JPH0487185A - Driver circuit for inverter type microwave oven - Google Patents

Abstract

PURPOSE:To accomplish a power supply circuit using a low-voltage DC power supply, with which a high output is assured at a low cost, by varying the ON time of a switching element by a control means periodically and continuously wherein the predetermined max. On time is observed as the upper limit. CONSTITUTION:A driver circuit for inverter type microwave oven is equipped with a push-pull voltage type inverter circuit 2 which converts the output power of an independent type DC power supply 1 into a high frequency electric power, a booster transformer 3 for the supply voltage, and a voltage doubler half-wave rectifying circuit 4 which rectifies the output of this booster transformer 3, and with the output from the last named circuit 4 a magnetron 5 is driven. Switching element drive circuits 9a, 9b and a control circuit 10 are provided as a control means to vary periodically and continuously the On time of switching elements 8a, 8b of the abovementioned circuit 2 in push-pull system (or bridge system), wherein the predetermined max. On time is used as the upper limit. This permits accomplishing a power supply circuit using a low- voltage DC power supply, with which a high power utilization factor and a high output are assured at a low cost.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an inverter microwave oven drive circuit that converts an independent low-voltage DC power source (for example, a storage battery) into a high-voltage, high-frequency current, rectifies this using a voltage doubler rectifier circuit, and supplies power to a magnetron. .

[Conventional technology]

In recent years, various electrical and electronic devices that are normally used with commercial AC power sources and can also be used outdoors have been developed. When used outdoors, it is necessary to drive electric/electronic equipment with an independent low-voltage direct current power source such as 12V or 24V such as a storage battery for an automobile. Attempts are also being made to use inverter microwave ovens, which are currently widely used, outdoors. The configuration of a typical conventional inverter microwave oven is shown in FIG. Inverter microwave ovens use commercial power (100V,
The AC power obtained from 50/60Hz) is converted to DC power by a rectifier circuit. This DC power is increased in frequency by a -6 resonance inverter circuit and boosted by a step-up transformer. The transformer output is rectified by a voltage doubler rectifier circuit and used to drive the magnetron. When using the above inverter microwave oven with a low voltage DC power supply, as shown in Figure 8, a DC/AC inverter is installed between the low voltage DC power supply and the inverter microwave oven to control the output of the low voltage DC power supply. A DC/AC inverter was used to convert the AC power to 100V, 50/60Hz, the same as a commercial AC power supply, and this AC power was used to operate an inverter microwave oven.

[Problem to be solved by the invention]

As mentioned above, when using an inverter microwave oven with a low voltage DC power supply, the method of inputting AC power to the inverter microwave oven using a DC/AC inverter
Since power conversion is performed twice between the C/A c inverter and the inverter circuit of the inverter microwave oven, there is a problem that the power utilization rate is extremely low. Also, since two inverters are required, the cost of the power supply circuit)
It also becomes more expensive. Although it is theoretically possible to change the specifications of a conventional inverter microwave oven by directly connecting a low-voltage DC power supply to the single-stone resonant inverter power supply circuit, Requires a switching element with very large current capacity. A switching element having such a current capacity is currently unavailable or extremely expensive. The present invention was made in view of the current situation, and
The purpose is to provide an inexpensive, compact, and high-output power supply circuit that uses a low-voltage DC power supply as a power source, and to suppress uneven heating of the load (food) by varying the magnetron input power. An object of the present invention is to provide a drive circuit for an inverter microwave oven that can efficiently utilize the electrical energy of an independent DC power supply with limited electrical capacity.

[First Means for Solving the Problems] The inverter electronic lens drive circuit of the present invention includes a push-up type or Brisono type circuit having a plurality of switching elements, and an on-time of the switching elements.
an inverter circuit equipped with a control means that can periodically and continuously vary the maximum ON time set in advance, a step-up transformer whose primary winding is supplied with alternating current from the inverter circuit; It features a voltage doubler rectifier circuit that is connected to the secondary winding and supplies power to the magnetron.

