JPH0594869A - Drive circuit for inverter microwave oven - Google Patents

Abstract

PURPOSE:To make a microwave over small in size and light in weight by letting the primary winding of a step-up transformer and the group of switching elements form a closed loop, and connecting the connecting points of the group of the elements and the center tap of the primary winding of the transformer to a low voltage capacitor. CONSTITUTION:A DC power supply 1 is connected to the group of switching elements 8a and 8b via a choke coil 11 and a low voltage capacitor 12, and the other end of the power supply is connected to the primary winding center tap 3c of a step-up transformer 3. When the power MOSFET of the group of the elements 8a is turned on, current flows to the voltage double rectifier of the secondary circuit 4 of the transformer 3 and the closed loop of a magnetron 5, so that electric energy is supplied to the magnetron 5. And when the MOSFET of the group of the elements 8a is turned off, electric energy stored in the transformer 3 is regenerated to the power supply 1 while being supplied to the magnetron. And high frequency electric power is supplied to the magnetron 5 with the aforesaid action repeated. By this constitution, no DC/AC inverter is required, a drive circuit inexpensive and high in output can thereby be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for an inverter microwave oven which converts a low-voltage DC power supply into high-voltage high-frequency power and supplies the power to a magnetron.

[0002]

2. Description of the Related Art In recent years, various devices have been developed in consideration of outdoor use of electric and electronic devices that have been used as commercial AC power sources, and are now widely used at home in inverter microwave ovens. Attempts have also been made to use it outdoors.

A circuit configuration of a conventional general inverter microwave oven is shown in FIG. In this circuit, commercial power supply (AC100
AC power obtained from V, 50/60 Hz) is first converted into DC power by a rectifier circuit. This DC power is made into a high frequency by the one-stone resonance type inverter circuit and boosted to several kV by the boosting transformer. The transformer output is rectified by a voltage doubler rectifier circuit and supplies power to the magnetron.

On the other hand, when the above-mentioned inverter microwave oven is used outdoors, it is necessary to use a limited energy source such as a DC 12V or 24V storage battery. Therefore, for example, as shown in FIG. By providing a DC / AC inverter between a low-voltage DC power source such as the above and a circuit used in an inverter microwave oven which uses the AC 100V as a power source, and by converting the DC low voltage into AC 100V by this DC / AC inverter. , Was driving an inverter microwave oven.

[0005]

When the inverter microwave oven is used with a storage battery or the like, the method of inserting the DC / AC inverter between the DC / AC inverter and the AC / DC must be performed twice. Therefore, the utilization rate of electric power is low. In addition, the cost is increased due to the addition of the DC / AC inverter.

Therefore, an inverter circuit for directly driving the magnetron using a low-voltage DC power supply such as a storage battery is required. To meet this demand, the applicant of the present invention has proposed an inverter microwave oven suitable for the low-voltage DC power supply. A drive circuit has already been proposed (Japanese Patent Application No. 2-200689 and Japanese Patent Application No. 2-2).
00690).

By the way, in the conventional inverter circuit, the PWM method is generally used in which the duty ratio of the on / off time of the switching element is controlled to control the output power. However, the above method has a drawback that switching loss becomes large because there is a period in which both the current and the voltage change sharply when the switching element is turned on and off. As one means for solving this drawback, there is a method of using a series resonance circuit composed of a capacitor and a coil.
In this case, the current waveform when the switching element is on becomes a sine wave due to the series resonance circuit, and the current or voltage crosses at almost zero when the switching element is on / off. Therefore, switching loss can be significantly reduced.
However, while the resonance frequency of the series resonance type inverter is uniquely determined, the PWM control changes the duty to control the output power.
It becomes difficult to change the output power without impairing the above characteristics. On the other hand, there is a method of controlling the frequency by changing it, but it has a drawback that the size of the transformer becomes large because it is necessary to design the transformer at the lowest frequency at which it operates. The present invention has been made in view of the above,
The purpose of this is to propose a drive circuit for an inverter microwave oven as previously proposed (Japanese Patent Application No. 2-200689 and Japanese Patent Application No. 200689).
-200690), a low-voltage DC power supply is used as a power supply, and an inexpensive and compact high-output and high-efficiency drive circuit for an inverter microwave oven is provided. Furthermore, a drive circuit capable of easily controlling output power To provide.

