JPH03269992A - Electronic oven - Google Patents

Abstract

PURPOSE:To prevent generation of an abnormal high voltage of the secondary winding of a boosting transformer immediately after the driving of a magnetron by varying the switching frequency of the switching element of a switch control circuit, in relation to the output voltage of a voltage generating circuit. CONSTITUTION:When an AC power source 1 is connected, the voltage rectified by a rectifier bridge 2 charges a smoothing capacitor 4. And the secondary voltage of a transformer 21 for control is rectified by diodes D1 and D2, compared with the voltage of a control power source Vc in a comparator A1, and when the voltage of the power source 1 is made at the voltage Vc or higher, the voltage at the input side of a pulse width modulating circuit 30 is reduced. And as the input side voltage of the pulse width modulating circuit 30 is reduced, the switching frequency is increased, and the current of the primary winding 7a of a boosting transformer 7 is suppressed. Consequently, an abnormal high voltage induced in the secondary winding 2b can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a microwave oven that converts an alternating current voltage into a high frequency voltage and drives a magnetron based on the converted high frequency voltage.

[Conventional technology]

In order to make them more compact, recent microwave ovens have a structure in which switching elements are operated by switching to convert the AC voltage of the commercial power supply into high-frequency voltage, and the magnetron is driven in relation to the high-frequency voltage. It becomes.

FIG. 2 is a circuit diagram of the main parts of the microwave oven.

The AC type a1 voltage is full-wave rectified by a rectifier bridge 2, and the rectified voltage is smoothed by a smoothing circuit including a choke coil 3 and a smoothing capacitor 4. Then, the DC voltage that is the charging voltage of the smoothing capacitor 4 is applied to the primary 5VA7a of the step-up transformer 7 to which the resonant capacitor 6 is connected in parallel via the switching power transistor 5, and the secondary winding &%7b of the step-up transformer 7 is applied. A boosted high-frequency high voltage is obtained, and this high-frequency voltage is rectified by a voltage doubler rectifier circuit consisting of a capacitor 8 and a high-voltage diode 9.10, and the high voltage is applied between the heater H of the magnetron 11 and the anode A. It drives the magnetron 11.

3(a) and (bl) are the waveforms of the primary current and secondary current of the step-up transformer 6. As is clear from this figure, G
When the on period of the power transistor 5 is long, that is, when the switching frequency is low, the current in the secondary winding 7b of the step-up transformer 7, that is, the anode current of the magnetron 10 becomes large, and the microwave output increases.

By the way, immediately after the magnetron 11 is driven by applying a high voltage, the magnetron 11 does not oscillate because the magnetron 11 is not sufficiently heated by the heater. Therefore,
Step-up transformer 7-2 cheat sheet! 7b becomes open circuit, and the magnetron 11 and high voltage diode 9. A voltage that is abnormally higher than the voltage when the magnetron 11 is operating in oscillation is applied to lO.

In order to reduce such an abnormal high voltage that occurs immediately after the magnetron 11 is driven, Japanese Patent Application Laid-Open No. 62-66595 states that within a predetermined time immediately after the magnetron is driven, the high frequency output is reduced. A so-called soft start operation is performed in which the magnetron is operated at a constant frequency higher than the frequency at which it is operating in oscillation, and then switched to the frequency at which full power is supplied.

[Problem to be solved by the invention]

However, as described above, even if the switching frequency of the power transistor is fixed during soft start, as the primary winding voltage increases, the secondary winding voltage of the step-up transformer increases in proportion to it. Therefore, the insulation strength of the circuit connected to the secondary winding of the step-up transformer must be increased in consideration of the voltage, which increases the cost of the microwave oven and also makes it impossible to reduce its weight.

In view of this problem, the present invention aims to provide a microwave oven in which the voltage of the secondary winding of the step-up transformer does not increase even if the voltage of the primary winding of the step-up transformer increases immediately after the magnetron is driven. .

[Means to solve the problem]

The microwave oven according to the present invention converts a DC voltage obtained by rectifying the voltage of an AC power supply into a high-frequency voltage using a switching element that performs a switching operation, applies the converted high-frequency voltage to the primary winding of a step-up transformer, and applies the converted high-frequency voltage to the primary winding of a step-up transformer. In a microwave oven that drives a magnetron in relation to a high voltage generated in a line, a voltage generation circuit that generates a voltage related to the voltage of the AC power source, and switching of a switch element in relation to the output voltage of the voltage generation circuit. and a switch control circuit that controls a frequency, and immediately after driving the magnetron, the switching frequency is changed in relation to the output voltage of the voltage generating circuit.

[Operation] The voltage of the AC power source is converted into a high frequency voltage by operating the switching element. When a high frequency voltage is applied to the primary winding of the step-up transformer, a high frequency high voltage is generated in the secondary winding, thereby driving the magnetron. Immediately after the magnetron is driven, the switch control circuit changes the switching frequency of the switch element in relation to the output voltage of the voltage generation circuit.

