JPH03280389A - Inverter control circuit for magnetron drive - Google Patents

Abstract

PURPOSE:To prevent sudden influx of rush current to an inverter control circuit concerned, without providing any special protection measure on a great scale, and stabilize the operation by introducing two-step soft starting system, i.e., at the time of preheat start of magnetron and at the time of supplying power to the body CONSTITUTION:When a start switch SW is put on, a transistor Tr2 is turned off together with turning-on of a timer circuit 1 to cause the voltage level, which is fed to the deadtime control terminal of a PWM control IC 2, to sink gradually according to the time constant of a CR circuitry as a mild change means composed of a capacitor C and a resistance Rd, and thus the 1st step soft start takes place and the magnetron is preheated. When the output of the circuit 1 is put off after a certain period, the transistor Tr1 is turned off to sink again the voltage to be fed to the terminal or the IC 2 according to the time constant of the CR circuitry, and thus the 2nd soft start takes place to achieve prevention of sudden influx of rush current to the inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an inverter control circuit for driving a magnetron such as a microwave oven driven by a DC power source.

<Conventional technology> Conventional inverter microwave ovens use commercial power as a power source, and once AC 50/60Hz 100V is converted into a rectifier circuit,
This DC voltage is converted into a DC voltage, and then, by controlling the switching elements on and off using an inverter, this DC voltage is converted into a high frequency intermittent AC voltage, and then a high frequency transformer and a voltage doubler full wave rectifier circuit are used to convert the DC voltage into a high frequency intermittent AC voltage. The voltage is increased to 5KV and power is supplied to the magnetron, which oscillates and radiates microwaves to heat food, etc.

However, this magnetron has the characteristic that it does not emit microwaves unless it is heated to the required temperature.
Since no power is required until the magnetron warms up, no current flows from the inverter to the magnetron.

For this reason, a preheating circuit has conventionally been provided for the magnetron from the inverter, separate from the drive circuit that directly sends power to the magnetron. Moreover, in order to significantly reduce the number of circuit parts, the mainstream method is to use the same high-frequency transformer for this preheating circuit and the drive circuit that directly sends power to the magnetron.

When the inverter receives a start signal and starts switching, current flows through the preheating circuit, and the filament warms the magnetron to the required temperature (
During this time, it took about 1.5 seconds.

At this time, for the first time, the magnetron requires power to radiate microwaves, so the necessary power is sent from the inverter to the magnetron.

In this way, when the preheating circuit and the drive circuit are configured using the same high-frequency transformer, it takes a long time from when the inverter receives the start signal and starts switching until the power necessary to drive the magnetron is sent. This will result in a misalignment. Therefore, in the past, when the inverter was started, the switching elements started switching, first sending several tens of W of power to the preheating circuit, and then running for a few seconds to generate the power several times that amount (the power that was originally supposed to be supplied). will be supplied to the magnetron to drive it.

On the other hand, this inverter is conventionally equipped with a soft start circuit for gradually raising the output voltage at startup. This soft start circuit works to slowly widen the pulse ON time width of the PWM (pulse width modulation) switching control signal of the switching element when starting the inverter from a narrow point, thereby preventing damage to the switching element and output. Prevents voltage overshoot. That is, without the above soft start circuit, the PWM switching control signal (hereinafter referred to as P
The WM control signal) starts operating at the maximum ON width from the beginning. As a result, the terminal voltage of the output side capacitor is initially Ov
Coupled with this, a large charging current flows through this capacitor, causing the collector current of the switching element to exceed and damage it. Further, even if damage is avoided, this inrush current causes an open neat condition in which the output voltage momentarily jumps higher than the specified voltage.

<Problems to be Solved by the Invention> From the above, in conventional inverters, the above-mentioned soft start control is performed when the switching element starts switching at startup, but in reality, this control is performed using the above-mentioned magnetron. Due to the time lag when power is supplied to the preheating circuit, the PWM control signal is already at its maximum ON width when supplying power to the preheating circuit, and when supplying driving power to the original magnetron. As a result, the soft start control does not perform its function at startup to drive the magnetron, and a steep inrush current flows into the inverter, causing damage to switching elements and overshooting of the output power, especially in inverters that use low DC voltage. This has led to the occurrence of shoots.