[Effect]

Here, a case will be described in which a push-pull type circuit is provided as an inverter circuit of a drive circuit of an inverter microwave oven. An inverter circuit with a push-pull type circuit is composed of two switching elements. These two switching elements are turned on and off alternately to convert DC power into high-frequency power, and a step-up transformer and voltage doubler circuit are used to drive the magnetron. The voltage is increased to supply power to the magnetron. Note that the basic idea is the same even in the case of an inverter circuit equipped with a Brisono type circuit. Here, when the two switching elements are turned off at the same time and one switching element is turned on from the fco state (rest period), the voltage doubler capacitor is the leakage inductance of the step-up transformer, the capacitance of the voltage doubler capacitor of the voltage doubler rectifier circuit, and the circuit resistance. (However, the magnetron's resistance is excluded) It is charged with a current that draws an oscillating arc determined by the magnetron's resistance. The magnitude of the charging voltage of the voltage doubler capacitor is determined by the initial voltage of the voltage doubler capacitor and the length of the ON time of the switching element. Next, when the same switching element as above is turned off, the electromagnetic energy stored in the step-up transformer is supplied to the voltage doubler capacitor and regenerated into the power supply, resulting in a rest period. Next, after a rest period, when the other switching element is turned on, electrical energy is supplied to the magnetron with a current that draws an oscillating arc determined by the circuit resistance including the leakage inductance of the step-up transformer, the cavanti of the voltage doubler capacitor, and the resistance of the McNetron. be done. Here, the power supplied to the magnetron is determined by the voltage of the voltage doubler capacitor and the length of the ON time of the switching element. When the switching element is turned off, the electromagnetic energy stored in the step-up transformer is supplied to the magnetron and regenerated into a power source. The above switching operation is repeated and the Macnetron oscillates microwaves. Note that the output of the magnetron increases in proportion to the length of the switching element on time. Therefore, if the on-time of the switching element is continuously and periodically varied with the preset maximum on-time of the switching element as the upper limit, the electrical energy supplied to the magnetron changes with the output at the maximum on-time as the peak. Here, for convenience, the Macnetron output at the maximum on-time of the switching element will be referred to as the peak output value. Heating a load (food) with a magnetron utilizes the fact that the microwaves emitted by the magnetron are applied to the food, absorbed, and the energy is converted into heat inside the food. The food generates heat here because it contains water, which has an extremely high dielectric constant that absorbs microwaves and converts them into heat. Therefore, when the magnetron is continuously heated at its peak output value, the food is heated internally. More rapid fever, 100
Once the temperature rises to a certain temperature, the water in the food evaporates. This water evaporation results in energy loss. As mentioned above, if you heat food by periodically varying the output of the magnetron with the above peak output value as the upper limit,
With the same heating time as continuous heating at peak output, the heat generation of the food is gradually and efficiently progressed from the center to the periphery, making it possible to suppress energy loss or uneven heating due to water evaporation. Furthermore, the microwave oven of the present invention assumes an independent DC power supply with a limited capacity, and does not continuously drive the magnetron at the peak output value, but instead fluctuates the magnetron with the peak output value as the upper limit. This has the advantage of reducing the average discharge current and allowing more power to be extracted from the power source.

