JPH04208075A - High frequency heater - Google Patents

Abstract

PURPOSE:To reduce damage of an element due to switching at the zero points of a voltage Vce by comparing the output of a DC converter for converting a power source voltage into a predetermined constant voltage with an applied voltage of a switching element in the case of detecting a zero point of a voltage to be applied to the element. CONSTITUTION:In order to switch at the points when a voltage Vce becomes zero, a drive controller 12 compares the Vce with the output voltage Vout of a DC converter 22, and conducts an element after a predetermined time from a point B where the Vce is crossed at the Vout from large to small. The output Vout of the converter 22 and the voltage Vce of a switching element 5 are input to a trigger generator 15. The generator 15 compares both the signals, and generates a trigger signal at a point A where the Vce is crossed at the Vout from large to small. A synchronizing signal is output from a synchronization oscillator 16 so as to turn ON the element 5 after a predetermined time upon synchronization with the trigger signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a high frequency heating device using a DC power source such as a battery, and particularly to a high frequency heating device equipped with a resonant inverter circuit.

BACKGROUND OF THE INVENTION Conventionally, various devices have been developed that are compatible with battery-powered DC power systems. Among them, high-frequency heating devices are suitable for use in vehicles because of their short cooking times, safety, and high efficiency. Therefore, a high-frequency heating device was developed that is compatible with the power sources of cars and ships.

Figure 4 shows the circuit configuration of a conventional high-frequency heating device.
In the figure, a hatchery 1 and a smoothing capacitor 2 constitute a DC power supply 3. The output of the DC power supply 3 is connected to an inverter circuit 6 comprising a resonant capacitor 4, a semiconductor switching element 6, and the like. Since the DC power source 3 is converted to high frequency by the inverter circuit 6, the transformer 7 can be made smaller. The high voltage obtained in the secondary winding 8 by the transformer 7 is further boosted by the voltage doubler rectifier circuit 9 and applied between the cathode and anode of the magnetron 10. On the other hand, low voltage power is supplied from the tertiary winding 11 of the transformer 7 to the cathode filament of the magnetron 10.

When the cathode filament is sufficiently heated and a high voltage is applied between the anode and cathode, the magnetron 10 starts oscillating and generates high frequency waves.

The semiconductor switching element 5 is driven by a drive control circuit 12. In order to reduce the loss of the switching element 5, the voltage Vce applied to the semiconductor switching element 5 is made into a sine wave by a resonant circuit composed of the resonant capacitor 4 and the primary inductance component 14 of the transformer 7, and the voltage V
The drive signal 13 is turned on and off at the timing when ce becomes zero. When the ON signal of the drive signal 13 is turned on at the time when the voltage CE of the semiconductor switching element 5 becomes zero,
The loss is the lowest. For this reason, Den! The trigger generation circuit 15 uses the voltage Vin of 1 and the semiconductor switching element voltage Vce.
A trigger signal is generated at the point where Vce crosses Vin from large to small. Synchronous oscillation is performed to turn on the semiconductor switching element 5 after a predetermined time from the trigger signal.
r! ], 6 are operated, and the on-period is determined by a pulse width modulation circuit 19 based on a foot strike signal 18 obtained by rectifying and amplifying the voltage across the detection resistor 17 so as to maintain the anode current at a predetermined value. The output of the pulse width modulation circuit 19 is current-amplified by the drive circuit 20 and the switching element 5
The drive signal becomes Vd13.

FIG. 5 shows the voltage V applied to the semiconductor switching element 5.
ce, the waveform of the drive signal V d13, and the power supply voltage Vi
Indicates n. In FIG. 5, a semiconductor switching element 5
After Vce is turned off, voltage Vce is generated and becomes zero at point A due to the characteristics of the resonant circuit. In order to detect point A, the voltage V
ce is compared with the electric B voltage Vin, a point B where Vce crosses Vin from large to small is detected, and the semiconductor switching element 5 is turned on after a predetermined time from point B. In this way, since the switching operation is performed when the voltage is zero, the loss of the element due to switching can be significantly reduced.

Problems to be Solved by the Invention However, such a configuration has the following problems.

If there are large fluctuations in the voltage of the DC source such as a battery,
When Vin is used as a reference voltage for detecting the zero point of the semiconductor switching element voltage Vce, there is a problem that the reference voltage is not stable and it is not possible to detect the optimum switch-on timing with less loss. That is, as shown in FIG. 6, when the taX pressure increases, the switching element may become conductive before the switching element voltage becomes zero. For this reason, the switching loss of the element for performing a switching operation with the voltage VL applied to the element increases, and the heat generated by this loss increases the temperature rise of the device. For this reason, there were problems in that the cooling device for cooling the device was thick and the reliability of the device was significantly reduced due to heat generation.

Means for Solving the Problems In order to achieve the above objects, the present invention provides a direct current power source composed of a battery or the like, a semiconductor switching element,
a resonant capacitor connected in parallel or series to the semiconductor switching element; a transformer that boosts the power source converted to a high frequency by the semiconductor switching element;
A magnetron connected to the secondary side of the transformer, a DC voltage converter that converts the output of the DC power source into a predetermined DC power source, and a voltage applied to the semiconductor switching element and the output of the DC voltage converter are successively compared. death,
The configuration includes a drive control circuit that drives the semiconductor switching element in synchronization with these cross signals.

