JPH06140145A - High frequency heating device - Google Patents

Abstract

PURPOSE:To stably control a high frequency heating device when output radio wave is minimized, or the load just after start is unstable. CONSTITUTION:A high frequency heating device has an inverter circuit formed of a resonance circuit consisting of a resonance capacitor 12 and a booster transformer 9, and a semiconductor switch element 10; a magnetron 4; a detecting circuit 19 for detecting that the voltage of the resonance circuit is less than a standard voltage; and a control part 6 for driving the semiconductor switch element 10 on the basis of the signal of the detecting circuit 19. Further, a capacitor 16 for interrupting the DC portion of the resonance voltage is provided to supply the resonance voltage to the detecting circuit 19 through the capacitor 19.

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 such as a microwave oven.

[0002]

2. Description of the Related Art A conventional high-frequency heating device will be described with reference to FIG. The commercial power supply 1 is converted into direct current by a diode bridge 7, and a resonance circuit including a resonance capacitor 11 and a step-up transformer 9, an inverter circuit including a semiconductor switch element 10 and a diode 12 connected in antiparallel thereto, and a step-up transformer 9 are provided. The output voltage of the semiconductor switch element 1 is rectified by a voltage doubler rectifier circuit composed of a high voltage diode and a high voltage capacitor to supply power to the magnetron 4.
The high frequency output is adjusted by controlling 0 on / off.

Next, a method of controlling the semiconductor switch element 10 will be described with reference to FIG. The voltage of the commercial power supply is rectified by the diode bridge 7 and the smoothing capacitor 8, and the rectified DC voltage is divided by a resistor (hereinafter referred to as signal 1) and the resonance voltage is divided by a resistor (hereinafter referred to as signal). Call 2). This signal 1, 2
Is input to the zero-cross detection circuit 19, and the signal 2 [(FIG. 4 (b)-(a)] is larger than the signal 1 [FIG. 4 (b)-(b)] as shown in FIG. 4 (b). Therefore, when the signal 2 becomes smaller than the signal 1 (hereinafter, it is referred to as detecting a zero cross), the zero cross detecting circuit 19 generates a pulsed voltage.The sawtooth waveform of the sawtooth wave generating circuit 20 is generated by the pulse voltage. Voltage of Fig. 4 (c)-
It begins to descend like (a). Further, the sawtooth wave generating circuit 20 is configured such that the sawtooth wave starts to rise again when it falls to a certain voltage as shown in FIGS. 4C to 4B. Further, the reference voltage generated by the reference voltage generation circuit 21 (FIGS. 4 (c)-(a)) and this sawtooth wave are compared by the comparison circuit 22, and when the sawtooth wave is lower than the reference voltage, FIG. 4 (d). A signal is given to the drive circuit 23 as shown in FIG. The drive circuit 23 gives an ON signal to the semiconductor switch element 10 by this signal. By repeating this operation, the semiconductor switch element is repeatedly turned on and off, and the inverter circuit supplies power to the magnetron through the voltage doubler rectifier circuit.

When the reference voltage set by the reference voltage generating circuit 21 is changed, the sawtooth wave generating circuit 20 is changed.
The time during which the reference voltage is higher than the sawtooth wave generated changes due to the change in the width of the ON signal to the semiconductor switch element 10. As a result, the magnitude of the electric power output from the inverter circuit changes, and the magnitude of the output radio wave can be adjusted.

[0005]

However, in the conventional configuration as described above, even when the output power is reduced, the zero-cross detection circuit detects the zero-cross and stably supplies the ON / OFF signal to the semiconductor switch element, In order for the high-frequency heating device to operate stably, it is necessary to lower the coupling coefficient of the step-up transformer to some extent. However, if the booster transformer is low-coupled as described above, the regenerative current in the resonance period becomes larger than that in the case of high coupling as shown in FIG.
It becomes necessary to increase the exciting current flowing to the secondary side.
Therefore, the loss in the step-up transformer is increased, and the voltage stress, the current stress and the loss of the semiconductor switch element are increased. Although it is conceivable to increase the coupling of the transformer to solve this problem, as shown in Fig. 6 (b) when the output power is reduced or the load is unstable immediately after the high frequency heating device is started. Signal 2 is signal 1
There is a problem that it becomes impossible to reliably detect the zero-cross because the state does not change from the larger state to the smaller state, and it becomes difficult to perform stable control.

