JPS5831716B2 - Oscillation circuit of induction heating cooker - Google Patents

Description

[Detailed description of the invention] The present invention relates to an oscillation circuit for an induction heating cooker.

The oscillation circuit of a conventional induction heating cooker is constructed as shown in FIG.

In other words, the induction heating cooker uses the heating coil L.

Transistor Q1 for the series circuit with capacitor C8 and capacitor C8
is turned on to supply power from the power sources V+ and V, and the transistor Q2 is turned on to release the energy stored in the series circuit, thereby causing an oscillatory current to flow through the heating coil Lo to heat the cooking pot.

And heating coil L taken out by current transformer OT in order to control on/off of transistors Q+ and Q2.

A pulse signal is formed by detecting the phase of the current, and based on this pulse signal, the monostable multi-bi break MM
M2 forms a drive signal with a predetermined time width and serves as a drive circuit.

On/off control of transistors Q+ and Q2 is performed via Dl and D2.

In this case, the time width of the drive signal is determined by the time constant elements R,1,O and R2,C2 of the monostable multi-bibreaks MM, MM2.

However, this time constant element has considerable variation, and the time width of the drive signal becomes uneven.

For this reason, the heating coil. There is a high possibility that power supply will be performed that does not meet the resonance conditions of the series circuit of the transistors T1. This causes problems such as temperature rise at T2 and variations in power consumption among products.

An object of the present invention is to provide an oscillation circuit for an induction heating cooker that can carry out current flow that meets the resonance conditions of a series circuit of a heating coil and a capacitor.

An embodiment of the present invention will be described below with reference to FIGS. 2 to 4.

FIG. 2 is a block diagram of a main part of an embodiment of the present invention, in which transistor Q1. A circuit portion forming a drive signal for driving Q2 is shown.

In this circuit, the Xerox point is obtained by detecting the phase of the detection signal of the current transformer OT, and the period from when the level of the waveform exceeds a certain value until it becomes lower than that value is obtained by detecting the voltage.

That is, phase detection circuit PD1. The zero cross point is detected by PD2 and the flip-flops FF, FF2 are set, and the voltage detection circuits VD, , VD2 detect the point where the signal level drops and the flip-flops FF1 . Reset FF2.

Therefore, the drive circuits Qt and D2 respectively turn on the transistors Qt and Q2 from the zero cross point until the signal level once exceeds a predetermined value and then falls below that level.

Here, a NAND circuit NAN inserted between the output end of the phase detection circuit PD2 and the input end of the flip-flop FF2
A start signal S8 is provided to this NAND circuit NAND.
is given by heating coil L by turning on transistor QI when the circuit starts.

This is to charge the series circuit of the capacitor C8 and the capacitor C8.

Figure 3 shows the detailed configuration of the circuit in Figure 2.
is the phase detection circuit, VD is the voltage detection circuit ○and,
Each of these detection circuits PD and VD is provided with a separate circuit ii.
c! The detection operation is performed based on the current signal given from T,, OT2.

FIG. 4 is a time chart showing signals of various parts of the circuit of FIG. 3, and the operation of the circuit of FIG. 3 will be explained based on these signals.

To start this circuit, first, a starting signal Ss is applied as one input of the NAND circuit NAND.

The starting signal Ss is always at a high level (as indicated by 17).
(hereinafter simply referred to as "H"), and becomes low level (hereinafter simply referred to as "Ln") when a signal is applied.

As a result, the output 19 of the NAND circuit NAND becomes "Hl" regardless of whether the signal 18 is "H" or "L".

As a result, the flip-flop FF2 is set and the output a is given to the transistor Q1, which in turn powers the heating coil L.

and capacitor C8 (Fig. 1) are charged.

This charging waveform becomes the output of the current transformer OT.

First, in the voltage detection circuit VD, a signal ``H'' is applied to the transistor Q13 while the output of the secondary terminal of the current transformer CT2 exceeds the operating level of the Zener diode ZD. (11, A signal [phase] having a level relationship opposite to that of "'L" is formed and applied to the inverter IN5 to obtain an inverted signal 0.

This signal 0 is differentiated by a differentiating circuit including a capacitor C2, and only the negative pulse O is taken out, and the inverter I
Converted to positive pulse 0 at N7 and sent to flip-flop FF
2 to reset flip-flop FF2.