[0008]

A drive circuit for an inverter microwave oven according to the present invention comprises two switching element groups in which one or more switching elements are connected in parallel, and control means for performing on / off operations of the switching elements. A push-pull inverter circuit, a step-up transformer that is supplied with high-frequency alternating current from the push-pull inverter circuit to the primary winding to generate high voltage in the secondary winding, and a secondary winding of the step-up transformer. A voltage doubler rectifier circuit that is connected and supplies power to a magnetron composed of a high voltage diode and a high voltage capacitor, a low voltage capacitor that is connected in parallel to a low voltage DC source that is a power source, and a low voltage capacitor that is connected to one end of the low voltage capacitor. It is composed of a choke coil connected in series with a low-voltage DC source, and has the primary winding of the step-up transformer and the two switches described above. The switching element group is connected to form a closed loop, and the connection point of the two switching element groups and the center tap of the primary winding of the step-up transformer are connected to both ends of the low voltage capacitor. ..

Further, the control means in the push-pull inverter circuit is configured so that the on-time is in relation to the relationship between the oscillation cycle of transient oscillation of the output current waveform of the push-pull inverter circuit and the on-time of the switching element. It is characterized in that it is set so as to fall within approximately one half cycle to one cycle of the vibration cycle.

Further, there is provided two or more low-voltage capacitors connected in parallel to the low-voltage DC source, and switching means for inserting and disconnecting the circuit connected in series to the low-voltage capacitor, and controlling the switching means. And a switching circuit for controlling the output power without changing the timing of the zero current switching by changing the capacity of the low voltage capacitor.

[0011]

When the two switching element groups in the push-pull type inverter circuit are turned off at the same time (inactive period) and one of the switching element groups is turned on, the high voltage capacitor becomes the leakage inductance of the step-up transformer, the capacitance of the high voltage capacitor, and the circuit resistance. It is charged with a current that draws an arc of vibration determined by (excluding the magnetron resistance). The magnitude of the charging voltage of the high voltage capacitor is determined by the initial voltage of the high voltage capacitor and the on-time length of the switching element group. Next, when the same switching element group as described above is turned off, the electromagnetic energy stored in the step-up transformer is regenerated by the power source while being supplied to the high-voltage capacitor, and there is a rest period.

Next, after the rest period, when the other switching element group is turned on, the current draws an arc of oscillation determined by the leakage inductance of the step-up transformer, the capacity of the high-voltage capacitor, and the circuit resistance including the resistance of the magnetron. Electric energy is supplied to the magnetron. Here, the electric power supplied to the magnetron is determined by the voltage of the high voltage capacitor and the length of the ON time of the switching element group.
Then, when the switching element group is turned off, the electromagnetic energy stored in the step-up transformer is regenerated by the power source while being supplied to the magnetron. The above switching operation is repeated and the magnetron is driven so as to oscillate high frequency power.

The current waveform of the switching element oscillates at a natural frequency determined by a circuit constant, that is, an inductance component, a capacitance component in the main circuit, and a circuit resistance. By adjusting the inductance value, the capacitance value, or both. If the ON time of the switching element is set in a period from one half cycle to one cycle of the natural frequency, zero current switching can be performed, the transition loss becomes very small, and the switching loss can be reduced.

