JPS6142395B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating device, and particularly to an induction heating device with an additional small object load circuit.

Conventionally, induction heating equipment uses a knife,
There was a risk of getting burned by touching a small load such as a fork without knowing that it was being heated, so if such a small object is placed on the mounting table, the heating operation will automatically stop. A so-called small object load detection circuit is installed. As an example, as shown in Fig. 1, the input level to the inverter 1 is detected by the current transformer 2, the power consumed by the heating coil 3 is known, and the timing circuit 4 is operated according to the level. There is a device that controls the oscillation circuit 5 that drives the inverter 1.
With such a configuration, the inverter can be driven intermittently to detect the load on small objects, and when the load is changed to an appropriate one, it can automatically return to normal operation.

On the other hand, power control in this type of induction heating device can be roughly divided into frequency control type and duty control type.

The former controls the power by changing the oscillation frequency of the inverter, and the latter controls the power by changing the operating duty while keeping the inverter's oscillation frequency unchanged.

In the case of a duty control type induction heating device, the duty cycle is generally from several seconds to several tens of seconds, and if a small object load detection circuit using the above-mentioned timing circuit is added to this duty control type, this small object load detection circuit can be used. It is necessary to consider the timing and synchronization relationship with the duty cycle.

Furthermore, it is known that switching noise is generated when duty control is turned on and off, which may cause interference with radio waves and the like.

The present invention proposes a synchronization relationship between the duty cycle and the timing for detecting the load on small objects, and a method for eliminating switching noise, which will be described in detail below.

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

1 is an AC power supply, 2 is a rectifier circuit, and 3 is an inverter. An SCR inverter can be used. 4 is an induction heating coil to which the output of the inverter 3 is applied, and 5 is a pot made of iron-based metal such as a magnetic material that is placed on the coil 4 and heated. Reference numeral 6 denotes an oscillation circuit that drives the inverter 3 to oscillate, and in the case of an SCR inverter, a pulse generation circuit that applies a trigger pulse to the gate of the SCR is used. 7 is a current transformer installed in the power supply line 1, and 8 is a small load detection circuit that discriminates the magnitude of the input current detected by this current transformer 7 and generates an output when there is a small load or a small current at no load. It detects the input current for each duty, and when a small object load is detected, the detection output becomes a duty ion and then outputs a small object load detection signal with a slight time delay (approximately 100 msec). Reference numeral 9 denotes a variable duty cycle oscillation circuit, which is configured to oscillate at a cycle of approximately 10 seconds, and controls the on/off time of the inverter 3. In other words, when the output is high, the on time is increased,
When the output is low, increase the off time. Reference numeral 10 denotes a power supply zero voltage synchronous pulse generation circuit that receives the AC power supply 1 and generates an output when the voltage is zero, and is connected to AND gates 11 and 12. The output of the variable duty cycle oscillation circuit 9 is directly applied to one of these AND gates 11, and the output of the variable duty cycle oscillation circuit 9 is applied directly to the other 12 through an inverter 13. 14
is a first R-S flip-flop which is set by the gate output of one AND gate 11 and reset by the gate output of the other AND gate 12, and 15 is a second R-S flip-flop which functions as a memory circuit. , the set output _Q1 of the first R-S flip-flop 14 is inputted to the set input terminal _S2 via the differentiating circuit 16, and the output of the small object load detection circuit 8 is inputted to the reset input terminal _R2 . be done. 17 is an AND gate whose inputs are the set outputs Q ₁ and Q ₂ of the first and second R-S flip-flops 14 and 15; 18 is an inhibit circuit controlled by the output of this AND gate 17; In the presence of the signal, this inhibit circuit 18 does not operate. This inhibition circuit 18 controls the oscillation circuit 6, and causes it to normally oscillate when the output of the AND gate 17 is at a high level.

Next, the operation of the device having such a structure will be explained using the waveform diagram in FIG. First, to explain from the waveform diagrams, waveform A is the output of variable duty cycle oscillation circuit 9, and waveform B is the output of zero voltage synchronous pulse generation circuit 1.
0 output, waveform C is the output of the first flip-flop 14, waveform D is the output of the differentiating circuit 16, waveform E is the output of the small load detection circuit 8, waveform F is the output of the second R-S flip-flop 15, waveform G is The AND gate 17 output is shown.

