JPH0416916B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an induction heating cooker used in general households.

Conventional configuration and its problems Conventionally, in induction heating cookers that use an inverter to generate an alternating magnetic field of superaudible frequency to induction heat a metal pot, the heating coil constitutes a resonant circuit of the inverter. The operating frequency varied greatly depending on the material and shape of the pot. For adjusting the output, there are two typical methods: a duty control method in which the heating state is intermittent and the average power is adjusted according to the intermittent time ratio, and a frequency control method in which the output is adjusted by changing the ultra-audible frequency. be. In the former case, there was an inconvenience that the voltage fluctuated due to the power source impedance during intermittent operation. On the other hand, in the latter case,
In order to achieve a wide output adjustment range, it is necessary to vary the operating frequency over a wide range, and when the frequency fluctuations due to the pot mentioned above are taken into account, the operating speed of the switching semiconductor reaches its limit, so only a narrow output adjustment range can be obtained.

Also, because the frequency fluctuates greatly, the fundamental wave of interference noise also fluctuates, and may cause interference if it matches a specific frequency of another device (for example, although it is not the fundamental wave, it may be a radio broadcast frequency or a remote control device). frequency, etc.).

Furthermore, by placing multiple induction heating cookers close together,
Or in equipment with multiple heating parts,
Unless the pot conditions and output settings of each burner are the same, the operating frequency will be different, and the leakage magnetic flux from each heating coil will be superimposed on the pot, causing a beat sound due to the frequency difference to be generated in the pot, which may be heard by the user. It also caused some discomfort.

OBJECTS OF THE INVENTION In order to solve such problems, the present invention aims to provide users with an induction heating cooker that always operates at a constant frequency.

Composition of the Invention The induction heating cooker of the present invention includes a resonant circuit including a heating coil, a switching semiconductor connected to the resonant circuit and excited, and a filter circuit including a choke coil and a capacitor connected between a power source and the resonant circuit. The control circuit includes a drive circuit that operates the inverter circuit at a constant frequency and a setting circuit that changes the constant of the filter. Therefore, the power can be varied.

DESCRIPTION OF EMBODIMENTS Hereinafter, an embodiment of the present invention will be described based on the accompanying drawings. FIG. 1 is a circuit diagram showing one embodiment, in which the rectified output terminal of a full-wave rectifier 2 connected to a commercial power supply 1 is composed of a series circuit of a saturable reactor 3 and a filter capacitor 4, which constitute a chiyoke coil. A filter circuit 6 is connected. Connected to the output terminal of the filter circuit 6 are a parallel circuit of a heating coil 7 and a resonant capacitor 8, and a series circuit of a diode 10 connected in anti-parallel to an NPN transistor 9, which together constitute an inverter circuit 11. There is. The secondary side terminal of the power transformer 12 connected to the commercial power source 1 is connected to the a and b terminals of the control circuit 13, and supplies low voltage power to the control circuit 13. The control circuit 13 includes a setting circuit 14 and a drive circuit 15, and the setting circuit 14 supplies a DC bias current to the control winding 3a of the saturable reactor 3 via output terminals c and d.
Here, the saturable reactor 3 has the configuration of a general magnetic amplifier, that is, it is composed of a main winding 3b and a control winding 3a wound on the same core. The inductance of the main winding 3b can be changed by the bias magnetic field in the core caused by the main winding 3a. When the DC bias magnetic field reaches a value sufficient to saturate the core, the inductance of the main winding 3b is no longer increased by the core, and is reduced to a value equivalent to the inductance of the air core. on the other hand,
In the absence of a bias magnetic field, the inductance of the main winding 3b has a core value, which is dramatically larger than the above-mentioned inductance with an air core. In this way, the inductance of the chiyoke coil can be varied by the DC control current from the control circuit 13.

Also, one drive circuit 1 constituting the control circuit 13
5 detects the terminal voltage of the transistor 9 from the input terminal e, and supplies a pulse output having a constant repetition period as a drive signal to the base and emitter of the transistor 9 via the terminals f and g, thereby causing the inverter 11 to oscillate. is under control.

