KR930011813B1 - Control circuit of inverter microwave oven - Google Patents

Abstract

The circuit prevents the damage of switching element when a high voltage is applied. The circuit comprises an AC power supplying section, a bridge diode, a smoothing section for smoothing a pulsating DC voltage, an inverter for converting the output of the smoothing section to high voltage, a halfwave voltage section, a magnetron for generating high frequency by supplying heating power and driving power, a switching element driving section, a saw tooth wave generator, a pulse generating section, a protection circuit of the switching element and a static power output control circuit.

Description

Inverter Microwave Control Circuit

1 to 2 are conventional circuit diagrams.

3 is a circuit diagram of the present invention.

4 to 5 are operation waveform diagrams of the magnetron.

* Explanation of symbols for main parts of the drawings

1: Commercial AC power supply 2: Bridge diode

3: smoothing part 4: inverter

5: half-wave voltage division 6: magnetron

7: switching element driver 8: pulse generator

9: switching element protection circuit 10: constant power output control circuit

11: sawtooth wave generator R ₁ -R ₁₇ : resistance

C ₁ -C ₆ : Capacitor D ₁ -D ₁₀ : Diode

OP ₁ -OP ₃ : Operational Amplifier

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter microwave oven, and more particularly, to a control circuit of an inverter microwave oven that is suitable for preventing a high voltage from being applied to a switching element of an inverter due to high impedance during initial driving of a magnetron.

A configuration of a conventional inverter microwave circuit is described with reference to FIGS. 1 to 2 as follows.

First, FIG. 1 is a smoothing part consisting of a commercial AC power supply unit 1, a bridge diode 2 for full-wave rectifying a commercial AC voltage, a coil L ₁ , and a capacitor C ₁ , and smoothing the full-wave rectified voltage. (3) and a resonant capacitor (C ₂ ), a damping (Damping) diode (D ₁ ), a switching element (Q ₁ ) and a boosting transformer (T ₁ ) to input a smoothed voltage to convert to a high voltage A half-wave voltage unit 5 connected to an inverter 4 and a secondary coil of the boosting transformer T ₁ to half-double the output high voltage of the inverter 4 to provide the driving voltage of the magnetron; Switching for driving the magnetron 6 generating the high frequency and receiving the filament heating voltage and the driving voltage from the boosting transformer T ₁ and the half-wave voltage unit 5 and the switching element Q ₁ of the inverter 4. It comprise a drive element (7), a sawtooth wave generator 11 and an operational amplifier (OP _1), the switching A pulse generator 8 for generating a pulse signal which is a drive signal of the self drive unit 7, an input current detector 15 for detecting current of a commercial AC power supply, and an output voltage detector for detecting an output high voltage of the inverter 4 16, an operational amplifier OP ₂₀ , a resistor R ₂₁ -R ₂₂ , a diode D ₂₀ , a capacitor C ₂₀ , and a reference voltage source 12a and an output of the input current detector 15. Input current control unit 12 to adjust the signal to a constant value, the operational amplifier (OP ₂₁ ), resistor (R ₂₃ ) (R ₂₄ ), diode (D ₂₁ ), capacitor (C ₂₂ ) (C ₂₃ ) and reference An output voltage control unit 13 comprising a voltage source 13a and configured to adjust the output voltage of the output voltage detection unit 16 to a constant value, and a differential amplifier consisting of transistors Q ₂₀ and Q ₂₁ . Among the output signals of the current control unit 12 and the output voltage control unit 13, a small current required by the commercial power supply unit 1 is selected. Is composed of a selection circuit 14 for outputting a pulse width (Width) control signal to the pulse generator (8).