First, when a normal load, that is, the pot 5 is placed on the induction heating coil 4, when the power is turned on at time _t0 , the variable duty cycle oscillation circuit 9 starts up and starts oscillating from time _t1 . do. On the other hand, the zero voltage synchronization pulse generation circuit 10 outputs a zero voltage synchronization pulse B when detecting zero voltage of the AC power supply 1. Therefore, the first flip-flop 1
4 set output _Q1 is duty cycle signal A
is synchronized with zero voltage synchronization pulse B, resulting in C. At this time, since a relatively large current flows through the power supply line 1, no output is output from the small load detection circuit 8, and therefore the second R-S flip-flop 15 is set by the output of the differentiating circuit 16. Generates output Q ₂ . When this set output _Q2 exists, the set output C of the first flip-flop 14 is input to the inhibit circuit 18 via the AND gate 17, and the oscillation circuit 6 is operated. That is, when the output G of the AND gate 17 is at a high level, the oscillation circuit 6 operates, and when it is at a low level, it stops. Output control is performed by changing this operation and stop time.

Next, if the pot 5 is removed before time t ₂ and small items such as knives and forks are placed on the heating coil 4,
After time _t2, the AND gate 17 outputs, the oscillation circuit 6 operates, and the inverter 3 is driven. However, since the load is small, the input current is small, and the small load circuit 8 emits an output pulse at time _t3 , which is about 100 msec later than the duty ion timing _t2 .
This output pulse causes the second flip-flop 1
5 enters a reset state and closes the AND gate 17. Therefore, the inhibiting circuit 18 operates and stops the oscillation circuit 6, and the inverter 3 stops driving. Thereafter, until the small object such as a knife or fork is removed, the inverter 3 is driven for a short period of approximately 100 msec and repeats small object detection for each duty. If the small object is removed before time _t4 and the pot 5 is placed again, no small object load detection output is generated after time _t4 , and therefore the second flip-flop 15 The set state is maintained by the output, the AND gate 17 is opened, and the oscillation circuit 6 operates in oscillation at a predetermined duty cycle.

As is clear from the above description, the present invention is configured such that the small object load detection signal is obtained with a slight time delay after the duty ion signal is output, so the duty cycle and the small object load detection operation can be synchronized. You can take it. Therefore, the small object detection operation can always be performed every duty cycle, and there is no need to supply more power to the small load than necessary for load detection, or to prevent the small object load detection operation from being performed for a long period of time. This inconvenience will be resolved. Also, to control on/off near the zero voltage of the power supply,
It is possible to suppress interference to radio waves caused by switching noise.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the existing small object load detection configuration, FIG. 2 is a block diagram showing the configuration of the induction heating device of the present invention, and FIG. 3 is a signal waveform diagram for explaining its operation. Inverter, 6 oscillation circuit, 7 current transformer, 8 small load detection circuit, 9 variable duty cycle oscillation circuit, 10
14 and 15 are flip-flops, and 18 is an inhibit circuit, respectively.

Claims (1)

[Claims] 1. An AC power source, a rectifier circuit that rectifies the power source,
an inverter driven by the output of the rectifier circuit, a control circuit that controls oscillation of the inverter, and an induction heating coil to which the output of the inverter is applied;
a small load detection circuit that detects when the load heated by the induction heating coil is a small load or no load; a variable duty cycle oscillation circuit that variably controls the drive time of the inverter; a voltage synchronous pulse generation circuit, the duty ion signal from the duty cycle variable oscillation circuit is synchronized with the pulse from the zero voltage synchronous pulse generation circuit,
Furthermore, the detection operation by the small object load detection circuit is performed in synchronization with the oscillation operation of the variable duty cycle oscillation circuit, and the synchronization relationship is such that there is a slight time delay after the duty ion signal is output, and the small object load detection signal is output. An induction heating device characterized by being in a relationship where it outputs.