JPS6127875B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating method suitable for induction heating a cooking pot or the like.

The operating coil (corresponding to the load) that performs induction heating work is used to perform this type of induction heating.
There are two types of inverter control systems: a series inverter control system in which the coil is connected in series with the capacitor, and a parallel inverter control system in which the operating coil is connected in parallel with the capacitor. For example, a pot placed near the operating coil was heated by induction. In addition, in order to control the power of induction heating in circuits based on each of the above inverter control methods, the load power is controlled by varying the power supply voltage using AC phase control or so-called rectification control, which performs phase control while rectifying, using switching elements such as triaxes and thyristors. was in control.

However, in the conventional device described above, since the load power changes in proportion to the square of the power supply voltage, there is a problem that the load power becomes unstable with respect to fluctuations in the power supply. In addition, the conventional type had a configuration in which the energy accumulated in the operating coil was not returned to the power supply, resulting in poor power efficiency and
Further, there were other problems such as a surge voltage several times higher than that in steady state being generated when there is no load. Furthermore, as described above, load power control has been performed by alternating current phase control or rectification control, which has resulted in problems such as an increase in apparent power and a deterioration in overall efficiency.

The present invention has been made in view of the above-mentioned circumstances, and in addition to eliminating the above-mentioned drawbacks of the conventional switching elements, it is possible to reduce the cost of the switching elements by reducing the withstand voltage rating of the switching elements used, and it also enables self-limiting frequency control. The purpose of the present invention is to provide an induction heating system that allows a load current to flow smoothly and continuously, and that also allows power control to be performed without deteriorating efficiency.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows its main circuit. In the figure, 1 is an operating coil that performs an induction heating action on a load (for example, a cooking pot) 2. One end of this coil 1 is connected to the positive electrode of a DC power supply 3, and the other end is connected to a collector of a switching element 4, Power supply 3 via emitsuta
is connected to the negative pole side of the Switching element 4
As such, a transistor is used here, but
GTO etc. may also be used. A diode 5 having the illustrated polarity is connected in parallel to the switching element 4, and a capacitor 6 forming a resonant circuit together with the operating coil 1 is also connected in parallel. A current transformer 7 for detecting load current is arranged at one end of the operating coil 1 . A radio frequency interference prevention circuit 8 is inserted on the AC input line side of the DC power supply 3.

FIG. 2 shows an example of a frequency self-limiting control circuit for controlling the switching element 4. As shown in FIG. That is, the resistor 11 connected to both ends of the current transformer 7
One end is diode 12, resistor 13, variable resistor
VR, resistor 1 through resistor 14, diode 15
1 is connected to the other end. The other end of this resistor 11 is connected to one end of the resistor 11 via a diode 16, a resistor 17, and a diode 18. A movable contact piece of the variable resistor VR is connected to the base of a transistor 22 via diodes 19 and 20 and a resistor 21. The base of this transistor 22 is grounded via a resistor 23. transistor 2
The collector of No. 2 is connected to the +Vcc ₁ power supply via a resistor 24 and to the input of an inverter 26 via a capacitor 25. The input section of this inverter 26 includes a resistor 27 and a diode 2.
8 in parallel to +Vcc ₁ power supply. The output of inverter 26 is connected to the reset terminal of flip-flop circuit 29. The J and K terminals of this flip-flop 29 are +Vcc
It is connected to the power supply and the CK terminal is grounded.
The set terminal of flip-flop 29 is connected to the output terminal of inverter 30, and the inverting output terminal is connected to the base of transistor 32 via resistor 31. The base of this transistor 32 is connected to the +Vcc ₁ power supply via a resistor 33,
The emitter is also connected to the +Vcc ₁ power supply.
The collector of transistor 32 is grounded via resistor 34, and the collector is connected to capacitor 3.
5. It is connected to the base of a transistor 38 via a resistor 36 in parallel and further via a resistor 37 in series. The base of this transistor 38 is a resistor 39
The emitter is also grounded directly. The collector of the transistor 38 is connected to the +Vcc ₂ power supply via the primary coil of a pulse transformer 40 and a resistor 41. One end of the secondary coil of the pulse transformer 40 is connected to the base of the switching element 4 via a resistor 41, and the other end is connected to the emitter of the switching element 4. Further, both ends of the pulse transformer 40 are connected to a diode 4.
2. Switching element base consisting of resistor 43.
Connected to emitter reverse voltage control circuit.