FIG. 2 is a variation of the configuration of FIG. 1. In FIG. 1, instead of the input current control unit 12, the output voltage control unit 13, and the selection circuit 14, diodes D ₂₀ and capacitors C ₂₁ , First and second voltages comprising a diode D ₂₁ and a capacitor C ₂₃ and smoothing the voltage of the current detecting transformer CT and the output voltage detecting unit 16 constituting the input current detecting unit 15, respectively. A selection circuit comprising a smoothing unit (17) (18) and a diode (D ₂₂ ) (D ₂₃ ) for selecting and outputting a larger value among the output signals of the first and second voltage smoothing units (17) (18). 19, the operational amplifier OP ₂₂ , the resistors R _25- R ₂₆ , the reference voltage source 20a, and the capacitor C ₂₄ , and control the output signal of the selection circuit 19 to a predetermined value. The pulse width control signal generator 20 is provided to the pulse generator 8.

The operation of the conventional configuration will be described in detail as follows. First, in the case of FIG. 1, the input current control unit 12 detects the input current of the commercial AC power supply unit 1 and thereby controls the output of the microwave oven, which is detected by the current detection transformer CT. the input to the terminal, and the operational amplifier (OP ₂₀₎ has it non-inverting (+) terminal and the input current to the diode (D ₂₀₎ and in which the smoothing via a capacitor (C _21), and then the operational amplifier (OP ₂₀₎ inversion of the () The control is performed to a predetermined value by amplifying the secondary compared with the input reference voltage, and then input it to the base of the transistor Q ₂₀ of the selection circuit 14 which is a differential amplifier.

In addition, the output voltage controller 13 may indirectly measure a signal obtained by reducing the waveform of the voltage across the primary coil of the boosting transformer T _{1, that} is, the voltage waveform across the secondary coil (both voltage waveforms of the switching element). .) Is smoothed through the diode (D ₂₁ ) capacitor (C ₂₃ ) and then input to the inverting terminal of the operational amplifier (OP ₂₁ ).

The operational amplifier OP ₂₁ compares it with a reference voltage source input to the non-inverting terminal and controls the resultant amplifier to a predetermined value by amplifying the secondary. The operational amplifier OP ₂₁ is then applied to the base of the transistor Q ₂₁ of the selection circuit 14 which is a differential amplifier. Enter it. The output signals of the input current control unit 12 and the output voltage control unit 13 turn on / off the transistors Q ₂₀ and Q ₂₁ of the selection circuit 14, respectively. By adjusting the pulse width control signal voltage input to the non-inverting terminal of the operational amplifier (OP ₁ ) of)) to prevent the application of a voltage higher than a predetermined value at both ends of the switching element (Q1).

In the case of FIG. 2, the first voltage smoothing unit 17, the selection circuit 19, and the pulse width control signal generator 20 operate in the same manner as the input current control unit 12 of FIG. The pulse width control signal of the pulse generator 8 which is the drive signal of the switching element Q ₁ is output by the input current.

In addition, the second voltage smoothing unit 18, the selection circuit 19, and the pulse width control signal generator 20 operate in the same manner as the output voltage control unit 13 of FIG. 1 and according to the detected output voltage. Do not apply more than the constant voltage set at both ends of Q ₁ ). However, the prior art had the following disadvantages. That is, it is difficult to obtain the constant power output of the inverter by mixing the detected input current and the voltage of both ends of the switching element detected by the secondary coil of the boost transformer. This is because the magnetron on the output side has different characteristics. In addition, due to the high impedance during the initial driving of the magnetron, a high voltage may be applied to both ends of the switching device, thereby causing the switching device to be damaged.

The present invention is to eliminate the above disadvantages, by using the current flowing through the boost transformer secondary side coil and the magnetron, respectively, it is possible to safely drive the switching element during the initial operation of the magnetron as well as independent of the change in input voltage It is an object of the present invention to provide an inverter microwave oven capable of outputting a constant power.

