JPH0124353B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an induction heating device, and particularly to a control circuit thereof.

(b) Conventional technology Conventionally, a transistor inverter device has been used in a household induction heating cooker, which is an application of induction heating. Transistor inverter circuits that perform high power conversion at ultrasonic frequencies utilize LC resonant circuits, but their control methods are quite limited by the LC resonant circuits.

That is, in the past, when the heating coil 1 and resonant capacitor 2 of the inverter circuit shown in FIG. Since it was driven by a transistor, if the oscillation frequency was increased by shortening the ON time of the transistor, the amplitude of the collector voltage of the transistor would become smaller, and the collector voltage would not become zero, making it impossible to obtain the ON timing of the transistor. It was hot. For this reason, for example, Japanese Patent Publication No. 58-36473 discloses a method of comparing the DC power supply voltage for supplying power to the inverter circuit with the collector voltage and supplying the base drive current to the transistor after a predetermined time delay.

(c) Problems to be solved by the invention However, in this method of driving a transistor by comparing the DC power supply voltage and the collector voltage of the transistor, if the oscillation frequency of the inverter is changed to adjust the output, Correspondingly, the collector voltage in each oscillation changes and the fall state of this collector voltage changes over time, whereas the DC power supply voltage does not change. Therefore, although the appropriate timing to apply base current to the transistor varies depending on the oscillation frequency of the inverter, in reality the delay time until the base current is applied is constant, so depending on the oscillation frequency the transistor The drawback was that the switching loss was large and the heating efficiency was poor.

(d) Means for Solving the Problems The present invention provides a control circuit that variably controls the input to the inverter by adjusting the ON time length of the switching element, and a control circuit that variably controls the input to the inverter by adjusting the ON time length of the switching element, and a voltage between the terminals of the switching element when the resonance circuit resonates. a resonance detection means for detecting that has become below a predetermined value, and after detection by this resonance detection means,
The switching element is activated after a predetermined time delay.
Generate a timing signal to turn on
ON timing generation means is provided, and the delay time in the ON timing generation means is adjusted according to the control state of the control circuit.

(E) Effect: Turns ON depending on the input control status of the above control means.
Since the delay time of the timing generation means is adjusted, the switching element turns ON when the voltage between the terminals of the switching element becomes zero or when the voltage between the terminals becomes the minimum.
It can output timing signals.

(F) Embodiment Figure 1 is a circuit diagram of the induction heating device of the present invention, in which 5 is a commercial AC power source, 6 is a rectifier circuit for full-wave rectification of this commercial AC power source, and 7 is an input terminal of this rectifier circuit 6. This is a tied chiyoke coil. 8 is a bypass capacitor connected to this chiyoke coil 7,
9 and 10 are a heating coil and a resonant capacitor connected in series between the chiyoke coil 7 and the input terminal of the rectifier circuit 6; 11 is the resonant capacitor 10;
a switching transistor provided in parallel with
A diode 12 is connected in antiparallel to the transistor 11, and the bypass capacitor 8 to the diode 12 constitute an inverter circuit 13. 14 is the above inverter circuit 13
shows the control circuit that controls the operating state of the
The collector voltage (collector-emitter voltage) of the transistor 11 is detected at the terminals 15 and 16, and the integrated voltage (average voltage) of this collector voltage is generated.The collector voltage is compared with the integrated voltage to determine whether the collector voltage is the same. A voltage is generated between terminals 17 and 18 after a predetermined time delay after the voltage becomes lower than the integrated voltage, and a base current is supplied to transistor 11.

