JPS6131954B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and more specifically, when a high frequency inverter circuit starts oscillating, it starts from a frequency higher than a set oscillation frequency, that is, from an input state lower than a set input, The present invention relates to an induction heating cooker in which the frequency is then gradually decreased to increase the input, and at the same time, the frequency is stabilized at a predetermined appropriate frequency value set according to the type of load.

A certain period, for example, one second is one cycle,
In an induction heating cooker that uses an on/off control system that repeatedly oscillates and stops the inverter at an appropriate rate during this period, the input is adjusted by sliding a slide switch marked with 1, 2, 9, 10. This is done by moving a lever. For example, if the slide lever position is "5", the high frequency inverter circuit continues to oscillate for 0.5 seconds and then stops for the next 0.5 seconds. Repeat this to get 50% input. In this way, it is possible to adjust linearly from "1", ie, 10% input, to "10", ie, 100% input. In this case, the oscillation frequency of the inverter circuit is set to a constant value, and usually that value is a value that provides a maximum input of, for example, 1300W.

In such an induction heating cooker, when the inverter circuit oscillates, a high-frequency alternating magnetic field generated from a heating coil in the inverter inductively heats the pot, which is a metal cooking utensil for heated objects.
By the way, in such a pot, a high frequency alternating magnetic field from a heating coil generates minute vibrations at a frequency substantially equal to the inverter oscillation frequency. Since the oscillation frequency of an inverter is generally 20KHz or higher, such minute vibrations do not cause audible noise during oscillation. Furthermore, when the inverter stops oscillating, the oscillating current in the inverter undergoes damped oscillation and the power supplied to the pot gradually decreases, making it difficult to generate audible sounds. However, when the inverter starts oscillating, the state of the power supplied to the wave is
A startup noise is generated due to the sudden change from 0W to inverter oscillation at the maximum input power state. When the on/off control period is 1 second, this occurs every second, resulting in an unpleasant noise for the user. Cooking pots made of several types of materials, such as stainless steel, are easy to input, such as enameled pots.
If the inverter is set to accept an input of 1,300W, and the above-mentioned special pot is placed on it, an input of approximately 1,800W will be input, which will destroy the switching elements that make up the inverter circuit.

The present invention provides an input compensation function that prevents input fluctuations due to differences in load materials and obtains a stable input value, and also sets the oscillation frequency to a high level when the inverter circuit starts oscillating, thereby reducing the input voltage. By starting from a low frequency and gradually lowering the frequency to a predetermined value and increasing the input, the above-mentioned noise generation is also prevented.

Experimental examples of the present invention will be described in detail below with reference to the drawings.

In Fig. 1, 1 and 1 are power supply terminals into which a commercial AC power supply is input, 2 is a rectifier circuit consisting of a diode bridge, 3 is a chiyoke coil, and 4 is a filter capacitor. A pulsating voltage that is obtained by full-wave rectification of alternating current appears. 5 is an induction heating coil, 6 is a resonant capacitor connected in series to the coil 5, 7 is a switching element connected in parallel to the resonant capacitor 6, and a gate control switch (hereinafter abbreviated as GCS) is used. 8 is a diode connected in antiparallel to the GCS 7, and the high frequency inverter circuit 10 is constituted by the circuit shown by the dotted line in the figure. 11 is a cooking pot made of iron-based metal.
Further, Vcon is the terminal voltage of the filter capacitor 4, and a voltage of several volts generated by the constant voltage power supply circuit 12 is applied via the diode 13, so that the pulsating current waveform from the rectifier circuit 2 of voltage
Only around 0V, a waveform is drawn that is raised to a constant voltage of several volts. VGCS is the anode terminal voltage of GCS7. Reference numeral 14 denotes a GCS drive circuit that applies an on or off pulse to the gate of the GCS 7 to cause the high frequency inverter circuit 10 to oscillate at a predetermined oscillation frequency, for example, about 25 KHz, and outputs a positive signal as an on pulse and a negative signal as an off pulse.
15 is an on-pulse generation circuit that operates to cause the drive circuit 14 to generate an on-pulse; 16 is an on-pulse generation circuit;
This is an off-pulse generation circuit that operates to generate off-pulses in the drive circuit 14, and both generate high-frequency pulses of about 25KHz. Reference numeral 17 denotes an on/off control circuit that outputs an on signal and an off signal that determine the oscillation period and stop period of the high frequency inverter circuit 10. 18 is a slide switch that changes the ratio of the oscillation continuation period and the stop period; 19 is a starting circuit that issues an inverter circuit starting signal in synchronization with the oscillation start signal from the on/off control circuit 17; 20 is a
The current transformer 21 that detects the current flowing in the AC power line before the rectifier circuit 2 receives a signal proportional to the magnitude of the load detected by the current transformer 20, and when an excessive current is detected,
It is an input compensation circuit that operates to reduce the input to the load.

FIG. 2 shows a specific circuit of the main part of the present invention. The on-pulse generation circuit 15 includes a flip-flop consisting of two NAND gates 22 and 23, a NAND gate 24, and a comparator 25. A starting signal generated from the starting circuit 19 is input to the NAND gate 23 via an inverter 26, and its output is driven. It is given to the circuit 14 as a GCS gate-on pulse. The NAND gate 24 uses the ON and OFF signals generated from the ON/OFF control circuit 17 as one input, and uses the output of the comparator 25 as another input. The side reference input terminal of the comparator 25 has a voltage Vcon
However, the voltage VGCS is also input to the side signal input terminal.

The off-pulse generation circuit 16 includes a time constant circuit 3 consisting of a comparator 27, a resistor 28, and a capacitor 29.
0, a time constant circuit 36 consisting of a transistor 31 that controls charging and discharging of the time constant circuit 30, resistors 32, 33, 34, 50, and a capacitor 35, a resistor 37 and a transistor 38 inserted in parallel with the capacitor 35; Series circuit, above transistor 3
8 includes a transistor 39 that controls opening and closing of the transistor 8. A time constant circuit 36 is connected to the side reference input terminal of the comparator 27.
The output of the time constant circuit 30 is applied to the side signal input terminal, and the output is input to the NAND gate 22. The opening and closing of the transistor 31 is controlled by the output of the NAND gate 22. The transistor 39 is made conductive by an on signal issued from the on/off control circuit 17, and is closed by an off signal. The transistor 38 has the collector output of the transistor 39 applied to its base, is off when the transistor 39 is on, and is on when the transistor 39 is off. The resistor 40, capacitor 41, and diode 42 constitute an initial state setting circuit, and the output thereof is input to the NAND gate 22.

The input compensation circuit 21 includes a comparator 43, and the input current detected by the current transformer 20 is converted into a DC voltage via a rectifier circuit 44 and a smoothing capacitor 45, and is input to the signal input terminal on that side. . A reference voltage obtained by dividing the constant voltage +V by resistors 46, 47, and 48 is applied to the side reference input terminal of the comparator 43, and its output is as follows.
The signal is applied to the time constant circuit 36 in the off-pulse generation circuit 16 via the resistor 49, and changes the time constant.

Next, the operation of the above configuration will be explained based on FIGS. 3 to 6.

Immediately after the power is turned on, the output of the initial state setting circuit is
After a predetermined time delay from the L (low) state, the H
(high) state. When the oscillation is stopped,
Since the output of NAND gate 24 is H, the inputs of NAND gate 22 are H, H, L, NAND gate 2
The input of 3 becomes H, H, H.

When an on signal is output from the on/off control circuit 17 and a start signal is output from the start circuit 19 at the same time, the start signal changes the inverter 26 output to L and the NAND gate 23 output to H, causing the drive circuit 14 to oscillate. An on pulse is input to start. From this, a gate-on signal is applied to GCS7. Due to the L conversion of the NAND gate 23 output, the NAND gate 22 output changes to H, and the transistor 3
Block 1. As a result, capacitor 29 and resistor 2
A charge current flows through 8, and its charge pressure level increases. When this voltage level reaches the reference level of the comparator 27, the output of the comparator 27 becomes
The output of the NAND gate 22 changes from L to H, and the output of the NAND gate 23 changes from H to L. As a result, the drive circuit 14
sends a negative signal to GCS to turn off GCS7.
Give to gate 7. When GCS7 is turned off,
A voltage VGCS appears at the GCS anode terminal, and this voltage is compared by a comparator 25 with a similar waveform voltage obtained by dividing the smoothing capacitor 4-terminal voltage Vcon. When the voltage VGCS drops to around 0V, the output of the comparator 25 changes from L to H. This H level signal is pulsed by a capacitor 49, and
The signal is input to the NAND gate 24 as a level on pulse. Therefore, the input of NAND gate 24 is
H, H, therefore the output becomes L, changing the output of the NAND gate 23 from L to H. From this, the drive circuit 14 applies a positive signal to the gate of the GCS 7 in order to turn it on. By repeating such ON/OFF pulses, the high frequency inverter circuit 10 continues to oscillate while the ON signal is being issued from the ON/OFF control circuit 17.
Subsequently, when the ON signal from the on/off control circuit 17 is cut off, the output of the NAND gate 24 is fixed at the H level, and the output of the NAND gate 23 is also fixed at the L level. As a result, high frequency inverter circuit 1
Oscillation of 0 is prohibited.

Next, the input adjustment function will be explained. The period from when the GCS 7 is turned on until it is turned on is determined by the time constant of the time constant circuit 30 and the side reference level of the comparator 27. When the on/off control circuit 17 is emitting an off signal, the transistor 39 is off and the transistor 38 is on, so the resistor 37 is connected in parallel to the capacitor 35, and the output level of the time constant circuit 36 is ,
It is determined by resistors 32, 33, 34, 37, and 50. Let this level be V _L1 . When the on/off control circuit 17 changes to an on signal, the transistor 39 is turned on, the transistor 38 is turned off, and the resistor 37 is placed in an open state. As a result, the output level of the time constant circuit 36 is
The value is determined to be 4.50, which is higher than the previous value. Let this level be _VH .

Immediately after the ON signal is issued from the ON/OFF control circuit 17, the voltage level is the same as that of the resistors 32 and 3.
With a time constant determined by 3, 34, 50 and the capacitor 35, the voltage level gradually changes from V _L1 to V _H. The above time constant is about 50 msec. From this, when the oscillation starts, the side reference level of the comparator 27 is the VL ₁ level, so the time when the comparator 27 changes from H to L, that is, the off-pulse output time, becomes earlier, and therefore the oscillation frequency of the inverter circuit 10 increases, and the input supplied to the load decreases. The input gradually increases from this low input state, and after about 50 msec, a steady oscillation state is reached. FIG. 4 shows the relationship between the on signal and the reference levels V _L1 and V _H.

To give an example of an experiment, when heating an enamel pot with 1300W, the frequency is set to about 25KHz, but when the above adjustment function works, the initial oscillation frequency is about 33KHz.
It starts at Hz, then decreases and becomes constant at about 25KHz.

Next, the operation of the input compensation circuit 21 will be explained.
When the input current increases beyond a predetermined value, the input level detected by the current transformer 20 exceeds the reference level of the comparator 43, and its output becomes
Change from H to L. Therefore, the side reference level of the comparator 43 is the resistor 49, 32, 33, 34, 50.
The output _timing of the off-pulse output from the comparator 27 is advanced,
The oscillation frequency of the inverter circuit 10 increases and therefore the input to the load decreases. Input compensation circuit 2
1 operates near the peak value of the power supply voltage Vcon, as shown in FIG. 5, and its operating period changes depending on the input difference depending on the type of cooking pot.

FIG. 6 shows the signal waveform of the reference level on the comparator 27 side when both the input adjustment functions of the input compensation circuit 21 and the off-pulse generation circuit 16 operate, and for reference, the signal waveform when only the input adjustment function operates (dotted line waveform). , the case where only the input compensation circuit 21 operates (dotted chain line waveform) are shown, respectively. When both work in this way, after oscillation starts, the input adjustment function works first, resulting in a high frequency state (low input state).
The frequency gradually decreases (input increases), and when it reaches the frequency set by the input compensation circuit 21, it is maintained at this frequency from then on, the input is kept constant, and any further excessive input is prevented. be done.

To give an example of an experiment, if a stainless steel pot is placed on a cooker that has been adjusted to input approximately 1300W (at this time frequency of 25KHz) to the enamel pot, the input would normally be approximately 1800W, but the input The frequency is raised to approximately 29KHz by the action of the compensation circuit.
This frequency value corresponds to approximately 1300W. When the input adjustment function operates at the same time as such an input compensation circuit, the oscillation frequency starts at about 33KHz, then gradually decreases and stabilizes at 29KHz.

As explained above, the induction heating cooker of the present invention has the following features:
When the high-frequency inverter circuit starts oscillating, it is operated at an oscillation frequency higher than the steady oscillation frequency and the input is lowered, and then gradually the steady oscillation frequency or the input compensation circuit is activated. Since the frequency is made to match the set frequency, the input does not fluctuate depending on the material of the pot, and a stable input can always be obtained. In addition, when the high-frequency inverter starts oscillating, the ON time of the switching element is shortened to increase the oscillation frequency, and then the ON time of the switching transistor is gradually increased to lower the oscillation frequency. Since the input power applied to the pot does not change abruptly but changes gradually, the unpleasant noise that occurs at about 1 second intervals, which conventionally occurs at the start of oscillation, is also prevented from occurring. Note that the low input period at the beginning of oscillation is several tens of meters.
Since it is a short period of sec, there is no risk that the total input will decrease.

It has been known for a long time to provide an input compensation circuit, but since such an input compensation circuit applies input to a load once and detects the current flowing at that time, a large current will necessarily flow at least once. ,
Although it is impossible to perform complete input compensation,
According to the present invention, at the initial stage of oscillation, the input is entered from a low input state, and then the input compensation circuit operates.
Such problems never occur.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a main circuit diagram, and FIGS. 3 to 6 are signal waveform diagrams for explaining the operation. 10...High frequency inverter circuit, 14...Drive circuit, 15...On pulse generation circuit, 16...Off pulse generation circuit, 17...On/off control circuit, 19...Start circuit, 21...Input compensation circuit.

Claims (1)

[Claims] 1. A high-frequency inverter circuit including an induction heating coil, a resonant capacitor, and a switching element, a switching element drive circuit that applies high-frequency on/off pulses to the switching element to cause it to oscillate, and an on-pulse that provides a signal to the drive circuit to turn on the switching element. a generator circuit, an off-pulse generator circuit that provides a signal for turning off the switching element to the drive circuit, an on/off control circuit that performs input control by varying the ratio of the oscillation period and stop period of the high-frequency inverter circuit, and the high-frequency inverter circuit. The off-pulse generator is equipped with an input compensation circuit that detects the input current to the load, determines the input to the load, and controls the oscillation frequency of the high-frequency inverter circuit so that the input remains constant regardless of the material and size of the load. The circuit has an input adjustment function that advances the off-pulse generation timing only at the initial stage of generation of the on-signal outputted from the on/off control circuit, thereby increasing the oscillation frequency of the high-frequency inverter circuit, and then gradually reducing the frequency. , and the frequency after the step-down is configured to match the oscillation frequency set by the input compensation circuit when the circuit operates.