In particular, when using a +2V or 24V power source such as an automobile storage battery, the input current must be 40A or 80A in order to obtain the same food heating effect as a microwave oven powered by commercial electricity.
This effect is particularly significant because of the large current required.

As described above, when a circuit system is adopted in which the transformer of the preheating circuit is shared with the high frequency transformer for driving the magnetron, the magnetron requires preheating and has the characteristic that it does not oscillate (does not require Wi force) until it has warmed up. However, because of this, there is a time delay between starting the inverter and actually supplying power to the magnetron, so in conventional soft start circuits, soft start control when supplying power to the preheating circuit is difficult. However, the large amount of power required for the magnetron to oscillate is sent after the previous soft start operation is completed, resulting in problems such as steep inrush current flowing to the inverter, and preheating Although soft start control is in place for this purpose, it has no effect on the magnetron drive.

The present invention is intended to solve the above-mentioned conventional drawbacks,
By creating a soft start circuit that fully takes into account the characteristics of a magnetron, the output characteristics and reliability of the circuit can be improved, and an inverter can be constructed at a low cost without requiring protection measures against particularly large inrush currents. The purpose of this invention is to provide an inverter control circuit for driving a magnetron that can be used with higher utility value.

Means for Solving the Problems> In order to achieve the above object, the present invention applies a PWM control signal whose pulse on time width gradually changes to a switching element of an inverter that drives the magnetron when the magnetron is started. An inverter control circuit for driving a magnetron, which receives a dead time control signal, and generates and outputs a PWM control signal having an on-time width corresponding to the level of the dead time control signal. , a starting means, a timer means for counting a certain period of time upon receiving a start signal from the starting means, and depending on whether or not the timer means has measured the certain period of time in response to a signal from the timer means, a switching means for switching a dead time control signal to be input to the PWM control signal generating means between a first setting value and a second setting value; and a switching means for switching a dead time control signal to be input to the PWM control signal generating means from an initial value at startup. The present invention is characterized by comprising a gradual change means for gradually changing to the first setting value and further gradually changing from the first setting value to the second setting value.

<Function> Due to its characteristics, the magnetron needs to be preheated when it is started up. Power is first supplied to the preheating circuit, and once the magnetron has warmed up, power is sent to the magnetron. It is necessary to apply soft start control to gradually raise the output voltage to avoid a sudden rush current to the inverter. Therefore, in the present invention, power is supplied to the magnetron's preheating circuit during the first soft start operation, and when the preheating is finished and current flows into the magnetron, a large amount of power is gradually supplied to the magnetron during the second soft start operation. This prevents damage to the switching elements due to sudden inrush current flowing into the inverter.

The circuit of the present invention basically uses f) dead time control in WM switching control.

When the inverter is started, the dead time control signal to be given to the PWM control signal generation means is changed by the switching means into two stages, the first and second set values, depending on whether or not the timer means has timed a certain period of time. It will be done. The fixed time measured by this timer means is set to the time when preheating ends. And, by slow change means, dead time con!・[The J-ru signal gradually changes from the initial value to the first set value, and after the above-mentioned fixed time period indicating completion of magnetron preheating, gradually changes from the first set value to the second set value. Changes to Therefore, the PWM control signal is
The on-time width is gradually changed when the magnetron is preheated and when the magnetron is driven.

Since the dead time control mentioned above is a well-known fact, it will not be explained in detail here, but to briefly touch on it, by applying a voltage signal to the dead time control terminal of the f'WM control signal generating means, the As shown in Figure 4, it is possible to obtain a switching ON time proportional to the given voltage level.