【Example】

Hereinafter, a driving circuit for an inverter microwave oven according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention in which a push-pull type circuit is provided. As shown in Figure 1,
This inverter microwave oven includes a push-pull voltage inverter circuit (hereinafter referred to as an inverter circuit) 2 that converts DC power from an independent DC power source (for example, an automobile storage battery) into high-frequency power, and a step-up transformer 3 that steps up the power supply voltage. A voltage doubler half-wave rectifier circuit 4 is provided to rectify the output of the step-up transformer 3, and a magnetron 5 is driven by the output of the voltage doubler half-wave rectifier circuit 4. Step-up transformer 3-2
Power for heating the filament of the magnetron 5 is also supplied from the next side. The voltage doubler half-wave rectifier circuit 4 has a known configuration,
It includes two high voltage diodes 6a, 6b and a voltage doubler capacitor 7. The inverter circuit 2 includes two power MO5FET8
a, 8b, a switching element drive circuit 9a9b for driving the power MOSFETs 8a and 8b, and a control circuit 10. The drains of the power MOSFETs 8a and 8b are connected to one end 3a and the other end 3b of the primary winding of the step-up transformer 3, respectively, and the power MOSFETs
The sources of MOSFETs 8g and 8b are connected to each other, and the gates of power MOSFETs 8a and 8b are driven by the control circuit IO via the switching element drive circuits 9a and 9b, thereby controlling the primary side of the step-up transformer 3. The flowing current is switched at high speed. power MO
In place of SFETs 8a and 8b, a power transistor, I
A switching element such as a GBT may also be used. One end of the DC power supply I is connected to a connection point between the source of the power MOSFET 8a and the source of the power MOSFET 8b, and the other end is connected to the center tap 3C of the primary winding of the step-up transformer 3. FIG. 2 is a circuit diagram of the control circuit IO. As shown in the figure, the oscillation circuit 11 is connected to a toggle flip-flop I2 and a sawtooth wave generation circuit 13, the toggle flip-flop 12 is connected to two AND gates 15a15b, and the sawtooth wave generation circuit 13 is connected to a comparison circuit 14. It is connected to the AND gates 15a and 15b through the gate. The toggle flip-flop 12 outputs a two-phase divided signal using the output signal of the oscillation circuit 11 as a trigger. The two-phase divided signals are input to two AND gates 15a and 15b, respectively. On the other hand, the f2 oscillation output given to the sawtooth wave generation circuit 13 is converted into a sawtooth wave synchronized with the oscillation frequency of the oscillation circuit 11, and then input to the comparison circuit 4. Further, the comparison circuit 14 is given a switching element on-time setting value VT. This switching element on time setting value VT is
This is the voltage value given from the microcomputer 16 via the D/A converter 17, and is the power MO9 shown in FIG.
This is a voltage value that sets the period during which the FETs 8a and 8b are turned on and determines the output of the magnetron. The details of how to set the switching element on-time setting value VT will be described later. As mentioned above, the comparison circuit 14 includes the sawtooth wave generation circuit 1.
Sawtooth wave from 3 and switching element on time setting value V
T is input, and the output of the comparator circuit 14 is a sawtooth wave voltage level. It becomes high level during a period longer than the element on time setting value VT. In this way, the comparator circuit 14 modulates the 7 signals so as to have a preset one-onion time.
By ANDing the signal input to the ND gates 15a and +5b and divided into two phases by the toggle flip-flop 12, the power MO9FEs 78a and 8b are alternately driven while having a period in which the two power MO5FETs are turned off simultaneously. . The outputs of the AND gates 15a and 15b are applied to the gates of the power MOSFETs 8a and 8b via switching element drive circuits 9a and 9b, respectively. When the output of the AND gate 15a is at a high level, the power MOSFET 8a is turned on. Also, when the output of the AND game h15b is high level, the power M
O9FET8b is turned on. FIG. 3 is a diagram showing the operation timing of the control circuit 10. As shown in the figure, the outputs of the AND gates 15a and 15b are alternately high level, so the power MOS
FET 8a and 8b are also alternately turned on. Here, the switching element on-time setting value VT is set so that there is a period in which the outputs of the AND gates 15a and 15b are simultaneously at a low level, that is, a dead time. Note that the dead time is provided to protect two switching elements from turning on simultaneously and causing a short circuit. Here, a method of setting the switching element on-time setting value VT will be explained. The switching element on time setting value VT is
Control circuit 10 with a value that determines the switching element on time.
Set by As shown in FIG. 4(a), as the switching element on-time set value VT increases, the on-time of the switching element becomes shorter, and therefore the magnetron output also becomes lower. The lower limit value V T min of the switching element on-time setting value VT corresponds to a preset switching element maximum on-time T max. Then, the upper limit of the switching element on-time setting value VT ■T
max is set in advance within a range that does not exceed the maximum value of the voltage level of the sawtooth wave generating circuit 13, and the switching element on time at this time is set as Tm1n. And Fig. 4(a)
