JPS60172193A - Electromagnetic induction heater - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an electromagnetic induction heating device used as a cooker, etc. More specifically, the present invention relates to an electromagnetic induction heating device that is capable of precise overcurrent protection. This is what we propose.

[Prior art]

2. Description of the Related Art An electromagnetic cooker is known that uses an electromagnetic induction phenomenon to heat an iron pot for cooking. This type of cooker is equipped with an inverter circuit that includes an induction heating coil, and a high-frequency current is passed through this coil to inductively heat a pot placed on the coil to heat the food in the pot. However, a protection circuit is provided to prevent overcurrent from flowing to the switching elements of the inverter circuit when an unsuitable load such as an aluminum or copper pot is placed on the inverter circuit.

This protection circuit detects the input power and uses this to adjust the duty ratio of the switching signal that controls the conduction of the switching elements, but the difference between the resonant frequency unique to the load and the oscillation frequency of the inverter circuit has become large. In some cases, adequate protection was not provided.

〔the purpose〕

The present invention has been made in view of the above circumstances, and detects this difference in frequency based on the phase difference between the switching signal and the time when the direction of the current in the induction heating coil is reversed, and calculates the detected phase difference. It is an object of the present invention to provide an electromagnetic induction heating device in which the amount of change in duty ratio is reflected.

〔composition〕

The electromagnetic induction heating device according to the present invention includes a switching signal for reversing the direction of current in an induction heating coil, a switching signal for reversing the direction of the current, and a position at which the direction of the current is reversed. The present invention is characterized in that it includes a phase difference detection circuit and an input power detection circuit, and is configured to control the drive of an inverter circuit based on the phase difference and input power detected by these circuits.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof.

FIG. 1 shows an inverter circuit portion of an electromagnetic induction heating device according to the present invention. The direct current obtained by the full-wave rectifier 2 connected to the commercial frequency AC power supply 1 must be supplied to the single-ended pseudo-spool type inverter circuit 5 via the smoothing capacitor 3 and choke coil 4. be. The inverter circuit 5 includes a series resonant circuit including an induction heating coil 5a and a resonant capacitor 5b, and this series resonant circuit is connected in parallel with a switching transistor 5c and in antiparallel with a flywheel diode 5d. The switching transistor 5c has a collector connected to the connection point between the choke coil 4 and the induction heating coil 5a, and an emitter connected to the collector of another switching transistor 5e. The emitter of the transistor 5e is connected to the negative electrode side of the full-wave rectifier 2, and a diode 5f is connected in antiparallel to the transistor 5e. And transistor 5c,
A drive circuit 6.7 is connected to each of the base and emitter of 5e. This inverter circuit 5 is supplied with a pulse signal to turn on the transistor 5e from a control circuit (to be described later) via a drive circuit 7, and thereby the coil 5e is turned on.
A current flows in the direction of the arrow a, and the capacitor 5b is charged by this current. Next, after turning off the transistor 5e, the transistor 5e is
A pulse signal is given to turn on the capacitor 5b, thereby discharging the charge in the capacitor 5b and causing a current to flow in the opposite direction to the arrow mark through the coil 5a and the transistor 5c.

In this way, by alternately turning on the transistors 5e and 5c at a high frequency, a high frequency current flows through the coil 5a, which heats the pot 8 placed on the coil 5a, as well as the food inside the pot 8. It becomes possible. The current flows through the series resonant circuit between the induction heating coil 5a and the capacitor 5b.
In other words, a current transformer 9 for detecting input power is provided. As will be clear from what will be described later, 1 of the transistor 5c
Although the energization time of the cycle is constant, the transistor 5e
is controlled according to the input power detected by the current transformer.

FIG. 2 is a circuit diagram showing a part of the control circuit of the device of the present invention. The commercial frequency power supply 1 is rectified by a rectifier circuit 2', and a constant voltage is obtained by a Zener diode 41, thereby providing an oscillation frequency f of this electromagnetic induction heating device.

The oscillator 42 is driven to output a pulse signal.

The constant voltage is also applied to the primary winding 43a of the pulse transformer 43.
and a series circuit of a transistor 44, which is turned on by the oscillator 42 output V2'.
is turned off, and the third
Obtain the pulse signals to be applied to terminals α and β of the protection circuit shown in the figure.

The output of the rectifier circuit 2' is also applied to a series circuit of a primary winding 45a of a pulse transformer 45 and a transistor 46. The output vI' of the oscillator 42 is applied to this transistor 46 to turn it on and off, and the secondary winding 45b, A pulse signal is obtained at the tertiary winding 45c. Secondary winding 45b and transistor 5c
is given between the emitter and the base.

That is, this pulse transformer 45 functions as a drive circuit 6 for the transistor 5c. , the terminals at both ends of the tertiary winding 45c and the intermediate terminal are the terminals VH and VL of the protection circuit in FIG.
, VN.

In Fig. 3, the half-wave rectifier circuits 11 and 12 are provided so that the terminal VN is common, and the positive side terminal is connected to Vll and Vll.
, the negative terminal is set to VL, and the half-wave rectifier circuit is 11°12
The series output of is given to a constant voltage circuit using a Zener diode 13 to obtain +Vc and -Vs.

A transistor 14 and a capacitor 15 are connected between these two potentials.
A parallel circuit, a resistor, and a series circuit of a Zener diode are provided, and a terminal VL is connected to the base of the transistor 14. The potential V OFF at the connection point between the parallel circuit and the resistor is applied to the input terminal of the comparator 16 .

The capacitor 15 is charged while the transistor 14 is off, but the pulse output V1 of the frequency fo of the oscillator 42
', a pulse signal appears at the terminal Vl, which turns on the transistor 14, so that V OFF becomes the fourth
As shown in figure (a), about 2 out of 1 cycle are at a constant level,
2 becomes a waveform that attenuates in a hyperbolic manner, and this attenuation curve is determined by the discharge circuit constant.

