JPH02306570A - Induction heating cooker - Google Patents

Abstract

PURPOSE:To reduce input regardless of the kind of cookers so as to enable driving with minimum power by detecting the current of a resonance condensor only during operation of a semi-conductor switching element, and controlling a charging current using a control circuit. CONSTITUTION:A cooker 9 is heated via a resonance circuit comprising a semiconductor element 8 the operation of which is controlled by a control circuit 11, and a resonance condensor 7 and the like. The current of the resonance condensor 7 is detected only during operation of the switching element 8 and is output to the comparing circuit 15 of the control circuit 11, and, when the value exceeds a reference value, the excess value is output to a duty cycle setting circuit so that the control mode is switched to control of a duty cycle by which operation of the switching element 8 is repeatedly on and off for a predetermined period of time. Thereby input power is reduced and so driving with minimum power is enabled regardless of the kind of cookers, and noise level is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an induction heating cooker that is compatible with minimum power when human power is reduced.

2. Description of the Related Art A conventional induction heating cooker of this type has a current detection means 18 for detecting the current of a transistor 17, as shown in FIG.
The control circuit 19 compares a predetermined reference value with the output value of the current detection means 18 using a comparison circuit 2o, outputs a signal from the power setting circuit 21 to the timer circuit 22, and causes the drive circuit 23 to drive the transistor 17. A signal is issued to control the conduction time of the transistor 17 so that the current in the transistor 17 does not exceed a predetermined value.

Problems to be Solved by the Invention However, in the conventional configuration as described above, the transistor 1
The maximum limit value of the current value of 7 is determined by the current value of the transistor 17 at the maximum power set by the control circuit 19, so as the sword power is reduced, the transistor 17 turns ON as shown in FIG. At the instant of this, a pulsed charging current is generated from the smoothing capacitor 5 to the resonant capacitor 7, and this charging current flows up to the maximum limit value mentioned above. This charging current becomes a source of noise, which adversely affects wireless equipment, etc. Furthermore, when the input power is reduced,
In a pot where the charging current exceeds the maximum limit value, the current of the transistor 17 is controlled to be constant and the input power is increased, so it is not possible to always reduce the input power to the same level for any pot. There was a problem.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide an induction heating cooker equipped with a control circuit that can be driven with minimum power by reducing input power.

Means for Solving the Problems In order to achieve the above objects, the present invention provides a heating coil for inductively heating a pot, a resonant capacitor forming a resonant circuit with this heating coil, a semiconductor switching element, and an operation of this semiconductor switching element. and current detection means for detecting the current of the resonant capacitor and outputting a signal to the control circuit, and the control circuit detects the signal of the current detection means only when the semiconductor switching element is in operation. In the induction heating cooker, when the current of the resonant capacitor exceeds a predetermined value, the operation of the semiconductor switching element is switched to control the duty cycle of repeating ON-OFF for a certain period of time.

Operation In the induction heating cooker of the present invention, the heating coil and the resonant capacitor are excited by ON/OFF control of the semiconductor switching element by the control circuit as described above, and a high frequency current is generated and supplied to the heating coil. This heating coil generates a high frequency magnetic field, which inductively heats the pot.

The current detection means detects the current of the resonant capacitor and sends a signal to the control circuit according to the amount of current.

The control circuit detects only the output signal of the current detection means detected when the semiconductor switching element is turned on, compares this with a predetermined reference value, and when the predetermined reference value is even slightly exceeded, controls the operation of the semiconductor switching element. , the duty cycle is controlled by repeating ON-OFF for a certain period of time according to the amount of current of the semiconductor switching element, and the current of the resonant capacitor is controlled to be constant so that it does not exceed a predetermined current value, and the input power is constantly controlled. It is designed so that it can be narrowed down to a small size.

With the above configuration, especially when the input power is reduced to a small value, the current detection means detects the peak value of the pulse-shaped (time width of approximately 1 μs) charging current from the power supply to the resonant capacitor that is generated at the moment the semiconductor switching element is turned on. The current value is detected by a control circuit, and the current value is suppressed to a constant value, and the operation of the semiconductor switching element is switched to ON-OFF duty cycle control at a constant time.

Example Hereinafter, the drawings shown as one embodiment of the present invention will be explained.

In FIG. 1, 1 is a commercial power source, to which a rectifier 2 is connected. A capacitor 3 and a choke coil 4 forming a filter circuit are connected to the output of the rectifier 2, and a smoothing capacitor 5 is connected to the output of this filter circuit. The smoothing capacitor 5 is connected to a heating coil 6, a resonant capacitor 7, and a transistor as a semiconductor switching element 8 having a built-in diode, which are connected in parallel. A current transformer serving as current detection means IO is connected in series to the resonant capacitor 7. Reference numeral 9 denotes a pot, which is placed opposite the heating coil 6 and is heated by induction.

+1 is a control circuit, which turns the semiconductor switching element 8 ON-.
The OFF operation is controlled by varying the conduction time. The output signal of the current detection means 1O is input to this control circuit II.

The control circuit II includes a drive circuit 12 that drives the semiconductor switching element 8, and a drive circuit 12 that drives the semiconductor switching element 8.
Only when the output signal of the current detecting means IO is N, the signal from the drive circuit 12 is used to compare the output signal of the current detecting means IO with a predetermined reference value and outputting a signal to the timer circuit 13. When the value is exceeded, a comparator circuit 15 outputs a signal corresponding to the exceeded signal amount to the duty cycle setting circuit 14, and a timer circuit I sets the input power.
3, a power setting circuit 16 that outputs a signal to
5 and the output signal of the power setting circuit 16, the conduction time of the semiconductor switching element 8 is set, and the drive circuit 12
is a timer circuit 13 that outputs a signal to the drive circuit I2 so as to output a drive signal to the semiconductor switching element 8.
Then, in response to the signal from the comparison circuit 15, the operation of the semiconductor switching element 8 is changed from continuous oscillation to a duty period of repeating ON-OFF for a certain period of time according to the signal amount of the comparison circuit 15, and the timer circuit 13 It also includes a duty-cycle setting circuit 14 that outputs a signal.