It is possible to calculate the relationship between the switching element on time and the switching element on time setting value VT shown in FIG. 4(b), and to continuously and periodically set the switching element on time setting value VT as shown in FIG. 4(b). A program that can be used is stored in advance in the microcomputer 16 of the control circuit IO. The program built into the microcomputer I6 will be explained based on the flowchart shown in FIG. First, in step Sl, the magnetron oscillation time T, the operation period Ta at the switching element maximum on time Tmax, and the switching element minimum on time Tmi
Initialize the operating period Tb at n. Next, in step S2, it is determined whether or not the oscillation of the Macnetron 5 has started, and when it is determined that the oscillation is anchored, the process proceeds to step S3. When it is determined that oscillation has not started, the process returns to step S2. In step S3, the timer starts counting down the magnetron oscillation time T. Next, proceeding to step S4, the magnetron oscillation time T
The magnetron oscillation time T
If it is determined that has become zero, the process proceeds to step S5 and the oscillation of the magnetron 5 is stopped. If it is determined that the magnetron oscillation time T has not become zero, the process advances to step S6. In step S6, time t is set to zero. Next, the process proceeds to step S7, where the switching element maximum on time Tma
A lower limit value VTmin of the switching element on-time setting value VT corresponding to x is calculated. Next, the process proceeds to step S8, where the lower limit value VT of the switching element on-time setting value VT is
min is output to the D/A converter 17 to start counting up at time t. Next, the process proceeds to step S9, and if it is determined that the operating period Ta has reached the switching element maximum on time T max (set in step S1) at time t, the process proceeds to step S10. If it is determined that the operating period Ta has not elapsed, the process returns to step S8. In the step S10, the time t is set to zero. Next, the process proceeds to step Sll, where the switching element is set to the minimum ON state. Upper limit value V T of switching element on time setting value VT corresponding to time Tm1n
Calculate max. Next, the process proceeds to step S12, where the upper limit value VTmax is output to the D/A converter 17, and counting up of time t is started. Next, step SI3
Proceeding to step St, time t is the operating period Tb at the switching element minimum on time T min set in step St.
If it is determined that the time has elapsed, the process returns to step S4. If it is determined that the time t has not elapsed by the driving period Tb, the process returns to step SI2. By executing the above program, a switching element on-time setting value corresponding to a desired switching element on-time can be determined, and as a result, variable control of the output of the Macnetron becomes possible. Therefore, heat generation in the food is gradually and efficiently transferred from the center to the periphery, making it possible to suppress moisture evaporation and uneven heating. Next, the operation of this embodiment will be explained. Power MO9PET
From the state where both 8a and 8b are off, the power M
When OSFET 8b is turned on, the secondary side circuit of the step-up transformer 3 includes the high-voltage capacitor 7, the high-voltage diode 6a, one end 3e of the secondary winding of the step-up transformer 3, and the other end 3d of the secondary winding.
A current flows through the closed loop of the voltage doubler capacitor 7, and the voltage doubler capacitor 7 is charged. Note that the magnitude of the charging voltage of the voltage doubler capacitor 7 is as follows:
Initial voltage of voltage doubler capacitor 7 and power MO'5FET
It is determined by the length of on time of 8a and 8b. Next, when the same power MO6FET 8b as above is turned off again, the electromagnetic energy stored in the step-up transformer 3 is supplied to the voltage doubler capacitor 7 and regenerated to the power supply l.
The period moves to a period in which the two power MO8PETs 8a and 8b are turned off simultaneously. Next, when the power MO6FET 8a is turned on, the secondary side circuit of the step-up transformer 3 includes the high-voltage diode 6b, the voltage doubler capacitor 7, and one end 3d of the secondary winding of the step-up transformer 3, 2
A current flows through the other end 3e of the next winding and the closed loop of the magnetron 5, and the magnetron 5 is supplied with electrical energy. Here, the power supplied to the magnetron 5 is determined by the voltage of the voltage doubler capacitor 7 and the length of on-time of the power MO8FETs 8a and 8b. And power MOSFET
When 8 a is turned off, the electromagnetic energy stored in the step-up transformer 3 is supplied to the magnetron 5 and the power source l is turned off.
will be regenerated. The above operations are repeated, and the magnetron 5 continues to oscillate high frequency power. The voltage doubler capacitor 7 has a current similar to the drain current waveform of the power MOSFET 8b, which draws an oscillating arc determined by the leakage inductance of the step-up transformer 3, the capacitance of the voltage doubler capacitor 7, and the circuit resistance (excluding the resistance of the magnetron 5). The magnetron 5 is charged with a train current waveform of the power MOSPET 8a that draws an oscillating arc determined by the leakage inductance of the step-up transformer 3, the capacitance of the voltage doubler capacitor 7, and the circuit resistance (including the resistance of the magnetron 5). Electrical energy is supplied in the form of a current waveform. Note that in this embodiment, the switching element off time setting value V
T is determined by the microcomputer 16, but
It is also possible to obtain the same effect by periodically varying the switching element on-time set value VT using a timer circuit or the like. Further, an embodiment of a drive circuit for an inverter microwave oven using a bridge type circuit is shown in FIG. As shown in FIG. 6, this inverter microwave oven uses a bridge type inverter circuit (hereinafter referred to as
