JPH0495386A - High frequency heating cooking device - Google Patents

Abstract

PURPOSE:To maintain the high frequency output of a magnetron constant by detecting the anode current of the magnetron with an anode current detecting means, and controlling a width of the on-time of a switching element in the direction for keeping the anode current at a constant value on the basis of the detecting signal. CONSTITUTION:During the oscillating operation of a magnetron 21, the anode current of the magnetron 21 is detected by a current transformer 22, and this detected current value I22 is averaged by a current averaging circuit 29, and an error amplifying circuit 30 compares the voltage signal V29 corresponding to a mean anode current value with the standard voltage signal V31 to output the differential voltage signal V30 corresponding to a difference thereof. An on-time width determining circuit 32 controls a time width (pulse width) of the base signal S32 so that as the differential voltage signal V30 become larger, a width of on-time of a switching transistor 15 becomes shorter.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention comprises a frequency conversion circuit that converts a commercial power source into a high frequency power source by controlling the on/off period of a switching element. The present invention relates to a high-frequency cooking device in which the AC output of a circuit is boosted by a step-up transformer and then fed to a magnetron.

(Prior Art) This type of conventional high-frequency cooking apparatus has a problem in that when the commercial power supply voltage, which is the input voltage, fluctuates, the high-frequency output of the magnetron also fluctuates, resulting in poor cooking performance.

(Problem to be solved by the invention) In order to eliminate the conventional problems, it is necessary to detect the anode current of the magnetron and adjust the on-time width of the switching element of the frequency conversion circuit so that the anode current maintains a constant value. It is conceivable that the high-frequency output of the magnetron can be kept constant by controlling this.

However, when the on-time width of the switching element is controlled so that the anode current is constant as described above,
As the commercial power supply voltage decreases, the input current increases,
Thermal stress on semiconductor elements such as rectifier circuits and switching elements that constitute a frequency conversion circuit increases, and if the thermal stress becomes excessive, it will adversely affect the life span of the semiconductor elements. Furthermore, as the input current increases, the primary current of the step-up transformer also increases, so the temperature of the secondary coil also increases.
It becomes overheated. Moreover, if the input current becomes excessive, safety devices such as indoor wiring circuit breakers will be activated, and other electrical equipment will also be adversely affected.

The present invention was made in consideration of these circumstances, and its purpose is to provide a high-frequency cooking device that can keep the high-frequency output of the magnetron as constant as possible while preventing the input current from becoming excessive. It is about providing.

[Structure of the Invention] (Means for Solving the Problems) The high-frequency cooking device of the present invention is provided with a frequency conversion circuit that converts a commercial power source into a high-frequency power source by controlling the on/off period of a switching element, and this frequency conversion A step-up transformer is provided which receives the alternating current output from the circuit on its primary side and boosts the voltage, a magnetron is provided to which the output of the secondary side of the step-up transformer is given, and an anode current detection means is provided to detect the anode current of this magnetron. A control circuit is provided for controlling the on-time width of the switching element in the frequency conversion circuit so that the anode current maintains a constant value based on a detection signal from the anode current detection means, The present invention is characterized in that it has a limiting means that limits the time width to be constant at the maximum on-time width when the time width exceeds a set maximum on-time width.

During operation, the anode current of the magnetron is detected by an anode current detection means, and based on the detection signal, the on-time width of the switching element is controlled so that the anode current maintains a constant value, thereby controlling the high-frequency output of the magnetron. control to keep it constant.

Then, the limiting means of the control circuit limits the input current to be constant at the maximum on-time width when the on-time width of the switching element exceeds the preset maximum on-time width. It is possible to keep the high frequency output of the magnetron at a constant level while preventing it from becoming excessive.

(Example) An example of the present invention will be described below with reference to the drawings.

In FIG. 1, 1.2 is a power supply terminal to which commercial power is supplied, and these are connected to an AC input terminal of a rectifier circuit 4 in a frequency conversion circuit 3. This rectifier circuit 4 is constructed by connecting diodes 5 to 8 as semiconductor elements in a bridge manner, and a filter circuit 11 consisting of a series circuit of a choke coil 9 and a capacitor 10 is connected between its DC output terminals.
is connected. The frequency conversion circuit 3 is for converting the commercial power supplied between the power supply terminals 1 and 2 into a high frequency power supply, and its main element is an oscillation circuit that resonates with the primary coil 12a of the step-up transformer 12 in parallel with the capacitor 9. A series circuit with a resonance capacitor 13 is connected, a diode 14 is connected in parallel to the resonance capacitor 13, and the collector and emitter of a semiconductor element as a switching element, such as a switching transistor 15, are connected to the anode and cathode of the diode 14. has been done.