The cathode side of the diode 16 is connected to the base of a transistor 48 via diodes 44, 45, 46 and a resistor 47. The base of the transistor 48 is grounded via a resistor 49, and the emitter is directly grounded. Further, the collector of this transistor 48 is connected to +
Connected to Vcc ₁ power supply and further directly connected to inverter 51
is also connected to the input end of the This inverter 5
1 is connected to the input end of an inverter 53 via a capacitor 52, and this input end is connected to a resistor 5.
4. Connected to +Vcc ₁ power supply via diode 55 in parallel. The output terminal of the inverter 53 is connected to one input terminal of the NOR circuit 56, the other input terminal of this NOR circuit 56 is connected to the starting pulse supply terminal 57, and the output terminal of the NOR circuit 56 is connected to the input terminal of the inverter 30. It is connected to the.

Next, the operation of the circuit having the above configuration will be explained with reference to the timing chart of FIG. 3 as appropriate. First, a starting pulse is applied to the terminal 57, and the flip-flop 29 is set via the NOR circuit 56 and the inverter 30. Then, a signal is applied to the base of the switching element 4 via the transistors 32, 38, pulse transformer 40, etc., and the element 4 is turned on.
As a result, the load current i _L of the operating coil 1 (=collector current icc of the element 4) starts to flow linearly at a rise rate of di _L /dt≈Edc/L. However, here, Edc is the voltage of the DC power supply 3, and L is the inductance of the operating coil 1. The load current i _L (=icc) is detected by the current transformer 7, and at the time _t1 when it reaches the set current I _p , the movable contact of the variable resistor VR has a resistor 1
3, 14 and the adjustment ratio of the variable resistor VR, the collector of the transistor 22 becomes a low potential, which resets the flip-flop 29 via the inverter 26, and the base current i _B of the switching element 4 increases. is reversed to turn off the element 4. In the main circuit, a resonant circuit determined by the capacitor 6 and the operating coil 1 causes a charging current IC ₁ to flow through the capacitor 6, and after this has finished flowing, it is reversed and a discharge current IC ₂ flows. After the time t ₂ when this ends, the current i _D continues to flow through the diode 5 due to the energy stored in the operating coil 1 and decreases, approaching zero (t ₃ ). Period during which this diode current i _D flows (t ₂ - _{t 3} )
An appropriate current value is detected by the current transformer 7, a pulse is obtained from the circuit of the transistor 48 and the inverters 51 and 53, the flip-flop 29 is set, and the above series of operations is repeated and maintained.

In the above operation, the time t ₁ ~ when the element 4 is off
At t ₂ (Th), a collector voltage Vcc close to a sine wave is applied to the element 4, and the peak value of this voltage Vcc is applied to the element 4.
_p is determined by the following formula.

V _p = {π/2 (T/Th-1) + 1} Edc ... (1) Also, the period Th during which the resonant currents ic ₁ and ic ₂ due to the operating coil 1 and capacitor 6 flow is determined by the following formula. .

Th=π√ ...(2) However, in the above equations (1) and (2), T is the load current i _L
, L is the inductance of the coil 1, and C is the capacitance of the capacitor 6. Therefore, if the peak value of the collector current of the switching element 4 (the peak value of the load current) _Ip is controlled to a constant value, and an appropriate capacitor 6 is connected, the repetition frequency of self-control is determined by the inductance L of the operating coil 1. become what is done.