Inverter circuit) 52, a transformer 53 that boosts the power supply voltage, and a voltage doubler half-wave rectifier circuit 54 that rectifies the output of the step-up transformer 53. Driven. Power for heating the filament of the magnetron 55 is also supplied from the secondary side of the step-up transformer 53 . The voltage doubler half-wave rectifier circuit 54 has a known configuration and includes two high voltage diodes 56a, 56b and a voltage doubler capacitor 57. The inverter circuit 52 includes four power MOSFETs.
(metal oxide semiconductor field effect transistors) 588 to 58d, four high-speed diodes 59a to 59d for protection of the four power MO8FETs, and the power MOSFET 58a.
~Switching element drive circuit 60 that drives 58d
a, 60b, and a control circuit 61. The drains of the power MOSFETs 58a and 58c are connected to the positive electrode of the DC power supply 51, and the drains of the power MOSFETs 58a and 58c are
The sources of 58b and 58d are connected to the negative electrode of DC power supply l. Above power MO9FETs 58a and 58
The sources of c are power MO9FETs 58b and 58, respectively.
connected to the drain of d. In addition, step-up transformer 5
One end 53a of the primary winding 3 is a power MO8FET 58c
and the other end 5 of the primary winding of the step-up transformer 53.
3b is the source of power MO5FE'T58a and power M
It is connected to the connection point of the train of OSFET 58d. In addition, the high speed diode 59a=d is a power MOSFET.
21. to T58a-d, respectively. connected in parallel. Power MOSFETs 58a to 58d as switching elements
The gates of the switching element drive circuits 60a, 60
By being driven by the control circuit 61 via the step-up transformer 53, the current flowing through the primary side of the step-up transformer 53 is switched at high speed. In addition, as switching elements, instead of the power MO8FETs 58a to 58d, iGBT
A switching element such as an insulated gate bipolar transistor (insulated gate bipolar transistor) may also be used. The control circuit 61 and the switching element drive circuits 60a and 60b in this case have the same configuration as the control circuit 11 and the switching element drive circuits 10a and 10b of the embodiment using the push-pull type circuit described above. Ma-
Furthermore, the method of setting the switching element on-time setting value VT is also the same as that of the drive circuit of the inverter microwave oven using the above-mentioned circuit of the Bunyupuru type, so the explanation thereof will be omitted. Next, the operation of this embodiment will be explained. Inverter circuit 52
When the power MOSFETs 58c and 58d are turned on from the state in which all of the power MOSFETs 58a to 58d are turned off, the secondary circuit of the step-up transformer 53 is connected to the high-voltage capacitor 57, high-voltage diode 56a1, and one end of the secondary winding of the step-up transformer 53. 53d, a current flows through the closed loop of the other end 53c of the secondary winding, and the voltage doubler capacitor 57 is charged. The magnitude of the charging voltage of the voltage doubler capacitor 57 is determined by the initial voltage of the voltage doubler capacitor 57 and the on-time length of the power MOSFETs 58a to 58d as switching elements. Next, again the same power MOSPET 58c and 58 as above
When d is turned off, the electromagnetic energy stored in the step-up transformer 53 is supplied to the voltage doubler capacitor 57, and
The path between one end 53b of the primary winding of the step-up transformer 53, the high-speed diode 59a, the DC power supply 51, the high-speed diode 59b, and the other end 53a of the primary winding of the step-up transformer 53 is the power supply 5.
1 and all power MOSPETs 58a to 5
8d is turned off at the same time. Next, when the power MOSFETs 5.8a and 58b are turned on, the secondary side circuit of the step-up transformer 53 is connected to the high-voltage diode 56b, the voltage doubler capacitor 57, and the step-up transformer 53's secondary circuit.
A current flows through the closed loop of the magnetron 55, with one end 53c of the secondary winding and the other end 53d1 of the secondary winding, and electrical energy is supplied to the magnetron 55. Here magnetron 55
The power supplied to the MOSFETs is determined by the voltage of the voltage doubler capacitor 57 and the on-time length of the power MOSFETs 58a to 58d. Then, when the power MO8FETs 58a and 58b are turned off, the electromagnetic energy stored in the step-up transformer 53 is supplied to the magnetron 55, and one end of the primary winding 53a1 of the step-up transformer 53 is connected to the high-speed diode F'59.
c. The period moves to a period in which the DC power supply 51, the high-speed diode 59d1, the other end 53b of the primary winding of the step-up transformer 53 is regenerated into the power supply 1, and all the power MO9FETs 58a to 58d are turned off at the same time. The above operations are repeated, and Macune 1 and Ron 5 continue to oscillate high-frequency power. The voltage doubler capacitor 57 has a power MOS FE that draws an arc of vibration determined by the leakage inductance of the step-up transformer 53, the capacitance of the voltage doubler capacitor 57, and the circuit resistance (excluding the resistance of one magnetron 55).
The magnetron 55 is charged with the same current waveform as the drain current waveform of the T 58c and 58d, and the magnetron 55 has the leakage inductance of the step-up transformer 53 and the voltage doubler capacitor
7 capacitance, circuit resistance (however, magnetron 55
power MO8 that draws an arc of vibration determined by
Electrical energy is supplied with a current waveform similar to the drain current waveform of FETs 8a and 8b. Although the DC power source of the drive circuit of the inverter microwave oven in the above two embodiments is assumed to be an independent DC power source, it may be a DC power source of a vehicle such as an automobile.