The switching transistor 15 of the frequency conversion circuit 3 has an on/off period controlled by a control circuit 16, as described later, so that the switching transistor 15 is connected to the step-up transformer 12.
A high frequency current is generated in the primary coil 12a. Further, both terminals of the secondary coil 12b of the step-up transformer 12 are connected to the anode and cathode of the magnetron 21 via a voltage doubler rectifier circuit 20 consisting of diodes 17, 18 and a capacitor 19, and the secondary coil 12c is connected to the anode and cathode of the magnetron 21. connected to the cathode. A current transformer 22 serving as anode current detection means is provided in the current-carrying path on the anode side of the magnetron 21. On the other hand, an on-timing detection circuit 25 consisting of a series circuit of resistors 23 and 24 is connected in parallel to the primary coil 12a of the step-up transformer 12, and resistors 26 and 27 are connected between the DC output terminals of the rectifier circuit 4.
A power supply voltage detection circuit 28 consisting of a series circuit is connected.

Now, turn on the switching transistor 15.

The specific configuration of the control circuit 16 that performs OFF control will be described with reference to FIG. 2. 29 is a current averaging circuit;
The input terminal of this is connected to the output terminal of the current transformer 22, and the detected current from the current transformer 22 is rectified and smoothed for one cycle of the commercial power supply voltage, and its average anode current value is is output as a voltage signal V2g. 30 is an error amplification circuit, one input terminal of which is connected to the output terminal of the current averaging circuit 29, and the other input terminal connected to the output terminal of the reference voltage signal setting circuit 31. In this case, the reference voltage signal setting circuit 31 sets a preset reference voltage signal 31 (for example, the magnetron 2
1 output cuff 00W), and
The error amplification circuit 30 receives the voltage signal V29 and the reference voltage signal V3.
□ outputs a voltage difference signal V3(]. 32 is an on-time width determining circuit, one input terminal of which is connected to a resistor 2 which is the output terminal of the on-timing detection circuit 25.
3.24, and the other input terminal is connected to the output terminal of the error amplification circuit 30. The on-time width determining circuit 32 is for determining the on-start timing and on-time width of the switching transistor 15, and is based on the voltage signal V2S from the on-timing detection circuit 25 and from the error amplification circuit 30. The base signal S of the pulse width based on the difference voltage signal ■3°
Outputs 32.

The output terminal of the on-time width determining circuit 32 is connected to a first input terminal of an AND circuit 33, and the output terminal of the AND circuit 33 is connected to the base of the switching transistor 15. 34 is a voltage averaging circuit;
This is because one input terminal is connected to the common connection point of the resistors 26 and 27 which are the output terminals of the power supply voltage detection circuit 28, and the detected voltage signal V28 from the power supply voltage detection circuit 28
is averaged by smoothing it for one cycle of the commercial power supply voltage, and the average voltage signal V34 is output. 35 is a voltage range comparison circuit, the input terminal of which is connected to the output terminal of the voltage averaging circuit 34. This voltage range comparison circuit 35 determines whether the commercial power supply voltage is within a usable range (for example, a range of 80 V or more and 120 V or less) from the average voltage signal V34 from the voltage averaging circuit 34, and when it is within the range, A stop signal S35 that is at a high level and at a low level when it is out of range is output. The output terminal of the voltage range comparison circuit 35 is connected to the second input terminal of the AND circuit 33. On the other hand, 36 is a maximum on-time width setting circuit as a limiting means, and its input terminal is connected to the on-timing detection circuit 25.
is connected to the output terminal of the

This maximum on-time width setting circuit 36 has a timing based on the voltage signal VH from the on-timing detection circuit 25 and has a pulse width of a preset maximum on-time width in one cycle of on and off of the switching transistor 15. A signal S36 is output. The output terminal of the maximum on-time width setting circuit 36 is connected to the third input terminal of the AND circuit 33.

Next, the operation of this embodiment will be explained with reference to FIGS. 3 to 6.

