JPH04215287A - High frequency heating apparatus - Google Patents

Abstract

PURPOSE:To decrease the heating output for a magnetron after starting this magnetron, by increasing the operating frequency of an inverter circuit when the voltage of an alternating current being supplied has increased, so as to prevent an excessively high voltage from being applied to the magnetron before starting the same. CONSTITUTION:When a power transistor 5 is turned on or off in response to a control signal from a drive circuit 23, an inverter circuit INV generates a high-frequency voltage having a specified operating frequency. This high-frequency voltage is inputted into an elevation transformer 7 to cause a voltage to be induced to occur in each of a secondary winding 7b and a heater winding 7c. Thus, an electric current flows into a heater H of a magnetron 11 to heat this magnetron 11. If this heating temperature is equal to or higher than a specified temperature, a current transformer CTV detects a large magnitude of current and alters the switching period of the transistor 5 to thereby vary the operating frequency of the circuit INV. Then, the heating output for the magnetron 11 is controlled to a specified level of output, whereby the magnetron 11 is started at a specified temperature. Thereafter, an electric current corresponding to the operating frequency of the circuit INV flows for heating the magnetron.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency heating device that converts an alternating current voltage into a high-frequency voltage and drives a magnetron with the converted high-frequency voltage.

0002

[Prior Art] In order to reduce the size of a high-frequency heating device that uses a magnetron as a heating source, an inverter circuit converts an alternating current voltage into a high-frequency voltage, and the converted high-frequency voltage is boosted by a step-up transformer. The structure is such that the magnetron is driven by the By changing the operating frequency of the inverter circuit, the heating output of the magnetron can be continuously controlled. Such a high frequency heating device is disclosed in, for example, Japanese Patent Application Laid-Open No. 1-1445.
This high-frequency heating device is disclosed in Publication No. 93, and uses an input current control method that controls the input current of a rectifier circuit that rectifies the AC voltage to a predetermined value so that the heating output does not change even if the AC voltage fluctuates. is adopted.

By the way, when driving a magnetron,
Since no anode current flows between the anode and the heater until the magnetron is activated, almost no input current to the rectifier circuit flows until the magnetron is activated. Therefore, if the operating frequency of the inverter circuit is controlled to be lowered in order to control the input current to a predetermined value, a high voltage will be induced on the secondary side of the step-up transformer, and an excessive voltage will be generated between the anode of the magnetron that is not activated and the heater. There is a risk of damage due to voltage being applied. For this reason, the operating frequency of the inverter circuit is increased until the magnetron starts up to suppress the voltage applied between the magnetron's anode and the heater, a so-called soft start. Thereafter, when the magnetron is sufficiently heated by the magnetron's heater and the magnetron is activated, an input current corresponding to the operating frequency of the inverter circuit flows to the rectifier circuit. If this input current is equal to or greater than a predetermined value, it is determined that the magnetron has started, and control is enabled to lower the operating frequency of the inverter circuit so as to obtain a predetermined heating output, and the soft start is canceled.

0004

[Problem to be Solved by the Invention] However, when performing a soft start, if the operating frequency of the inverter circuit is kept constant at a high level, if the AC voltage increases during the period until the magnetron starts up, Accordingly, the voltage applied between the anode of the magnetron and the heater increases, and there is a problem that an excessive voltage is applied between the anode of the magnetron and the heater, which may cause damage. In view of this problem, the present invention provides the following steps before the magnetron starts up.
It is an object of the present invention to provide a high frequency heating device in which an excessive voltage is not applied to a magnetron even if the alternating current voltage fluctuates.

[0005]

[Means for Solving the Problems] A high-frequency heating device according to the present invention converts a DC voltage obtained by rectifying an AC voltage into a high-frequency voltage using an inverter circuit with a variable operating frequency, and drives a magnetron with the converted high-frequency voltage. The high-frequency heating device includes a voltage detection unit that detects that the AC voltage exceeds a predetermined value, and is configured to increase the operating frequency of the inverter circuit when it is detected that the AC voltage exceeds the predetermined value. It is characterized by

[0006]

[Operation] The magnetron is driven by the high frequency voltage generated by the inverter circuit. When it is detected that the AC voltage supplied to the inverter circuit with variable operating frequency has exceeded a predetermined value, the operating frequency of the inverter circuit increases. When the operating frequency of the inverter circuit increases, the voltage applied between the magnetron anode and the heater is suppressed. Therefore, even if the AC voltage fluctuates, an excessive voltage is not applied to the magnetron until the magnetron starts up.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below with reference to drawings showing embodiments thereof. FIG. 1 is a block diagram of main parts of a high-frequency heating device according to the present invention. Bridge 2 rectifies AC voltage from commercial power supply 1
The rectified voltage is full-wave rectified and smoothed by a smoothing circuit consisting of a choke coil 3 and a smoothing capacitor 4. A DC voltage, which is the charging voltage of the smoothing capacitor 4, is applied to the primary winding 7a of a step-up transformer 7, which has a resonant capacitor 6 connected in parallel, through a switching power transistor 5, which has a snubber diode SD connected in parallel. Thus, a boosted high frequency voltage is obtained at the secondary winding 7b.

