JPH07240276A - Microwave oven - Google Patents

Abstract

PURPOSE:To shorten the time required for the magnetron of a microwave oven to heat its cathode and start oscillating at the time of starting drive. CONSTITUTION:When a magnetron 13 is started to drive, the voltage of an AC power source 1 is detected by a voltage detecting circuit 22 connected to the secondary side of a stepdown transformer 21. A main control circuit 19 sets the length of the ON time of an IGBT 9, at the length according to the voltage detected by the voltage detecting circuit 22, and the ON and OFF of the IGBT 9 is controlled by the length of the above ON time. As a result, the voltage induced at the secondary side of a booster transformer 8 does not receive the influence of the variation of the AC power source 1, being maintained at an about a constant high voltage, and as a result, the cathode of the magnetron 13 is heated rapidly, and the time to start the oscillation is reduced. And even though the voltage of the AC power source 1 is increased, no abnormal high voltage is applied to electron parts to compose the IGBT 9 and a magnetron driving circuit 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microwave oven provided with an inverter device for controlling on / off of a switching element to convert an AC power supply frequency into a high frequency.

[0002]

2. Description of the Related Art In a microwave oven of this type, a high frequency output of an inverter device is supplied to the primary side of a step-up transformer, and a magnetron drive circuit connected to the secondary side of the step-up transformer heats the cathode of the magnetron. A high DC voltage is applied to the magnetron. In such a microwave oven, the voltage induced on the secondary side of the step-up transformer can be controlled by changing the on-time of the switching element of the inverter device.

By the way, since the magnetron is a kind of vacuum tube, even if a high voltage DC voltage is applied, the oscillation operation does not start unless the cathode is warm. In order to shorten the time until the start of this oscillating operation, it is sufficient to lengthen the on time of the switching element at the start of magnetron drive so that the voltage induced on the secondary side of the step-up transformer becomes high. The voltage applied to the electronic parts of the magnetron drive circuit connected to the secondary side of the step-up transformer is the input voltage of the inverter device (the voltage of the commercial AC power supply).
Therefore, when the voltage of the commercial AC power supply becomes high, a high voltage is applied to the electronic components of the switching element and the magnetron drive circuit, which shortens the life thereof. Therefore, conventionally, the energization time of the switching element is set to a relatively short fixed time so that an abnormally high voltage is not applied to the switching element or the electronic component even if the voltage of the commercial AC power supply fluctuates.

[0004]

However, as described above, when the ON time of the switching element of the inverter device is shortened, when the voltage of the commercial AC power source is low, the voltage induced on the secondary side of the step-up transformer is also low. Therefore, it takes time for the cathode to warm up when the magnetron is started to drive, and there is a problem that the time for the magnetron to start the oscillation operation becomes long.

The present invention has been made in view of the above circumstances, and an object thereof is to shorten the time until the cathode warms up and the oscillation of the magnetron starts when the magnetron starts to be driven. An object of the present invention is to provide a microwave oven to which no voltage is applied.

[0006]

The microwave oven of the present invention comprises:
An inverter device that rectifies an AC power supply and switches its output by a switching element to supply it to the primary side of a step-up transformer, and is connected to the secondary side of the step-up transformer, heats the cathode of the magnetron, and outputs the secondary side output. A magnetron drive circuit that rectifies and drives the magnetron,
It comprises a voltage detection means for detecting the voltage of the AC power supply, and a control means for ON / OFF control of the switching element, wherein the control means sets the length of the ON time of the switching element at the start of driving the magnetron to the voltage. It is characterized by setting the value based on the detection voltage of the detection means to control the switching element to be turned on and off.

Further, the microwave oven of the present invention is provided with a current detection means for detecting a current flowing through the magnetron, and the control means detects the length of the ON time of the switching element after the oscillation of the magnetron is detected by the current detection means. It can be configured to change based on the current.

Further, in the microwave oven of the present invention, the control means sets the on-duty ratio for the on-off cycle of the switching element to a value based on the detection voltage of the voltage detection means at the start of driving of the magnetron, and the on-duty ratio thereof. It may be configured to control the length of the on-time of the switching element by the duty control according to.

Further, a step-down transformer for stepping down the voltage of the AC power supply and supplying it to electronic parts is provided, and the voltage detecting means detects the voltage of the AC power supply by detecting the voltage on the secondary side of the step-down transformer. Can be configured to.

