JPH056316B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a drive circuit for a magnetron used as heating means in a microwave oven, and more specifically, it proposes a drive circuit for a magnetron that uses a high-frequency transformer and is capable of cold start. be.

A technique is known in which a magnetron is driven by a circuit using a high-frequency transformer instead of a circuit using a low-frequency leakage transformer, thereby making it possible to reduce the size and weight and adjust the output. This rectifies the commercial frequency power supply, converts it to high frequency using an inverter, applies this high frequency to a step-up transformer, obtains a high frequency high voltage, and supplies this to the magnetron via a voltage doubler rectifier circuit. This is what was done. Then, in order to stabilize the input power, the input power of the inverter is detected, and when it increases (or decreases), control is performed to shorten (or lengthen) the time during which the switching elements of the inverter are conductive.

Conventionally, when such a high frequency driving method is used, the current of the filament of the magnetron's cathode is supplied by a transformer provided separately from the step-up transformer. This is because it was necessary to rely on a hot start method in which the magnetron is started with the filament in a preheated state.

The impedance of a magnetron is higher before oscillation than during steady oscillation, and if the power is turned on under such conditions, the voltage will be stepped up transiently due to the residual magnetic flux of the step-up transformer, the power switch turn-on phase, the magnitude of the input voltage, etc. The magnetic flux in the transformer's iron core became saturated, causing an excessive excitation rush to flow, which made it impossible for the unpreheated magnetron to start oscillating.

Therefore, it is conceivable to suppress the excitation rush by increasing the frequency of the inverter when the power switch is turned on, in other words, by shortening the time during which the switching elements are conductive. On the other hand, when performing a cold start, the tertiary winding of the step-up transformer is used as the power source for the filament, as described later in the circuit of the present invention.By increasing the frequency of the inverter, the inductance of the power source for the filament is can no longer be ignored, the filament current decreases, and as a result, the time it takes for the magnetron to oscillate becomes longer.In short, cold start using a high-frequency drive method has not been put to practical use until now.

The present invention has been made in view of the above circumstances, and has a control characteristic that shortens the conduction time of the switching elements as the input is smaller in the range where the input to the inverter is less than a predetermined value, thereby making it possible to perform a cold start. It is an object of the present invention to provide a magnetron drive circuit that can maintain input power stably after reaching a steady state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof.

FIG. 1 is a schematic circuit diagram of a magnetron drive circuit according to the present invention, in which a commercial frequency power source 10 is connected to a rectifier circuit 11, and its output is connected to capacitors 12 and 1.
4 and a coil 13, and the high frequency high voltage of the secondary winding 16s of the step-up transformer 16 is applied to a half-wave voltage doubler rectifier circuit consisting of a capacitor 23, a diode 21, and a varistor 22. This rectified output is applied to the magnetron 24, and the inverter includes a chiyoke coil 15, a primary winding 16p of a step-up transformer 16, a resonant capacitor 17, a flywheel diode 18, a gate-off (GTO) type controlled rectifier, etc. It consists of a switching element 19 and a switching control circuit 25 that controls on/off of the switching element 19, and an input circuit 2.
5d, the detected current of CT26, which is a current transformer provided on the negative side output line of the rectifier circuit 11, is inputted. Further, the tertiary winding 16t of the step-up transformer 16 is used as a power source for the filament of the magnetron 24.

FIG. 2 is a circuit diagram showing this switching control circuit 25. As shown in FIG.