The boosted high frequency voltage is rectified by a voltage doubler rectifier circuit consisting of a high voltage capacitor 8 and high voltage diodes 9 and 10, and the rectified high voltage is applied between the anode A of the magnetron 11 and the heater H. Further, the voltage of the heater winding 7c of the step-up transformer 7 is applied to the heater H of the magnetron 11. Anode A of magnetron 11 is grounded. The smoothing capacitor 4, the resonant capacitor 6, the primary winding 7a, and the power transistor 5 constitute an inverter circuit INV.

The circuit connecting the commercial power source 1 and the rectifier bridge 2 is provided with a current transformer CT for detecting the input current of the rectifier bridge 2, and a resistor R10 is connected between its output terminals. A series circuit of a diode D0 and a resistor R11, the anode of which is connected to the resistor R10, is connected in parallel to the resistor R10. The connection between the resistors R10 and R11 is grounded, and the connection between the diode D0 and the resistor R11 is connected to the positive input terminal + of the comparator CP.

The commercial power source 1 is one of the control transformers 20.
It is connected to the secondary winding 20a, and its secondary winding 20b
is connected to the AC input side of the rectifier bridge 21. The voltage at the positive DC output terminal 21a of the rectifier bridge 21 is the control power supply VCC. The negative side DC output terminal 21b of the rectifier bridge 21 is grounded. One end of the secondary winding 20b of the control transformer 20 is connected to a diode D1.
The anode of the diode D2 and the other end thereof are connected to the anode of the diode D2, and the cathodes of the diodes D1 and D2 are connected in common and connected to one end of the resistor R1. The other end of the resistor R1 is connected to the base of a transistor Q1 whose emitter is grounded via a Zener diode ZD whose cathode is connected to it, and the cathode of the Zener diode ZD is grounded via a resistor R2. . The collector of transistor Q1 is resistor R3
is connected to the input side of a pulse width modulation (PWM) circuit 22 through a resistor R8, and this input side is connected to a control power supply VCC through a resistor R7 and a capacitor C.
It is grounded through a parallel circuit with 1.

The negative input terminal of the comparator CP is connected to a resistor R5.
It is connected to the control power supply VCC via a resistor R6, and grounded via a resistor R6. The output terminal of comparator CP is resistor R4
It is connected to the input side of the pulse width modulation circuit 22 via. The output side of the pulse width modulation circuit 22 is connected to the input side of a drive circuit 23 for driving the power transistor 5, and the output side thereof is connected to the base of the power transistor 5. The pulse width modulation circuit 22 is configured to generate and output a pulse with a high (low) frequency when the level is low (high) in accordance with the level of the input voltage. The drive circuit 23 is a pulse width modulation circuit 22
The pulse input from the power transistor 5 is amplified to a control signal level capable of driving the power transistor 5.

[0012] Next, the operation of the high frequency heating device constructed as described above will be described in detail. AC voltage from a commercial power source 1 is rectified by a rectifier bridge 2, and the rectified voltage is applied to an inverter circuit INV via a choke coil 3. When the power transistor 5 is caused to perform a switching operation by a control signal from the drive circuit 23, the inverter circuit INV generates a high frequency voltage at a predetermined operating frequency. This high frequency voltage is input to the primary winding 7a of the step-up transformer 7. As a result, the secondary winding 7b of the step-up transformer 7 and the heater winding 7
A voltage is induced at c. A current flows through the heater H of the magnetron 11 due to the voltage of the heater winding 7c, and the magnetron 11 starts to be heated. On the other hand, the voltage induced in the secondary winding 7b is voltage doubled and rectified by the high voltage diodes 9 and 10, and the rectified voltage is applied between the anode A of the magnetron 11 and the heater H. Here, if the heating temperature of the magnetron 11 has not reached the predetermined temperature, no anode current flows through the magnetron 11, and therefore the output of the current transformer CT is extremely small.

