JPH0453078B2 - - Google Patents

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to a magnetron drive circuit.

(b) Prior Art FIG. 3 shows a high frequency drive circuit for driving a magnetron at a high frequency of about 20 KHz, as found in Japanese Patent Application Laid-Open No. 59-194378. The voltage of the commercial power supply 10 is rectified by a rectifier circuit 11, and its output is passed through a low-pass filter consisting of capacitors 12 and 14 and a coil 13, and then passed through a coil 15, a primary winding 16p of a step-up transformer 16, a capacitor 17, and a diode 1.
8. It is applied to an inverter circuit consisting of a switching element 19 such as a transistor or a thyristor, and a switching control circuit 20. Since the capacitors 12 and 14 are used as low-pass filters, their capacitance is small and the commercial power supply 10 can be sufficiently smoothed. It's not a thing. The switching element 19 is turned on and off by a high frequency pulse signal outputted from a switching control circuit 20.

Unlike in the case of a commercial power source, when an inverter circuit is used, a high voltage is generated in the secondary winding 16s of the step-up transformer 16 according to each switching waveform by turning the switching element 19 on and off. It is applied to a half-wave voltage doubler rectifier circuit 22 consisting of a capacitor 23 and a diode 21, and its output is applied to a magnetron 24 to excite it. In addition, the tertiary winding 16t of the step-up transformer 16
serves as a power source for the cathode heater of the magnetron 24.

The switching control circuit 20 receives signals from an input circuit 20a that detects the output voltage of the rectifier circuit 11, that is, the input voltage of the inverter circuit, and a resistor 25 that detects the output current of the current circuit 11, that is, the input current of the inverter circuit. input circuit 20d
, a signal conversion circuit 20b that receives signals from these input circuits 20a and 20d, and a drive circuit 20c that responds to the output of the signal conversion circuit.

Further, the power supply circuit 60 is a constant voltage circuit that inputs the voltage of the commercial power supply 10 at the same time as power is turned on to the rectifier circuit 11 to generate an input voltage for the switching control circuit 20.

The switching control circuit 20 is based on Japanese Patent Application Laid-open No. 59-
Since it is explained in detail in the publication No. 194378, the detailed explanation will be omitted here, but to briefly explain, first, the signal conversion circuit 20b is connected to the input circuit 20.
It forms a high frequency signal based on the signals from the input circuits 20a and 20d, and when the signals from the input circuits 20a and 20d change from large to small, the high level period of the high frequency signal is changed from short to long and output. . The drive circuit 20c controls the switching element 19 on and off using the high frequency signal.

The inverter circuit of the high frequency drive circuit will now be described with reference to the signal waveform diagram of FIG. 4. Note that the switching control circuit 20 sends a pulse signal as shown in FIG. 4a to the switching element 19.
It is assumed that the pulse from time t10 to time t11 is the first pulse given to the switching element 19.

When the switching element 19 is turned on by a pulse between time t10 and t11, a primary current i1 flows through the coil 15 and the primary winding 16p of the step-up transformer 16, as shown in FIG. The flow follows an increasing trend in the direction of the arrow.

Then, when the pulse generation stops at time t11, the switching element 19 is turned off. Then, between the above time t10 and t11, the coil 15
The energy stored in the primary winding 16p causes a resonance state between the coil 15, the primary winding 16p, and the capacitor 17, resulting in a nearly cosine wave of 1.
As the secondary current i1 flows as a resonant current, a substantially cosine wave resonant voltage _vc as shown in FIG. 4b is generated at the connection point between the primary winding 16p and the capacitor 17.

Then, when a pulse is generated again at time t13, the switching element 19 becomes in a state where it can be turned on, but from time t12 to t15, in the resonance state, the coil 15 and the primary winding 16
Since the diode 18 is in the on state to release the energy stored in p, the switching element 19 is reverse biased and is not in the on state. When the release of energy ends at time t15, the switching element 19 is turned on, and the primary current i1 again flows in the direction of the arrow in FIG. 3, following an increasing trend. Then the above time t11
The operations from then on are repeated.

(c) Problems to be Solved by the Invention Here, focusing on the time during which the switching element 19 is in the on state, the switching element 1
The second and subsequent ON state of 9 is between time t15 and t13, whereas the first ON state is from time t10.
t12 and is only long from time t12 to t15.

Therefore, if the switching element 19 is first turned on at a time when the instantaneous value of the voltage of the commercial power supply 10 is high, the primary current i1 flowing during this on state becomes very large, and therefore The resonant voltage v _c generated when the switching element 19 of
A high voltage is generated in the secondary winding 16s of the diode 2.
1 and the capacitor 23 may be destroyed. Moreover, the resonant current also increases, and there is a possibility that the diode 18 may be destroyed.

Therefore, an object of the present invention is to prevent destruction of each of these elements.

(d) Means for Solving the Problems The present invention is configured to obtain a high frequency voltage by an inverter circuit having a switching element which is controlled on and off by a switching control circuit, and to boost this high frequency voltage and apply it to the magnetron. The magnetron drive circuit is characterized by comprising a switching start circuit that causes the switching control circuit to turn on the switching element for the first time at a point in time when the instantaneous value of the input voltage of the inverter circuit is low.

(E) Effect According to the present invention, the first turning-on operation of the switching element of the inverter circuit is performed at a point in time when the instantaneous value of the commercial power supply voltage applied to the inverter circuit is low.

(F) Embodiment FIG. 1 is a circuit diagram of a main part showing an embodiment of the present invention.