Function The high-frequency heating device of the present invention detects the zero point of the voltage applied to the switching element by comparing the output of a DC-DC converter that converts the power supply voltage into a predetermined constant voltage with the voltage applied to the switching element. Therefore, even if the power supply voltage Vin fluctuates, the switching timing will not be delayed or early, so that the switching loss of the element is small and stable operation is possible.

Furthermore, even if the power supply voltage fluctuates somewhat, it has little effect on the switching operation of the inverter circuit, so the smoothing capacitor C2 can also be made smaller.

Embodiment Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings.

FIG. 1 shows a circuit diagram of the high frequency heating device of the present invention. Fourth
Components that are the same as those in the figures are given the same reference numerals.

In FIG. 1, the unidirectional power supply 3 is composed of a battery =■ and a smoothing capacitor 2. The output of the unidirectional power supply 3 is converted into a high frequency output by an inverter circuit 6 composed of a resonant capacitor 4, a switching element 5, and a regeneration diode 21. The high-frequency output is boosted by a step-up transformer 7, and a high-voltage high-frequency voltage is generated in the secondary winding 8.

A voltage doubler rectifier circuit 9 is connected to the secondary winding 8 to rectify and boost the output of the secondary side and apply it to the magnetron 10 . On the other hand, power is supplied from the tertiary winding 11 of the transformer 7 to heat the cathode filament of the magnetron 10.

A voltage Vce as shown in FIG. 2 is generated in the switching element 5 by the function of the resonant circuit composed of the primary winding 14 of the transformer and the resonant capacitor 4.

Here, in order to perform a switching operation when the voltage Vce becomes zero, the drive control circuit 12 compares Vce with the output voltage Vout of the DC converter 22, and compares Vce with the output voltage Vout of the DC converter 22,
The element is made conductive after a predetermined time from point B where e crosses Vout from large to small. ' The output Vout of the DC converter 22 and the voltage Vce of the switching element 5 are input to the trigger generation circuit 15.

The trigger generation circuit 15 compares both signals and determines that Vce is Vou.
A trigger signal is issued at point A where t crosses from large to small. A synchronizing signal is output from the synchronous oscillator 16 so as to turn on the switching element 5 after a predetermined time in synchronization with the trigger signal. In addition, during the on period, the pulse width modulation circuit 19 is controlled by a feedback signal 18 obtained by rectifying and amplifying the voltage across the detection resistor 17 so as to maintain the anode current at a predetermined value.
It can be determined by

That is, constant output control of the high-frequency heating device is performed by performing pulse width modulation that shortens the on-period when the anode current is large and lengthens the on-period when the anode current is small. The output of the pulse width modulation circuit is current-amplified by the drive circuit 20 and becomes the drive signal V d13 for the switching element 5.

FIG. 2 shows the switching element voltage Vce and the drive signal Vd.
The waveform of is shown. In FIG. 2, Vout remains stable even when the power supply voltage varies from inmax to Virvin, so the timing to the trigger signal generation point does not vary. Therefore, switching can be performed reliably at the zero point.

Therefore, the loss of the switching element may be extremely small. Therefore, it is possible to realize a high-frequency heating device with a simple cooling configuration and stable quality.

FIG. 3 shows another embodiment of the high frequency heating device of the present invention.

Components that are the same as those in FIG. 1 are given the same reference numerals, and explanations thereof will be omitted.

In FIG. 3, a step-down regulator type DC voltage converter 25 consisting of a resistor 23 and a Zener diode 24 is shown.
forms a constant voltage source Vout2. Here, the Zener voltage of the diode 24 is set to the minimum value of the t'fAt pressure Vin. With this configuration, the power supply signal for synchronization becomes constant at Vout2, and stable operation of the inverter circuit 6 is possible. Further, a circuit for stabilizing the power supply signal can be easily constructed by simply connecting the Zener diode 24 and a resistor.

Effects of the Invention As described above, the high frequency heating device of the present invention provides the following effects.

In order to switch (1) at the zero point of pressure Vce,
Less element loss.

(2) Since the source voltage converted by a voltage converter is used for drive control synchronization, even if the source changes, there is little effect on the inverter operation. Therefore, the power supply smoothing capacitor C2 can be made smaller. For this reason,
An inexpensive high-frequency heating device can be realized.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a high-frequency heating device for DC power supply according to a first embodiment of the present invention, FIG. 2 is a circuit waveform diagram of the same device, and FIG. 3 is a diagram of a high-frequency heating device according to another embodiment of the present invention. circuit diagram,
FIG. 4 is a circuit diagram of a conventional high-frequency heating device, and FIGS. 5 and 6 are circuit waveform diagrams of the same device. 3...DC power supply, 4...Resonant capacitor, 5...Semiconductor switching element, 7...
...Transformer, 10...Magnetron, I2
... Drive control circuit, 22 ... DC voltage converter. Name of agent: Patent attorney Akira Okaji and two others 3-DC power supply U-11 DC voltage] N-7 1. ! ! !

Claims (2)

[Claims] (1) A DC power source, a semiconductor switching element, a resonant capacitor connected in parallel or series to the semiconductor switching element, a transformer that boosts the power source converted to a high frequency by the semiconductor switching element, and a secondary transformer of the transformer. A magnetron connected to the side, a DC voltage converter that converts the output of the DC power source into a predetermined DC power source, and a voltage applied to the semiconductor switching element and the output of the DC voltage converter are compared, and a cross signal thereof is generated. A high-frequency heating device comprising a drive control circuit that drives the semiconductor switching element in synchronization with. (2) The high-frequency heating device according to claim 1, wherein the DC voltage converter is a step-down regulator.