Thereafter, when the heating temperature of the magnetron 11 reaches a predetermined value, the magnetron 11 is activated. Then, an anode current flows between the anode A of the magnetron 11 and the heater in accordance with the operating frequency of the inverter circuit. Then, the output of the current transformer CT increases. After the magnetron 11 is activated in this manner, even if the voltage of the secondary winding 7b of the step-up transformer 7 is increased, an excessive voltage is not applied to the magnetron 11, thereby changing the switching period of the power transistor 5. By changing the operating frequency of the inverter circuit, the heating output of the magnetron 11 can be controlled to a predetermined heating output.
The heating operation will be performed by

Now, as mentioned above, the input current of the rectifying bridge 2 before and after starting up the magnetron 11 is transferred to the current transformer CT.
A voltage corresponding to the detected output is generated across the resistor R11 and input to the positive input terminal + of the comparator CP. The negative input terminal of the comparator CP is connected to the control power supply VCC.
A reference voltage obtained by dividing the voltage between resistors R5 and R6 is input, and comparator CP compares both input voltages. If the output of the current transformer CT is small before the magnetron 11 is started, the output voltage of the comparator CP is low, which causes the charging voltage level of the capacitor C1 to become low. The charging voltage is input to the pulse width modulation circuit 22, which generates a pulse with a small duty, that is, a pulse with a high frequency, and inputs it to the drive circuit 23. The drive circuit 23 amplifies the signal to a signal level capable of driving the power transistor 5 and supplies it to the pulse transistor 5 to control switching of the power transistor 5. As a result, the operating frequency of the inverter circuit INV increases, and when the magnetron 11 is not activated, an excessive voltage applied to the magnetron 11 is suppressed to protect the magnetron 11 from overvoltage.

Thereafter, when the magnetron 11 is activated, the input current of the rectifier bridge 2 increases in accordance with the operating frequency of the inverter circuit INV, as described above, and the output of the current transformer CT increases. Then, the output voltage of the comparator CP becomes high, the charging voltage level of the capacitor C1 becomes high, and the pulse width modulation circuit 22 generates a pulse with a large duty, that is, a low frequency. The switching of the power transistor 5 is controlled, the operating frequency of the inverter circuit is lowered, the voltage of the secondary winding 7b of the step-up transformer 7 can be increased, and the soft start of the magnetron 11 is canceled.

On the other hand, the AC voltage of the commercial power source 1 is input to the control transformer 20, and the secondary voltage is applied to the diode D.
1 and D2 for full wave rectification. When this full-wave rectified voltage exceeds the Zener voltage of the Zener diode ZD, that is, when the AC voltage fluctuates and exceeds a predetermined value determined by the Zener voltage, the transistor Q1 is turned on. Then, the charging voltage of the capacitor C1 is discharged through the transistor Q1, the charging voltage level of the capacitor C1 decreases, and the pulse width modulation circuit 22 outputs high frequency pulses in the same way as when the output voltage of the comparator CP decreases. As a result, the switching of the power transistor 5 is controlled and the operating frequency of the inverter circuit is increased. Therefore, if the AC voltage increases during the period until the magnetron 11 starts up, the increase in the voltage applied to the magnetron 11 is suppressed accordingly.

Further, after the magnetron 11 is activated, an increase in the heating output of the magnetron 11 is suppressed as the AC voltage increases. Conversely, when the AC voltage decreases and the voltage of the secondary winding of the control transformer 20 becomes less than the Zener voltage, the transistor Q1 is turned off and the charging voltage level of the capacitor C1 is set to a high state. keep. As a result, the pulse width modulation circuit 22 does not generate pulses with a high frequency, but generates pulses with a frequency corresponding to the output of the current transformer CT to control the operating frequency of the inverter circuit INV.

Therefore, even if the AC voltage increases during the period until the magnetron 11 is activated, an excessive voltage will not be applied between the anode A and the heater H of the magnetron 11. Furthermore, even if the AC voltage increases after the magnetron 11 is activated, the heating output of the magnetron 11 does not increase. It goes without saying that this embodiment is not limited to microwave ovens, but can be widely applied to other heating devices using magnetrons.

[0019]

As described in detail above, according to the present invention, even if the alternating current voltage increases during the period until the magnetron is activated, an excessive voltage is not applied to the magnetron in response to the increase. Furthermore, even if the AC voltage increases after the magnetron is started, the heating output of the magnetron does not increase. Therefore, the present invention has an excellent effect of significantly improving the reliability of the high-frequency heating device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of main parts of a high-frequency heating device according to the present invention.

[Explanation of symbols]

1 Commercial power supply 3 Choke coil 5
Power transistor 6 Resonant capacitor 7 Step-up transformer 11 Magnetron 20
Control transformer 22 Pulse width modulation circuit CT Current transformer D1, D2 Diode ZD Zener diode Q1
Transistor CP comparator

Claims (1)

[Claims] 1. A high-frequency heating device that converts a DC voltage obtained by rectifying an AC voltage into a high-frequency voltage using an inverter circuit with a variable operating frequency, and drives a magnetron with the converted high-frequency voltage, wherein the AC voltage exceeds a predetermined value. 1. A high-frequency heating device, comprising: a voltage detection section that detects that an alternating current voltage exceeds a predetermined value;