[0010]

In the microwave oven of the above means, the voltage of the AC power source is detected by the voltage detecting means at the start of driving the magnetron. Then, the control means sets the length of the ON time of the switching element based on the detected voltage of the voltage detection means, and controls the ON / OFF of the switching element by the length of the ON time. For this reason, the voltage applied to the switching element and the induced voltage on the secondary side of the step-up transformer are controlled so as to be substantially constant despite the change in the AC power supply voltage. Therefore, even if the AC power supply voltage becomes high, an abnormally high voltage is not applied to the switching elements and the electronic components of the magnetron drive circuit, and even if the AC power supply voltage drops, the voltage induced on the secondary side of the step-up transformer. Does not decrease and the magnetron cathode warms up quickly.

When the magnetron starts oscillating, the current detecting means detects the current flowing through the magnetron. Then, the control means changes the length of the ON time of the switching element based on the detection current of the current detection means. As a result, even if the voltage of the AC power supply fluctuates, the power supplied to the magnetron and thus the output of the magnetron can be maintained constant.

Further, in the microwave oven of the above-mentioned means, the voltage detection means detects the voltage of the AC power supply by detecting the voltage on the secondary side of the step-down transformer, so that the voltage detection means and the control means are The constituent electronic components can be insulated from the AC power supply side.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1 showing the electrical configuration, an inverter device 2 as a frequency conversion means for converting the frequency of the AC power supply 1 into a high frequency is connected to a commercial AC power supply 1. The inverter device 2 includes a rectifying circuit 3, a smoothing circuit 4, and an inverter main circuit 5.

The rectifying circuit 3 is composed of diodes connected in bridge, and full-wave rectifies the AC voltage. The smoothing circuit 4 includes a choke coil 6 and a smoothing capacitor 7, and converts the voltage including the pulsating current that is full-wave rectified by the rectifying circuit 3 into a DC voltage. The inverter main circuit 5 includes a primary coil 8a of a step-up transformer 8 between both terminals of a smoothing capacitor 7.
And IGBT (Insulated Gate B) which is a switching element.
ipola Trasistor) 9 is connected in series, and primary coil 8a
The resonance capacitor 10 is connected in parallel with the
A flywheel diode 11 is connected in parallel with T9.

A magnetron drive circuit 12 is connected to the secondary side of the step-up transformer 8. This magnetron drive circuit 12 is one of the secondary coils 8b and 8c provided on the secondary side of the step-up transformer 8 and is one of the secondary coils 8b.
And a cathode of the magnetron 13 is connected to form a heater circuit, and the other secondary coil 8c is connected to an anode and a cathode of the magnetron 13 via a voltage doubler rectifier circuit 14 as a high voltage rectifier. In this case, the voltage doubler rectifier circuit 14 connects one terminal of the secondary coil 8c to the magnetron 1
3, the cathode of the magnetron 13 is connected to the other terminal of the secondary coil 8c via the diode 15 and the capacitor 16 having the polarities shown in the drawing, and the capacitor 16
And a diode 17 having the illustrated polarity is connected in parallel with the secondary coil 8c.

When the cathode is warmed, the magnetron 13 starts an oscillating operation and supplies microwaves to a heating chamber (not shown) to heat the food. This magnetron 1
3 is oscillated, the current flowing through the magnetron 13 is detected by the current detecting means, specifically, the current transformer 18 as the anode current detecting means for detecting the anode current of the magnetron 13, and the detection signal is detected by the main control circuit 19. Given to. The main control circuit 19 is mainly composed of a microcomputer, and receives a menu selection signal from a menu selection switch provided on an operation panel (not shown), a cooking time signal from a time setting switch, a start signal from a start switch, etc. The operation of the microwave oven is controlled based on the input signals and the program stored in advance.

Now, the IGBT of the inverter main circuit 5
9 is on / off controlled by a gate signal given from the inverter control circuit 20. And this IGBT
By turning on and off 9, the primary coil 8a of the step-up transformer 8
High-frequency current flows through. The magnitude of this high-frequency current is IG
The IGBT 9 changes depending on the length of the on-time, and the larger the length of the on-time of the IGBT 9, the larger the current flows.
As a result of the increase in the counter electromotive force generated in the secondary coil 8b,
The voltage induced in 8c is also high. And
The inverter control circuit 20 receives the on-time command signal from the main control circuit 19 as described later during the on / off control of the IGBT 9, and supplies the high-level signal to the gate of the IGBT 9 for the commanded on-time to turn on the IGBT 9. To operate.

