KR100361027B1 - Microwave oven - Google Patents

Abstract

The present invention relates to a high voltage converter for converting an AC power source to generate a high voltage, and to a microwave oven for supplying a high voltage from the high voltage converter to generate an electromagnetic wave, the rectifying and smoothing unit rectifying and smoothing a commercial AC power source. Wow; An inverter unit converting the DC power rectified and smoothed from the rectifying and smoothing unit into an AC power of high frequency based on an input control signal; And a state detecting unit for detecting an abnormal state of the inverter unit, providing the control signal to the inverter unit to regulate an output AC power supply, and adjusting the control signal to reduce the output of the inverter unit based on the abnormal signal from the state detecting unit. It characterized in that it comprises a control unit. Accordingly, the second high voltage can be obtained by rectifying the rectifying element and converting it into direct current (DC) and then switching it to a high frequency (about 20 Khz) through a semiconductor oscillation method. The inverter unit can change the secondary high voltage from low output to high output in analogy according to the switching, so that the heat supplied to the food can be finely adjusted to improve the taste of the food. In addition, the HVT can be reduced in weight, making it easy to carry and install, and reduce manufacturing costs.

Description

Microwave oven {MICROWAVE OVEN}

The present invention relates to a microwave oven, and more particularly, to a microwave oven having an HVT (high voltage transformer), an MGT device and an inverter unit.

In general, a microwave oven obtains a second high voltage by directly applying a commercial AC power source to the primary side of an iron core type HVT (high voltage transformer). The operating principle of the microwave oven is that when the cathode filament provided in the magnetron is heated and a high voltage of about 4,000 volts is applied, the magnetron oscillates to radiate microwave energy. Microwave energy is converted into thermal energy when it encounters a water or a water-containing object (ie, food ingredients, etc.) in a closed space, and cooking can be performed by the characteristics of the microwave oven.

7 is a circuit diagram schematically showing the configuration of a microwave oven according to the related art. As shown in the figure, the conventional microwave oven is connected to the power supply 51, the HVT 53 for generating a high voltage by the power supplied through the power supply, and the high voltage generated in the HVT 53. MGT device 55 for heating the food contained in the cooking chamber of the microwave by generating electromagnetic waves, a relay (57) for controlling the supply of voltage and frequency, etc., and a power supply device (51). When power is supplied from the control unit, the control unit 59 controls the HVT 53, the MGT device 55, and the relay 57.

By such a configuration, when power is supplied from the power supply device 51 and the relay 57 is energized by the control of the controller 59, a current flows to the primary side of the HVT 55, whereby the HVT 55 ) Voltage is induced on the secondary side. At this time, the secondary winding of the HVT 55 is provided with a voltage of several volts for heating the filament of the MGT device 55 and thousands of volts for the oscillation of the MGT device. On the other hand, since the direct current should be applied to the cathode of the MGT apparatus 55, rectification and smoothing means for performing rectification and smoothing are provided.

By the way, in such a conventional microwave oven, since the core material of the core HVT 53 is a silicon steel sheet used in a general transformer, the weight is very heavy and bulky, so handling is inconvenient and manufacturing cost increases. In addition, since the number of turns of the secondary winding of the HVT 53 has to be increased for the high output of the HVT 53, the size of the HVT 53 becomes larger.

In addition, such a conventional microwave oven, in adjusting the output, it is impossible to perform an analog control to continuously control from a low output to a high output, so that the power of the maximum rated output supplied from the power supply device 51 is turned on. The duty cycle control method is used to control the ratio between the off time and the off time. This duty cycle control method utilizes the reason that a short on time and a long off time result in a low output, while a long on time and a short off time result in a high output.

However, in the case of using such a duty cycle control method, since the temperature variation provided to the cooking target is very large, the cooking efficiency may be lowered, thereby reducing the taste of the food.

Accordingly, an object of the present invention is to control the secondary high voltage, that is, the output of the high voltage can be continuously changed analogously to generate a high output, thereby improving the cooking efficiency and improve the taste of the food It relates to a microwave oven.

Still another object of the present invention is to reduce the size and weight of the high voltage generator, thereby facilitating handling and installation, and reducing the manufacturing cost.