In order to achieve the above object, the present invention provides an inverter-driven microwave oven circuit, comprising: a switching element driver for driving a switching element of an inverter, a pulse generator for generating and providing a pulse signal at a predetermined interval for driving the switching element driver; When the output voltage of the secondary side auxiliary coil of the boost transformer in the inverter is detected to be higher than a predetermined value, the switching maintains a pulse width control signal for reducing the width of the output pulse of the pulse generator for a predetermined time and outputs it to the pulse generator. When the voltage induced by the element protection circuit and the current flowing through the anode of the magnetron is detected to be greater than or equal to a predetermined value, a pulse width control signal for reducing the width of the output pulse of the pulse generator is maintained for a predetermined time. And a constant power output control circuit for outputting.

This will be described below with reference to the accompanying FIG. 3.

3 is a smoothing part comprising a commercial AC power supply 1, a bridge diode 2 for full-wave rectifying a commercial AC voltage, and a coil L ₁ and a capacitor C ₁ and smoothing the full-wave rectified pulsating DC voltage. (3) and a resonant capacitor (C ₂ ), a damping diode (D ₁ ), a switching element (Q ₁ ) and a boost transformer (T ₁ ), and inputs the output signal of the smoothing part (3) to a high voltage. An inverter 4 to be converted, a half-wave double voltage unit 5 connected to the secondary side of the boost transformer T ₁ to half-back double the output voltage of the inverter 4, and two of the boost transformer T ₁ . A magnetron 6 that generates high frequency by being connected to the vehicle side and the half-wave voltage unit 5 and supplied with filament heating power and driving power, and a switching element driver for driving the switching element Q _{1 in} the inverter 4. 7, the sawtooth wave generator 11, the operational amplifier (OP ₁ ), the resistor (R _2- R ₄ ) and the capacitor (C ₄ ) And a pulse generator 8 for outputting a pulse signal of a predetermined width to the switching element driver 7, a comparator 9a and a feedback unit 9b, and an operational amplifier OP ₂ and a resistor ( R ₅ -R ₁₁ ), a diode (D ₄ -D ₆ ), and a capacitor (C ₅ ), which are connected between the secondary coil of the boost transformer (T ₁ ) and the pulse generator (8), both ends of the switching element. A switching element protection circuit 9 for outputting a pulse width control signal so that the output pulse width is reduced to the pulse generator 8 when the voltage of the secondary auxiliary coil indicating the voltage is higher than or equal to a predetermined value; ) And feedback (10 _b ) and consists of operational amplifier (OP ₃ ), resistor (R ₁₂ -R ₁₆ ), diode (D ₈ -D ₁₀ ) and capacitor (C ₆ ) and anode of magnetron (6) When the induced voltage is detected by the current flowing in the predetermined value or more, the pulse width control signal is output to the pulse generator 8 so that the output pulse width is reduced. It is configured to output constant power control circuit 10 for. Its operation is as follows.

First, the electrical characteristics of the magnetron 6 generating high frequency have almost no current flowing below the threshold voltage as shown in FIG. 5, and when the threshold voltage is exceeded, the current flowing rapidly increases and the power consumption increases nonlinearly.

That is, in the state where the filament which is the initial driving state of the magnetron 6 is not heated, there is almost no current flowing even when a large voltage is applied between the anode and the cathode of the magnetron 6.

4a to 4c show the voltage waveform between the anode and the cathode of the magnetron 6 and the anode of the magnetron 6, divided into before and after the filament heating time (about 2 seconds in the embodiment) about the time axis t. Current waveforms and power consumption waveforms of the magnetron 6 are shown, respectively.

Therefore, the current does not flow to the anode of the magnetron 6 before the filament heating time of the initial driving time, but the secondary auxiliary coil of the boost transformer T ₁ has the same waveform as the resonance voltage waveform at both ends of the switching element Q ₁ . At this time, when the voltage of the auxiliary coil rises above a certain value, the output of the operational amplifier OP ₂ , that is, the comparator of the switching element protection circuit 9, that is, the pulse width control signal appears in a low state, so that the operation of the comparator of the pulse generator 8 is performed. The non-inverting input terminal of the amplifier OP ₁ is applied with a signal voltage of a level slightly lower than before.

