JPH0345519B2 - - Google Patents

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to an induction heating cooker.

(b) Conventional technology Induction heating cookers rectify commercial AC power and convert it into DC voltage such as pulsating current, and this DC power causes an inverter to oscillate at a high frequency of about 20 to 40kHz, which generates electricity. A high frequency alternating current is applied to an induction heating coil to generate a magnetic field, 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.
Such an induction heating cooker is disclosed in, for example, Japanese Patent Application Laid-open No. 55-
No. 136492.

(c) Problems to be Solved by the Invention Incidentally, in such an induction heating cooker, the reference level that determines the ON time of the ON switching element is maintained by a capacitor for stabilization. The oscillation is started by lowering the signal and shortening the ON time of the switching element to prevent a sudden current from flowing through the switching element. For this reason, the capacitor had to be discharged to lower the above reference level after the oscillation stopped and before the restart started, but when restarting due to small object detection, etc., the oscillation restarts immediately after the oscillation stops. There may be cases where In such a case, there was a problem in that the capacitor was not sufficiently discharged and the overcurrent at the time of starting the oscillation was not sufficiently prevented.

(d) Means for Solving the Problems The present invention has been made in view of the above points, and provides a time constant for determining the ON time length of the switching element by comparing its output with the above reference level. The time constant of the circuit is changed to reduce the time constant.

(e) Effect With the above configuration, the ON time length of the switching element can be quickly switched, and even when oscillation is started immediately after oscillation has stopped, the ON time length of the switching element can be reliably kept short. It becomes possible to start oscillation with a small switching element current.

(F) Embodiment Figure 1 shows the main circuit of this type of cooker including an inverter, 1 is an AC power supply, 2 is a power switch, 3 is a rectifier circuit, 4 and 5 are high frequency coils and smoothing capacitors. It constitutes a filter circuit. 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. Switching element 1
0, 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. Figure 2 shows
This is a circuit block diagram for driving and controlling the main circuit 12, in which 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 object load, and appropriate load, and 15 is a self-excitation circuit. This is an on-signal generating circuit used when oscillating, and applies an on-signal to the switching element 10 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 when to turn off 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. Reference numeral 18 denotes a latch circuit which is activated when the load detection circuit 14 detects an unsuitable load or a small load, and inhibits the NAND gate 19 in the next stage.
9 is canceled by inputting a start signal. 20 is a waveform shaping circuit, and 21 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 the 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 cookers with this type of configuration, the number of loads
Since kW of power is added, inverter 6 has several
A high frequency current of 10A flows, switching element 10
This is the cause of thermal destruction. In particular, when the inverter 6 starts oscillating, the current flowing through the switching element 10 increases, reaching a peak of about 60 A or more, as shown in waveform E' (switching element off signal) and IL' (load current) in Figure 5B. . This means that at startup, the load current flows from the 0 point for T ₀ time, whereas in other periods, the load current flows from the negative position to T ₀
This is because time passes. Therefore, as shown in the waveform shown in Figure 5 (b), since the current flows from the 0 point at startup for T ₀ time, its peak is as large as 60 amperes, and the reverse current is also large, resulting in a long negative period. The peak value that appears again on the positive side also becomes smaller. Such an abnormally large current that appears at startup and the small current that follows this is a cause of startup noise.

The present invention has been made in view of these circumstances, and as shown in FIG.
is provided to forcibly shorten the on-time of the switching element 10 immediately after startup to prevent excessive current from flowing through the element 10. Embodiments of the present invention will be described below based on FIG. 4 and FIG. 5A. 22
is a start-up protection circuit, in which the start-up signal A generated from the start-up signal generation circuit 13 is passed through the capacitor 23 and the resistor 2.
The signal is applied to the base of the transistor 25 through a differentiating circuit consisting of 4 circuits. Therefore, transistor 2
Immediately after the activation signal is generated, the "H" level signal B appears at the collector of the circuit 5. On the other hand, frequency control circuit 17
includes a comparison circuit 26, and the signal from the input power setting circuit 16 is applied to the input terminal thereof as a voltage level signal. In other words, input power setting circuit 1
6 outputs a pulse signal whose output time width is changed according to the set input, and this pulse signal is
The signal is converted into a potential level signal through an integrating circuit consisting of resistors 27, 28, 29 and a capacitor 30, divided by dividing resistors 31 and 32, and inputted to the input terminal of a comparison circuit 26 as a reference level signal. Here, the dividing resistors 31 and 32 are arranged so that when no output is generated from the input power setting circuit 16, the oscillation frequency is approximately
The reference level of the comparator circuit 26 is maintained so that it does not fall below 20 kHz, thereby preventing the generation of audible noise. The input terminal of the comparison circuit 26 is also connected to the activation signal generation circuit 13 via a resistor 33 and a diode 34, and is supplied with the activation signal A. Therefore, during the period when the "L" level activation signal is generated, the reference level signal C is slightly lowered.

