JPS6142391B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and particularly to a control circuit thereof.

Conventionally, in induction heating cookers whose power source is a high-frequency inverter device using power switching semiconductors, the material of the metal pot or the size of the pot changes greatly, resulting in extremely large load fluctuations. The disadvantage was that the usage conditions were very strict. In particular, in induction heating cookers, in order to control the heating output to a desired level, it has been common to detect the input current and perform feedback control so as to achieve the output set by the user. However, if the input current is simply controlled to a predetermined level, load fluctuations or fluctuations in the input power supply voltage will place a large load on the power switching semiconductor, increasing losses and increasing temperature. There were drawbacks such as destruction of the switching semiconductor. The present invention eliminates the above drawbacks and provides a control circuit for an induction heating cooker that detects load fluctuations and input power supply voltage fluctuations and controls conduction of a power switching semiconductor.

Hereinafter, one embodiment of the present invention will be described in detail according to the drawings.

FIG. 1 shows an embodiment of an inverter device for an induction cooking device according to the present invention, and FIG. 2 shows an embodiment of a block diagram of a control circuit according to the present invention. FIG. 3 is a characteristic diagram of the inverter circuit shown in FIG. 1 with respect to power supply voltage fluctuations and load fluctuations, and FIG. 4 is a specific example of a part of the control circuit according to the present invention.

In FIG. 1, an AC voltage is applied from a low frequency AC power source 1 to a rectifier circuit 2 and converted to a DC voltage to constitute a DC power source. An inverter circuit 3 is connected to the DC output side of the rectifier circuit 2. The inverter circuit 3 converts DC power into high frequency power, and has an input capacitor 30 connected between the DC power lines, and a series connection of a heating coil 31 and a switching semiconductor 32 connected in parallel with the input capacitor 30. A damper diode 33 is connected antiparallel to the switching semiconductor 32, and a resonance capacitor 34 is connected in parallel to the heating coil 31. The resonance capacitor 34 operates in the same way even if it is connected in parallel with the switching semiconductor 32. The switching semiconductor 32 is a power switching element that can be turned on and off, such as a power transistor or a gate turn-off thyristor (abbreviated as GTO). The control circuit 4 includes input detection terminals 40a and 40b connected to the input current detection current transformer 40 of the inverter circuit 3, an input DC voltage detection terminal 41a,
Forward voltage detection terminal 4 of switching semiconductor 32
1b, a common terminal 41c connected to one side of the DC power supply, output terminals 42a and 42b connected to the control terminal of the switching semiconductor 32, a variable resistor 43 operated by the user, and an input setting terminal that is a connection terminal thereof. 43a and 43b. The operation of the inverter circuit 3 is in a quasi-class E mode, and after the diper diode 33 becomes conductive, the switching semiconductor 32 is made conductive, and when the switching semiconductor 32 is turned off, the LC resonance of the heating coil 31 and the resonant capacitor 34 is activated. As a result, the forward voltage, so-called flyback voltage, of the switching semiconductor 32 rises in a sinusoidal manner, and is characterized by low turn-on loss and low turn-off loss.