However, in the above circuit of the present invention, Th/T〓45~
The capacitance C of the capacitor 6 is selected to be approximately 55%, and the peak value V _p of the collector voltage of the switching element 4 is set to the power supply voltage Edc.
The breakdown voltage rating of the switching element used is reduced by 2.3 to 3 times or less.
By lowering this withstand voltage rating, it is possible to reduce the cost of the switching element, and the saturation voltage V _CE (sat) is lowered, resulting in smaller losses.
It also has the advantage of reducing high frequency switching loss. In general, the capacitance L of the operating coil 1 varies significantly (usually by 30 to 40%) depending on the size and material of the object to be heated (for example, a cooking pot), the alignment state (the state of attachment between the pot and the operating coil), etc. However, in this circuit, by performing self-limiting frequency control corresponding to changes in the inductance L of the operating coil 1, the load current connects and flows smoothly, and control can be performed efficiently. By the way,
When controlled at a constant frequency from an external circuit, when the base signal of the next cycle is given during the cycle Th (t ₁ to t ₂ ) of the resonant currents ic ₁ and ic ₂ , the discharge current due to the charge of the capacitor 6 flows through the switching element. It flows into the collector of element 4, causing a large loss and possibly destroying element 4. Furthermore, when the base signal of the next cycle is applied after time t ₃ after the diode current i _D becomes zero, the load current i _L becomes an intermittent current and becomes an asymmetrical waveform of positive and negative, resulting in a decrease in efficiency and the above-mentioned As in the case described above, the loss due to the discharge current of the load of the capacitor 6 becomes large, and the switching element 4 may be destroyed. Particularly when applied to cooking pots, open loads may occur, and it is important to avoid the above phenomenon.

In addition, in the conventional induction heating method, the load power was configured to change approximately in proportion to the square of the power supply voltage, so there was a problem that the load power became unstable due to fluctuations in the power supply. Load current i _L (=
icc) is constant, the load power only changes by a factor of twice the power supply voltage, and therefore a nearly constant load power can be obtained even with power supply fluctuations. In addition, since the energy stored in the operating coil 1 is fed back to the power supply through the damper diode 5, this feedback current becomes part of the load current, increasing efficiency. Whereas a surge voltage that is several times higher than that generated occurs, this circuit operates stably with a low increase of about 10 to 30%. In addition, to control the load power, the second
By adjusting the movable contacts of the variable resistor VR shown in the figure, the set current I _p (MAX) is set as I _p1 , I _p2 , I
By reducing the peak current with _p3 ... or increasing it conversely, the load current i _L can be changed to I _RMS (MAX), I
The load power is controlled by changing _RMS2 , I _RMS3 , and so on. In this way, the maximum rated value
The load power can be stably varied from 100% to about 20%.

As explained above, according to the present invention, the main circuit corresponding to FIG. In addition, the load power is adjusted by changing the current level instead of using power control that increases the apparent power as in the past, making it possible to provide an induction heating method with good power efficiency. It is something.

[Brief explanation of the drawing]

The figures show one embodiment of the present invention, in which Fig. 1 is a main circuit diagram, Fig. 2 is a frequency self-limiting control circuit diagram, Fig. 3 is a timing chart for explaining the operation, and Fig. 4 is a timing chart for explaining the operation.
The figure is a timing chart for explaining power control. 1... Operating coil, 4... Switching element, 5...
Diode, 6...Capacitor, 7...Current transformer, 29...Flip-flop, 40...Pulse transformer, VR...Variable resistor.

Claims (1)

[Claims] 1. An operating coil for induction heating of a load, a switching element connected in series to this coil, a capacitor connected in parallel to this element and forming a resonant circuit together with the operating coil, and a switching element connected in parallel to the switching element. a connected rectifying element, a load current detecting means flowing through the operating coil, and generating a control signal to turn off the switching element when the load current detected by the means reaches a predetermined peak value; When the load current flowing through the rectifying element decreases to a predetermined value due to the charging/discharging operation of the resonant circuit, a control signal is generated to turn on the switching element, and the load current is changed in one direction and the opposite direction at a predetermined peak value. The present invention is characterized by comprising: a frequency self-limiting control circuit that controls the switching element so that the switching element is alternately connected to the current direction; and means that is provided in this circuit and changes the detection level of the load current according to the supplied power. Induction heating method.