<Embodiment> FIG. 1 shows an embodiment of an inverter control circuit for driving a magnetron according to the present invention. This circuit is the start switch SW1
1 timer circuit, 1 transistor! , 1'r2, resistor Ra, 1111% RC%, and capacitor c1 resistor Rd. Below, the operation of this circuit will be explained in Section 1.2.
This will be explained with reference to the figures. First, when the starting switch SW as the starting means is in the OFF state, the reference voltage V ref is applied to the dead time control terminal of the PWM control IC2 as the PWM control signal generating means because the transistor Tr2 is ON. Due to the characteristics of dead time control mentioned earlier, PWM
The control IC 2 does not output a PWM control signal (pulse signal). Therefore, this starting switch SW
and transistor 'r r 2 constitute a starting circuit. Next, when the starting switch SW is turned on, the transistor Tr2 is turned off, and at the same time, the timer circuit 1 serving as a timer means is turned on and a time limit is started. At this time PW
Since Trl is in the on state, the voltage level input to the dead time control terminal of M control IC2 is set to a voltage that is obtained by dividing the reference voltage Vrer into Rc/(Ra+Rc) using a slow loop composed of capacitor C and resistor 11d. It gradually decreases according to the time constant of the CR circuit as the changing means. This is the first stage soft start,
As shown in Figure 3.4, the pulse-on time width of the PWM control signal output from the control IC gradually expands to a predetermined on-time width in proportion to the fluctuation in the voltage level of the dead time control terminal. This realizes soft start control.

Next, after turning on the start switch SW (timer circuit 1
When a predetermined period of time (after T is turned on) has elapsed, the output of timer circuit l is turned off and transistor T is turned on.
By turning off r l, the voltage level manually applied to the dead time control terminal of PWM control lc2 changes from the reference voltage V ref to Re/(Ra+11c
+Rb), which again gradually decreases according to the time constant of the CR circuit. Here, the reference voltage ref and the resistors 1'la, Rc, and Rh constitute a voltage level circuit, and the transistor Trl constitutes a switching means for switching the voltage level in response to a signal from the timer circuit l.

As a result of this control, as shown in Figure 2, the PWM control signal that has expanded to a certain ON time width during the soft start of the first stage is further expanded to a predetermined ON time width during which the inverter normally operates, and is then applied to the second stage. A soft start operation will be performed.

By using this circuit as described above, the first stage soft start operation supplies power to the magnetron heater, and once the magnetron has warmed up, the second stage soft start operation is performed when the magnetron requests input current. By doing so, it is possible to prevent a sudden rush current from flowing into the inverter.

The present invention can be implemented not only by hardware such as the circuit example described above, but also by software processing using a microcomputer or the like.

In the above embodiment, the soft start is performed in two different stages, but it may be performed continuously.

<Effects of the Invention> As described above, according to the present invention, since a two-step soft start is performed, for example, when it is necessary to flow a large current into the inverter using a DC low voltage power source (such as an automobile battery), Therefore, it is possible to realize an inverter for driving a magnetron for a microwave oven, etc., which can operate stably without causing a steep inrush current to flow into the circuit when starting the magnetron.

In addition, when using a 12V or 24V power source such as an automobile storage battery, a large input current of 40Δ or 80A is required to obtain the same food heating effect as a microwave oven powered by commercial power. The effects of the present invention are significant.

Furthermore, according to the present invention, it is not necessary to take any large-scale protection measures against transient inrush current, which leads to cost reduction. It can be applied to an extremely wide range of applications.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is a characteristic diagram of the time-dead time control signal of the above embodiment, and Fig. 3.4 is a characteristic diagram of the dead time control signal and the dead time control signal. FIG. 3 is a diagram showing a relationship with a PWM control signal. l...Timer circuit, 2...PWM control IC. Trl Tr2・Transistor, Ra, Rb, Re, Rd...Resistor, C...Capacitor.

Claims (1)

[Claims] (1) An inverter control circuit for driving a magnetron that outputs a PWM control signal whose pulse on time width gradually changes to a switching element of an inverter that drives the magnetron when the magnetron is started, and which outputs a dead time control signal. a PWM control signal generating means for generating and outputting a PWM control signal having an on-time width corresponding to the level of the dead time control signal; a starting means; and a timer means for measuring time, and upon receiving a signal from the timer means, the PW according to whether or not the timer means has timed the predetermined time.
switching means for switching the dead time control signal to be input to the M control signal generating means between a first set value and a second set value; 1. An inverter control circuit for driving a magnetron, comprising: gradual change means for gradually changing the first setting value to a second setting value.