Next, a detailed explanation of the control circuit 13 will be given using the embodiment shown in FIG. A diode 1 is connected to the input terminals a and b connected to the secondary winding of the power transformer 12.
After being rectified and smoothed by 01 and a capacitor 102, a stabilized power supply is configured by a resistor 103 and a constant voltage diode 104, and supplies power to a setting circuit 14 and a drive circuit 15. The setting circuit 14 includes a resistor 105,1
07, and a variable resistor 106 whose output is set by the user.
It consists of a voltage divider circuit, a PNP transistor 109, and a constant current circuit consisting of a resistor 108 for setting the emitter current.The voltage arbitrarily set with the variable resistor determines the base voltage of the transistor 109, which is determined by the emitter resistor 108. A current flows through the collector to the control winding 3a of the saturable reactor 3 connected to the output terminals c, d. The transistor 109 has a grounded-emitter configuration and has a sufficiently large DC amplification factor, so the transistor 109 operates at a constant current according to the base voltage, and is stable against variations in the resistance value of the control winding 3a. The flowing current is constant, allowing stable control. Furthermore, since the AC component flowing through the main winding is induced in the control winding 3a, a capacitor 110 is connected in parallel to absorb the induced voltage.

The drive circuit 15 includes two NPN transistors 11
4,118, the pulse output of a constant period is inverted by the NPN transistor 124, and when it is turned off, the collector resistor 12
The current flowing through transistor 3 is supplied to the base terminal of transistor 9 via output terminal f. Furthermore, the collector voltage of the transistor 9 is connected to the input terminal e and the resistor 11.
9 to the base of the transistor 122, and the collector of the transistor 122 is connected in parallel to the collector of the transistor 124. Therefore, while the transistor 9 is off and the collector voltage is generated, the base current is not generated. Not supplied.

To explain the above operation using the waveform diagrams of each part in Fig. 3, Fig. 3 shows the collector voltage waveform of the transistor 9, Fig. 3b shows its collector current waveform, and Fig. 3c shows the above-mentioned unstable multi-channel waveform. The collector output of the transistor 124, which is turned on and off by the output of the vibrator, is shown. At this time, the period T ₁ of the pulse output is determined by the time constant of the multivibrator and is always constant. On the other hand, as shown in FIG. 3D, the collector output of the transistor 122 is on when the collector voltage of the transistor 9 is generated, and the transistors 122 and 124 are connected in parallel.
During the seventh period T ₃ of the transistor 124, which overlaps with the period T ₂ in which the transistor 122 is on, no output is output from the terminal f.

That is, after the voltage of the transistor 9 becomes zero due to resonance between the heating coil 7 and the resonant capacitor 8, the base current of the transistor 9 is supplied from the drive circuit 15 at a constant period T ₁ (T ₁ - T ₂ ). be done.
The solid lines in A and B of FIG. 3 indicate the high output setting, where the current in the control winding 3a is large and the inductance of the main winding 3b is small, so when the transistor 9 is conductive. , capacitor 4
so that the electric charge is not discharged through the heating coil 7,
It charges quickly from the power source. On the other hand, the dashed line is
This shows the low output setting, and since the current in the control winding 3a is small and the inductance is increasing,
Substantially, the impedance of the power source becomes high, and even when the transistor 9 is conductive, sufficient current does not flow through the heating coil 7, and the induction heating output is set low.

FIG. 4 shows a waveform diagram illustrating the operation of the drive circuit when the material of the pot to be heated by induction changes. FIGS. 4A and 4B show the voltage and current waveforms of the transistor 9, respectively, where the broken line indicates a normal iron-based pot and the solid line indicates a non-magnetic pot such as aluminum or stainless steel. As can be seen from the explanation in FIG. 3, the output period of the multivibrator is constant regardless of the pan, and the transistor 124 is also constant as shown in FIG. 4C.

However, the output of the transistor 122, which is switched by the voltage of the transistor 9, is as shown in FIG.
As shown in d', it changes depending on the pot, and as a result, the conduction period of the transistor is automatically corrected depending on the pot material, etc. within a certain period.