FIG. 3 is a circuit diagram of the main part of the control circuit 14, and 19 is a rectifier circuit 6 from the commercial AC power supply 5.
A current transformer detects an input current applied to the current transformer, and the detected current is rectified by a diode 20 and charged into a capacitor 21. 22 is a variable resistor that sets the input to the inverter circuit 13;
3 is the charging voltage of the capacitor 21 and the variable resistor 2
A first comparator is shown which compares the voltage levels from 2 to 2, and depending on its output, the voltage Vref held by capacitor 24 is adjusted. The current transformer 19 to the capacitor 24 constitute a control circuit that adjusts the input to the inverter. 25 is a second terminal which receives the above Vref at its input terminal.
The output of the comparator is a drive signal for the transistor 11. Reference numeral 26 denotes a charging/discharging capacitor that is charged with a constant voltage Vcc via a resistor 27 and a diode 28, and discharged via a resistor 29. And this capacitor 26,
Turned on by resistor 29 and second comparator 25
It forms timing generation means. Further, the charging voltage of the charging/discharging capacitor 26 is applied to the input terminal of the second comparator 25. 30 is a third comparator in which the anode of the diode 28 is connected to the input terminal and the cathode is connected to the input terminal, and its output terminal is connected to the resistor 27 via the resistor 31, and
9 to the charge/discharge capacitor 26. Reference numeral 32 denotes a fourth comparator that receives the resonant voltage V _cE of the collector of the switching transistor 11 and the integrated voltage Vc of this resonant voltage, where V _cE <
Outputs “H” signal when _VcE > Vc
When this happens, an “L” signal is output. That is, this fourth comparator 32 serves as resonance detection means.
33 is a differentiating circuit that receives the output of this fourth comparator 32, and 34 is a transistor inserted between the output terminal of the third comparator 30 and the output terminal of the second comparator 25, which receives the signal transmitted from the differentiating circuit 33. Turns on.

In such an induction heating device, the cooking utensils are first placed close to the heating coil 9, and when the power is turned on,
When Vcc rises, the charging/discharging capacitor 26 is charged via the resistor 27 and the diode 28. At the start of oscillation, a starting pulse is input to the differentiating circuit 32 from a starting pulse generating circuit (not shown). As a result, a pulse is applied from the differentiating circuit 33 to the transistor 34, and this transistor 34 is turned on for a predetermined period of time. For this reason, the resistor 27,
31 connection point voltage V _B rapidly decreases and becomes lower than charging voltage V _A of charging/discharging capacitor 26 . That is,
The input terminal voltage of the third comparator 30 becomes lower than the input terminal voltage. As a result, the output of the third comparator 30 becomes a low level, and the charge in the charge/discharge capacitor 26 is discharged via the resistor 29. As a result, when the charging voltage V _A decreases and becomes lower than the holding voltage Vref of the capacitor 24, the output Vp of the second comparator 25 becomes "H" level, turning on the switching transistor 11. Thereafter, the discharge of the charge/discharge capacitor 26 progresses, and when V _A falls below V _B and the output of the third comparator 30 becomes "H", the charge/discharge capacitor 26 is charged with the Vcc voltage via the resistor 27 and the diode 28. is started. As a result of this charging, the charging voltage V _A of the charging/discharging capacitor 26 rises and becomes higher than the above-mentioned Vref, the output Vp of the second comparator 25 becomes "L", and the switching transistor 11 becomes
It will be turned off. After that, the charging/discharging capacitor 26 is charged. Turning off these switching elements
Thereafter, due to the resonance of the heating coil 9 and the resonant capacitor 10, the collector voltage of the switching transistor 11 oscillates in a waveform that rises greatly and then falls again. A voltage V _cE obtained by dividing this voltage is input to the input terminal of the fourth comparator 32, and a voltage Vc obtained by dividing this resonant voltage and integrating and smoothing is input to the input terminal of the fourth comparator 32, and V _cE is input to the input terminal of the fourth comparator 32.
When the voltage drops below Vc, this fourth comparator 32
The output V _D rises to “H” and the differentiating circuit 33
sends a pulse to transistor 34 and turns it on for a predetermined time
urge As a result, the charging/discharging capacitor 26 starts discharging again in the same manner as described above, and when the voltage V _A of the charging/discharging capacitor 26 falls below Vref, the second comparator 25 output Vp becomes "H" again, and the switching transistor 11 turns off. Turned on.
Further, in such a device, the Vref is adjusted by comparing the set level of the variable resistor 22 with the charging level of the capacitor 21 according to the input current using the first comparator 23. Therefore, when the level of the variable resistor 22 is set low, the charging voltage of the capacitor 24 becomes low, and the switching transistor 11 is turned on.
As the period length becomes shorter, the oscillation output of the inverter circuit 13 becomes lower. On the other hand, when the variable resistance level is set high, the charging voltage of the capacitor 24 also becomes high, the ON period of the switching transistor 11 becomes long, and the oscillation output of the inverter circuit 13 becomes high.