The positive output line of the rectifier circuit 11 is connected to an input circuit 25a formed by connecting two resistors in series, and the potential at the connection point between the resistors is set to the output voltage of the rectifier circuit 11, in other words, the input voltage of the inverter. A representative signal is given as a negative input of the multiplier 30. The detected current of the CT 26 provided on the negative output line of the rectifier circuit 11, that is, the output current of the rectifier circuit 11 or the input current of the inverter, is made to flow through the input circuit 25d consisting of a current limiting resistor, and is applied to other inputs of the multiplier 30. I try to give it as such. The multiplier 30 provided at the input stage of the signal conversion circuit 25b uses, for example, a general-purpose multiplier XR-2208 manufactured by EXAR, and its external resistor 30a adjusts the offset, and the resistor 30b adjusts the multiplier of the internal operational amplifier. A signal obtained by multiplying the product of both inputs by this multiplier is obtained at its output terminal. This multiplier 30
The output V is given to the +input terminal of the operational amplifier 31 and the -input terminal of the operational amplifier 32. FIG. 3 shows the relationship between the output of the multiplier 30 and the outputs of the operational amplifiers 31 and 32.
V ₃₁ has a characteristic that increases as the output of the multiplier 30 increases, and the output V ₃₂ of the operational amplifier 32
It shows a characteristic that decreases as the output increases, and both characteristics have a point where they intersect. operational amplifier 31,3
The upper and lower limits of the two outputs are limited by limit circuits 33 and 34, and are applied to a low level signal selection circuit 37 consisting of diodes 35 and 36, etc., and the lower voltage selected here is sent to the + input terminal of a comparator 47. Given. This comparator 47 determines the time width of the output pulse signal of this switching control circuit 25, that is, the time when it is at a high level, and charges the capacitor 41 extremely linearly with a constant current circuit 40 consisting of a resistor and a transistor. Then, this charging voltage V ₄₁ is determined by the comparator 47.
It outputs a high level output while the signal applied to the + input terminal is at a higher level than the signal applied to the - input terminal. The output of this comparator 47 is applied as a set input S to an R-S flip-flop consisting of NAND gates 43, 43, the Q output of the R-S flip-flop is applied to a drive circuit 25c, and the output is connected to a resistor 44 and a capacitor. 45 parallel circuits to the base of a transistor 46 connected to a terminal of a capacitor 41. When this transistor 46 is turned on, the capacitor 41 will be discharged. On the other hand, the comparator 48 determines the time period during which the output pulse of the switching control circuit 25 is at a low level, and the capacitor 49
Its negative input is the charging voltage, and its positive input is the output of a voltage dividing circuit such as a resistor 50. The output of this comparator 48 is applied to the R-S flip-flop as its reset input R. Further, the terminal of the capacitor 49 is grounded via the transistor 51.
The Q output of the R-S flip-flop is applied to its base.

The drive circuit 25c is a buffer C-MOSIC52,
53 at the input stage, and its output passes through a surge prevention circuit 54 using a diode to the transistor 5.
5, 56, 57, 58, 59, etc., and the amplified output is provided to the switching element 19. The base input of the transistor 55 is C-coupled using a capacitor 61, and is designed to shorten the rise time of the input pulse signal (square wave). diode 62
The resistor 63 is also provided to increase speed. Transistors 57, 58 and 59, 6
0 is a connection that constitutes a complementary circuit. Further, a coil 64 connected to an intermediate node between the transistors 59 and 60 and a resistor 65 connected thereto are provided for current limiting.