The present invention is intended to solve the above problems,
By increasing the coupling of the step-up transformer, reducing the output power, and even when the load immediately after start-up is unstable, the zero-cross is reliably detected and the on / off signal to the semiconductor switch element is stably provided. The purpose is to ensure the stability of control.

[0007]

A voltage of a resonance circuit composed of a resonance capacitor and a step-up transformer and a semiconductor switch element, a magnetron driven by an output of the step-up transformer, and a voltage of the resonance circuit is equal to or lower than a reference voltage. And a control unit for driving the semiconductor switch element based on a signal of the detection circuit, and a capacitor for cutting off the direct current component of the resonance voltage is provided, and the capacitor is provided via the capacitor. The resonance voltage is supplied to the detection circuit.

[0008]

The present invention has the following functions due to the above configuration.

An inverter circuit composed of a resonance circuit composed of a resonance capacitor and a step-up transformer and a semiconductor switch element, a magnetron, a detection circuit, a control unit, and a capacitor for cutting off the direct current component of the resonance voltage are provided. Since the resonance voltage is supplied to the detection circuit via the capacitor, this capacitor is charged and discharged by the resonance voltage, so the input voltage of the detection circuit via the capacitor is positive until the resonance voltage reaches the maximum value. It shows a value and shows a negative value from the maximum value to the minimum value. Since the reference voltage has a positive value, the voltage input to the detection circuit via the capacitor surely changes from the state higher than the reference voltage to the state lower than the reference voltage. This ensures that the detection circuit can generate a signal.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS.

FIG. 1 is a circuit diagram showing the entire structure. 1
Is an external power source such as a commercial power source or a generator, and supplies power to the high frequency heating device 2 from the outside. High frequency heating device 2
Is composed of a power conversion unit 3 that converts the power supplied from the external power supply 1 and a magnetron 4 that receives the power converted by the power conversion unit 3 and generates microwaves.
In addition, the power conversion unit 3 includes a diode bridge 7, a smoothing capacitor 8, a step-up transformer 9, a semiconductor switch element 10, a diode 11 connected in antiparallel to the semiconductor switch element 10, a resonant capacitor 12, and a voltage doubler rectifier circuit. It comprises a converter 5 and a control unit 6 for driving the semiconductor switching device 10. The control unit 6 cuts off the direct current component of the resonance voltage applied to both ends of the resistors 14 and 15 for dividing the voltage of the smoothing capacitor 8 and the semiconductor switching device 10. Capacitor 16 and resistors 17, 18 and zero-cross detection circuit 1
9, sawtooth wave generation circuit 20, reference voltage generation circuit 21,
It is composed of a comparison circuit 22 and a drive circuit 23.

Next, the operation will be described with reference to FIG. Figure 2
(A) is a resonance voltage waveform applied to both ends of the switch element 10, (b) is a voltage waveform input to the zero-cross detection circuit, and (c) is an output of the sawtooth wave generation circuit 20 and a reference voltage generation. The output waveform of the circuit 21 is the same as (d).
Is the output waveform of the comparator 22. In (b), when the resonance voltage divided by the resistors 17 and 18 via the capacitor 16 changes from a state larger than the voltage divided by the resistors 14 and 15 to a state smaller, the zero-cross detection circuit generates a pulsed voltage. To do. When the pulsed voltage rises, the sawtooth wave generated by the sawtooth wave generation circuit 20 begins to fall as shown in FIGS. Further, the sawtooth wave generation circuit 20 is configured so that when the sawtooth wave reaches a certain constant value, the sawtooth wave starts rising again. This sawtooth wave (FIG. 2 (c)-(a)) and the reference voltage set by the reference voltage generation circuit 21 (FIG.
(C)-(b)) is compared by the comparator 22, and FIG.
The comparator 22 produces a signal when the sawtooth wave is below the reference voltage as seen in (d). Then, the drive circuit 23 gives an ON signal to the semiconductor switch element 10 by this signal. By repeating this operation, the high frequency heating device 2 generates a microwave.