FIG. 2 is a block diagram of a control circuit according to the invention. The basic oscillation control method of the inverter circuit 3 is to convert the input DC voltage V _DC and the forward voltage V _CE of the switching semiconductor 32 into the voltage comparison circuit 4.
In addition to 4, when V _CE becomes lower than V _DC , a synchronizing pulse V _t is generated and the pulse width control circuit 45
A synchronizing pulse V _t is added to the pulse width control signal V _p of the switching semiconductor 32 to generate the pulse width control signal V p. The output pulse width of the pulse width control circuit 45 is controlled by a pulse width setting signal V _S . The signal V _P is applied to a drive circuit 47 via a gate circuit 46, and the drive circuit 47 amplifies it into an electrical signal that can sufficiently control the conduction of the switching semiconductor 32. Gate circuit 4
6 is controlled by an oscillation start/stop circuit 48 to turn on/off the pulse width control signal V _P to stop oscillation. An output detection circuit 49 is connected to the forward voltage detection terminal 41b, and the switching semiconductor 32
is converted into a voltage signal approximately proportional to the peak value of the forward applied voltage. The operation is the same whether the heating coil 31 current or the switching semiconductor 32 forward current is used. The output signal V _CP of the output detection circuit 49 is applied to a first comparison amplifier circuit 50 , where it is compared and amplified with the output signal V _CS of the voltage setting circuit for the flyback voltage V _CE and applied to an OR circuit 52 . Voltage setting circuit 51 is controlled by input DC voltage V _DC . The signal from the current transformer 40 is applied to an input current detection circuit 53 and converted into a voltage signal corresponding to the input current of the inverter circuit 3.The signal from the current transformer 40 is then converted into a voltage signal corresponding to the input current of the inverter circuit 3. It is compared with and amplified and added to the OR circuit 52. The input setting circuit 55 can vary its input current setting signal using a variable resistor 55 that can be operated by the user.
Input current can be set to desired level. The OR circuit 52 gives priority to the signal level of either the first or second comparison amplifier circuit. The output signal of the OR circuit 52 is transmitted to the pulse width control circuit 4 through a pulse width limiting circuit 56 consisting of a voltage limiting circuit.
Add the pulse width setting signal V _S to 5. Figure 3 shows
This shows how the input current of the inverter circuit changes depending on the power supply voltage when the power supply voltage and flyback voltage are not fed back in the inverter circuit 3.
A normal iron pot load, B, is a non-magnetic stainless steel (SUS304) pot load. Non-magnetic stainless steel pots, aluminum-clad pots, and copper-clad pots tend to reduce the equivalent inductance of the heating coil 31, lower the heating coil impedance, and increase the heating coil current, resulting in an increase in output power. be. In Fig. 3, in the iron pot A, the input current is _IO at a power supply voltage of 100V.
By adjusting the pulse width limiting circuit 56, the power supply voltage
Below 100V, the conduction pulse width of the switching semiconductor 31 is limited, and the input current decreases approximately in proportion to the power supply voltage. Further, when the power supply voltage is 100V or more, the input current is limited to a constant value I _O and pulse width controlled. However, in a pot that tends to lower the impedance of the heating coil, even if the power supply voltage decreases, the input current remains at a constant value I _O , and the pulse width of the switching semiconductor 31 widens.
The LC section generates a large amount of heat, and if the power supply voltage drops, the rotation speed of the fan motor, which is the cooling device, will drop, making effective cooling impossible.

Therefore, in order to compensate for load fluctuations, we not only need to detect a signal corresponding to the output of an inverter circuit, such as the flyback voltage of a switching semiconductor, and perform feedback control, but also detect the power supply voltage and, if the power supply voltage drops, Narrow the pulse width of the switching semiconductor. FIG. 4 shows a specific embodiment of the present invention, and shows a forward voltage detection terminal 41b.
Therefore, the resistors 490a and 490b are connected in series, and the series connected body of the integrating circuit consisting of the diode 491, the smoothing capacitor 492, and the resistor 493 is connected to the resistor 49.
Connect in parallel with 0b. smoothing capacitor 49
2 is a voltage approximately proportional to the forward direction peak voltage of the switching semiconductor 32, and this voltage signal V _CP is applied to the operational amplifier (abbreviated as operational amplifier) 50 of the first comparison amplifier circuit 50.
Add to + input of 0. The output signal V _CS of the voltage setting circuit 51 is applied to the negative input of the operational amplifier 500 via the input resistor 501, and the gain is adjusted by the feedback resistor 502. If the output signal V _CS of the voltage setting circuit 51 is constant, the flyback voltage V _CE
The output voltage of the comparison amplifier circuit 50 changes approximately in proportion to . The voltage setting circuit 51 has an input DC voltage detection terminal 41a connected to resistors 510a and 510b.
are connected in series, and a Chener diode 511 is connected in parallel with the resistor 510b. The voltage across the Chener diode 511 just clips when the AC power supply voltage is in a steady state, for example 100V, and below the steady state voltage, a voltage approximately proportional to the power supply voltage is generated. A smoothing capacitor 512 is connected in parallel to the resistor 510b to reduce ripples in the input DC voltage.
The cathode terminal of the Chener diode 511 is connected to the base of a transistor 513, and the collector of the transistor 513 is connected to the DC low voltage +V _CC of the control circuit via a collector resistor 514.
A resistor 515 is connected to the emitter of the transistor 513.
A parallel circuit consisting of a capacitor 516 and a capacitor 516 are connected to perform impedance conversion. The voltage V _CS generated across the resistor 515 is approximately equal to that of the Chener diode 51.
1, and above the steady power supply voltage, it becomes a constant value, and when it falls below the steady value, it takes a value corresponding to the power supply voltage. Therefore, the set voltage V _CS of the first comparison amplifier circuit 50 decreases as the power supply voltage decreases, and serves to lower the forward voltage V _CE of the switching semiconductor 32 to be compared in accordance with the power supply voltage.
The output signal of the first comparison amplifier circuit 50 is applied to the OR circuit 52, and the OR circuit 52 outputs the voltage of the first comparison amplifier circuit 50 or the voltage of the second comparison amplifier circuit 54.
Either one of the high voltage signals is connected to the diode 52.
The diode OR connection of 0a and 520b adds it to a smoothing circuit consisting of a capacitor 521 and a resistor 522. As the voltage of the capacitor 521 becomes higher, the conduction pulse width of the switching semiconductor 32 becomes narrower. When the flyback voltage V _CE increases or the input DC voltage decreases, the output signal of the first comparator amplifier circuit 50 becomes high and the conduction pulse width of the switching semiconductor 32 is narrowed. When the AC power supply voltage exceeds a steady value, the output voltage V _CS of the voltage setting circuit 51 is clipped to a constant value, but the forward voltage of the switching semiconductor 32 also increases, so the first comparison amplifier circuit 5
0 output voltage rises and the switching semiconductor 32
By narrowing the conduction pulse width of the switching semiconductor 3
The forward peak voltage of No. 2 is controlled so as not to exceed a predetermined level.