Accordingly, the on time of the switching element Q ₁ is reduced, so that the peak-peak value of the resonant voltage waveform decreases and the voltage between both ends of the switching element Q ₁ decreases, but the non-inverting terminal has a diode D ₆ and a resistor R. _10), and continued for a capacitor (C ₅₎ positive feedback (positive feedback) it is because a capacitor (C ₅₎ a predetermined time (carried about 10sec) in the example of until discharged through the charge is a resistance (R ₁₀ charge) by the The output of the operational amplifier OP ₂ , which is a comparator, is kept low. After the discharge is completed, the output of the operational amplifier OP ₂ is in an open state, and as a pulse width control signal, the divided voltage by the reference voltage source resistor R ₇ (R ₈ ) is transmitted through the non-inverting terminal. The pulse generator 8 outputs a square wave having a large width to the switching element driver 7 so that the switching element Q ₁ has a large on time by outputting the non-inverting terminal of the operational amplifier OP ₁ of FIG. .

When the voltage between both ends of the switching element Q ₁ , that is, the secondary coil of the boosting transformer T ₁ is over a certain voltage, the initial filament heating time in which the high impedance is applied by repeating the operation as in the above case (for about 2 seconds) By limiting the size of the voltage across the switching device (Q ₁₎ for thereby protecting the switching element (Q _1). After the filament of the magnetron 6 is heated, according to the anode current size of the magnetron 6, the constant power output control circuit 10 operates like the switching element protection circuit 9 so that a certain amount of the anode of the magnetron 6 is heated. It controls the flow of current.

Therefore, as in the prior art, the constant constant power is output more stably than the output control by the input current of the commercial AC power supply unit 1 on the input side. Here, the role of the positive feedback portion 9a for charging and discharging operations including the diode D ₆ , the resistor R ₁₀ , and the capacitor C _{5 in the} switching device protection circuit 9 is as follows.

That is, the voltage size of the secondary coil of the boost side transformer T ₂ that senses the voltage across the switching element Q ₁ varies by the oscillation frequency (about 25KHZ-50KHZ), and thus the switching element Q ₁ . The fluctuation in the on-operation time can be avoided by operating the switching element Q ₁ for a predetermined time in an incomplete region, thereby preventing the occurrence of sound noise.

As described above, the present invention has the following effects.

First, despite the variation of the voltage input from the commercial AC power supply unit, it is possible to output a constant power by the operation of the constant power output control circuit according to the output current of the magnetron.

Second, in the initial operation, the switching element protection circuit is operated by the operation of the switching element protection circuit according to the detection of the voltage of the secondary coil of the boosting transformer, such as the voltage of both ends of the switching element, thereby preventing damage to the switching element.

Third, acoustic noise can be prevented by the feedback circuits provided on the output and the input sides of the operational amplifiers, which are comparators of the switching element protection circuit and the constant power output control circuit.

Claims (2)

An inverter driven microwave circuit, comprising: a switching element driver for driving a switching element in an inverter, a pulse generator for generating and outputting a pulse signal at a predetermined interval for driving the switching element driver, and a voltage at both ends of the switching element; The switching element protection circuit outputs a pulse width control signal to the pulse generator for a predetermined time according to the voltage value of the secondary coil of the boost transformer in the inverter, and the voltage value induced by the current flowing through the anode of the magnetron. And a constant power output control circuit for outputting a pulse width control signal to the pulse generator for a predetermined period of time. The switching element protection circuit and the constant power output control circuit each compare the secondary auxiliary coil voltage value of the boosting transformer and the voltage value induced by the anode current of the magnetron with the reference voltage value. And a feedback unit connected between the input / output side of the comparator and a feedback unit for returning the output signal of the comparator to the input side for a predetermined time period.