As a result, the primary current of the step-up transformer is suppressed immediately after the magnetron is driven, and even if the secondary winding voltage becomes high, the
No abnormal high voltage is generated in the next winding.

〔Example〕

The present invention will be described in detail below with reference to drawings showing embodiments thereof.

FIG. 1 is a circuit diagram of a main part of a microwave oven according to the present invention.

AC power a1, which is a commercial power supply, is connected to the AC input side of the rectifier bridge 2, and a series circuit of a choke coil 3 and a smoothing capacitor 4 is connected to the DC output side. A series circuit of a primary winding 7a of a step-up transformer 7 to which a resonant capacitor 6 is connected in parallel and a transistor 5 serving as a switching element is connected in parallel to the smoothing capacitor 4. The secondary winding 7b of the step-up transformer 7 is connected to a series circuit consisting of a high-voltage capacitor 8 and a diode 9 whose anode is connected to the high-voltage capacitor 8. The cathode of the diode 9 is connected to the anode A of the magnetron 11. and grounded. Heater winding of step-up transformer 11! 7c is connected to the heater H of the magnetron 11, and a diode 10 whose anode is connected to the heater H is interposed between one end of the heater H and the anode of the diode 9.

Further, in the middle of the circuit connecting the cathode of the diode 9 and the anode A of the magnetron 11, there is a secondary winding 7.
A current transformer 20 is provided to detect the current of b.

The AC power supply l is connected to the primary side 21a of the control transformer 21, and its secondary side 21b is connected between the AC input terminals 22a and 22b of the rectifying bridge 22.

A capacitor C1 is interposed between the DC output terminals 22c and 22d of the rectifying bridge 22, and a series circuit of a transistor T□ and a capacitor C2 is connected in parallel to the capacitor C. A resistor R1 is interposed between the collector and the moose of the transistor T, and its base is connected to the DC output terminal 22d and grounded via a Zener diode ZD connected to the cathode. The voltage of the control power supply VC is output from the emitter of the transistor T1.

At each end of the secondary side 21b of the two control transformers,
The anodes of the diodes D, D2 are connected separately, and the common connection part where the cathodes of the diodes Dr and Dz are connected in common is connected to one end of the series circuit of the resistors R1 and R2. The other end of the circuit is grounded.

The connection between the resistors R1 and R2 is connected to the negative input terminal - of the comparator A. The positive input terminal of the comparator A+ is connected to the connection between resistors R3 and R4, and one end of the series circuit of resistors R3 and R4 is connected to the control power supply Vo.
is connected and the other end is grounded.

and diodes D,,D2, and resistors R,,R,.

R3, R, and comparator A1 constitute a voltage generation circuit that outputs a rectified voltage related to the voltage of AC power supply 1.

A resistor R6 is interposed between the two ends of the coil 1 of the current transformer 20, and a series circuit of a diode D3 and a resistor R7 having an anode connected thereto is connected in parallel to the resistor R6, A capacitor C3 is connected in parallel to the resistor R7, and a common connection between the resistors R6, R7 and the capacitor C3 is grounded.

The cathode of the diode D3 is connected to the positive input terminal + of the comparator A2. The negative input terminal - of the comparator A2 is connected to the connection between the resistors R8 and R9, the control type RVc is connected to one end of the series circuit of the resistors R11 and R9, and the other end is grounded.

The output of the comparator A2 and the output of the comparator A are applied to the input terminal of a pulse width modulation circuit 30, which is a switch control circuit, through a resistor R5° and a resistor R1, respectively. The input side of the pulse width modulation circuit 30 is a capacitor C4.
It is grounded via a resistor R8, and connected to a control power source V via a resistor R8. This pulse width modulation circuit 30 is configured to output a rectangular wave pulse, and the frequency of the rectangular wave pulse is changed in relation to the voltage applied to its input terminal. That is, as the voltage at the input terminal decreases, the frequency of the rectangular wave pulse increases. The output of the pulse width modulation circuit 30 is applied to a drive circuit 31 that drives the transistor 5, and the output of the drive circuit 31 is applied to the base of the transistor 5.

Next, the operation of the microwave oven configured as described above will be explained.

Now, when the AC power source 1 is connected, the voltage rectified by the rectifier bridge 2 is smoothed by the choke coil 3 and the smoothing capacitor 4, and the smoothing capacitor 4 is charged.

On the other hand, the voltage of the AC power source 1 is applied to the rectifier bridge 22 via the control transformer 21, and the rectified voltage charges the capacitor C1 and the transistor T.

is turned on by the base voltage given by the Zener diode ZD, and the control power supply ■. voltage is generated.

Further, the secondary voltage of the control transformer 21 is connected to the diode D,
, D2, and provides a voltage related to the voltage of AC type #1 to the negative input terminal - of comparator A1. As a result, comparator A compares that voltage with the voltage of control type #vc, and the voltage of AC power supply 1 becomes control power supply ■. When the voltage exceeds , the voltage at the output terminal becomes negative, and the input voltage of the pulse width modulation circuit 30 decreases. That is, the input side voltage of the pulse width modulation circuit 30 changes in relation to the voltage fluctuation of the AC power supply 1.

