JPS6054756B2 - induction heating cooker - Google Patents

Description

Detailed Description of the Invention The present invention relates to an induction heating cooker, and in particular detects the voltage and current applied to the induction heating coil, calculates the power consumed by the load, and compares it with a preset input level. The purpose is to perform power control so that the set power is always supplied.

Conventionally, as a power control method for this type of induction cooker,
It is known to do this by detecting the load current flowing through an induction heating coil and increasing or decreasing the peak of this load current; however, such methods do not ensure that the current accurately reflects the power delivered to the load. Therefore, it was difficult to ensure accurate power control. That is, when heating a cooking pot made of a metal material containing iron, the input power to the cooking pot changes because the magnetic properties and inherent ohmic resistance vary depending on the iron content or temperature changes. When detecting this input power and making it match the preset power, if only the load current was detected, the input power would not be reflected correctly, making complete power control impossible.

The present invention makes it possible to accurately control the power actually supplied to the cooking pot by correctly detecting the input power to such a load and operating it to match a predetermined power level.

Embodiments of the present invention will be described in detail below with reference to the drawings.

In Fig. 1, reference numeral 1 indicates an induction heating cooker, and the top plate 2 is made of a heat-resistant insulating substrate such as a ceramic plate on the upper surface.
A cooking pot (not shown) serving as a load is placed on top of the cooking pot. The cooking pot is made of metal containing iron, such as iron or stainless steel. 3 is a plug to which AC power is supplied;
4 is a power switch, 5 is a slide knob for adjusting input power,
6 is an input power display section consisting of a light emitting diode etc., W(
(watts) is displayed as a 4-digit number.

When the detected power and the set power match on this display unit 6,
The value is displayed digitally. Figure 2 A, B, and C are
7 shows a commercial AC power supply, 4 a power switch, 8 a rectifier circuit formed by bridge-connecting diodes, 9 a choke coil, 10 a smoothing capacitor,
Reference numeral 11 denotes an induction heating coil, which is wound into a flat plate shape and placed close to the back surface of the top plate 2.

12 is a switching element connected in series with the gate induction heating coil 11, and in this embodiment, a semiconductor switching element G
CS (Gate, Controlled Switch) (Product name)
are using.

13 is a flywheel diode connected in antiparallel between the anode and power sort of GCS12, 14 is a resonant capacitor, and these induction heating coil 11, GCSl2
, a flywheel diode 13, and a resonant capacitor 14 constitute an inverter 15.

20 is a cooking pot arranged close to the induction heating coil 11;
1 and R2 are dividing resistors connected in parallel to the smoothing capacitor 14, and the voltage V1 is taken out from the dividing point.

R, , R4 are dividing resistors connected in parallel to the resonant capacitor 14, and voltage V2 is taken out from the dividing point. 21 is a current transformer that detects the current flowing through the induction heating coil 11.

FIG. 3 shows the current and voltage waveforms of each part of the inverter. During heating operation, the voltage ε across the resonance capacitor 1 Ω diode 13 and the GCSl 2 increases due to the operation of the Emizu resonance circuit. Charge is transferred and received by the smoothing capacitor 10.

When the voltage V6 becomes <0, the gate signal VG of the GCSl2 is switched on and the current 1L flowing through the induction heating coil 11 starts to increase, but this current value is detected by the current transformer 21. The voltage is converted to the reference voltage V. Current 1 corresponding to . When it reaches, the gate signal VO is switched to negative and GCS12 is turned off. As a result, the circuit operation becomes ■λ resonance again, and the same operation is repeated thereafter and oscillation continues. When the cooking pot 20 is placed on the top plate 2 in such a cooker, the inherent ohmic resistance of the pot and the winding resistance of the induction heating coil 11 are equivalent to a series circuit of the induction heating coil 11 and the resistor, and Since the winding resistance of the heating coil 11 is very small, the input power to the load can be detected by detecting the voltage (2) and current h across the induction heating coil 11. That is, since the instantaneous human power p(t) is expressed as, the power P during one oscillation period (T) can be obtained by integrating equation 1 over a period of T as follows: -)'--1'.

Rewriting equation 2, we get Here, R is the inherent ohmic resistance of the load.

It can be seen from equation 3 that the input power can be controlled by controlling the current 1L(t).

Therefore, when the human power is small, it is sufficient to increase I(t), but this I(t) is the reference voltage V. In such a case, the voltage V. It would be better to increase the level of . On the other hand, if you want to decrease the input power, use the voltage V. All you have to do is lower it. Next, 2nd B and C
The input power control circuit will be described in detail based on the diagram.

