JPH0546077B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency magnetron drive circuit.

The magnetron used as a heating means in a microwave oven is generally driven by connecting a voltage doubler rectifier circuit to the secondary circuit of a low frequency leakage transformer and applying the rectified output to the magnetron to excite it. However, high-frequency drive circuits have recently been developed that allow the transformer to be made smaller and lighter, provide stable magnetron output regardless of voltage fluctuations in the commercial frequency power supply, and allow more precise output adjustment. ing. This circuit uses an inverter to obtain a high-frequency voltage, feeds it to a voltage doubler rectifier circuit, and excites the magnetron with the rectified output.Output can be adjusted by adjusting the output frequency of the inverter. . However, this circuit has the disadvantage that the switching elements of the inverter may be destroyed for reasons described later.

The present invention solves this problem and provides a drive circuit that can stably excite a magnetron.

The present invention will be specifically explained below.

FIG. 1 shows a power supply circuit for the magnetron 14, and the wiring of the circuit itself is the same as that of a conventional high-frequency drive circuit. The commercial frequency power supply 13 is rectified by the full-wave rectifier circuit 1, and the pulsating current that is the rectified output is
It consists of a capacitor 2 connected between the positive and negative poles of the full-wave rectifier circuit 1, a low-frequency choke 3 with one end connected to the positive pole, and a capacitor 4 connected between the other end of the low-frequency choke 3 and the negative pole of the full-wave rectifier circuit 1. It is applied to the inverter via a low pass filter. This low-pass filter is for preventing noise from propagating from the inverter side to the commercial frequency power supply 13 side.

The inverter controls the high-frequency choke 5, the primary winding 6p of the step-up transformer 6, the resonant capacitor 7, the flywheel diode 8, the switching element 9 such as a GTO (gate turn-off) type controlled rectifier, and the on/off of the switching element 9. Control circuit 1
5, a high frequency choke 5, a primary winding 6p and a resonant capacitor 7 are connected in series,
The pulsating flow can be applied through the low-pass filter. A flywheel diode 8 and a switching element 9 are connected in parallel to the resonant capacitor 7. The inductance L ₁ of the high-frequency choke 5 and the inductance L ₂ of the primary winding 6p are matched with the magnetron 14 and, as will be described later, take into account the resonant period of the resonant circuit consisting of these and the resonant capacitor 7 (capacitance value C). Determined by The switching element 9 is repeatedly turned on and off by a switching control signal periodically generated by the control circuit 15, and the resonance capacitor 7 connects the high frequency chain 5 and the primary winding 6p when the switching element 9 turns from on to off. The energy stored in the capacitor 7 resonates with them, and a resonant voltage is generated across the resonant capacitor 7. Therefore, a high frequency alternating current flows through the primary winding 6p, and a high frequency high voltage is generated at the secondary winding 6s of the step-up transformer 6. 2
The next winding 6s has a capacitor 10 and a diode 11.
A half-wave voltage doubler rectifier circuit consisting of a varistor 12 and a varistor 12 is connected, and its rectified output
It is connected so as to be applied between the anode and cathode of No. 4. Note that the tertiary winding 6t of the step-up transformer 6 serves as a heater power source for the cathode of the magnetron 14.

Now, a half-wave doubled voltage as shown in Figure 2 is applied to the magnetron 14, but the magnetron does not oscillate over the entire half-wave voltage, but rather has a threshold voltage (determined by the magnetron's anode voltage). )
It will not oscillate below. In other words, as shown in Figure 2, there is no oscillation during period b ₁ when the voltage is below the threshold voltage during boosting, and after period a when the voltage is above the threshold voltage, period b when the voltage is below the threshold voltage. ₂ also does not oscillate.

This is the same in conventional circuits using low frequencies, but when using high frequencies, the following problem occurs regarding period _b2 . That is, in the low frequency range, the magnetron can be equivalently considered to be a series circuit of a battery and a resistor. Here, the battery indicates the existence of a threshold voltage, and the resistor indicates that the input to the magnetron is purely resistive. However, in a high frequency region, the circuit exhibits properties not like such a simple equivalent circuit, but as an equivalent circuit that also includes a capacitance-like element with a predetermined discharge time constant. In FIG. 2, when period a has passed, the magnetron stops oscillating and its capacitance is charged. And next step-up transformer 6
The resonant capacitor 7 starts discharging on the primary winding 6p side, thereby charging the capacitor 10 on the secondary winding 6s side. When the charge stored in the magnetron 14 is discharged during such a period, a voltage is induced on the primary winding 6p side due to the discharge, and as a result, the resonance voltage waveform becomes as shown in FIG. The voltage will be distorted in such a way that the voltage drop will be delayed during the drop. When the resonant voltage waveform is distorted in this way, the period of the resonant voltage becomes longer than the designed value, and the switching element 9 is turned on while the capacitor 7 has charge remaining. As a result, both ends of the resonant capacitor 7 are short-circuited, and a large current flows through the switching element 9, such as a GTO type control rectifier.
This will destroy it.

The inventor of the present application determined the distortion of this resonant voltage through experiments and found that the shorter the resonant period and the larger the resonant voltage, the more likely the distortion occurs. This phenomenon is considered to be caused by a delay in the rise of conduction of the diode 11 on the side of the secondary winding 6s. In other words, if the resonance period is short or the resonance voltage is large, the gradient of voltage change is large, and the rise characteristics of the diode 11 are relatively slow. Therefore, when the resonant period is short or the resonant voltage is large, the secondary side equivalent circuit when the charged charge of the magnetron 14 is discharged is as shown in FIG. resistance 11'
Therefore, the discharge current flows to the secondary winding 6s and affects the resonant voltage. Therefore, by setting the resonant period and resonant voltage of the resonant circuit to below predetermined values determined by the characteristics of the diode 11, etc., it is possible to eliminate distortion of the resonant voltage waveform.

The present invention has been made based on this knowledge, and FIG. 5 shows the resonance period (√( _{1 +2} )),
In addition, the vertical axis shows the resonant voltage, and the region where the resonant voltage waveform is distorted and the region where the resonant voltage waveform is not distorted are shown. so that L ₁ ,
It defines circuit constants such as L ₂ and C.

As described above, the magnetron drive circuit according to the present invention defines the resonant circuit of the inverter based on its resonant period and resonant voltage. Since the present invention is characterized in that it is configured to operate in conjunction with the above, it is possible to realize a drive circuit that can prevent the switching elements of the inverter from being destroyed and can stably excite the magnetron.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a magnetron drive circuit according to the present invention, Figs. 2 and 3 are waveform diagrams for explaining distortion of the resonant voltage waveform, Fig. 4 is an equivalent circuit diagram, and Fig. 5 is a graph showing a resonance voltage waveform distortion-free region. 5...Low frequency choke, 6...Step up transformer,
7... Resonance capacitor, 9... Switching element, 14... Magnetron.

Claims (1)

[Scope of Claims] 1. In a magnetron drive circuit that excites a magnetron using high frequency and high voltage obtained from an inverter having a resonant circuit, the resonant circuit has a resonant voltage waveform defined based on its resonant period and resonant voltage. What is claimed is: 1. A magnetron drive circuit characterized in that the magnetron drive circuit is configured to operate in relation to the resonance period and resonance voltage in the distortion-free region of the resonance voltage waveform in the distortion-free region.