1 is a schematic block diagram of a microwave inverter device according to the present invention;

FIG. 2 is a schematic block diagram of a half bridge inverter circuit of FIG. 1;

3 is a detailed circuit diagram of each part of FIG. 1;

4 is a graph showing potentials and waveforms of respective parts shown in FIG. 2;

5 is a graph showing a waveform in which a direct current is superimposed on a power factor improving source signal;

6 is a graph showing the operating characteristics of the over resonance detection unit;

7 is a circuit diagram of a microwave oven according to the related art.

<Explanation of symbols for the main parts of the drawings>

1: Low voltage off part 2: D / A conversion part

3: On-off and soft start part 4: Output control part

5: over resonance detection part 6: resonance part

7: power supply unit 8: rectifier device

22, 23: switching element 24: HVT (high voltage transformer)

27: switching unit 30: inverter unit

40: control unit

According to the present invention, the object of the present invention is a rectifying and smoothing unit for rectifying and smoothing a commercial AC power source in a microwave oven having a high voltage converter for generating a high voltage and an MGT device that receives a high voltage from the high voltage converter to generate electromagnetic waves. Wow; An inverter unit converting the DC power rectified and smoothed from the rectifying and smoothing unit into an AC power of high frequency based on an input control signal; And a state detecting unit for detecting an abnormal state of the inverter unit, providing the control signal to the inverter unit to regulate an output AC power supply, and adjusting the control signal to reduce the output of the inverter unit based on the abnormal signal from the state detecting unit. It is achieved by a microwave oven comprising a control unit.

The control unit may include a D / A converter converting the control signal into an analog signal; An output controller for controlling and outputting a frequency of the control signal converted by the D / A converter; It is preferable to have an oscillator for varying the period of the signal output from the output control unit to input to the inverter unit.

The state detection unit is an over resonance detection unit for detecting an abnormal over voltage of the inverter unit, and the abnormal over voltage is more effectively fed back to an input terminal of the output control unit. The output control unit increases in frequency according to an abnormal signal provided from the over resonance detection unit. It is effective to re-output the generated oscillation signal to the oscillator. The control unit may further include an on-off and a soft start unit for controlling oscillation on / off and soft start of the oscillation unit according to the input control signal.

The control unit may further include a low voltage off unit for outputting a stop signal to the on / off and soft start unit and the D / A converting unit when abnormal power is input through the power supply unit.

Preferably, the control unit branches the control signal and inputs the control signal to the D / A converter and the on / off and soft start unit.

Preferably, the oscillator has a switching unit that switches the DC power source to an AC power source, and the oscillating unit oscillates when an external resistor and a capacitor are connected to generate a gate pulse of the switching unit.

Preferably, the switching unit is a pair of switching power elements, and the oscillation unit is provided with an oscillation frequency of the formula Fo = 1 / (1.4 * (external resistance +75) * capacitor).

The output controller is more effective to use the drain-source resistance characteristics of the field effect transistor to change the external resistance value.

The on-off and soft-start portions preferably use the drain-source resistance characteristics of the field effect transistor for soft start.

The low voltage off unit may be configured in the form of an AND logic logic in which a transistor is connected in series with a photocoupler, and an AC signal is effectively transmitted to an input terminal of the output control unit through a DC offset capacitor.

The HVT is preferably composed of a ferrite core with low high frequency loss.

Hereinafter, with reference to the accompanying drawings will be described in detail.

1 is a schematic block diagram of a microwave inverter device according to the present invention, Figure 2 is a schematic block diagram of the half-bridge inverter circuit of FIG. As shown in these figures, the present microwave oven generates electromagnetic waves by the power supply unit 7 that receives power from the outside, the HVT 24 that generates a high voltage, and the high voltage generated by the HVT 24. MGT apparatus 25, rectification and smoothing section 8 for rectifying and smoothing the power supplied from power supply section 7, control signal input section 26 for inputting control signals, and primary side of HVT 24 Inverter 30 is connected to the coil and rectifies the control signal through the rectifying and smoothing unit 8 to input to the secondary coil of the HVT, and provides a control signal to the inverter unit 30 to regulate the output AC power supply. It has a control part 40.

