JPS6116633Y2 - - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to an induction heating cooker.

Induction heating cookers convert commercial low-frequency alternating current into direct current using a rectifier circuit and smoothing circuit, drive an inverter to oscillate with this DC power supply, and apply the resulting high-frequency alternating current to a work coil to generate a magnetic field. This magnetic field is placed near the coil and applied to the magnetic pot to heat it by induction.

The present invention aims at reducing the switching loss of the switching transistor in this type of induction heating cooker and increasing the speed of inverter oscillation. An embodiment will be described below with reference to the drawings. FIG. 1 shows a circuit diagram, in which 1 is a commercial low-frequency AC power supply, and 2 is a rectifier circuit formed by bridge-connecting four diodes, which receives the AC power supply 1 as an input.
_C1 is a capacitor connected to the output side of the rectifier circuit 2, and a high-frequency bypass capacitor is used, which has high impedance for commercial AC frequencies (50/60Hz) and low impedance for high frequencies. S is a switch, WC is a work coil connected to the positive electrode side of the power supply section, one end of which is configured by the rectifier circuit 2, through the switch S, and CT is a current detection device that detects the load current flowing through the work coil WC. The circuit is, for example, a current transformer. 5
is a trigger circuit into which the pulsating current signal VCC ₂ stepped down from the AC power supply 1 via the transformer T ₁ and the rectified and smoothed DC voltage VDD are input, and immediately after the pulsating current signal VCC ₂ starts rising from zero. It is configured to generate a trigger pulse. 6 is an amplifier circuit that inputs this trigger pulse, and the pulsating current signal is
Inputs VCC ₂ and DC voltage VDD. Furthermore, the output of the current transformer CT is also added to this amplifier circuit 6, and its output signal is amplified. This amplifier circuit 6 has a built-in one-shot multivibrator, which operates in response to a trigger pulse input and when the current I _L detected by the current transformer CT rises from zero, and outputs an arbitrarily set time. Occur. Reference numeral 7 denotes a drive circuit to which the output signal of the amplifier circuit 6 is applied, and supplies a signal to the base of a switching transistor to be described later to drive it. Note that the amplification factor of the signal amplified by the amplifier circuit 6 is set so that the switching transistor is turned off near zero voltage. 8
is an inverter that receives a signal from the drive circuit 7 and oscillates.

An implementation circuit of the inverter 8 and the drive circuit 7 will be explained below based on FIG. GTR is a switching element, such as a transistor, whose collector is connected to the other end of the work coil WC, and whose emitter is connected to the negative electrode side of the power supply section. In addition to transistors, a GTO (gate turn-off thyristor) can also be used as the switching element. The work coil WC is spirally wound, and an insulating top plate 3 is placed adjacent to the work coil WC.
is arranged, and furthermore, a magnetic pot 4 is placed on this top plate 3. Therefore the work coil
The magnetic field generated by the WC passes through the top plate 3 and is applied to the pot 4. _C2 is a low capacitance resonant capacitor connected in parallel with the switching transistor GTR, and _D1 is a diode connected in anti-parallel with the switching transistor GTR. The drive circuit 7 has a switching transistor Q to which the signal from the amplifier circuit 6 is inputted to its base and makes it conductive, and a circuit for voltage-current conversion between the collector of this transistor Q and the pulsating current power supply VCC ₁ . A series circuit consisting of the primary coil of the transformer T ₂ connected to the primary coil, a discharging resistor R ₁ and a reverse current blocking diode D ₂ connected in parallel with the primary coil, and further connected in parallel with the resistor R ₁ . It is equipped with a discharge capacitor _C3 connected to. The secondary coil of the above transformer _T2 is a transistor
It is connected between the base of GTR and the emitter. _R2 is a resistor inserted between the base of the transistor GTR and the secondary coil, and serves to protect the transistor GTR and control switching speed. _D3 is an accelerating diode connected in parallel to this resistor _R2 and in a direction opposite to the base-emitter current direction of the switching transistor GTR. D ₄ and R ₃ are diodes and resistors for forced inversion of the switching transistor GTR, and are connected in series between the base and emitter of the transistor GTR.