FIG. 2 is a waveform diagram showing the timing of the output signal of the drive circuit 12 and the current waveform of the resonant capacitor 7. When the power is narrowed down to a small value, when the transistor 17 is turned on as in the conventional example shown in FIG. When the charging current of the resonant capacitor 7 becomes larger than a predetermined reference value, the control circuit II controls the semiconductor switching so that the charging current of the resonant capacitor 7 becomes the predetermined reference value when the semiconductor switching element 8 is turned on. The conduction time of the element 8 is increased and controlled.

In the above configuration, when trying to reduce the input power by reducing the conduction time of the semiconductor switching element 8 to the minimum power set by the power setting circuit 16, if the coupling state between the pot 9 and the heating coil 6 is good, , the damping coefficient of the resonant circuit of the heating coil 6 and the resonant capacitor 7 increases, and the resonance energy also decreases, and most of the resonant current is consumed by the resistance of the heating coil 6, causing resonance at the moment the semiconductor switching element 8 turns on. The voltage across the capacitor 7 is smaller than the voltage across the smoothing capacitor 5. Therefore, at the moment when the semiconductor switching element 8 turns ON, the smoothing capacitor 5 tries to make the voltage across the resonance capacitor 7 the same potential as the voltage across the smoothing capacitor 5
A charging current flows from the resonant capacitor 7 to the resonant capacitor 7. Since the time width of this charging current is very short, about 1 μs, if this charging current value is large, it becomes a noise source in the broadcast band and causes interference to wireless equipment and the like.

Therefore, in the control circuit 11, when the charging current to the resonant capacitor 7 is about to exceed a predetermined reference value, the comparison circuit 15 compares the signal detected by the current detection means 10 with a predetermined reference value. , outputs a signal to the timer circuit 13 to increase the power by increasing the conduction time of the semiconductor switching element 8 so that the charging current to the resonance capacitor 7 does not exceed a predetermined reference value. Here, the timer circuit 13 outputs a signal to the drive circuit 12 so as to lengthen the conduction time of the semiconductor switching element 8.

As a result, the input power continues to increase until the charging current to the resonant capacitor 7 reaches a predetermined reference value. If the input power increases, the predetermined minimum power cannot be obtained. outputs a signal corresponding to the difference between the charging current to the resonance capacitor 7 and a predetermined reference value to the duty cycle setting circuit 14.

The duty cycle setting circuit 14 determines that the average value of the input power is determined by the power setting circuit 16 according to the signal amount of the comparison circuit 15.
The duty at a certain time is set to the minimum power of
A signal is output to the timer circuit 13 to turn on and off the semiconductor switching element 8 at a periodic time (for example,
5 seconds cycle, 2 seconds ON, 3 seconds 0FF).

Finally, the peak value of the charging current to the resonance capacitor 7 is controlled to be constant at a predetermined reference value, and the input power is also controlled to the duty cycle setting circuit 14 according to the peak value of the charging current to the resonance capacitor 7. By repeating the ON-OFF cycle, the power of the power setting circuit 16 can always be reduced to the minimum power.

Therefore, the charging current of the resonant capacitor 7 can be reduced, and noise can be suppressed to a low level. Furthermore, a wide margin can be provided for the loss when the semiconductor switching element 8 is turned on and the forward bias ASO region. Furthermore, noise can be suppressed to a low level and the same minimum power can be maintained no matter what pot 9 is used.

In the above embodiment, the current detecting means lO is a current transformer, but it is not limited to this, and any means capable of detecting current may be used. For example, a resistor may also be used, which can quickly achieve the effects of the present invention. It is.

Effects of the Invention As described above, the present invention detects the current of the resonant capacitor and uses the control circuit to suppress the charging current to a low level, thereby reducing noise and reducing the loss when the semiconductor switching element is turned on. However, the noise level can be lowered. Further, a sufficient margin can be provided for the forward bias AS of the transistor. Furthermore, when the charging current of the resonant capacitor is detected and exceeds a predetermined reference value, it will turn ON-Off.
Since the switch is switched to F duty control, there are effects such as being able to always control the same minimum power for any pot.

[Brief explanation of the drawing]

Fig. 1 is an electric circuit diagram of an induction heating cooker showing an embodiment of the present invention, Fig. 2 is a waveform diagram showing the timing of the output signal of the drive circuit of the embodiment and the current of the resonant capacitor, and Fig. 3 is FIG. 4 is a waveform diagram showing the timing of the output signal of the drive circuit and the current of the resonant capacitor in the conventional example.

Claims (1)

[Claims] (1) A heating coil that inductively heats a pot, a resonant capacitor that forms a resonant circuit with this heating coil, a semiconductor switching element, a control circuit that controls the operation of this semiconductor switching element, and detects the current of the resonant capacitor. and current detection means for outputting a signal to the control circuit, the control circuit detecting the signal of the current detection means only when the semiconductor switching element is in operation, and when the current of the resonant capacitor exceeds a predetermined value. An induction heating cooker, characterized in that the operation of the semiconductor switching element is switched to control the duty cycle of repeating ON-OFF for a certain period of time.