The controller 40 is a D / A converter 2 for converting an external control signal into an analog signal, and an output controller 4 for controlling and outputting a frequency of a signal converted and input by the D / A converter 2. And an oscillation section 21 for varying the period of the signal output from the output control section 4 and inputting it to the inverter section 30. The control unit 40 also controls the oscillation on / off and soft start of the oscillation unit 21 according to the over resonance detection unit 5 for detecting an abnormal state of the resonance voltage of the inverter unit 30 and the input control signal. When the abnormal power is input through the off and soft start unit 3 and the power supply unit 7, the low voltage off unit outputting a stop signal to the on and off and soft start unit 3 and the D / A converter unit 2 ( 21) is further included. Here, the output control unit 4 controls the frequency of the abnormal signal detected by the over resonance detection unit 5 and outputs it to the oscillation unit 21 again. In addition, the control unit 40 branches and inputs the control signal input from the control signal input unit 26 to the D / A conversion unit 2 and the on / off and soft start unit 3.

As shown in FIG. 2, the inverter unit 30, that is, the half-bridge inverter circuit, has a reactor 9 and capacitors 603 and 604 that prevent external emission of noise of power input from the power supply unit 7. Switching elements 22 and 23 which have a resonator 6 to allow high frequency current to flow through the resonator 6 according to the pulse waveform received from the oscillator 21 between the resonator 6 and the oscillator 21. It is prepared.

3 is a detailed circuit diagram of an inverter device for a microwave oven according to the present invention. As shown in this figure, the microwave oven generates high-frequency energy to heat the object inside the microwave, and the high voltage circuit unit and the MGT device 25, and generates a secondary voltage to the MGT device 25 (HVT) 24), a resonator unit 6 connected in series with the primary coil of the HVT 24 to perform a resonance action, and a switching unit 27 for switching a DC power source to an AC power source. The switching unit 27 has a pair of switching power devices 22 and 23.

The inverter device includes an output controller 4 for controlling the output of the HVT 24, an oscillator 21 whose oscillation frequency is varied by a change in the resistance value of the output controller 4, an on-off and a soft start (SOFT) of the inverter. Abnormal on-voltage generated from the on-off and soft-start unit 3 for realizing START), the D / A converter 2 for converting a pulse width modulation (PWM) signal received from the outside into an analog signal, and the resonator unit 6 And an over-resonance detector (5) for protecting the switching elements (22, 23) of the switching part (27) from the low voltage off part (1) for stopping the inverter operation when the input voltage is extremely low.

The commercial AC power source 7 is a power source SOURCE for supplying power to the inverter device, and the commercial AC power source 7 is connected with a rectifier element 8 and the rectifier element 8 is a commercial AC power source 7. Convert to DC. The reactor 9 and the smoothing capacitor 10 are connected to the rectifier element 8, and the reactor 9 and the smoothing capacitor 10 prevent external emission of inverter noise. In addition, the voltage attenuation resistor 19 and the other smoothing capacitor 20 connected to the rectifying element 8 receive a high DC voltage of approximately 310V level rectified by the rectifying element 8 to a semiconductor driving power supply at about 15V level. Lower it.

Since the HVT 24 applied to the inverter device according to the present invention is driven at a high frequency (about 20 Khz) through semiconductor oscillation, a ferrite core having very low high frequency loss is used. The HVT 24 using the ferrite core has a volume of about 1/4 and a weight of about 1/20 or less as compared to conventional iron core HVT. In addition, since the HVT 24 in the present invention is driven at a high frequency through semiconductor oscillation, it is not necessary to increase the number of turns of the secondary coil.

By such a configuration, in the present invention, when the power is initially supplied to the microwave oven or waiting for use, the inverter device is stopped because no signal is given to the input terminal of the photocoupler 18. When the operation of the inverter device is stopped, the oscillation of the inverter device is stopped. In order for the inverter device to start oscillation, a continuous PWM waveform is applied to the input terminal P1 of the photocoupler 18.

The PWM waveform applied to the photocoupler 18 of the present invention turns on (starts oscillation) the inverter device, and the oscillation frequency of the oscillation device is changed according to the pulse width change of the PWM waveform, thereby adjusting the output of the inverter device. Play a role.

First, in the inverter device according to the present invention, the on-off operation and the output adjusting portion will be described in detail.