22 is an operational amplifier, and the voltage signal 1 is input to the 8 input terminals.
Further, a voltage signal V2 is inputted to each of the eight input terminals via a resistor R5.

The output of this operational amplifier 22 is amplified by an amplification factor set by resistors R5 and R6. This signal V4 is connected to the resistor R
7 to the logarithmic converter 23 at the next stage to obtain an output V5.

Here, K is Boltzmann's constant, q is the amount of charge of the electron, and α
The current amplification factor 1E3 is the reverse current between the base and emitter of the transistor 24.

25, the current detected by the current transformer 21 is converted into a voltage signal V3 via dividing resistors R8 and R9, and is inputted to a logarithmic converter to obtain an output V6.

The above signal V5 and signal V6 are connected to resistors RlO and RlO, respectively.
Both are applied to the e input terminal of operational amplifier 26 via RIO. This operational amplifier 26 is used as an adder and obtains an output V. This addition signal ■7 is
The signal is input to the antilogarithmic converter 27 at the next stage, and an output V8 is obtained.
From equations 4 to 8 above, this output signal ①8 is input to the integrator 28 and integrated over a period of T.

This integrator 28 includes a capacitor C1 and an analog switch 30 for discharging.1. ▼Claw 1! A signal V is obtained as the output of the integrator 28. This signal V9 is amplified via the next-stage operational amplifier 29 and converted into a voltage signal that can be compared with the voltage signal Vp set by the operation of the slide knob 5. Thus, as the output of the operational amplifier 29, the signal VlO
get. From formula 9, @, 5, here, A = t.
O is applied to the four input terminals of operational amplifier 30 shown in FIG. 2C.

The e input terminal of the operational amplifier 30 has a variable resistor Rl whose voltage value changes according to the operation of the input power setting slide knob 5.
8 outputs V, are added. 31 is an operational amplifier that inputs the signal ■, to the 1 input terminal and the signal VlO to the o input terminal, 32 and 33 are inverters, and 34 is the operational amplifier 3.
This is an AND gate in which the respective outputs of 0 and 31 are input via inverters 32 and 33, respectively, and the analog switch 35 is controlled to open and close by the output.

This analog switch 35 connects the signal VlO to a comparator 36.
When the output of the AND gate 34 is an H level signal, the switch 35 is opened and the signals V and O are applied to the four input terminals of the comparator 36. A stepped voltage signal from a ladder circuit 37 is applied to eight input terminals of the comparator 36. The ladder circuit 37 includes a binary counter 39 which inputs the clock pulse CK from the clock oscillator 38, and a resistor Rl at the output of this counter 39.
6, Rl8, . . . and sequentially output higher potential level signals from the lower potential level. 40 is a W-adic counter that receives and counts clock pulses CK from the clock oscillator 38, and together with the binary counter 39, the timer 4
Counting starts with a rising signal of 1 and is reset with a falling signal.

The signal output period of this timer 41 is set to be at least longer than the period during which the number of clock pulses corresponding to the number of levels of the output of the ladder circuit 39 is output. 42 is a latch circuit that stores the contents of the decimal counter 40, and is operated by the output of the comparator 36.

43 is a decoder that converts the contents of the latch circuit 42 into a code, and 6 is the display section that displays the contents of this decoder 43.

In this way, when the binary counter 39 and the w-adic counter 40 start counting at the output of the timer 41 and the potential output from the ladder circuit 37 reaches the signal 10 or higher, the comparator 3
6 is inverted from the H level to the L level, and the contents of the W-adic counter 40 at this time are held in the latch circuit 42. The storage contents of this latch circuit 42 are stored in a decoder 43.
The power is displayed on the display unit 6 via the power display. Thereafter, the binary and w-base counters 39 and 40 are reset by the fall of the output signal of the timer 41 and return to the initial state. 4
4, a signal obtained by dividing the power signal Vs by resistors Rl, , R2O, R2l is input to the 4 input terminal, and an operational amplifier 30
The operational amplifier 45 inputs the output of the operational amplifier 44 to the θ input terminal, and is a comparator that inputs the output Vll of the operational amplifier 44 to the 4-input terminal and the detection signal VcT from the current transformer 21 to the θ input terminal. is input to the reset terminal of the RS flip-flop 46.

The set output of the RS flip-flop 46 is the acceleration resistor R2. and gate drive circuit 4 via capacitor C2.
7, and when the set output is inverted to the L level, the transistor 48 in the drive circuit 47 is made conductive, the voltage -Vcc is outputted as the gate signal (2), and the GCS12 is turned off.
49 is a terminal to which the signal V2 is input; when 2 becomes a negative value, the base potential of the transistor 50 decreases, turning it off.