Here, the inverter control circuit 20 has an I
The collector potential of the GBT 9 and the positive potential of the smoothing circuit 4 are applied. Then, the inverter control circuit 20
In the ON / OFF control of the IGBT 9, the both potentials are compared, and the timing at which the ON operation of the IGBT 9 is started is a time point at which a predetermined time has elapsed from the time point when the potential of the collector of the IGBT 9 becomes lower than the positive potential of the smoothing circuit 4. . As a result, the IGBT 9 is turned on in the state where no voltage is applied to the collector, so that the switching loss is reduced.

On the other hand, a primary coil 21a of a step-down transformer 21 is connected to the AC power source 1. The output of the secondary coil 21b of the step-down transformer 21 is rectified by a rectifier circuit (not shown) and given to electronic circuits such as the main control circuit 19 and the inverter control circuit 20.
Therefore, the step-down transformer 21 constitutes a direct current power supply for an electronic circuit together with a rectifier circuit (not shown). The voltage induced in the secondary coil 21b of the step-down transformer 21 is detected by the voltage detection circuit 22 as a voltage detection means. Since the voltage induced in the secondary coil 21b has a proportional relationship with the voltage of the AC power supply 1, the voltage detection circuit 22 detects the voltage of the AC power supply 1 from the voltage of the secondary coil 21b. It is given to the main control circuit 19.

The main control circuit 19 is arranged so that the AC power supply 1 detected by the voltage detection circuit 22 is detected when the magnetron 13 is driven.
The ON time of the IGBT 9 is set on the basis of the voltage of, and the set time is given to the inverter control circuit 20 as an ON time command signal, and during the oscillation operation of the magnetron 13, the anode current of the magnetron 13 detected by the current transformer 18 is set. The on-time of the IGBT 9 is set on the basis of the set time, and the set time is used as an on-time command signal for the inverter control circuit 2
Give to 0. Then, the inverter control circuit 20 gives the high level signal to the gate of the IGBT 9 only for the time corresponding to the ON time command signal.

Here, the voltage Vin of the AC power source 1, the IGBT
FIG. 2 shows the relationship between the ON time TON of 9 and the collector-emitter voltage VCE of the IGBT 9 (hereinafter, collector voltage VCE). From the figure, the collector voltage VCE increases as the voltage Vin of the AC power supply 1 increases and the ON time TON of the IGBT 9 increases.
It is understood that it will be higher. That is, when the ON time TON of the IGBT 9 is set to be short, for example, 11.5 μsec, the voltage Vin of the AC power supply 1 is 110V, 100V, 9V.
At 0V, the collector voltages VCE are 650V and 6V, respectively.
It becomes 00V and 550V. On the contrary, when the ON time TON of the IGBT 9 is set to be long, for example, 15 μsec, when the voltage Vin of the AC power supply 1 is 110V, 100V and 90V, the collector voltages VCE are 750V and 700V, respectively.
It becomes 650V.

On the other hand, the higher the collector voltage VCE is, the higher the voltage induced on the secondary side of the step-up transformer 8 is. Therefore, at the start of driving the magnetron 13, the cathode thereof is warmed faster, and the magnetron 13 oscillates faster. To start. However, if the collector voltage VCE is too high, I
The life of the GBT 9 and the voltage doubler rectifier circuit 14 is shortened. Therefore, assuming that the maximum allowable voltage of the collector voltage VCE that does not adversely affect the life of the electronic components of the IGBT 9 and the voltage doubler rectifier circuit 14 is 650 V, for example, when the voltage Vin of the AC power supply 1 is 110 V, 100 V, 90 V,
The ON time TON of the IGBT 9 is 11.5 μsec,
If set to 13 μsec and 15 μsec, the collector voltage VCE becomes the maximum allowable voltage of 650 V in either case,
The cathode of the magnetron 13 can be warmed quickly without shortening the life of the IGBT 9 and the voltage doubler rectifier circuit 14. FIG. 3 shows the relationship between the voltage Vin of the AC power supply 1 and the ON time TON of the IGBT 9 as described above.

The voltage Vin of the AC power source 1 and the IG
The relationship with the ON time TON of the BT9 is stored in the memory of the main control circuit 19, and the main control circuit 19 uses the voltage Vin of the AC power supply 1 detected by the power supply detection circuit 22 to turn on the IGBT9. TON is searched, and the searched time is given to the inverter control circuit 20 as an on-time command signal.