When the PWM waveform is not applied to the on / off and soft start section 3 through the photocoupler 18, the transistor 306 is connected to the base by the pull up resistor 302 and the smoothing capacitor 303. ) Bias is applied and the state is turned on. As a result, the gate potential of the field effect transistor 310 becomes the lowest, and the resistance between the drain and the source of the field effect transistor 310 becomes infinite. Therefore, the result of having the capacitor 311 separated from the oscillation part 21 is stopped, and oscillation of the oscillation part 21 is stopped. That is, the inverter OFF state is maintained.

When the PWM waveform is applied to the on-off and soft-start unit 3 through the photocoupler 18, the base bias of the transistor is released through the direction diode 301, and the transistor 306 is turned off. . Zener diode 304 then blocks the base residual bias of transistor 306 so that transistor 306 is completely off. When the transistor 306 is turned off, the VCC voltage is gradually charged to the smoothing capacitor 308 through the pull-up resistor 305 and the gate resistor 307. Therefore, while the drain-source resistance of the field effect transistor 310 gradually decreases, the oscillation capacitor 311 and the oscillator 21 are coupled. When the oscillation unit 21 and the oscillation capacitor 311 are combined, oscillation is finally started. The medium for coupling and separating these two elements is the drain-source resistance of the field effect transistor 310 as described above. When the drain-source resistance is high, such as when the field effect transistor 310 is initially turned on, the oscillation frequency becomes high due to the equivalent capacity of the capacitor 311 being equivalent, and conversely, the field effect transistor ( When 310 is turned on, the drain-to-source resistance becomes a negligible low value, and oscillation of the entire capacitance of the capacitor 311 is achieved.

As described above, the inverter output decreases when the oscillation frequency is high, and when the inverter starts oscillation, it starts at the highest frequency as possible, and the output is minimized, and then the frequency is gradually lowered to the desired output state. This consideration is called soft start. The soft-start characteristic is a very important factor directly related to the lifetime of the inverter device. In the present invention, the soft-start has realized this by using the drain-source resistance characteristic of the field effect transistor.

In consideration of the output control portion, the oscillator 21 structurally connects the external resistor RT and the capacitor CT to generate the gate pulses of the switching elements 22 and 23, where RT = resistance. 404 / {resistance 403 + drain-source resistance 402}, CT = capacitor 311.

In addition, as can be seen from the equation Fo = 1 / (1.4 * (RT + 75) * CT) for determining the oscillation frequency of the oscillator 21, in order to change the oscillation frequency, an external resistance value may be changed. The inverter device also uses the drain-source resistance characteristic of the field effect transistor 402 to change the external resistance value.

Changing the oscillation frequency has an important purpose to improve the power factor in addition to the purpose of controlling the inverter output as described above. Without consideration of power factor improvement, the output of the inverter, i.e., the secondary high voltage of the HVT 24, is sized in proportion to the supply voltage. Since the supply voltage is a pulse wave waveform rectifying commercial AC power, the secondary high voltage also has the same waveform as the pulse wave waveform, and the magnetron oscillator tube has a top dead center of the secondary high voltage (90 ° and 270 ° of the commercial AC signal). Operate only in the vicinity. In addition, near zero crossing (0 ° and 180 ° of the commercial AC power supply), the operation is stopped due to the low secondary high voltage.

This not only results in a very deteriorating efficiency of electrical energy, but also seriously affects the life of the magnetron oscillator tube. Therefore, the operation of the magnetron oscillation tube is ideally to have a load characteristic similar to the resistance possible over the entire AC power waveform. Detailed operation of the power factor will be described later.

When the PWM waveform is applied to the input terminal of the photo coupler 18, the analog voltage value P2 of the D / A converter 2 is determined according to the high, low (HI, LO) ratio of the PWM waveform. When the high ratio (P2) of the PWM waveform is high (P2), the voltage is lowered, so that the resistance value between the drain and source of the field effect transistor 402 increases, so that the oscillation frequency is lowered, thus increasing the output of the inverter. The pull-up resistor 201 is for the gate bias voltage of the field effect transistor 402, and the resistor 203, the capacitor 204, and the resistor 205 are π-type filter, which are analogous to the PWM waveform given by the digital quantity. The voltage is converted into a voltage and applied to the field effect transistor 310 through the gate resistor 401.

