JPS584433B2 - Chiyorikinoyuudoukanetsuhoshiki - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating system for a cooking appliance that uses a series type inverter in which an operating coil and a generator are connected in series, and is frequency self-controlled.

Conventionally, the induction heating method using a frequency differential series inverter is known, but when the load placed on the operating coil section (the object to be heated, for example, a cooking pot) changes,
The inductance of the operating coil varies significantly depending on the metal material and size of the load, the matching state with the operating coil, etc., and the resonant frequency of the series circuit of the operating coil and the capacitor varies significantly.

Therefore, in the case of a frequency control type in which a fixed frequency ignition pulse is externally applied to the switching element (Sirisk, etc.) used in a series type invert, the reverse bias time of the switching element always changes depending on the load condition. When the load is light, the reverse bias time is short and commutation fails, causing unstable operation.

Furthermore, if the reverse bias time of the switching element is increased in order to eliminate this problem, the apparent power increases, causing a decrease in efficiency.

The present invention has been made in view of the above-mentioned circumstances, and can improve power efficiency by eliminating unstable operation of the series inverter even when there are load fluctuations, and can prevent short-circuit accidents in the DC power supply due to malfunction of switching elements in the series inverter circuit. It is an object of the present invention to provide an induction heating method for a cooker that can prevent the above problems.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of the main circuit. That is, the positive terminal 1 of the DC power source is connected to one end of the series resonant circuit 3 via the thyristor T1, and the negative terminal 2 of the DC power source is connected to the other end of the series resonant circuit via the resistor 7a.

The series resonant circuit 3 is constructed by connecting an operating coil 5 and a capacitor 6 in series to perform an induction heating operation on a load 4. An example of the load 4 is a cooking pot.

Both ends of the series resonant circuit 3 are connected via a thyristor T2 which discharges the charge of the capacitor 6.

A resistor 8a is connected in parallel to the thyristor T2, and a charging circuit for a capacitor 10a is connected between one end of the resistor 8a and the power supply terminal 1 via a resistor 9a.

The anode of a thyristor T3 is connected to the connection point between the capacitor 10a and the resistor 9a, and the cathode side of the thyristor T3 is connected to the negative end of the resistor 7a.

This resistor 7a is for detecting overcurrent flowing between power supply terminals 1 and 2, and a variable resistor V is connected in parallel to this resistor 7a.
R11 is connected.

A movable terminal of variable resistor VR11 is connected to the gate side of thyristor T3.

In addition, reactor L.

is the resistance R for di/dt suppression and dv/dt suppression.

and capacitor C0 is for dv/dt suppression.

FIG. 3 shows a control circuit for obtaining firing pulses for each of the thyristors T1 and T2.

That is, from terminals 11 and 12 connected to terminals 8 and 9 of the current transformer 7, a voltage VÅ corresponding to a positive half-wave and a voltage VB corresponding to a negative half-wave are obtained via a full-wave rectifier circuit 13.

The electric current EEVA is applied to the gate of the transistor Tr1, is amplified and shaped into a waveform, and is then connected to the capacitor C1 + resistor V.
The differential output is given to the differentiating circuit 14 consisting of R1jR1, and the differential output is waveform-shaped by the transistor Tr2.
5, the waveform is shaped by the transistor Tr3, and its output is inverted by the inverter 16 and supplied to the NAND circuit 17.

Similarly, the voltage VB is supplied to the NAND circuit 21 via the transistor Tr6, the differential circuit 18, the transistor Tr7, the differential circuit 19, and the transistor Tr8A inverter 20, which correspond to those described above.

The variable resistor vR1 of the differentiating circuit 14 is a transistor Tr.
A differentiating circuit 18 consisting of a capacitor C2A resistors vR2 and R2, which is used to change the width of the output waveform after 2.
The variable resistor vR2 is for changing the width of the output waveform after the transistor Tr7.

The output of the NAND circuit 17 is given to the transistor Tr4, and further passed through the transistor Tr5A transformer 22 to the first
A firing pulse is sent to the gate of thyristor T2 in the figure.

Similarly, the output of the NAND circuit 21 is the transistor Trg, T
rlO,}Thyristor T1 in FIG. 1 via lance 23
Sends an ignition pulse to the gate of the

However, the gate of the transistor TrlO is also connected to the starting pulse supply terminal 24, so that when a starting pulse is supplied thereto, the transistor TrlO and the transformer 21 send out a firing pulse for the thyristor T1.

The input of the NAND circuit 17 is connected to the collector of the transistor Tr6, and the input of the NAND circuit 21 is connected to the collector of the transistor Tr1 in the circuit shown in Fig. 2, so that firing pulses are generated in the same phase in the thyristors T1 and T2. If there is a chance, the commutation will fail and the DC power supply terminals 1 and 2 will be short-circuited, which is dangerous. Therefore, during the energization period of thyristor T1 or T2, the NAND circuit 17 and 21 will send an ignition pulse to the thyristor on the other side. This is to prevent this from occurring.

FIG. 4 is an operational waveform diagram of the circuits shown in FIGS. 1 and 3, and particularly shows the firing pulse generation process of the thyristor T2.

Next, the operation of each of the above circuits will be explained.

First, immediately before starting, in a state where no residual voltage exists in the capacitor of the resonant circuit 3, a starting pulse to the terminal 24 in FIG. 3 provides an ignition pulse to the thyristor T1.

