JPS6342396B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and in particular, by oscillating the inverter at a high oscillation frequency, that is, at a low input, at the initial stage of power-on, it is possible to prevent excessive current from flowing to switching elements and stabilize the oscillation mode. It is something that has been transformed.

FIG. 1 shows a block diagram of an induction heating cooker according to an embodiment of the present invention, in which 1 is an AC power supply, 2 is a power switch, 3 is a rectifier circuit formed by bridge-connecting diodes, 4 is a filter circuit, and a chain coil 5 and Consisting of a smoothing capacitor 6, this capacitor 6
The capacitance of is set small, and the voltage obtained from this is a pulsating voltage that is hardly smoothed.

Reference numeral 7 denotes an induction coil to which direct current power is supplied through the filter circuit 4. The induction coil 7 is wound into a flat plate and is placed close to the back surface of a top plate 8 made of a ceramic plate or the like. 9 is a cooking pot made of iron, stainless steel, etc. placed on the top plate 8; 10 is a switching element connected in series with the induction coil 7;
In this embodiment, a GCS (gate controlled switch) (trade name) is used. 11 is GCS
A flywheel diode 10 is connected in antiparallel between the anode and cathode, and 12 is a resonant capacitor. The induction coil 7, GCS 10, flywheel diode 11, and resonant capacitor 12 constitute an inverter 13. This inverter 13 is of an operation type called a self-limiting inverter, and starts oscillating in response to a starting signal provided by a starting signal generating circuit 14, and thereafter continues oscillating in response to a gate signal issued from a gate signal generating circuit 15. 16 is an inverter drive circuit that applies a positive or negative signal to the gate of the GCS 10 to turn it on and off; 17 is a flip-flop that controls the inverter drive circuit 16 and is operated by a start signal and a gate signal. 18 is a rectifier circuit to which the AC power supply voltage is applied via a step-down transformer 19; 20 is a constant voltage circuit to which the voltage rectified by this rectifier circuit 18 is input via a smoothing capacitor 21; DC voltage ±
V _B (±12V) and a stabilized DC voltage Vcc (8V) are taken out from the output side. Here, the voltage ±V _B is used as a drive power source for the inverter drive circuit 16 and a control power source for the timing circuit 22,
The voltage Vcc is supplied to the activation signal generation circuit 14, the gate signal generation circuit 15, and the timing circuit 22 as a driving power source. Here, the start signal generation circuit 14
, which provides the first starting signal for the inverter 13, oscillates at a predetermined period, for example, a 0.4 second period, and once the oscillation starts, its operation is prohibited until the oscillation stops. Further, the gate signal generation circuit 15 generates a trigger pulse for self-controlled oscillation and gives an on/off signal to the gate of the GCS 10. To explain its function, after the inverter 13 is started by the initial start signal, the GCS 10 side induction coil voltage Vc
Set flip-flop 17 by
A signal is applied to the gate of 10 to make it conductive. Thereafter, when the output voltage of the current transformer CT wound between the cathode and ground becomes larger than the input side voltage V _A of the induction coil 7 due to conduction of the GCS 10, the flip-flop 17 is reset and the GCS
Block 10. As a result, the energy stored in the induction coil 7 is charged into the resonant capacitor 12, and subsequently, the capacitor 12 starts discharging. When this discharge ends, the energy accumulated in the induction coil 7 during this period is transferred to the diode 1.
flows through GCS10, this discharge ends, and GCS10
When the anode-cathode voltage changes from negative to positive, the GCS 10 becomes conductive again and the next oscillation cycle begins. Note that during the conduction period of the diode 11, the voltage Vc becomes negative, and this negative signal causes
When an on signal is applied to the gate of the GCS 10 and the voltage between the anode and cathode changes to positive, it becomes conductive. In this way, self-controlled oscillation continues, and its oscillation frequency is about 20 to 40 KHz.

The timing circuit 22 uses the voltage V _B as a control voltage and oscillates at different frequencies in response to changes in this voltage, and its output is applied to the flip-flop 17 via an OR gate 23 together with the output of the gate signal generation circuit 15.

