputs an "L" level pulse E' at a high frequency to maintain the oscillation of the inverter. This minimum reference level signal
C'(V+) is selected to be a value higher than the minimum detectable level of the comparison circuit 38, and is set so as to obtain a minimum input of 200W.

Figure 5 A, B, and C show the relationship between input power and input setting range due to differences in pot material. , compatible with 1200W and 200W, respectively. B is when using a pot with a three-layer structure of 18-8 stainless steel - iron - 18-8 stainless steel, and the input can be adjusted continuously from 225W to 1200W, and the minimum input is 200W. Can be done. C is when using a 18 stainless steel pot, and the input can be continuously adjusted from 200W to 1100W. By the way, conventionally, in the case of B, the input was 200W.
If it is not possible to obtain and the input is set to the minimum, oscillation may stop, and in the case of C,
The minimum input had dropped to about 100W.

As described above, in the induction heating cooker of the present invention, a voltage level signal corresponding to the minimum input is given to the frequency control circuit by resistance division, so even if the material of the pot is 18 stainless steel, the minimum input is It is possible to always match the set value, and it is possible to eliminate the conventional variations in input values due to differences in materials. Also, when using a load made of 18-8 stainless steel, iron, and 18-8 stainless steel, the output of the input power setting circuit remains at the "L" level and no signal is input to the frequency control circuit. Even in such a case, the frequency control circuit can operate normally by applying the minimum voltage level, and the heating operation can be continued with the minimum input.

[Brief explanation of the drawing]

Fig. 1 is a general circuit diagram of the main circuit including an inverter in an induction heating cooker, Fig. 2 is a block diagram including a control section, Fig. 3 is a circuit diagram of an embodiment of the present invention, and Fig. 4 A and B. 5 is a waveform diagram, and FIG. 5 A, B, and C are characteristic diagrams showing the relationship between input and input setting range. 6... Inverter, 12... Main circuit, 16...
Input power setting circuit, 17...frequency control circuit.

Claims (1)

[Claims] 1. In an induction heating cooker that has a high-frequency inverter including an induction heating coil and a switching element, and whose output is controlled by varying the oscillation frequency of the inverter, an input power setting circuit that outputs a signal corresponding to an arbitrarily set input power. , a frequency control circuit that receives the output of the input power setting circuit and drives the switching element ON and OFF to oscillate the inverter at an oscillation frequency corresponding to the output; The present invention is characterized in that ON time compensation means for compensating for the minimum ON time is provided, so that when the input power setting circuit is set to a low input, at least a predetermined minimum input can be obtained regardless of the material of the load. Induction heating cooker.