When the capacitance value of the low voltage capacitor is further changed from this state, the average value of the output current changes due to the change of the waveform shape due to the change of the oscillation condition of the output current waveform without breaking the zero current switching state. However, since the output power is proportional to the average output current value, the output power of the inverter is controlled.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The driving circuit for an inverter microwave oven according to the present invention, the circuit output improvement, and the circuit loss reduction will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a power supply circuit diagram of a first embodiment of the present invention. This power supply circuit rectifies the output of a push-pull voltage type inverter circuit 2 for converting the DC power of a low voltage DC power supply (for example, a battery for automobiles) 1 into high frequency power, a step-up transformer 3 for stepping up the power supply voltage, and a step-up transformer 3. The voltage doubler half-wave rectifier circuit 4 is provided, and the magnetron 5 is driven by the output of the voltage doubler half-wave rectifier circuit 4. The filament heating power supply of the magnetron 5 is also supplied from the secondary side of the step-up transformer 3.

The voltage doubler half-wave rectifier circuit 4 is composed of two high voltage diodes 6a and 6b and a high voltage capacitor 7.

The inverter circuit 2 is a power MOSFET for switching one or more direct currents at high speed.
(Metal Oxide Semi-conductor Field Effect Transist
2 sets of switching element groups 8 connected in parallel
a, 8b, switching element drive circuits 9a, 9b for driving the switching element groups 8a, 8b, and control circuit 1
0 and.

The drains of the power MOSFETs forming the switching element groups 8a and 8b are respectively connected to one end 3a and the other end 3b of the primary winding of the step-up transformer 3 to form these two switching element groups 8a and 8b. The sources of the power MOSFETs that are connected to each other are connected to each other, and the power M that constitutes the switching element groups 8a and 8b
The gate of the OSFET is the switching element drive circuit 9
a and 9b, respectively.

The switching element groups 8a and 8b composed of power MOSFETs are driven by the control circuit 10 and the switching element drive circuits 9a and 9b, and the current flowing through the primary side of the step-up transformer 3 is switched at high speed.

The DC power source 1 has a source connected to the switching element group 8a and a switching element group 8b via a choke coil 11 having one end connected in series, and a low voltage capacitor 12 connected in parallel to the DC power source 1 and the choke coil 11.
Is connected to the connection point with the source of the
Is connected to the center tap 3c of the primary winding.

Next, the operation of this embodiment will be described. When the power MOSFETs of the switching element group 8b are turned on while the power MOSFETs of the switching element groups 8a and 8b are both off, the secondary side circuit of the step-up transformer 3 has a high-voltage capacitor 7, a high-voltage diode 6a, and a step-up transformer 3. The current flows through the closed loop of the one end 3e of the secondary winding and the other end 3d of the secondary winding to charge the high voltage capacitor 7.

Next, when the power MOSFETs of the same switching element group 8b as described above are turned off again, the step-up transformer 3
The electromagnetic energy stored in the power supply 1 is regenerated while being supplied to the high-voltage capacitor 7, and the power MOSFETs of the two switching element groups 8a and 8b are simultaneously turned off.

Next, the power M of the switching element group 8a
When the OSFET is turned on, the secondary side circuit of the step-up transformer 3 has a high-voltage diode 6b, a high-voltage capacitor 7, one end 3d of the secondary winding of the step-up transformer 3, the other end 3e of the secondary winding, and a current in the closed loop of the magnetron 5. The magnetron 5 is supplied with electric energy. When the power MOSFETs of the switching element group 8a are turned off, the electromagnetic energy stored in the step-up transformer 3 is regenerated by the power source 1 while being supplied to the magnetron 5. The above-described operation is repeated and high frequency power is supplied to the magnetron 5.

At this time, the high-voltage capacitor 7 has circuit inductance, circuit capacitance, and circuit resistance in the main circuit, that is, leakage inductance of the step-up transformer 3, high-voltage capacitor 7 and low-voltage capacitor 12, capacitance, switching element ON resistance, and wiring resistance. Switching element group 8 that draws an arc of vibration determined by circuit resistance such as
It is charged with a current waveform similar to the drain current waveform of the power MOSFET of b, and the leakage inductance of the step-up transformer 3 and the high voltage capacitor 7 are connected to the magnetron 5.
And a power MOSFET of the switching element group 8a that draws an arc of vibration determined by the capacitance of the low-voltage capacitor 12, the switching element ON resistance, the wiring resistance, and other circuit resistance
Electrical energy is supplied with a current waveform similar to the drain current waveform of.