Next, referring to FIG. 3, the operation of one embodiment of the present invention will be described. VCC ₁ is _the low frequency AC frequency (50/60
Hz) signal, which includes a high frequency component a of approximately 20KHz. VCC ₂ is a pulsating current signal with a low frequency AC frequency obtained from the AC power supply 1 via the step-down transformer T ₁ , and VDD is a DC voltage obtained by rectifying and smoothing the pulsating current, which is applied to the trigger circuit 5 and the amplifier circuit. 6. V _T represents the trigger pulse generated at the initial time point A of the signal VCC ₂ , I _L represents the high frequency load current flowing through the work coil WC, and VCE represents the collector potential of the switching transistor GTR.

First, to explain the operation related to the AC frequency signal, when the switch S is closed, the pulsating current signal VCC ₂
When a trigger pulse V _T is applied from the trigger circuit 5 to the drive circuit 7 via the amplifier circuit 6 at the initial rising time A, this trigger pulse V _T is amplified and applied to the switching transistor GTR, making it conductive. Due to the conduction of the transistor GTR, a self-oscillation operation, which will be described later, is started and continues until time B, when the signal VCC ₂ ends at falling. At this point B,
Since the pulsating current signal VCC ₂ is small, the amplification factor of the amplifier circuit 6 decreases and oscillation stops. Thereafter, when a trigger pulse V _T is generated at time A of the signal VCC ₂ , oscillation is started again, and the same operation is repeated thereafter. Here, an explanation will be added regarding the trigger pulse V _T . After the oscillation of the pulsating current signal VCC ₂ stops at time B, if the trigger pulse V _T is not output at the next time A, the load current I due to the low frequency voltage VCC ₁ will decrease because the capacitance of the resonant capacitor C ₂ is small. Almost no current flows in _L , and if this continues, the transistor GTR will become conductive again and oscillation will not start. Therefore, it is necessary to repeatedly generate this trigger pulse V _T at the initial rise time of the pulsating current signal VCC ₂ .

Next, the operation of self-oscillation in high frequency components will be explained. Now, the switch S is closed at an appropriate time between times A and B of the pulsating flow signal VDD. At this time, the switching transistor
GTR is in off state and resonant capacitor C ₂
shall be in a completely discharged state. By closing switch R, the load current I _L changes to the work coil.
WC→resonant capacitor _C2 →capacitor _C1 →switch S→flow rapidly through work coil WC.
Therefore, this current I _L is the current transformer C _T
When the current I _L detected at rises from zero, the trigger circuit 5 is activated, and a positive feedback current flows to the base of the transistor GTR, making it conductive. Let this transistor GTR on time be _T1 . This time _T1 is the signal output time of the one-shot multivibrator included in the amplifier circuit 6. During this time, the voltage VC becomes zero, the load current I _L gradually increases, and the work coil
Energy is stored in WC. After time _T1 has elapsed, a reverse bias is applied between the base and emitter of transistor GTR, turning off transistor GTR. During the following time _T2 , the energy stored in the work coil WC is discharged (at this time, the polarity of the work coil is positive on the collector side of the switching transistor), and the collector voltage V _C of the transistor GTR increases. Therefore, this voltage V _C charges the resonant capacitor C ₂ . work coil WC
At time T ₃ after completion of discharge, the resonant capacitor C ₂ is now discharged (at this time, the polarity of the resonant capacitor is positive on the collector side of the switching transistor), and the load current I _L changes from the resonant capacitor C ₂ → Energy flows through a closed circuit consisting of work coil WC → switch S → capacitor C ₁ → resonance capacitor C ₂ , and energy is supplied to work coil WC again. Further after the above discharge is completed, the time
At _T4 , the energy of the work coil WC is discharged, and the load current I _L flows through a closed circuit consisting of the work coil WC, the switch S, the capacitor _C1 , the diode _D1 , and the work coil WC. Here, voltage V _C is clamped by diode D ₁ . At the subsequent time T ₁ , the load current I _L changes due to the oscillating current of the work coil WC from the work coil WC to the resonant capacitor C ₂ to the capacitor C ₁ to the switch S
flows through the work coil WC, and this load current I
_L is detected by the current transformer CT near zero, the trigger circuit 5 is activated, turns on the switching transistor GTR, and the oscillation operation is started again. The above oscillation operation generates the pulsating flow signal VCC ₂
It continues until point B.