Next, the operation of the above configuration will be described. In order to perform heating and cooking, the user operates the menu selection switch to select the desired menu and sets the cooking time with the time setting switch, and the start switch (not shown).
When is operated, the main control circuit 19 sets an ON time according to the voltage Vin of the AC power supply 1 detected by the voltage detection circuit 22, and supplies this to the inverter control circuit 20 as an ON time command signal. On the other hand, the inverter control circuit 20
A high level signal and a low level signal are alternately applied to the gate of the IGBT 9 so that the ON time TON of the IGBT 9 becomes a time commanded by the main control circuit 19 to control the IGBT 9 on and off.

By the ON / OFF control of the IGBT 9, a high frequency current flows through the primary coil 8a of the step-up transformer 8 and a high frequency voltage is induced in the secondary coils 8b and 8c. Then, the voltage induced in the secondary coil 8b is applied to the cathode of the magnetron 13, whereby a current flows through the cathode and the cathode heats up. The voltage induced in the secondary coil 8c is rectified by the voltage doubler rectifier circuit 14 and boosted by a factor of two before being applied between the anode and cathode of the magnetron 13.

During the on / off control of the IGBT 9 as described above,
When the voltage Vin of the AC power supply 1 changes, the main control circuit 19 sets the ON time TON according to the voltage after the change in the same manner as described above, and the inverter control circuit 20 sets the AC power supply 1 after the change. The IGBT 9 is ON / OFF controlled so that the ON time TON corresponding to the voltage Vin of is. As a result, the collector voltage VCE of the IGBT 9 is controlled so as to always maintain a constant maximum allowable voltage (650V in this embodiment) despite the fluctuation of the voltage Vin of the AC power supply 1. Therefore,
The voltage induced on the secondary side of the step-up transformer 8 is also maintained at a constant high voltage that does not adversely affect the electronic components of the voltage doubler rectifier circuit 14, and as a result, the voltage Vin of the AC power supply 1 is increased.
Even if is low, the cathode of the magnetron 13 will warm up quickly. And when the cathode warms up, magnetron 1
3 starts an oscillating operation and supplies microwaves into a heating chamber (not shown) to heat food.

Now, when the magnetron 13 starts the oscillating operation, a current flows through the anode of the magnetron 13,
The current transformer 18 gives a current detection signal to the main control circuit 19. As a result, the main control circuit 19 causes the magnetron 13
Recognizes that the oscillator has started oscillating, and thereafter, the voltage detection circuit 2
Instead of the detection voltage of 2, the ON time is set according to the detection current of the current transformer 18 and given to the inverter control circuit 20. By such feedback control, the main control circuit 19 controls the on-time of the IGBT 9 so that the anode current of the magnetron 13 is constant, and as a result, the output of the magnetron 13 is controlled to be constant, and good food heating is achieved. The constant output control, which is a prerequisite for cooking, can be achieved.
In this case, the magnitude of the anode current can be changed according to the selected menu, for example, a high current value is set for a normal food warming menu and a low current value is set for a defrosting menu.

As described above, according to this embodiment, the ON time TON of the IGBT 9 is set at the start of driving the magnetron 13.
When the voltage Vin of the AC power supply 1 is lowered by the configuration in which the length is set according to the voltage Vin of the AC power supply 1,
Since the ON time TON of the IGBT 9 can be lengthened to prevent the voltage on the secondary side of the step-up transformer 8 from decreasing, the cathode of the magnetron 13 can be warmed quickly,
The time required for the magnetron 13 to oscillate can be shortened. Moreover, when the voltage Vin of the AC power supply 1 becomes high, the ON time TON of the IGBT 9 can be shortened to prevent the voltage on the secondary side of the step-up transformer 8 from rising abnormally. It is possible to prevent an abnormally high voltage from being applied to the electronic components of the voltage rectifier circuit 14 and adversely affecting its life.