FIG. 2 shows the power MOSFE of this embodiment.
It is a figure which shows an example of the current waveform which flows into T. The fact that the circuit output power can be improved will be described in detail with reference to FIG.
As described above, the current waveform oscillates at the natural frequency determined by the leakage inductance of the step-up transformer 3, the capacitances of the high voltage capacitor 7 and the low voltage capacitor 12, and the circuit resistance, and the natural frequency F can be expressed by the following equation.

[0027]

[Equation 1]

Here, L is the leakage inductance of the step-up transformer, C is the capacitance of the high-voltage capacitor, R is the circuit resistance, n is the step-up transformer turn ratio, and the power M is within a range from one half cycle to one cycle of this waveform.
L in the above formula to match the on-time of OSFET,
When the values of C and R are set (1 / 2F≤TON≤1 / F),
It can be seen that the drain current when the MOSFET is off becomes almost zero and zero current switching becomes possible, so that the occurrence of transition loss can be suppressed as much as possible and the switching loss can be reduced. Also, especially on time is half
By making the period equal to (1 / 2F = TON), the current in the ON period of the power MOSFET (the integrated value in the ON period of the current waveform) becomes almost maximum as shown in FIG. Can be maximized. If the natural frequency is too long or too short as compared with the on time, the current value during the on period becomes small as shown in FIGS. 2 (b) and 2 (c).

On the other hand, by utilizing the fact that the current waveform draws an arc of oscillation, by changing the circuit constant,
The current value is controlled by changing the natural frequency of the current waveform during the on period. This effect can be obtained by changing any of the circuit constants, that is, circuit capacitance, circuit inductance, and circuit resistance. Here, the greatest effect can be expected as an example, and it can be easily realized. As another method, a method of switching the capacity of the low voltage capacitor using a relay contact and its effect will be described.

FIG. 3 is a power supply circuit diagram of the second embodiment of the present invention. The difference from the first embodiment is that three low-voltage capacitors (12a, 12b, 12c) are inserted in parallel, two of which are connected to the main circuit via relays 13a, 13b. This relay 13a, 1
A switching circuit 14 for controlling ON / OFF of 3b is added. For example, if the capacitances of these three low-voltage capacitors are 4.7 μF, 10 μF, and 22 μF, respectively, and the switching circuit 14 controls the ON / OFF of the relay contacts, the input capacitor capacitance will change depending on the ON / OFF states of the relay contacts 13a and 13b. 36a when both 13a and 13b are on.
7µF, 26.7µ when 13a is off and 13b is on
When F, 13a is on and 13b is off, 14.7μF,
When both 13a and 13b are off, the value is 4.7 μF. As a result, the circuit capacitance in the main circuit changes, and the current waveform supplied to the magnetron is shown in FIGS.
It changes like the actually measured waveforms shown in FIGS. Here, the average current value can be obtained from the integrated value of the current waveform, and in each case, 146.3 mA, 93.6 mA, 58.
It becomes 9mA and 31.1mA. Also, since the power supplied to the magnetron is proportional to the above current average value, the power supplied to the magnetron, that is, the power control of the microwave oven can be easily controlled by switching the input capacitor capacity as described above, and the power in normal PWM control Instead of changing the switching-on time as in the control,
As shown in FIG. 7, since the period of the current waveform is changed while keeping the switching on time as it is, power control can be performed without impairing the feature of performing zero current switching. Therefore, the power control does not reduce the efficiency.

The same operation can be performed by changing the capacity of the high-voltage capacitor, but switching with a contact in the high-voltage circuit is difficult in terms of cost and technology, and the capacity of the low-voltage capacitor is gradually changed. Alternatively, the present embodiment is characterized in that it is possible to change it continuously.