Next, the operation of the drive circuit 7 will be explained with reference to FIG. In the oscillation operation, when the transistor Q is conductive (period T ₁₁ ), an electromotive force VC ₂ is induced on the secondary side by the primary voltage VC ₁ of the transformer T ₂ . This voltage VC ₂ causes a current IB ₁ to flow to the base of the switching transistor GTR via the resistor R ₂ , making it conductive. When transistor Q shuts off (period
T ₁₂ ), the current IB ₂ flows in the opposite direction due to the back electromotive force generated on the secondary side of the transformer T ₂ . This current _IB2 is generated by discharging the charges stored between the base and emitter of the switching transistor GTR, and at the end of this discharge, the switching transistor GTR is completely cut off.

The time delay (period T ₁₂ ) between when the transistor Q shuts off and when the switching transistor GTR shuts off is a cause of impediments to increasing the oscillation frequency, but this invention solves this problem with the following configuration. are doing. In other words, in order to make the tracking of the switching transistor GTR more sensitive, the current
All you have to do is increase IB ₂ and make the residual energy on the secondary side of the transformer T ₂ almost zero. Therefore, in the present invention, a resistor R ₃ and a diode D ₁ are connected between the base and emitter of the transistor GTR.
However, a diode _D4 is further provided in parallel with the resistor _R2 . Now the base of the transistor GTR
When the second voltage between the emitters ends, the residual energy on the secondary side of the transformer T ₂ is transferred to the diode D ₄ and the resistor R ₃
and discharge rapidly through diode D ₃ .
Therefore, the conduction time _T12 of this current _IB2 is shortened, and the timing of shutting off the switching transistor GTR is brought forward. Note that the dashed line curve in the figure shows the operating curve according to the conventional example, and is indicated by the IB ₂ energization time T ₁₂ '.

In this way, the energy in the transformer T ₂ is quickly and completely discharged, so the next time transistor Q conducts, sufficient current will flow through the primary side of the transformer T ₂ and the voltage VC ₁ will have a large value. Because it will be,
If induced on the secondary side, the base current IB ₁ will also have a sufficient value, and as a result, the current IB ₂ can also have a sufficient value. This allows the on/off operation of the switching transistor GTR to closely follow the on/off operation of the transistor Q.

As explained above, according to the present invention, since the cutoff response operation of the switching transistor is accelerated, the oscillation frequency of the inverter can be reduced from the current 30 KHz.
to approximately 60KHz, making it possible to heat 18-8 stainless steel pots, which had previously been considered an inefficient load.

[Brief explanation of the drawing]

Fig. 1 is a circuit block diagram of an embodiment of the induction heating cooker of the present invention, Fig. 2 is a specific circuit diagram of the drive circuit in the embodiment, and Figs. 3 and 4 are explanations of the operation of the embodiment. FIG. 3...Top plate, 4...Magnetic pot, 5...
...Trigger circuit, 6...Amplification circuit, 7...Drive circuit, CT...Current transformer, _T1 , _T2 ...Transformer.

Claims (1)

[Scope of utility model registration request] A low frequency pulsating current power source, a work coil and a switching transistor connected in series with this pulsating current power source, a resonant capacitor and a diode respectively connected in parallel with this switching transistor, and generating a trigger pulse of a predetermined period. In an induction heating cooker including a trigger circuit and a drive circuit that operates in response to the generation of a trigger pulse from the trigger circuit and drives the switching transistor, the drive circuit becomes conductive in response to the generation of the trigger pulse. a transistor, and a transformer with a primary coil connected between the collector (emitter) of this transistor and the pulsating current power supply,
The secondary coil of this transformer is connected to the base of the switching transistor through a parallel circuit of a resistor and a diode, and a diode and a resistor are connected between the base and emitter of the switching transistor to forcibly invert the transistor. An induction heating cooker characterized by connecting a series circuit.