JPH0810629B2 - Microwave heating device power supply - Google Patents

Description

TECHNICAL FIELD The present invention relates to a power supply for a microwave heating device which operates at a frequency higher than a commercial frequency.

[Prior Art] A power supply for operating a magnetron used in a microwave heating device is conventionally composed of a transformer, a capacitor, a diode, and the like which operate at 50 Hz or 60 Hz.

As a recent trend, it has been considered to make the power supply smaller and lighter and to make it easier to control the output. An example of this kind is shown in Japanese Patent Laid-Open No. 60-250588. However, the consideration of the characteristics peculiar to magnetrons is still insufficient.

[Problems to be solved by the invention]

When the switching power supply is used to drive the magnetron, the power supply cannot supply power to the magnetron smoothly. Most of the causes are that the characteristics of the magnetron have non-linear characteristics in which the current I _{M of} the magnetron is not proportional to the voltage V _M of the magnetron, as shown in FIG. Therefore, there is a problem that the switching power supply side needs to have a characteristic matched to the magnetron.

[Means for solving problems]

A choke coil is inserted in the voltage doubler rectifier circuit of the switching power supply, and the power supply to the magnetron can be smoothly supplied by the function of the capacitor for double voltage rectification.

[Action]

By setting the resonance cycle of the circuit obtained by the choke coil added to the voltage doubler rectifier circuit and the capacitor in the voltage doubler rectifier circuit to twice the switching cycle, the current waveform flowing in the voltage doubler rectifier circuit has a rounded shape. It is possible to decrease the peak current of the magnetron and lengthen the current flowing time.

〔Example〕

A switching power supply for a microwave heating device is shown in FIG.

1 is a magnetron, 2 is a switching element, 3 is a snubber capacitor, 4 is a high voltage transformer, 5 is a diode, and 6 is a capacitor. Reference numeral 7 is a choke coil added to the voltage doubler rectifying circuit which is a feature of the present invention.

The operation of the choke coil 7 will be described below with reference to the drawings.

Further, FIG. 2 shows the relationship between the current and the voltage in the circuit of FIG. 1 including the presence or absence of a choke coil.

I _C is a current flowing through the switching element 2, I _L is a current flowing through the voltage doubler rectifier circuit, I _M is a current flowing through the magnetron 1, and V _M is a voltage applied between the anode and the filament of the magnetron 1.

FIG. 2 (c) shows respective waveforms when the choke coil 7 is not provided in the voltage doubler rectifier circuit. Switching element 2
During the OFF period, the diode 5 becomes conductive and the capacitor 6 accumulates electric charge. When the switching element 2 is turned on, a voltage on the secondary side of the high voltage transformer is further applied to the voltage applied to the capacitor 6, a magnetron current I _M flows, the charge of the capacitor 6 is also discharged, and I _M rapidly decreases.

The problem with this circuit is that since a uniform current cannot flow through the entire ON period of the switching element 2, I _M max becomes large in order to obtain the required output, which is 1.5 to 2 times that when operating at a conventional commercial frequency. The current density of the magnetron cannot meet this. Further, during the OFF period, the capacitor 6 is charged through the diode 5, but a spike current flows due to the vibration of the circuit at this time, which causes noise.

Next, FIG. 2B shows the secondary side of the transformer 4 and the capacitor 6 in the voltage rectifying circuit of the switching circuit of FIG.
A choke coil 7 is provided between the choke coil 7 and
2 is a waveform when the inductance of the choke coil 7 is set under the condition of resonating at the switching frequency by the capacitor 6 and the capacitor 6. In this case, the resonated current can be passed only during the period when the magnetron current flows.
Therefore, the magnetron current peak value I _M max for obtaining the same output can be made lower than that in Fig. 2 (c), but the drawback of this circuit is that the current of the resonant circuit flowing during the OFF period of switching is smoothed. Therefore, it cannot be said that the spike-shaped current cannot be removed yet.

Therefore, the method according to the present invention and the waveform shown in FIG. 2 (a) will be described. The resonance frequency of the high voltage circuit when the diode 5 is turned on by the inductance of the choke coil 7 of FIG. 1, the capacitor 6 and the capacitor 3 is obtained, and this value is set to be twice the switching frequency. To do. The current flowing through the diode 5 during the OFF period does not reach the level where it oscillates in the vibration mode of the circuit, but due to the pulling-in phenomenon, it relatively approaches the multiple of the vibration mode of the circuit, making it difficult for the spike-shaped current to flow.

Regarding operating frequency and circuit constants Ask in. However, f = switching frequency (Hz), Le =
The inductance (H) of choke coil 7, C _S = capacity (f) of capacitor 3, C _H = capacity (f) of capacitor 6 and K = constant.

Since the circuit of the embodiment of the present invention utilizes the pull-in phenomenon, the resonance frequency of the circuit is doubled with respect to the switching frequency, but a value close to twice is also effective. According to the experiment, the value of K = constant should be in the range of 1.0 to 1.25.

〔The invention's effect〕

When the magnetron is operated by an inverter power supply, the current waveform becomes smooth, noise is reduced, stable operation is obtained, and the design margin for reducing the size and weight of the microwave oven is improved.

[Brief description of drawings]

FIG. 1 is a switching circuit diagram showing an embodiment of the present invention,
FIG. 2 (a) is a diagram for explaining the voltage and current waveforms of the present invention, FIGS. 2 (b) and 2 (c) are diagrams for explaining the voltage and current waveforms up to the present invention, and FIG. 3 is a magnetron voltage. FIG. 3 is a characteristic diagram showing a relationship between current and current. 1 ... Magnetron, 2 ... Switching element, 3 ...
Snubber capacitor, 4 ... High voltage transformer, 5 ... Diode, 6 ... Capacitor, 7 ... Choke coil.

Claims (1)

[Claims] 1. In a power source of a microwave heating device which is converted to a frequency higher than a commercial frequency by a switching element and constitutes a voltage doubler rectifying circuit to drive a magnetron, a choke coil is installed in the voltage doubler rectifying circuit, Microwave heating, characterized in that a resonance circuit is formed by the inductance of the choke coil and the capacitor of the voltage doubler rectifier circuit, and the constant of the resonance circuit is set to be twice the switching frequency. Equipment power supply.