Furthermore, after the fourth comparator 32 detects that the collector voltage of the switching transistor 11 has become lower than its integrated voltage, which is the comparison voltage, the switching transistor 11 is actually turned on.
The delay time until the charge/discharge capacitor 26 and the resistor 29 determine the time constant and the holding voltage of the capacitor 24 determine the delay time until the capacitor 24 is charged. That is, as Vref becomes higher, the delay time becomes shorter, and as Vref becomes lower, the delay time becomes longer. Therefore, when the ON period is long, the output is high, and V _cE falls quickly, the above delay time is short, and when the ON period is short, the output is low, and V _cE falls slowly, the delay time is short. The above delay time becomes longer. Therefore, even if the input setting level is changed from a high level to a low level, the switching transistor 11 is actually turned on when the collector is at zero voltage or minimum voltage.

Furthermore, in this embodiment, since the integrated voltage of the collector voltage of the switching transistor 11 is used as a level for comparing whether or not the collector voltage has become lower than a predetermined voltage, The comparison level changes, and the ON timing of the switching transistor 11 can be generated more stably. FIG. 4 shows operating waveforms when the ON period length of the switching transistor 11 of such a device is gradually shortened.

In this embodiment, the switching transistor 1
Although the integrated voltage of the collector voltage is used as a comparison level with the collector voltage of 1, this may be a constant voltage at a certain level, or may be a voltage between the heating coil 9 and the bypass capacitor 8, for example.

Furthermore, in this embodiment, a unidirectional switching transistor is used as a switching element in the inverter circuit, but as shown in FIG.
It is also possible to omit 2. That is, in a configuration in which zero voltage or minimum voltage of the voltage between the terminals of the switching element can be detected as in the present invention, the diode current after the resonance period, that is, the feedback current can be caused to flow through the bidirectional conducting element 35.

Incidentally, as such a bidirectional conducting element 35, the above-mentioned
In addition to FETs (including MOS type and junction type), SIT (static induction transistor), etc. can also be used.

(G) Effects of the Invention As described above, the induction heating device of the present invention reduces the input control level in the control circuit by reducing the delay time from when the resonance voltage becomes lower than a predetermined voltage to when the switching element is turned on by the resonance detection means. Since the oscillation output of the inverter circuit is changed, the switching element is always turned on when the voltage between the terminals is minimum or zero, even if the ON time of the switching element is changed.
An induction heating device with low switching loss and high heating efficiency is provided.

[Brief explanation of drawings]

Fig. 1 is a circuit block diagram of the induction heating device of the present invention, Fig. 2 is a circuit diagram of a general inverter circuit,
FIG. 3 is a circuit diagram of a control circuit, FIG. 4 is a waveform diagram for explaining the operation of the apparatus of the present invention, and FIG. 5 is a circuit block diagram of another embodiment of the induction heating apparatus of the present invention. 5... AC power supply, 6... Rectifier circuit, 8... Bypass capacitor, 9... Induction heating coil, 10...
... Resonance capacitor, 11 ... Switching transistor, 12 ... Diode, 13 ... Inverter circuit, 22 ... Variable resistor, 21, 24, 26 ...
... Capacitor, 23, 25, 30, 32 ... Comparator, 27, 29, 30 ... Resistor, 28 ...
Diode, 33... Differential circuit, 34... Transistor.

Claims (1)

[Claims] 1. A DC power source, an induction heating coil coupled to the DC power source, a resonant capacitor forming a resonant circuit together with the induction heating coil, and a switching element connected to the resonant circuit to generate a resonant current in the resonant circuit. In an induction heating apparatus having an inverter circuit that generates an oscillating current in the resonant circuit by controlling ON/OFF of the switching element, the ON time length of the switching element is adjusted to control the input to the inverter. A control circuit that performs variable control; a resonance detection means that detects that the voltage between the terminals of the switching element becomes lower than a predetermined voltage when the resonance circuit resonates; and a predetermined time delay after detection by the resonance detection means. ON timing generation means for generating a timing signal for turning on the switching element after the
An induction heating device characterized in that the delay in the ON timing generation means increases as the input control level in the control circuit decreases.