This turns off transistor Q1 and turns off heating coil L.

, the charging operation of the series circuit of capacitor C8 is stopped.

That is, charging is stopped when the accumulated energy of this series circuit reaches a predetermined level or more.

Therefore, by setting this level appropriately, charging can be stopped at an optimal timing.

Due to this charging stop, the output of the current transformer CT decreases and reaches the zero point, and in the phase detection circuit PD, the signal ■, which was previously "HII", becomes "(, l+", and as a result, the transistor Qr
t's correct output ■ is from ((L +1 to “H
Changes to ll.

This signal ■ is inverted by the inverter IN1 and becomes a signal ■ showing the same level change as the signal ■. It is differentiated by a differentiating circuit including the capacitor C1, and only the negative pulse ■' is taken out and passed through the inverter ■N3 to become a positive pulse. ■
becomes.

This positive pulse (2) is applied to the flip-flop FF1 to set it.

The output of this flip-flop FF is the transistor Q2
Heating coil L.

, discharges the stored energy in the series circuit of capacitor C8.

This causes the output of the current transformer CT to have opposite polarity.

Then, looking at the voltage detection circuit VD, when the signal at one terminal of the current transformer CT2 increases and exceeds the operating level of the Zener diode ZD2, a signal 0 is obtained which changes from "L?1" to "H", and the transistor Q14 ?The correct output [phase] changes from "H" to "L"? Changes to 1.

This signal 0 is inverted by the inverter IN6 and the signal [
The signal [phase] shows a level change similar to the phase], is differentiated by a differentiating circuit including a capacitor C4, and only its negative pulse is taken out and passes through an inverter N8 to become a positive pulse [phase].

This positive pulse [present] is applied to the flip roller 7'FF to reset it.

This turns off the transistor Q2 and turns off the heating coil L.

, the discharging operation of the series circuit of capacitor C8 stops.

As a result, in the phase detection circuit PD, the signal ■ appearing at one terminal of the current transformer CT1 is applied to the transistor Q1□ to become a signal ■, whose polarity is inverted by the inverter ■N2, and then applied to the differentiating circuit including the capacitor C2. Only the negative pulse of the differentiated output is taken out and inverted by an inverter IN4 to become a signal (2), which is further inverted by an inverter IN to become a signal [phase] which is applied to a NAND circuit NAND.

This signal @ is similar to the signal @, that is, the start signal Ss, and thereby sets the flip-flop FF2.

The subsequent operations repeat those described above. As described above, the present invention turns on the switching element at the zero-crossing point of the current in the series circuit of the heating coil and the capacitor to energize, and after the energizing current increases, the current flows before the next zero-crossing point at which it decreases by a predetermined amount. Since the switching element is turned on, the switching element is turned on and off at timings suitable for charging and discharging the series circuit.

Therefore, the operating conditions of the two switching elements can be made uniform, and even if the inductance of the heating coil changes due to the characteristics of the cooking pot, optimal energization control can be performed at all times.

[Brief explanation of the drawing]

FIG. 1 is an oscillation circuit diagram of a conventional induction heating cooker, FIG. 2 is a block diagram showing the main circuit of an embodiment of the present invention, and FIG.
The figure is a detailed circuit diagram of the same embodiment, and FIG. 4 is a time chart showing signals of each part of FIG. 3.

Claims (1)

[Claims] 1. Power is supplied from the power source to a series circuit having a heating coil and a capacitor through a first switching element.Then, the first switching element is turned off and a second switching element inserted in parallel to the series circuit is connected. A phase detection circuit for detecting the zero point of the current in the heating coil in an oscillation circuit for an induction cooking device in which an alternating current is caused to flow through the heating coil by repeatedly turning on and discharging the stored energy in the series circuit. a voltage detection circuit for detecting the point in time when the current in the heating coil reaches a predetermined level or more and drops below the level; and a voltage detection circuit for detecting the point in time when the current in the heating coil reaches a predetermined level or higher and turns on each of the switching elements by the output of the phase detection circuit. An oscillation circuit for an induction heating cooker, comprising a circuit for controlling turn-off of the switching elements based on the output of the voltage detection circuit.