FIG. 5 is a circuit diagram showing the configuration of an embodiment that performs more accurate control by detecting the input current and the voltage or current of the inverter circuit. A current transformer 16 is added for this purpose. FIG. 6 shows the control circuit 1 of the above embodiment.
3, the constant voltage power supply circuit is connected to the a and b terminals as in FIG. 2, and the setting circuit 14
and supplies power to the drive circuit 15. The setting circuit 14 converts the signal from the current transformer into a voltage using a resistor 130, and converts the signal from the current transformer into a voltage using a full-wave rectifier 131 and a smoothing capacitor 13.
2 to DC voltage, the collector voltage of transistor 9 is divided by resistors 133 and 134, and the high output voltage of each is connected to diodes 135 and 13.
6 and connected to the negative input terminal of the operational amplifier 139.

The other positive input terminal of the operational amplifier 139 is
Since the reference voltage is supplied by a resistive voltage divider circuit including a variable resistor 106 set by the user, the saturable reactor 3 is adjusted so that the higher value of the input current or the voltage of the inverter circuit 11 is equal to the set value. A control current flows through the control winding 3a. As a result, it is possible to control the settings so that excessive input does not occur, or excessive voltage or current is generated in the inverter circuit. It is possible to prevent such problems and improve reliability. In particular, for non-magnetic materials such as aluminum with low resistance, the collector current is expected to be excessive compared to a normal pot, as can be seen from the solid line waveform in Figure 4, so the transistor 9
The collector current of is detected by a current transformer (not shown),
It is also effective to suppress the peak value of the current of the transistor 9 by increasing the inductance of the saturable reactor 3.

The above has been explained based on the embodiments in FIGS. 1 to 6, but the transistor 9 and the diode 1
The same effect can be obtained by using a self-extinguishing switching element such as FET or GTO instead of 0, and the connection of the resonant capacitor 8 is different between Fig. 1 and Fig. 5.
The inverter operations are equivalent.

Effects of the Invention The induction heating cooker of the present invention always has a constant oscillation output regardless of load fluctuations or output settings. Since the oscillation frequencies are the same, beats due to frequency differences do not occur, and the user does not feel uncomfortable.

2. A conduction signal is generated when the voltage of the switching semiconductor becomes zero, and no loss occurs during turn-on, allowing the use of small-capacity elements.

3. A large output variable range can be obtained depending on the variable range of the inductance of the choke coil. That is,
A wide output variable range can be obtained without using high-speed elements in the switching semiconductor.

4. Since the frequency is fixed, noise interference will not occur if the fundamental frequency is chosen so that harmonics are not synchronized with the radio frequency.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an induction heating cooker according to an embodiment of the present invention, Fig. 2 is a detailed circuit diagram explaining the embodiment of Fig. 1, and Figs. Second
FIG. 5 is a circuit diagram showing an induction heating cooker according to another embodiment of the present invention, and FIG. 6 is a detailed circuit diagram explaining the embodiment of FIG. 5. 3...Saturable reactor, 6...Filter circuit, 7...Heating coil, 8...Resonant capacitor,
9...transistor, 11...inverter circuit,
14... Setting circuit, 15... Drive circuit.

Claims (1)

[Claims] 1. A resonant circuit including a heating coil, a switching semiconductor connected to the resonant circuit and excited,
It consists of an inverter circuit consisting of a filter circuit consisting of a chiyork coil and a capacitor between the power supply and the resonant circuit, and its control circuit, and the control circuit changes the constant of the chiyoke coil and a drive circuit that operates the inverter circuit at a constant frequency. An induction heating cooker comprising a setting circuit and varying electric power by changing a constant of the chiyoke coil. 2. The Chi York coil is composed of a saturable reactor with a control winding, and the control circuit has an input detection circuit that detects input and a state detection circuit that detects the voltage or current of the inverter circuit, and the control circuit has The induction heating cooker according to claim 1, wherein the value of the current flowing through the control winding of the saturable reactor is controlled by any output of the detection circuit. 3. When a terminal voltage of the switching semiconductor is generated, a pulse generation circuit that generates a pulse of a constant width and an inhibition circuit that generates a prohibition output while the terminal voltage of the switching semiconductor is generated are activated. The induction heating cooker according to claim 1, further comprising a drive circuit that supplies a drive signal to the switching semiconductor after the voltage becomes zero.