4 and 5 are graphs showing potentials and waveforms of respective parts according to FIG. 3. As shown in this figure, the operation of improving the power factor is such that the MGT device 25 does not operate only near a certain phase angle as described above, but has an even load characteristic over the entire AC signal section. This is only possible with purely resistive loads, but this is difficult to expect in nonlinear load structures such as the MGT device 25. Therefore, in order to operate the MGT device 25 of the nonlinear load structure with a linear load, a method of artificially correcting the operating voltage must be used.

The inverse correction of the operating voltage is easily expressed, which reduces the high voltage applied to the MGT device 25 near the phases 90 ° and 270 ° in which the MGT device 25 is most actively operated, and on the contrary, phase 0 ° when the activity is stagnated. In the vicinity of 180 ° is to increase the high voltage. As described above, it is another technical problem of the present invention that the high voltage applied to the MGT device 25 is corrected and supplied to the magnitude opposite to the load characteristic, so that current consumption close to the resistive load is achieved.

The diodes 11 and 12 are full-wave rectifier circuits for ac power waveform improvement and for acquiring the AC signal waveforms required for the operation of the low voltage off section 1, and the pulse currents obtained here are changed to low voltages by the attenuation resistors 13 and 14. It is transmitted to the gate of the output control unit 4 through the DC offset capacitor (17). The DC offset capacitor 17 transmits only an AC signal without breaking the gate bias voltage P4 of the output control unit 4, so that the field effect transistor 402 is always in the active region.

As shown in FIG. 5, a sine wave having a top dead center at the phase angles of 90 ° and 270 ° is applied to the magnitude of the voltage as shown in FIG. 5, and is inversely proportional to the field effect transistor. The drain-source resistance value of 402 is changed to vary the output of the inverter device. That is, at the phase angles of 90 ° and 270 °, the drain-source resistance of the field effect transistor 402 becomes minimum, so that the oscillation frequency of the oscillator 21 becomes the highest, and the inverter output becomes low.

In the present invention, a basic source for improving the power factor is obtained from an AC power source, and the power factor improving operation utilizes the drain-source resistance change of the field effect transistor.

The low voltage off unit 1 is applied to stop the operation of the inverter device when the AC input voltage is extremely low due to an abnormality or a lightning strike of the power line, and is used for the purpose of protecting various power devices. When the AC signal changed to the low voltage by 16 is charged to the smoothing capacitor 103 through the diode 101, and the voltage is equal to or less than the zener value of the zener diode 102, the transistor 104 is turned off. The oscillation (operation) of the inverter device is stopped by invalidating the PWM waveform applied to the photocoupler 18. Since the photocoupler 18 and the transistor 104 are connected in series with each other, these two elements become AND logic LOGIC to each other, and the result is turned off even if any one is turned off.

The over resonance detecting unit 5 is applied to the base BASE of the transistor 504 through the voltage divider resistors 601 and 505 when the resonance voltage generated by the resonator unit 6 is equal to or higher than a predetermined value. After charging the charging capacitor 502 is applied to the input terminal of the output control unit 4 through the diode 501. On the other hand, the output control part 4 controls the frequency of the abnormal signal detected by the over resonance detection part 5, and outputs it to the oscillation part 21 again. Here, the reason why the resonance voltage of the resonance unit 6 rises abnormally is because it is influenced by the surge noise coming in through the power line, and there is a possibility that various power devices may be damaged without considering the protection.

In the present invention, such an abnormal resonance voltage is impedance-converted through an emitter follower driving transistor to feed back to an input terminal of an output control device, thereby performing a closed loop operation.

6 is a graph showing the operating characteristics of the over resonance detection unit. As shown in the figure, the midpoint voltage P6 of the resonator device 6 before the start of the inverter operation, that is, when the inverter is stopped, starts oscillation most smoothly when V / 2. In other words, the most suitable soft start is realized. Here, V denotes a DC voltage applied to the collector of the switching power device 22 and the resonant capacitor 602 through the reactor 9, and when the commercial AC input is 220V, V is about 310V, so V / 2 It is about 155V.