Therefore, the collector potential of the transistor 50 rises, and this rising signal is pulsed through the CR differentiating circuit 51 and further inverted through the inverter 52 to become the R
It is input to the set input terminal of the S flip-flop 46. That is, the flip-flop 46 is set by the negative conversion of the signal 2, and its set output becomes H level, making the transistor 53 in the gate drive circuit 47 conductive, and the voltage +Cc is obtained as the signal VO. by GC
Sl2 is turned on. 53 is a flip-flop 46
A transistor operated by the reset output of R23,
C3 is a resistor and a capacitor that constitute a time constant circuit, 5
4 is a comparator which outputs an L level signal after a certain period of time determined by a time constant circuit after setting the flip-flop 46;
This is to reset the flip-flop 46, and the GCSl
It serves to set the maximum current conduction time of No. 2, that is, the minimum oscillation frequency of the inverter.

This minimum oscillation frequency is set at approximately 20 KHz to prevent the occurrence of loud noise. Next, the detected power signal ■,. The operation will be explained for three cases in which the set power level ■P can take. (1) Vl. =
For Vp, this is the case when the detected power and the set power are equal. At this time, operational amplifiers 30, 3
Both outputs of 1 are at L level, so the inverter 32
, 33 are both at H level. Therefore, the output of AND gate 34 also becomes H level, and analog switch 35 is closed. As a result, when the signal VlO is input to the comparator 36 and matches the output signal from the ladder circuit 37, the digital signal corresponding to the output of the ladder circuit 37 at this time is held in the latch circuit 42, and its contents are , is digitally displayed on the display unit 6, for example, as in the old 00W. At this time, since the signal input to the θ input terminal of the operational amplifier 44 does not change, the oscillation frequency also changes.
Oscillation at a constant frequency continues. (2) When VlO>Vp When the detected power level becomes larger than the set power level, the outputs of the operational amplifiers 30 and 31 become, so the output of the AND gate 34 becomes L level and the analog switch 35 turns off. No display is made on the display section 6.

On the other hand, the output Vll of the operational amplifier 44 is as follows.

Since VO>■p, the voltage signal VCT output from the current transformer 21 is the reference voltage V with which it is compared. decreases. Therefore, the timing at which the output of the comparator 45 is inverted to L level is earlier, and the reset of the flip-flop 46 and the turn-off of the GCS12 are also earlier. The oscillation frequency therefore increases and the input power decreases. In this way, when the input power decreases and matches the set power level, the current power is displayed on the display section 6, and oscillation continues at a constant frequency thereafter. (3) VlO
<Vp When the detected power level is smaller than the set power level, the case is opposite to the example (2) above, and the output of the operational amplifier 30 becomes L level and the output of the operational amplifier 31 becomes H level.

In this case as well, the AND gate 34 is not opened and therefore no display is made. In this case, the output Vll of the operational amplifier 44
is the reference voltage V of the comparator 45.

will rise. Therefore, the time when the output of the comparator 45 is inverted to the L level is delayed, and accordingly, the time when the GCS12 is turned off is also delayed. Therefore, the oscillation frequency decreases and the input power increases. In this way, when the increased input power reaches the set power level, the oscillation frequency becomes constant, and thereafter constant power supply is performed. Of course, the power at that time is displayed on the display section at the same time as the two power levels match.
Note that the power supply cannot be increased indefinitely, and is limited to a frequency of about 20 KHz as described above.

As explained above, the induction heating cooker of the present invention detects the voltage across the induction heating coil and the current flowing therethrough.
Since there is a device that calculates the power actually supplied to the load, it allows for much more accurate power control compared to conventional cookers that only detect the load current and control the power by controlling its peak value. can be done.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIGS. 2A, 2B, and 2C are circuit diagrams of the embodiment, and FIG. 3 is an operating signal waveform diagram. 1...Induction heating cooker, 2...Top plate, 4
...Power switch, 5...Slide knob, 6...
Input power display section, 12...GCSll5...Inverter.

Claims (1)

[Claims] 1. Voltage detection means for detecting the voltage applied to the induction heating coil, current detection means for detecting the current flowing through the induction heating coil, multiplication means for multiplying the output signals of the voltage detection means and the current detection means, and an output from the multiplication means. an integrating means for integrating the signal for a certain period of time and outputting a voltage signal corresponding to the electric power; a comparing means for comparing the output of the integrating means with a predetermined reference voltage level; An induction heating cooker comprising current control means for adjusting the period of current flowing through the coil.