【Effect of the invention】

As described above, according to the present invention, DC/AC
Since no inverter is used, it is possible to provide a power supply circuit that is inexpensive, has high power utilization efficiency, and has high output. Furthermore, since low-voltage DC power is directly converted into high-frequency current, the step-up transformer, which is the largest and heaviest of all power supply circuits, can be made smaller and lighter, allowing for more compact power supply circuits. I can figure it out. In addition, since the output of the Macnetron is varied to vary its strength, heat generation from the food is gradually and efficiently progressed from the center to the periphery, making it possible to suppress moisture evaporation and uneven heating. Furthermore, the inverter microwave oven of the present invention assumes that the power source is an independent DC power supply with a limited capacity, and compared to driving a magnetron at a constant peak output, the average discharge current of the independent DC power supply is Since the power consumption can be reduced, there is an advantage that a large amount of electric power can be extracted from the above-mentioned independent DC power supply.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an inverter microwave oven drive circuit using a push-pull type circuit according to an embodiment of the present invention;
FIG. 2 is a block diagram of the control circuit of the above embodiment, FIG. 3 is a waveform diagram of each control signal of the control circuit, and FIG. 4(a),
(b) is an explanatory diagram of the method of setting the switching element on-time setting value of the above embodiment, and FIG. 5 is a flowchart of the program built into the micro combi coater of the above embodiment.
Fig. 6 is a circuit diagram of an inverter microwave oven using a bridge type circuit according to an embodiment of the present invention, Fig. 7 is a circuit block diagram of a conventional inverter microwave oven, and Fig. 8 is a circuit diagram of an inverter microwave oven using a bridge type circuit according to an embodiment of the present invention. 1 is a diagram showing a method of driving a conventional inverter microwave oven. ■, 51... DC power supply, 2.52... Inverter circuit,
3.53...Step-up transformer, 4.54...Voltage doubler half-wave active current circuit, 8a, 8b, 58a, 58b, 58c, 58d・Power MO9FET. 9a, 9b, 60a 60b...Switching element drive circuit, 10.61...
・Control circuit.

Claims (1)

[Claims] (1) A push-pull type or bridge type circuit having a plurality of switching elements, and a control means that can periodically and continuously vary the on-time of the switching element with a preset maximum on-time as an upper limit. An inverter circuit, a step-up transformer through which alternating current is supplied to the primary winding from the inverter circuit, and a voltage doubler rectifier circuit connected to the secondary winding of the step-up transformer to supply power to the magnetron. An inverter microwave oven drive circuit featuring: