JPS6131953B2 - - 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 having a configuration in which the frequency is then reduced to a set oscillation state, and a set input is supplied to the 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 certain value, which is usually the maximum input power, for example, 1300W.
is the value obtained.

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 power supplied to the pot is
A startup noise is generated due to the sudden change from 0W to inverter oscillation at the maximum input power state. Since this occurs every second when the on/off control period is one second, it becomes an unpleasant noise for the user.

The present invention was developed based on the fact that such noise is caused by the maximum input being suddenly applied to the load when oscillation starts. The frequency is set high, and then the frequency is gradually lowered over a period of several tens of milliseconds to match the maximum input, and then fixed at this frequency for the duration of the oscillation, thereby preventing the above-mentioned startup noise from occurring. This is what I did.

Embodiments 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 commercial AC power is input, 2 is a rectifier circuit consisting of a diode bridge, 3 is a chiyoke coil, and 4 is a filter capacitor, which has almost no smoothing effect, so
A pulsating voltage produced by full-wave rectification of alternating current appears at the terminal. 5 is an induction heating coil, 6 is a resonant capacitor connected in series to this coil 5, 7 is a switching element connected in parallel to the resonant capacitor 6,
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.
Reference numeral 15 denotes an on-pulse generation circuit that operates to generate an on-pulse in the drive circuit 14, and 16 an off-pulse generation circuit that operates to generate an off-pulse in the drive circuit 14, both of which generate high frequency pulses of approximately 25 KHz. An on/off control circuit 17 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 startup circuit that issues an inverter circuit startup signal in synchronization with the oscillation start signal from the on/off control circuit 17; and 20 is a switch at the front stage of the rectifier circuit 2. A current transformer 21 that detects the current flowing in the AC power line receives a signal proportional to the magnitude of the load detected by the current transformer 20, and reduces the input to the load when an excessive current is detected. This is an input compensation circuit that operates as follows.

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 It is given to the drive circuit 14 as a GCS gate-on pulse. The NAND gate 24 receives the ON and OFF signals generated from the ON/OFF control circuit 17, and receives the output of the comparator 25 as its other 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, resistors 32, 33, 34, 50, and a capacitor 35 that controls charging and discharging of the time constant circuit 30. A resistor 37 and a transistor 38 are connected in series in parallel to the capacitor 35. circuit, the above transistor 38
A comparator 2 including a transistor 39 that controls opening and closing of the
The output of the time constant circuit 36 is input to the side reference input terminal 7 , 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 . transistor 31
is controlled to open and close by the output of the NAND gate 22. The transistor 39 is connected to the on/off control circuit 1
It becomes conductive when the ON signal is emitted from 7, and it is closed when the OFF signal is generated. 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 this comparator 43, and its output is sent to the off-pulse generating circuit 16 through a resistor 49. It is applied to the constant circuit 36 to change its 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 low.
(low) state with a predetermined time delay.
(high) state. When the oscillation is stopped,
Since the output of the NAND gate 24 is H, the inputs of the NAND gate 22 are H, H, L, and the inputs of the NAND gate 23 are 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 An on-pulse is input to start oscillation. This adds a gate-on signal 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 inputs of the NAND gate 24 are H, H, and therefore its output is L, changing the output of the NAND gate 23 from L to H. Accordingly, 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 becomes H.
level, and the NAND gate 23 output is also fixed to L. As a result, oscillation of the high frequency inverter circuit 10 is prohibited.

Next, the input adjustment function will be explained. The period from when the GCS 7 is turned off until it is turned off 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 issuing 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 26 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.
3, 34, 50 and the time constant determined by the capacitor 35, the voltage level gradually changes from V _L1 to V _H. The above time constant is about 50 msec. From this, since the side reference level of the comparator 27 is the VL ₁ level at the start of oscillation, the time when the comparator 27 changes from H to L, that is, the off-pulse output time is earlier, and therefore the oscillation of the inverter circuit 10 The frequency will increase and the input supplied to the load will drop. 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 a korin 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 45 is the resistor 49, 32, 33, 34, 50.
It is determined by The level becomes low V _L2 , and the comparator 27
The output timing of the off-pulse output is advanced, the oscillation frequency of the inverter circuit 10 increases, and the input to the load decreases. As shown in FIG. 5, the input compensation circuit 21 operates near the peak value of the power supply voltage Vcon, 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 the oscillation starts, the input adjustment function works and starts from a high frequency state (low input state), and the frequency gradually decreases (input increases), and the input compensation circuit 21 When the set frequency is reached, the frequency is maintained at this frequency, the input is made constant, and any further excessive input is prevented from being supplied.

To give an example of an experiment, if a stainless steel pot is placed on a cooker that has been adjusted to receive an input of approximately 1300W (frequency 25KHz) using an enameled 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. If the input adjustment function operates simultaneously with such an input compensation circuit, the oscillation frequency will initially be stabilized at approximately 33KHz.

As explained above, the induction heating cooker of the present invention has the following features:
When high-frequency inverter oscillation starts, oscillation is started with the ON time of the switching element shortened and the oscillation frequency raised, and the ON time of the switching transistor is gradually increased to lower the oscillation frequency. Since the applied input power does not change abruptly but changes gradually, the unpleasant noise that occurs at intervals of about 1 second, which conventionally occurs at the start of oscillation, is eliminated. Furthermore, since the input reduction period is a short period of several tens of milliseconds, there is no risk that the total input will decrease.

[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 generation circuit, an off-pulse generation circuit that provides a signal for turning off the switching element to the drive circuit, and an on/off control circuit that performs input control by varying the ratio of the oscillation period and the stop period of the high-frequency inverter circuit; The generation circuit has an input that advances the off-pulse generation period only at the initial stage of generation of the oscillation on signal output from the on/off control circuit, makes the oscillation frequency of the high frequency inverter circuit higher than the steady oscillation state, and then gradually reduces the frequency to the steady oscillation state. An induction heating cooker characterized by being provided with an adjustment function.