FIG. 2 shows a specific circuit diagram of the main parts of the embodiment of the present invention, and 15, 22, and 17 are the aforementioned gate signal generation circuit, timing circuit, and flip-flop, respectively. Note that the gate signal generation circuit 15
The circuit part related to voltage Vc is the gist of the present invention.
I've omitted it because it's irrelevant. COM ₁ is a comparison circuit, and the input terminal connects the current signal detected by the current transformer CT to resistors 24, 25 and diode 2.
The signal V _CT obtained by converting to voltage and dividing the voltage in step 6
is applied to the input terminal, and the voltage V _A is connected to the resistor 2
The signals obtained by dividing the voltage at 7 and 28 are added. In the timing circuit 22, COM ₂ is a comparison circuit similar to the above, and the output of a time constant circuit made up of a resistor 29 and a capacitor 30 is added to the input terminal. Voltage V _B is applied to this time constant circuit as a control voltage. A transistor 31 is connected in parallel with the capacitor 30 and is controlled on/off by the output of the flip-flop 17. Further, a reference signal obtained by dividing the constant voltage V _CC by resistors 32 and 33 is applied to the input terminal of the comparator circuit COM ₂ . 17 is two NAND gates NAND ₁ ,
In the aforementioned flip-flop consisting of NAND ₂ , the outputs of comparator circuits COM ₁ and COM ₂ are both input to NAND gate NAND ₁ . In the figure, the outputs of both comparison circuits COM ₁ and COM ₂ are wired-OR connected, so the OR gates as in FIG. 1 are not shown.

Next, the operation of the embodiment of the present invention will be explained using FIGS. 3 and 4. FIG. 4 shows the voltage-load current characteristics when the oscillation of the voltage V _B is stopped and when it is in steady oscillation. At the time K when the inverter 13 stops oscillating, that is, the inverter drive circuit 16 is not operating
The load current flows only through the control circuit and is in a high voltage state. On the other hand, when the inverter 13 is activated and starts oscillating, the load current drops to a low voltage state because most of the load current is spent driving the switching elements. This voltage drop gradually progresses due to the presence of the capacitor 21.
The steady state voltage is shown at time M.

In FIG. 4, first, at time _t1 , when the output of the NAND gate NAND ₁ changes to "L" by the activation pulse generated from the activation signal generation circuit 14, the inverter drive circuit 16 operates, and the GCS 10 turns on, causing oscillation. is started. As the NAND gate NAND ₁ changes to "L" level, the transistor 31 is turned off and the capacitor 30 is charged with the voltage _VB . When the reference level Vref on the input side of the comparator circuit COM ₂ is reached, the output of the comparator circuit COM ₂ changes to "L", inverts the flip-flop 17, and makes its output "H". This turns on the transistor 31, and the charge in the capacitor 30 is discharged. Oscillation is repeated in this manner, but immediately after time _t1 , the value of voltage V _B is large, so the frequency is high, and gradually decreases as time passes.

On the other hand, the comparison circuit COM ₁ of the gate signal generation circuit 15
, the voltage V _CT is input to its input terminal,
This level is compared with the reference level Vref on the input side, but since the detected current is small immediately after time _t1 , the output maintains the "H" level. However, at time t ₂ , when the voltage V _B drops to a substantially steady state and the oscillation frequency drops to the steady oscillation frequency, the comparator circuit COM ₂ input reaches the reference level Vref.
The output of COM ₁ reaches the reference level, and an "L" level pulse is generated at that output. Thereafter, the flip-flop 17 will be driven by the gate signal generation circuit 15.

In this way, the period from time t ₁ to t ₂ is
It is possible to oscillate at a high frequency to suppress the input to the load, while oscillating at a normal frequency after time _t2 . Such a configuration can maintain the control voltage at a high value and control the load when the inverter oscillates, for example, immediately after the power is turned on, when the load is too small, or when the inverter oscillates under an abnormal load such as a no-load state. Since the detection current is set to a high frequency, that is, a minute current, and prevents excessive current from flowing to the switching element, it can be said that it functions extremely well as a safety device. Furthermore, in the present invention, even if the oscillation is restarted after the power switch is turned ON, a soft start is performed.
Even if an induction heating cooker is configured with a duty control mechanism and a small object detection mechanism with a restart function, stable oscillation startup can be performed in a soft start state even when restarting.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of an embodiment of the present invention.
FIG. 3 is a characteristic diagram showing the relationship between control voltage and load current, and FIG. 4 is a signal waveform diagram for explaining the operation of the embodiment. 4...Filter circuit, 13...Inverter, 1
4... Start signal generation circuit, 15... Gate signal generation circuit, 16... Inverter drive circuit, 17...
Flip-flop, 22...timing circuit.

Claims (1)

[Claims] 1 Once oscillation starts, it becomes a power source for driving the switching element in an induction heating cooker including a self-controlled oscillation inverter that generates an oscillation trigger signal by itself to control opening and closing of the switching element to maintain oscillation. A switching element drive power supply, a constant voltage power supply serving as a power supply for the control circuit system, and a voltage from the switching element drive power supply are received, and when this drive power supply voltage rises, the conduction period of the switching element is shortened, and the above drive power supply voltage is increased. timing means for lengthening the conduction period of the switching element when the switching element falls.