The output of a time constant circuit 37 made up of a resistor 35 and a capacitor 36 is applied to an input terminal of the comparator circuit 26 . A transistor 38 is connected in parallel to the capacitor 36 , and the base of the transistor 38 is connected to the base of the waveform shaping circuit 20 .
Since a pulse signal synchronized with the inverter oscillation frequency of ~40kHz is added, it repeats on and off at the same frequency. Therefore, the time constant circuit 37
The output is a sawtooth wave shown in waveform D, and when this reaches the reference level, the comparator circuit 26 generates an "L" level pulse E, which is applied to the waveform shaping circuit 20 as a switching element 10 off signal.

In this configuration, when the activation signal A is first output, the reference level signal C at the input terminal of the comparator circuit 26 is held at a slightly low level during this output period. On the other hand, the "H" level signal of the transistor 25, which is output in synchronization with the fall of the activation signal, is transmitted to the resistor 35.
A new resistor 39 is added to the circuit 39, which acts to lower the time constant of the time constant circuit 37. Therefore, the charging of capacitor 36 is accelerated and its output becomes T ₀ for a short period of time
When the reference level is reached, the off signal E is output. Since the output B of the transistor 25 is at the "L" level in the next high frequency oscillation cycle, the resistor 39 is cut off and the time constant returns to its original state. This is why it took a little longer than last time.
is charged. At this time, the period t ₀ ' until the reference level is reached is longer than the previous period t ₀ . When the output of the start signal A is turned off, the reference level C rises, so it takes a longer period for the charge in the capacitor 36 to reach the reference level.
T ₀ ″ becomes longer, and the magnitude relationship between these periods becomes T ₀ <T ₀ ′<T ₀ ″. The waveform I _L shows the load current flowing through the induction heating coil 7, and at the initial stage of startup,
The value can be set to about 40A, which is much lower than the current of about 50A during normal oscillation.

(G) Effects of the Invention In the induction heating cooker of the present invention, when the inverter oscillates, the switching element conduction period is made shorter than the period corresponding to the preset input power level. It is possible to prevent an abnormally large current from flowing through the device, prevent destruction and deterioration of the element due to heat generation, and improve the reliability and safety of the cooker.

Furthermore, when starting up the inverter oscillation, the conduction period of the switching element is made shorter than the period corresponding to the preset input power level, and then gradually increased. can further prevent the flow of

Furthermore, when shortening the conduction period of the switching element in this way, the time constant of the time constant circuit that measures the conduction period of the switching element is lowered to start oscillation (after that, the time constant is ) of the switching element.
The ON time length can be quickly switched, and even when oscillation starts immediately after oscillation stops, the ON time length of the switching element can be reliably kept short.
It becomes possible to start oscillation in a state where the switching element current is small.

[Brief explanation of drawings]

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 FIGS. 5A and 5B are waveform diagrams. 6... Inverter, 12... Main circuit, 17... Frequency control circuit, 22... Start-up protection circuit.

Claims (1)

[Claims] 1. A high-frequency inverter including an induction heating coil and a switching element, a starting signal generation circuit that gives a starting signal to the switching element and turns it on, and a level signal corresponding to the setting input to the reference input terminal in synchronization with the inverter oscillation frequency. A frequency control circuit that has a comparison circuit that inputs the time constant circuit output that repeats charging and discharging to the signal input terminal, and outputs a switching element OFF signal when the time constant circuit output reaches a reference level, and when the start signal is being generated, the above comprising a start-up protection circuit that reduces the time constant of a time constant circuit and then gradually increases the time constant; and an on-signal generation circuit that detects the resonance state of the inverter after the switching element is turned off and provides an on-signal to the switching element; An induction heating cooker characterized in that immediately after inverter oscillation is started, the input is lowered and then the input is gradually increased.