Power supply circuit 60 is the same as in FIG. 3 and is shown in more detail. The output voltage of the commercial power supply 10 is stepped down to a voltage of, for example, 12V by a step-down transformer 61, and then full-wave rectified by a rectifier circuit 62. Then, this rectified voltage is applied to the electrolytic capacitors 63, 6
4, resistor 65, capacitor 66, transistor 6
7 and a Zener diode 68, a predetermined voltage Vde, for example, a voltage _V64 of 8V is obtained, and the switching control circuit 20 is driven by this voltage _V64 .

The switching start circuit 70 is a feature of the present invention, and is for setting the start time of the on/off control of the switching element 19 by the switching control circuit 20.

The voltage V ₆₄ of the power supply circuit 60 is applied to two resistors 71 and 72 connected in series and two resistors 74 and 75 also connected in series. resistance 7
The potential V ₇₃ at the connection point of 1 and 72 is the reverse breakdown voltage of this due to the connected Zener diode 73.
V ₇₃ th and is applied to one input terminal of the comparator 77. Also, the potential V ₇₆ at the connection point between the resistors 74 and 75 is the capacitor 76 connected thereto.
It is determined by , and is applied to the + input terminal of the comparator 77 . The comparator 77 is
+ Signal given to input terminal (i.e. potential V ₇₆ )
is the signal applied to one input terminal (i.e., the potential
V ₇₃ ) Outputs a high level signal when the level is higher than V 73 ).

The output of the comparator 77 is given to one input terminal of an AND gate 78, and the output of the signal conversion circuit 20b given to the terminal 80 is given to the other input terminal of the AND gate 78. Based on the output of the drive circuit 20c
performs on/off control of the switching element 19.

The operation of such a circuit will be explained below with reference to the signal waveform diagram of FIG. 2. The signal waveform diagram in FIG. 2 shows the state when the commercial power supply voltage is turned on to the power supply circuit 60, and the power is turned on at time 0. Furthermore, it is clear from the circuit diagram of FIG. 3 that at the same time, the voltage of the commercial power supply 10 is also applied to the inverter circuit.

As shown in FIG. 2a, the voltage V ₆₄ of the power supply circuit 60 rises stepwise from the time the power is turned on, and increases with time.
At _1.5t0 , a predetermined voltage Vde that can normally drive the switching control circuit 20 is reached. This is because the capacitance of the electrolytic capacitors 63 and 64 is large, so that the rectified voltage by the rectifier circuit 62 when the capacitors 63 and 64 are not present (the broken line in _FIG . This is because the capacitors 63 and 64 cannot be sufficiently charged.

The potential V ₇₃ applied to one input terminal of the comparator 77 is applied to the power supply circuit 60 as shown in FIG. 2b.
The voltage of V rises according to ₆₄ and Zener diode 7
When the reverse breakdown voltage V ₇₃ th of 3 is reached, this voltage
V ₇₃ th held.

On the other hand, the potential V ₇₆ applied to the + input terminal of the comparator 77 increases as the capacitor 76 is charged, as shown in FIG _.
The constants of the resistors 74 and 75 and the capacitor 76 are determined so that the potential is higher than the voltage V ₇₃ th in the vicinity.

Therefore, the output of the comparator 77 is as shown in FIG.
As shown in FIG. 2, since the signal is at a low level from the time the power is turned on until time _2t0 , the AND gate 78 is in the closed state and the switching element 19 is turned off by the drive circuit 20c. Then, at time _2t0 , the output of the comparator 77 becomes high level, and the AND gate 78 becomes open. At this time, as shown in Figure 2a, at 1.5to, the voltage V ₆₄
has reached the predetermined voltage Vde, so the switching control circuit 20 is normally driven. Therefore, the terminal 80 also outputs a high level signal near time 2to, and the on/off control of the switching element 19 by the drive circuit 20C starts.

By the way, time 2t ₀ is the lowest point of the instantaneous value of the rectified voltage by the rectifier circuit 62, as shown by the broken line in FIG. 2a. Here, as shown in FIG. 3, since the same voltage of the commercial power supply 10 as the voltage applied to the power supply circuit 60 is applied to the rectifier circuit 11, the rectified voltage by the rectifier circuit 11, in other words, the inverter The input voltage of the circuit is the rectifier circuit 62
The rectified voltage and phase are synchronized. Therefore, the lowest point of the instantaneous value of the rectified voltage by the rectifier circuit 62 is also the lowest point of the input voltage of the inverter circuit.

Therefore, the on/off control of the switching element 19 is started from the lowest point of the input voltage of the inverter circuit.

In addition, in the present invention, the switching control circuit 20, the power supply circuit 60, and the switching starting circuit 7
0 is not limited to the above example, and various changes are possible. For example, the switching start circuit 70 may be configured to directly detect the rectified voltage by the rectifier circuit 11.

Further, the switching starting circuit 70 has a voltage V ₆₄
Instead, the configuration may be such that the output voltage of the rectifier circuit 62 is detected.

(G) Effects of the Invention According to the present invention, since the on/off control of the switching element is configured to start from a point in time when the instantaneous value of the input voltage of the inverter circuit is low, the initial ON state time of the switching element is long. However, since a large current does not flow, the circuit components are not destroyed, and a highly reliable power supply circuit can be provided.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a main part of an embodiment of the present invention, FIG. 2 is a signal waveform diagram, FIG. 3 is a circuit diagram showing a typical example, and FIG. 4 is a signal waveform diagram. 19... Switching element, 20... Switching control circuit, 70... Switching start circuit.

Claims (1)

[Claims] 1. In a magnetron drive circuit configured to obtain a high frequency voltage by an inverter circuit having a switching element that is turned on and off by a switching control circuit, and to step up the high frequency voltage and apply it to the magnetron, 1. A magnetron drive circuit comprising a switching start circuit that turns on a switching element for the first time at a low instantaneous value of the input voltage of the inverter circuit.