The output of the current transformer 9 passes through the full-wave rectifier circuit 17 as shown in Fig. 4 (b).
This results in a pulsating flow signal ■I as shown in FIG. This signal Vl is applied to the integrating circuit 9 via the diode 18. The DC potential V19 obtained by integrating VI corresponds to the detected current of the current transformer 9,
Represents input power.

The current transformer 9 output is also connected to the diode 20. The signal is applied to a zero cross detection circuit 22 using an operational amplifier 21 or the like.

This circuit 22 detects the point in time when the current transformer output (alternating current) has a phase of 0°, and its output v1 is as shown in Fig. 4 (c).
It becomes as shown in . This signal (1) is applied to the input terminal of the operational amplifier 23 used as a monostable multi-pie break.

Terminals α and β to which the output of the secondary winding 43b of the pulse transformer 43 is applied are connected to the base and emitter of the transistor 24. The emitter of this transistor is at a potential of -V
s, and the collector is connected to the potential element Vc via a resistor. Since a pulse signal is applied between the terminals α and β, the collector potential VD2 becomes a pulse signal with a duty ratio of % as shown in FIG. 4(d). This signal VD2 is given to the differentiating circuit, and its differentiated output V2 (Fig. 4 (e)) is given to one input terminal of the operational amplifier 23.Then, the output Vo of the operational amplifier 23 becomes as shown in Fig. 4 (e). This is integrated by the capacitor 26.As will be explained in detail later, when Vo is a pulse signal with a duty ratio of 2, the integral output V26 is a potential VN between Vc and -Vs.
becomes. Since the capacitor 26 and the integrating circuit 19 are connected so that this potential VN is added to the integrating circuit 19, the output potential VIP of the integrating circuit 19 becomes V19+V26. This VIP (FIG. 4(A)) is applied to one input terminal of the comparator 16.

Then, the comparator 16 output VPW becomes as shown in FIG. 4 (G). In other words, the signal is at a low level only during the period in which VIP is higher than V OFF. This VPW is given to the three-man NAND gate 27. A potential VD2 is applied to this NAND gate 27 via an inverter. 28 is f. impark circuit 5 with a cycle sufficiently longer than
This is an R-S flip-flop for controlling heating by turning on and off the drive of the
The remaining staff are working alone. and NAND gate 27
Give the output to the drive circuit 7, drive circuit 7 output VD
R (Fig. 4(d)) is applied to the transistor 58. When the output of the NAND gate 27 is at a low level, VDR becomes a high level, and during this time, the transistor 5e is turned on.The R-S flip-flop When the 28 output is at a high level, ■D with a duty ratio of 2
The output of the NAND gate 27 can be at a low level while the signal 2 is at a high level, but this period is limited by the comparator output vpw. That is, NAND gate output (or VDR)
becomes high level (or low level) during the period of VOFF<VIP, and the transistor 5e is driven by a pulse signal with a duty ratio of % or less.And when the input power becomes high (low), Vl9 becomes As it rises,
The length of the period T becomes longer, and as a result, the input power is suppressed (increased), a constant input is secured, and the transistor 5f
This makes it possible to protect the

Now, depending on the pot 8, the phase of the current transformer 9 output may lag or lead the output phase of the oscillator 42. This is because the resonance frequency fL when the pot 8 is placed on the induction heating coil 5a is fL<fo.

This corresponds to fL>fo. When the phase lags (or advances) in this way, Vl lags (or advances) with respect to ■2, and for convenience of illustration, conversely, Vl
It is shown in FIG. 4 (E) by a dashed line (or a dashed double dotted line) that 2 leads (or lags) with respect to Vl. Then V
. The falling timing of changes as shown by the one-dot chain line (or two-dot chain line) in FIG.
(or become high)

Accordingly, the VIP also follows the dashed-dotted line (
Accordingly, the duty ratio of the VDR increases (or decreases) as shown by the dashed line (or dashed double dot line) in Figure 4 (H). ), which turns on the transistor 5e.

〔effect〕

The effects of such a configuration are as follows. That is, in the case of a phase-lead load where fL > ro, current has already begun to flow through the flywheel diode 5f antiparallel to the drive pulse VDR of the transistor 5e while it is low. Therefore, V19 and V O
This means that the effect of inputting the FF to the comparator 16 and controlling the current of the transistor 5e based on its output is weak.

However, according to this invention, in such a case, V
26 becomes high, and as a result, the duty ratio of VDH becomes small, which makes it possible to suppress the current flowing through the transistor 5e, and the switching elements of the inverter circuit can be safely protected regardless of the impedance of the load.

[Brief explanation of the drawing]

Fig. 1 is a schematic circuit diagram of the inverter circuit of the present invention, Fig. 2 is a circuit diagram of the main part of the control circuit of the device of the present invention, Fig. 3 is a circuit diagram of the protection circuit as well, and Fig. 4 is an explanation of its operation. This is a time chart for 5... Inverter circuit 5a... Induction heating coil 5
c, 5e... switching transistor 9...
Current transformer 16...Comparator 19...Integrator circuit 23.
...Operational amplifier 26...Capacitor patent Applicant: Sanyo Electric Co., Ltd. Agent Patent attorney: Noboru Kono

Claims (1)

[Claims] 1. In an electromagnetic induction heating device equipped with an inverter circuit including an induction heating coil, a circuit that detects a phase difference between a switching signal for reversing the direction of current in the induction heating coil and the time when the direction of the current is reversed. and an input power detection circuit, and is configured to control the drive of an inverter circuit based on the phase difference and input power detected by these.