In the switching control circuit 25 configured as described above, the output V of the multiplier 30 is proportional to the input voltage and input current values of the inverter, and has a value representative of the input power. When this output V is given to the operational amplifiers 31 and 32, the operational amplifiers 31 and 32
32 outputs output signals V ₃₁ and V ₃₂ at levels corresponding to the characteristics shown in FIG.
The lower of V ₃₁ and V ₃₂ will be input to the comparator 47. In other words, as shown in Figure 4, the relationship between the input power of the inverter and the input _V47 to the comparator 47 is upward in the range lower than the input power corresponding to the intersection of _V31 and _V32 , and in the higher range. This shows a downward-sloping characteristic. The comparator input V ₄₇ (V ₃₁
or V ₃₂ ) is connected to capacitor 4 by comparator 47.
It is compared with the charging voltage of 1. V ₄₇ is capacitor 4
1 charging voltage V ₄₁ is higher than comparator 47
The output, ie, the set input S of the R-S flip-flop, is at a high level, keeping its Q output at a high level and its output at a low level.
As a result, the transistor 51 is turned on and the transistor 46 is turned off. Eventually, when _V41 becomes higher than _V47 , the output of the comparator 47 goes low, and the R-S flip-flop reverses its state so that the Q output goes low and the output goes high. Therefore, transistor 46 is turned on to discharge the charge in capacitor 41, while transistor 51 is turned off and charging of capacitor 49 is started. While the + input of comparator 48 is higher than the charging voltage of capacitor 49, comparator 4
8 output, that is, the reset input R of the R-S flip-flop, is at high level, but capacitor 49
As charging progresses, R eventually turns to low level, which inverts the R-S flip-flop and causes Q
The output changes to high level and the output changes to low level. As can be understood from this behavior,
In the range where the input power of the inverter is lower than the value corresponding to the intersection of V ₃₁ and V ₃₂ , the high level time of the Q output of the R-S flip-flop changes to be longer or shorter depending on its magnitude. In a high range, the high level time of the Q output changes from short to long. The higher the input level of the comparator _V47 is, the longer the Q output is at the high level, that is, the longer the switching element 19 is conductive (see FIG. 4). Therefore,
When the input power of the inverter until the magnetron 24 starts oscillating is smaller than a predetermined value, the conduction time of the switching element 19 is controlled to be short. When the switching element 19 is turned on for a controlled conduction time, the current flowing to the inverter, that is, the input power increases in accordance with the conduction time. When the input power increases, switching element 1
The conduction time of V 9 becomes longer in accordance with the characteristics of the output signal V ₃₁ side, and by repeating such an operation, the input power of the inverter increases. As the input power to the inverter increases in this way, the filament of the magnetron 24 gradually heats up and starts oscillating. When the magnetron approaches a steady state and the input power of the inverter exceeds a predetermined value, the conduction time of the switching element 19 changes depending on the characteristics of the output signal V ₃₂ side, depending on whether the input power is large or small. can be changed to short or long.

By shortening the initial conduction time of the switching element 19 in this way, excitation rush current is prevented,
Then, due to the characteristics of the output signal V ₃₁ side, the conduction time of the switching element 19 becomes gradually longer, that is, by lowering the frequency of the inverter, the required filament current of the magnetron 24 is secured, and a cold start of the magnetron 24 is performed. In addition, when the magnetron 24 is in a normal oscillation state, the input power to the inverter can be stabilized.

As described above, the present invention shortens the conduction time of the switching elements of the inverter to prevent the excitation rush of the step-up transformer when the input power of the inverter is below a predetermined value, and shortens the conduction time according to the input power. Since the required filament power of the magnetron is obtained by controlling the length of the filament to gradually become longer, a cold start of the magnetron can be realized in a short time. Furthermore, if the input power of the inverter exceeds a predetermined value after driving the magnetron, the conduction time of the switching elements is controlled to be short or long depending on the magnitude of the input power of the inverter. Input power can be stabilized.
Therefore, it is possible to cold start the magnetron, and after the magnetron is driven, there is an excellent effect of providing a magnetron drive circuit that can stabilize the output of the magnetron.

[Brief explanation of the drawing]

FIG. 1 is a schematic circuit diagram of the circuit of the present invention, FIG. 2 is a circuit diagram of its main part, and FIGS. 3 and 4 are characteristic diagrams for explaining its operation. 19... Switching circuit, 24... Magnetron, 25... Switching control circuit, 30...
Multiplier, 31, 32... operational amplifier, 37... low level signal selection circuit.

Claims (1)

[Claims] 1. It is equipped with an inverter that detects the input and performs switching control, and uses the output of the inverter to heat the filament of the magnetron and apply a high frequency high voltage to the magnetron to drive the magnetron. The magnetron drive circuit has a circuit that generates a signal that controls the conduction time of the switching element of the inverter to be longer or shorter depending on the magnitude of the input, and a circuit that generates a signal that controls the conduction time of the switching element of the inverter to lengthen or shorten the conduction time of the switching element of the inverter depending on the magnitude or magnitude of the input. It is equipped with a circuit that generates a signal that controls the conduction time of the switching element to shorten or lengthen it. The magnetron drive circuit described above is characterized in that it is configured to control the conduction time of a switching element to be short or long depending on the magnitude or magnitude of the input.