Here, the resistor 1 is connected via the capacitor 16.
The signal divided by 7 and 18 and input to the zero-cross detection circuit 19 has a waveform obtained by differentiating the resonance voltage applied to both ends of the semiconductor switch element as shown in FIG. 2B, so that the resonance voltage has a maximum value. It shows a positive value until it reaches, and shows a negative value while it shifts from the maximum value to the minimum value. For this reason,
This signal reliably goes from a state larger than the voltage divided by the resistors 14 and 15 to a state smaller than the voltage. Therefore, the zero-cross detection circuit can output the signal reliably, and when the coupling of the step-up transformer is increased, the output power is reduced, and the control of the semiconductor switch element is stable even when the load immediately after start-up is unstable. It can be done.

Further, by adjusting the values of the capacitor 16 and the resistors 17 and 18, the time constant determined by these values can be adjusted to adjust the phase advance of the input signal of the zero-cross detection circuit with respect to the resonance voltage. . This phase is adjusted to compensate for the delay time of the output response in the control circuit (for example, the delay time from the output of the comparison circuit 22 to the drive circuit 23 giving an ON signal to the semiconductor switch element 10). be able to. For this reason,
Since the control circuit can be configured with elements that have a delay time in the output response, it is not necessary to reconfigure the control circuit with elements corresponding to the high frequency when the switching frequency is increased, and the high frequency can be obtained at low cost. It has an effect that a heating device can be provided.

[0015]

As described above, the high frequency heating apparatus of the present invention has the following effects.

It has an inverter circuit composed of a resonance capacitor composed of a resonance capacitor and a step-up transformer, and a semiconductor switch element, a magnetron driven by the output of the step-up transformer, a detection circuit and a control section, and cuts off the direct current component of the resonance voltage. In the case where the coupling coefficient of the step-up transformer is increased by providing a capacitor for supplying the resonance voltage to the detection circuit via this capacitor,
Even when the output power is reduced or the load immediately after startup is unstable, the zero cross can be detected with certainty, and the ON / OFF signal can be stably given to the semiconductor switch element. This has the effect of stabilizing the control of the inverter.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a high-frequency heating device according to an embodiment of the present invention.

FIG. 2A is a diagram showing a resonance voltage applied to both ends of a semiconductor switching element in the high-frequency heating device, and FIG. 2B is a diagram showing a voltage waveform input to a zero-cross detection circuit in the high-frequency heating device. The figure which shows the voltage waveform input into the comparator in a device. (D) The figure which shows the voltage waveform output from the comparator in the same high frequency heating device.

FIG. 3 is a circuit diagram of a conventional high frequency heating device.

4A is a diagram showing a resonance voltage waveform applied to both ends of a semiconductor switch element, FIG. 4B is a diagram showing a voltage waveform input to a zero-cross detection circuit, and FIG. 4C is a diagram showing a voltage waveform input to a comparator. d) Diagram showing the voltage waveform output from the comparator

5A is a diagram showing a resonance voltage applied to both ends of the semiconductor switching device when the coupling of the boosting transformer is high. FIG. 5B is a diagram showing a current flowing through the semiconductor switching device. FIG. 5C is a primary winding of the boosting transformer. Figure showing the exciting current flowing through the line (d) Figure showing the resonance voltage applied across the semiconductor switching element when the coupling of the step-up transformer is low (e) Figure showing the current flowing through the semiconductor switching element (f) The step-up transformer Diagram showing the exciting current flowing through the primary winding of

FIG. 6A is a diagram showing a resonance voltage applied to both ends of the semiconductor switch element when the zero cross of the high frequency heating device cannot be detected; FIG. 6B is a diagram showing a signal input to the zero cross detection circuit.

[Explanation of symbols]

 2 high-frequency heating device 4 magnetron 6 control unit 9 step-up transformer 10 semiconductor switching element 11 resonant capacitor 16 capacitor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuji Nakabayashi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Makoto Shibuya, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (1)

[Claims] 1. An inverter circuit comprising a resonance circuit composed of a resonance capacitor and a step-up transformer, and a semiconductor switch element, a magnetron driven by the output of the step-up transformer, and a controller for driving the semiconductor switch element. A configuration in which the control unit is provided with a detection circuit for detecting that the resonance voltage has become equal to or lower than a reference voltage and a capacitor for cutting off a direct current component of the resonance voltage, and the resonance voltage is supplied to the detection circuit via the capacitor. High frequency heating device.