On the other hand, when the current transformer 20 does not detect a current, no voltage is applied to the positive input terminal + of the comparator A2, the voltage at the output terminal of the comparator A2 becomes negative, and the input terminal voltage of the pulse width modulation circuit 30 decreases. Furthermore, when a current is detected, the negative level changes depending on the current. In other words, the input terminal voltage of the pulse width modulation circuit 30 changes in relation to the current change in the secondary winding 7b of the step-up transformer 7.

As a result, the pulse width modulation circuit 30 generates a rectangular wave pulse having a frequency related to the input terminal voltage and applies it to the drive circuit 31, and the drive circuit 31 generates a rectangular wave pulse having a frequency related to the input terminal voltage. A drive signal for driving transistor 5 is applied to the transistor 5. Then, the transistor 5 performs a switching operation at the frequency of the rectangular wave pulse.

When the transistor 5 is turned on, the voltage of the smoothing capacitor 4 is applied to the primary winding 7a of the step-up transformer 7,
The switching operation of transistor 5 causes the primary winding vA to
A high frequency voltage is applied to the secondary winding 7a, a high voltage which is the boosted primary winding voltage is induced in the secondary winding 7b, and a voltage obtained by rectifying the high voltage is applied to the magnetron 11.
1 will be driven.

Note that the switching frequency of the transistor 5 changes in relation to the input side voltage of the pulse width modulation circuit 30, that is, as the input side voltage of the pulse width modulation circuit 30 decreases, the switching frequency increases, and the switching frequency of the step-up transformer 7 increases. 1
The current in the next winding 7a is suppressed. In addition, abnormal high voltage induced in the secondary winding 7b due to the increased voltage of the primary winding 7a is suppressed.

In this way, when the driven magnetron 11 is sufficiently heated by its heater H and performs an oscillating operation, current begins to flow through the secondary winding 7b of the step-up transformer 7, which is detected by the current transformer 20. The negative voltage at the output terminal of comparator A2 changes in the direction of positive voltage, and the pulse width modulation circuit 3
0 input side voltage increases. As a result, the frequency of the square wave pulse decreases, and the switching frequency of transistor 5 decreases. Therefore, the current in the primary winding 7a increases, and the voltage induced in the secondary winding 7b increases, causing the magnetron 11 to oscillate at full output corresponding to the voltage of the AC power supply 1, thereby performing a soft start operation. finish.

As mentioned above, when the magnetron 11 does not oscillate and therefore the current transformer 20 does not detect current, the switching frequency of the transistor 5 becomes high, and the voltage induced in the secondary winding 7b of the step-up transformer 7 increases. However, in such a state, when the voltage of the AC power supply 1 increases, the input terminal voltage of the pulse width modulation circuit 30 further decreases, and the switching frequency of the transistor 5 further increases. Therefore, even if the voltage of the AC power source 1 becomes high immediately after the magnetron 11 is driven, it is possible to prevent the voltage of the secondary winding vA7b of the step-up transformer 7 from becoming high due to this.

Therefore, it is no longer necessary for the circuit connected to the secondary winding 7b to have an insulation strength that can withstand a high voltage higher than the high voltage that should be applied to the magnetron 11, and the cost and weight of the microwave oven can be reduced.

〔Effect of the invention〕

As detailed above, according to the present invention, the switching frequency of the switch element changes in relation to the voltage of the AC power supply, so that the secondary winding of the step-up transformer is in an open state immediately after the magnetron is driven. Even if the voltage of the AC power source becomes high, an abnormally high voltage will not be induced in the secondary winding.

As a result, the insulation strength of the circuit connected to the secondary winding of the step-up transformer can be lowered, resulting in an excellent effect of reducing the cost and weight of the microwave oven.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of the main part of the microwave oven according to the present invention, Fig. 2 is a circuit diagram of the main part of a conventional microwave oven, and Fig. 3 is a waveform diagram of the primary and secondary winding vA current of the step-up transformer. be. ■... AC power supply 2... Rectifier bridge 4... Smoothing capacitor 5... Transistor 7... Step-up transformer 11... Magnetron 20... Current transformer 2
1... Control transformer A,, AZ... Comparator 30... Pulse width modulation circuit 31... Drive circuit VC... At the time of control power supply Applicant

Claims (1)

[Claims] 1. A DC voltage obtained by rectifying the voltage of an AC power supply is converted into a high-frequency voltage by a switching element that operates, and the converted high-frequency voltage is applied to the primary winding of a step-up transformer, and the secondary winding is In a microwave oven that drives a magnetron in relation to a high voltage generated in a line, a voltage generation circuit generates a voltage related to the voltage of the AC power supply, and a switching element is switched in relation to the output voltage of the voltage generation circuit. 1. A microwave oven comprising: a switch control circuit for controlling a frequency; and immediately after driving the magnetron, the switching frequency is changed in relation to an output voltage of a voltage generating circuit.