Further, in this embodiment, the anode current is controlled to be constant after the magnetron 13 is oscillated, that is, the output of the magnetron 13 is controlled to be constant. The food can be cooked satisfactorily without the possibility of a period. Furthermore, in the present embodiment, the voltage detection circuit 22 detects the voltage of the AC power supply 1 by the voltage induced in the secondary coil 21b of the step-down transformer 21.
2 can be in a state of being insulated from the AC power supply 1 side. Therefore, unlike the case where the voltage dividing resistor circuit is connected to the AC power source 1 and the voltage of the AC power source 1 is detected by the voltage dividing resistor circuit, the voltage detecting circuit 22 is electrically connected to the voltage detecting circuit 22. There is no possibility that a high voltage on the side of the AC power supply 1 is applied to the main control circuit 19 and the inverter control circuit 20 that are present, and they can be protected from the high voltage.

The present invention is not limited to the above embodiment, but can be modified or expanded as follows. IGBT
The ON time length of the IGBT 9 is set such that the ON duty ratio for the ON / OFF cycle of the IGBT 9 is set to the ON duty ratio according to the voltage of the AC power supply 1, and the ON time length of the IGBT 9 is controlled by the ON duty ratio. Also good. Specifically, the on / off cycle of the IGBT 9 is set to 33 μse.
In the case of c, the on-duty ratio is 0.34 when the voltage of the AC power supply 1 is 110V, 0.35 when the voltage is 100V,
Set to 0.45 at 90V.

[0031]

As described above, in the microwave oven of the present invention, the length of the ON time of the switching element of the inverter device at the start of driving the magnetron depends on the voltage of the AC power source detected by the voltage detecting means. By setting the length to control the switching element on / off, a high voltage can be induced on the secondary side of the step-up transformer even when the voltage of the AC power supply is low, and the anode of the magnetron can be warmed quickly. Thus, the oscillation operation can be started quickly, and even in this case, the abnormal high voltage is not applied to the electronic components that form the switching element and the magnetron drive circuit.

In the microwave oven of the present invention, an anode current detecting means for detecting the anode current of the magnetron is provided, and after the oscillation of the magnetron is started, the length of the ON time of the switching element is changed according to the detection current of the anode current detecting means. The output of the magnetron can be controlled to be constant by adopting a configuration in which it is changed.

Further, in the microwave oven of the present invention, the voltage detection means detects the voltage of the AC power supply by detecting the voltage on the secondary side of the step-down transformer which steps down the voltage of the AC power supply and gives it to the electronic parts. As a result, the electronic component forming the voltage detecting means can be insulated from the AC power source side.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an IGBT on time, a collector voltage VCE, and an AC power supply voltage.

FIG. 3 AC power supply voltage and IG under constant collector voltage conditions
The figure which shows the relationship with the on time of BT

[Explanation of symbols]

1 is an AC power supply, 2 is an inverter device, 3 is a rectifier circuit, 5
Is an inverter main circuit, 8 is a step-up transformer, and 9 is an IGBT
(Switching element), 12 is a magnetron drive circuit,
13 is a magnetron, 14 is a voltage doubler rectifier circuit, 18 is a current transformer, 19 is a main control circuit (control means), 20 is an inverter control circuit (control means), 21 is a step-down transformer,
22 is a voltage detection circuit (voltage detection means).

Claims (4)

[Claims] 1. An inverter device for rectifying an AC power supply and switching its output by a switching element to supply the primary side of a step-up transformer; and a secondary side of the step-up transformer for heating a cathode of a magnetron, A magnetron drive circuit that rectifies a secondary output to drive a magnetron, a voltage detection unit that detects a voltage of the AC power supply, and a control unit that controls ON / OFF of the switching element, wherein the control unit is the A microwave oven for controlling ON / OFF of the switching element by setting a length of an ON time of the switching element to a value based on a detection voltage of the voltage detecting means at the start of driving of the magnetron. 2. A current detecting means for detecting a current flowing through the magnetron is provided, and the control means changes the length of the ON time of the switching element after the oscillation of the magnetron is started according to the detected current of the current detecting means. The microwave oven according to claim 1, wherein the microwave oven is configured as follows. 3. The control means sets the on-duty ratio for the on-off cycle of the switching element to a value based on the detection voltage of the voltage detection means at the start of driving of the magnetron, and the switching element is controlled by the duty control according to the on-duty ratio. The microwave oven according to claim 1 or 2, wherein the length of the on-time is controlled. 4. A step-down transformer for stepping down the voltage of the AC power supply and supplying it to electronic parts, wherein the voltage detecting means detects the voltage of the AC power supply by detecting the voltage on the secondary side of the step-down transformer. The microwave oven according to any one of claims 1 to 3, wherein the microwave oven is configured.