[0032]

According to the present invention, since it is not necessary to use a DC / AC inverter, it is possible to provide a low-cost drive circuit for an inverter microwave oven with high power utilization efficiency and high output. Furthermore, since the low-voltage DC power supply is directly converted to high-frequency current, the booster transformer, which is the largest and heaviest in the power supply circuit, can be made compact and lightweight, and the drive circuit can be made compact. ..

Further, a large circuit output can be obtained by controlling the on-time of the switching element and the oscillation cycle of the transient oscillation of the current waveform, and the reduction of switching loss due to zero current switching can be realized.

Further, by controlling the capacity of the low voltage capacitor, output power control can be realized without impairing the effect of zero current switching.

From the above, if the drive circuit of the present invention is used in a microwave oven, it can be used with power according to the contents of cooking such as normal cooking and food defrosting, and it is highly efficient. A microwave oven driven by a low-voltage DC power supply, which was not possible, will be realized. The present invention can be widely used for driving a magnetron.

[Brief description of drawings]

FIG. 1 is a power supply circuit diagram of a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a switching current waveform of the power MOSFET according to the first embodiment.

FIG. 3 is a power supply circuit diagram of a second embodiment of the present invention.

FIG. 4 is a diagram showing a current waveform supplied to the magnetron in the second embodiment.

FIG. 5 is a diagram showing a current waveform supplied to the magnetron in the second embodiment.

FIG. 6 is a diagram showing a current waveform supplied to the magnetron in the second embodiment.

FIG. 7 is a diagram showing a waveform of current supplied to the magnetron in the second embodiment.

FIG. 8 is a circuit block diagram of a conventional inverter microwave oven driven by a commercial power source.

FIG. 9 is a diagram illustrating a method for driving a conventional inverter microwave oven using a low voltage DC power supply.

[Explanation of symbols]

1: DC power supply 2: Inverter circuit 3: Boost transformer 4: Double voltage half-wave rectifier circuit 5: Magnetron 6: High voltage diode 7: High voltage capacitor 8a, 8b:
Switching element group 9a, 9b: Switching element drive circuit 10: Control circuit 11: Choke coil 12: Low voltage capacitor 13: Relay 14: Switching circuit

Claims (3)

[Claims] 1. A push-pull type inverter circuit comprising two switching element groups in which one or more switching elements are connected in parallel, and a control means for performing on / off operation of the switching element, and the push-pull type inverter circuit. A high-frequency AC from the power supply is supplied to the primary winding to generate a high voltage in the secondary winding, and power is supplied to the magnetron that is connected to the secondary winding of the boost transformer and is composed of a high-voltage diode and a high-voltage capacitor. And a low-voltage capacitor connected in parallel to a low-voltage DC source serving as a power source, and a choke coil connected to one end of the low-voltage capacitor and connected in series to the low-voltage DC source. The primary winding of the step-up transformer and the two switching element groups are connected to each other to form a closed loop. Driving circuit of an inverter microwave oven and the primary winding of the center tap of the switching element group of the connection point and the step-up transformer is characterized in that it is connected to both ends of the low-pressure condenser. 2. A control means in the push-pull inverter circuit controls the on-time with respect to the relationship between the oscillation cycle of transient oscillation of the output current waveform of the push-pull inverter circuit and the on-time of the switching element. 2. The drive circuit for an inverter microwave oven according to claim 1, wherein the drive circuit is set so as to be contained within approximately one half cycle to one cycle of the vibration cycle. 3. A switching means for inserting and disconnecting a circuit connected in series with the low-voltage capacitor, comprising two or more low-voltage capacitors connected in parallel to the low-voltage DC source, and controlling the switching means. 3. The drive circuit for an inverter microwave oven according to claim 2, further comprising a switching circuit that controls output power without changing timing of zero current switching by changing the capacity of the low voltage capacitor.