As described above, the present invention controls the forward voltage or forward current of the switching semiconductor to a constant value when the AC power supply voltage or the input DC voltage of the inverter circuit exceeds a steady value. However, when the input DC voltage drops, the forward voltage or forward current of the switching semiconductor is lowered depending on the input DC voltage. In the conventional method of controlling either the input current of the inverter circuit or the input current and the forward voltage of the switching semiconductor, when the inverter circuit input DC voltage drops, depending on the load, the switching The drawback was that the operating conditions for semiconductors deteriorated and the temperature rose significantly. However, as in the present invention, if the conduction of the power switching semiconductor is controlled according to three signals: the inverter circuit input current, a signal due to load fluctuation, and the input DC voltage of the inverter circuit, the power switching semiconductor There is little temperature rise, which is a feature that prevents it from being used under abnormal conditions. In particular, in pots with low induction heating output, it is possible to design the rated output, and even in pots with high induction heating output, there is no abnormal load on the power switching semiconductor, and the power switching is uniform regardless of the type of pot. You can get the same output. Therefore, this was a drawback of the conventional method. It is possible to eliminate the disadvantage that the induction heating output varies greatly depending on the type of pot, or the disadvantage that the usage conditions of the power switching semiconductor deteriorate.

Note that as an embodiment of the present invention, the output detection circuit 4
9 shows a method of detecting the voltage of the switching semiconductor, but the effect is the same whether the voltage of the heating coil, the current of the heating coil, or the current of the switching semiconductor is used.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an inverter device for an induction heating cooker according to an embodiment of the present invention, Fig. 2 is a block diagram of the same control circuit, and Fig. 3 is a power supply voltage fluctuation and load of the inverter circuit shown in Fig. 1. FIG. 4 is a specific circuit diagram of a part of the control circuit according to the present invention. 1... Low frequency AC power supply, 2... Rectifier circuit, 3...
...Inverter circuit, 30...Input capacitor, 3
1... Heating coil, 32... Switching semiconductor.

Claims (1)

[Scope of Claims] 1. Consists of an inverter circuit that converts DC power into high-frequency power and its control circuit, and the inverter circuit consists of a heating coil, a switching semiconductor connected in series with the heating coil, and a resonance capacitor. , the control circuit includes an input current detection circuit that detects the input current of the inverter circuit, and a predetermined circuit configured to prevent the current or voltage of a switching element or heating coil of the inverter circuit from exceeding an upper limit depending on the input voltage of the inverter circuit. a voltage setting circuit for setting a value; an output detection circuit for detecting the voltage or current of the switching semiconductor or the heating coil; a pulse width control circuit that controls the pulse width of the voltage setting circuit; and a comparison amplifier circuit that inputs the output signal of the voltage setting circuit and the output signal of the output detection circuit, and the comparison amplifier circuit controls the output signal of the output detection circuit and the voltage. An induction heating cooker that compares and amplifies a difference from a predetermined value set by a setting circuit, and operates the pulse width control circuit to narrow the pulse width when the output detection circuit exceeds an upper limit. 2. The induction heating cooker according to claim 1, wherein the output detection circuit detects a forward peak voltage or a forward current of the switching semiconductor. 3. The induction heating cooker according to claim 1, wherein the output detection circuit detects the voltage or current of the heating coil.