To set the voltage (P6) to the V / 2 level, the value of the pull-up resistor 602 should be equal to the sum of the resistors 601 and 505, but the resistor 505 is much smaller than the resistor 601. It was ignored because it was a value, and the same value as the resistance 601 was taken to apply a DC bias of V / 2 level to the midpoint P6 of the resonator 6.

The biggest feature of the inverter device for a microwave oven according to the present invention is to generate a high voltage through the semiconductor oscillation, the magnitude of the high voltage obtained here can be freely increased or lowered by varying the oscillation frequency. Lowering the oscillation frequency increases the secondary high voltage by increasing the resonance current, and increasing the oscillation frequency lowers the secondary high voltage. In addition, a stable voltage is output by detecting an abnormal state of the inverter and controlling the inverter.

Since the output of the microwave oven, that is, the output of the MGT device is proportional to the magnitude of the secondary high voltage, controlling the secondary high voltage means controlling the output of the microwave oven, which is not the inverter device according to the present invention. It can be seen that the type microwave oven is impossible.

As described above, according to the present invention, by configuring the output of the inverter device for a microwave to be continuously variable analog, it is possible to improve the cooking efficiency and improve the taste of the food. In addition, by adjusting the output voltage of the inverter device to be stable, it is possible to protect various power devices from overpower.

The present invention is easy to handle by miniaturizing and reducing the weight of the HVT, and compared with the conventional iron core type HVT, it is possible to realize a high output with a small amount of resources, thereby reducing the relative cost.

Claims (16)

In a microwave oven having a high voltage converter for generating a high voltage, and an MGT device that receives a high voltage from the high voltage converter to generate electromagnetic waves, Rectification and smoothing unit for rectifying and smoothing the commercial AC power; An inverter unit converting the DC power rectified and smoothed from the rectifying and smoothing unit into an AC power of high frequency based on an input control signal; And a state detecting unit for detecting an abnormal state of the inverter unit, providing the control signal to the inverter unit to regulate an output AC power supply, and adjusting the control signal to reduce the output of the inverter unit based on the abnormal signal from the state detecting unit. Microwave oven comprising a control unit. The method of claim 1, The control unit, A D / A converter converting the control signal into an analog signal; An output control unit for controlling and outputting a frequency of the signal converted and input by the D / A converter; And an oscillator for varying a period of the signal output from the output controller and inputting the signal to the inverter. The method of claim 2, And the state detection unit is an over resonance detection unit for detecting an abnormal overvoltage of the inverter unit, and the abnormal overvoltage is fed back to an input terminal of the output control unit. The method of claim 3, And the output control unit re-outputs the oscillation signal whose frequency is increased in accordance with the abnormal signal provided from the over resonance detection unit to the oscillator. The method of claim 2, The control unit, And an on-off and soft-start unit for controlling oscillation on-off and soft-start of the oscillator according to the input control signal. The method of claim 2, The control unit may further include a low voltage off unit for outputting a stop signal to the on / off and soft start unit and the D / A converting unit when abnormal power is input through the power supply unit. The method according to any one of claims 1 to 6, And the control unit branches the control signal to input the D / A conversion unit and the on / off and soft start unit. The method of claim 2, And the oscillating unit has a switching unit which switches the DC power to AC power. The method of claim 8, And the oscillator is oscillated when an external resistor and a capacitor are connected to generate a gate pulse of the switching unit. The method of claim 9, And the switching unit is a pair of switching power devices. The method of claim 10, The oscillator is a microwave oven, characterized in that the oscillation frequency is given by the formula Fo = 1 / (1.4 * (external resistance + 75) * capacitor). The method of claim 2, And the output controller uses a drain-source resistance characteristic of a field effect transistor to change the external resistance value. The method of claim 4, wherein And the on-off and soft-start portions use the drain-source resistance characteristics of the field effect transistor for soft start. The method of claim 5, The low voltage off unit is a microwave oven, characterized in that the transistor is connected in series with the photocoupler in the form of AND logic. The method of claim 2, The microwave oven is configured to transmit an AC signal to an input terminal of the output control unit through a DC offset capacitor. The method of claim 1, The HVT is a microwave oven, characterized in that consisting of a ferrite core with a low frequency loss.