Then, the thyristor T1 is ignited, and a series resonant current 11 flows through the path of the DC power supply terminal 1, the thyristor T1, the operating coil 5, the capacitor 6, the resistor 7, and the power supply terminal 2.

If the inductance of the coil 5 is L1 and the capacitance of the capacitor 6 is C, then the current width tw of 11 is tw
It is determined by =π√C-.

When this resonant current 11 reaches the maximum current value, the capacitor 6 is charged to approximately the DC power supply voltage E.

The above resonant current 11 is detected by the current transformer 7 inserted in the series resonant circuit 3, amplified by the transistor Tr1 and converted into a rectangular wave, and the phase corresponding to the end point of the resonant current 11 is detected. do.

An appropriate delay time td from this phase is obtained by the differentiating circuit 14 and transistor Tr2, and a pulse generated immediately after the elapse of the above delay time td is obtained by the differentiating circuit 15 and transistor Tr3, and the inverter 16, NAND circuit 17, transistor Tr4pTr, , } provides an ignition pulse to the thyristor T2 via the lance 22.

Here, an appropriate delay time td from the end point (near the zero point) of the resonant current 11 is the reverse bias time tq of the thyristor T1.
The reverse bias time must be set appropriately longer than the turn-off time of the thyristor T1 itself.

However, if it is too large, the reverse bias time becomes long and stable operation can be expected, but the apparent power becomes large and the efficiency is too low, so it is better to make the reverse bias time as close as possible to the turn-off time of the thyristor itself. .

Next, when the thyristor T2 is fired, a circuit of the capacitor 6, the operating coil 4 and the thyristor T2 is formed, and the charge in the capacitor 6 is discharged.

The current width tw of the discharge current 12 flowing at this time is approximately tw=
It is determined by πJLC.

This discharge current 12 is detected by a current transformer 7,
Hereinafter, the transistors Tr6 to 23 corresponding to the transistors Tr1 to 22 detect the phase corresponding to the end point of the current 12, and further lag time t from this phase.
The ignition pulse for the thyristor T1 is obtained from the pulse generated immediately after the lapse of d.

Note that the voltage v1 across the thyristor T1 becomes higher than the power supply voltage E by a voltage corresponding to the reverse bias period of the thyristor T2 due to reverse charging of the capacitor 6 due to the end point of the current 12.

Even if the voltage across the thyristor T2 is still v2, the above voltage v
The operation corresponds to 1.

Hereinafter, the series of operations for igniting the thyristor T1 and the series of operations for igniting the thyristor T2 described above are performed alternately, and the operations continue.

However, in the main circuit of FIG. 1, resistor 9a. The capacitor 10a is charged via the capacitor 10a.

In the event that an overcurrent flows between DC power supply terminals 1 and 2 due to an accident such as failure of commutation in thyristors T1 and T2,
Since a voltage greater than the normal operating state is generated across the resistor 7, the thyristor T3 is fired by this voltage.

Then, the charge in the capacitor 10a is discharged via the capacitor C1, the thyristor T3, and the resistor 8a, so a reverse bias is applied to the thyristor T2.
is turned on and the inverter operation stops, providing overcurrent protection.

FIG. 2 shows a modification of the main circuit shown in FIG.

The feature of this circuit is that an overcurrent flowing between power supply terminals 1 and 2 is detected by a current transformer 31. In this case, a voltage larger than that of the inverter circuit in the steady operating state is generated in the variable resistor 32, so a Zener diode is used. 33 becomes conductive, igniting the thyristor T3 and turning off the thyristor T2.

Since other operations correspond to those of the circuit shown in FIG. 1, corresponding parts are given the same reference numerals and explanations will be omitted.

As explained above, in the induction heating method of the present invention using a frequency self-limiting series type inverter, the resonant current flowing in the series resonant circuit is detected to obtain a constant reverse bias time for the thyristor in response to load fluctuations. This prevents the thyristor from failing in commutation and resulting in unstable operation, and also prevents the apparent power from increasing due to excessive reverse bias time, which improves the power efficiency of the circuit. This is something to look forward to.

Since the switching element of the first series inverter circuit is turned off when it malfunctions, it is possible to prevent short-circuit accidents of the DC power supply.

[Brief explanation of the drawing]

1 and 2 are examples of the main circuit used in the present invention, FIG. 3 is an example of the control circuit for the main turntable, and FIG. 4 is an operational waveform diagram thereof. 1, 2... DC power supply terminal, 3... Series resonant circuit, 4
・Rhyme, T1, T2, T3... Cyrisk, 7, 31
... Current transformer AvR11... Variable resistor, 7a, 8a, 9a... Resistor, 10a... Capacitor, 32... Variable resistor, 33... Zener diode.

Claims (1)

[Claims] 1. A DC power source, a series resonant circuit consisting of an operating coil vaporized capacitor connected between the positive and negative electrodes of this DC current and performing induction heating of the load, and a series resonant circuit that is energized to charge the capacitor of this circuit. a first switching element for discharging the capacitor charge of the series resonant circuit; and a second switching element for discharging the capacitor charge of the series resonant circuit; and a second switching element for discharging the capacitor charge of the series resonant circuit; The first and second circuits are arranged so that the operation of passing a current in one direction and the operation of passing a current in one direction after detecting that the current in the opposite direction of the series resonant circuit has become close to zero are performed alternately. a control circuit for controlling the second switching element; a means for detecting that an overcurrent has flowed between the positive and negative electrodes of the DC power source; and a control circuit for controlling the second switching element; An induction heating method for a cooker, characterized by comprising a circuit for turning off a switching element.