After the start of cooking, the switching transistor 15 is turned on,
The primary coil 12a of the step-up transformer 12 is turned off by the off control.
An oscillating current flows through the oscillating circuit consisting of the resonant capacitor 13 and the resonant capacitor 13, and FIG. 4 shows the high frequency voltage 1 and the high frequency current 11 induced in the primary coil 12a in this case. Such high frequency voltage V1 is further boosted by step-up transformer 12 and supplied to magnetron 21 to drive it. In this frequency conversion operation, the on-time width T1 of the switching transistor 15 is forcibly controlled by the base signal S3□ in accordance with the magnitude of the commercial power supply voltage, which will be described later, but the off-time width T2 of the switching transistor 15 is It is determined by the energy stored in the inductance of the step-up transformer 12 and the size of the resonant capacitor 13 during the on-time width.

That is, the off time width T2 of the switching transistor 15 is set until the time TO when the high frequency current 1 becomes approximately zero,
This time TO is also the time when the next period starts to turn on. The on time width determining circuit 32 is always on timing detecting circuit 25
receives a voltage signal V25 corresponding to the high frequency voltage V1 from
The time point To at which the high frequency current I becomes zero is determined from the voltage value Vo in this signal V25, and the timing for outputting the base signal S3□ is obtained.

On the other hand, during the oscillation operation of the magnetron 21, the anode current of the magnetron 21 is detected by the current transformer 22, this detected current value I22 is averaged by the current averaging circuit 29, and a voltage signal corresponding to the average anode current value is The error amplifier circuit 30 compares V2g with the reference voltage signal V3 and outputs a difference voltage signal V30 according to the difference. This difference voltage signal V30 becomes larger as the commercial power supply voltage applied between the power supply terminals 1 and 2 becomes higher, and in the on-time width determining circuit 32, the on-time width of the switching transistor 15 increases as the difference voltage signal V3o becomes larger. The base signal S
32 time width (pulse width). This prevents the anode current from increasing as the voltage increases.In other words, the anode current is controlled to decrease or increase as the commercial power supply voltage increases or decreases, thereby increasing the high frequency of the magnetron 21. The pressure is made constant. In addition, when the commercial power supply voltage (input voltage) is constant, the input current (input current of the frequency conversion circuit 3) with respect to the change in the ON time width during one cycle of ON and OFF of the switching transistor 15
The relationship between is as shown in FIG. 5, and when the ON time width during one cycle of ON and OFF of the switching transistor 15 is constant, the relationship of input current with respect to changes in input voltage is as shown in FIG. As shown.

In addition, in parallel with the above-described operation, the voltage range comparison circuit 35 receives the detected voltage 28 from the power supply voltage detection circuit 28 via the voltage averaging circuit 34, and the commercial power supply voltage is 80V.
When the voltage is outside the range of 120V or less, the stop signal S35 is set to low level and the AND circuit 33 is cut off.
The on/off operation of the switching transistor 15 is stopped, and the operation of the magnetron 21 is stopped. In this case, the lower limit value of 80V is determined because if the voltage is lower than this, the anode current of the magnetron 21 becomes excessive, and the upper limit value of 120V is determined to be the upper limit of the withstand voltage of the magnetron 21.

By the way, when the magnetron 21 is operated with a high frequency output of 700 W, for example, and the anode current is kept constant as described above, the input current to the frequency conversion circuit 3 becomes equal to the commercial power supply voltage ( If the current decreases below 95V in this state, the input current will exceed the permissible value of 15A, for example, as shown by the dotted line in Figure 3, so the switching transistor 15, etc. Problems such as excessive thermal stress being applied to semiconductor elements, overheating of the primary coil 12a of the step-up transformer 12, and activation of safety devices such as indoor wiring circuit breakers may occur.

Therefore, this embodiment operates as follows. That is, when the on-time width determining circuit 32 outputs the base signal S32 having a pulse width based on the differential voltage signal V30, at the same timing, the maximum on-time width setting circuit 36 outputs a pulse corresponding to the preset maximum on-time width. A width limit signal S36 is output. Both the base signal S3□ and the limit signal S36 are given to the AND circuit 33, so that the pulse width (time width) of the base signal S3□ becomes the limit signal S36.
If the pulse width is smaller than the pulse width of , the base signal S3□ is given priority and is applied to the base of the switching transistor 15 via the AND circuit 33. However, the base signal S
When the pulse width of 3□ becomes larger than the pulse width of the limit signal S36 (corresponding to when the input voltage becomes 95V), the limit signal S36 takes priority and the AND circuit 33
From this point on, no matter how large the pulse width of the base signal S32 becomes, the on-time width of the switching transistor 15 is constant to the pulse width of the limit signal S36. It will be restricted as follows. Therefore, when the input voltage drops below 95V, the input current decreases and does not exceed 15A, as shown by the solid line in FIG.

In this way, the input current to the frequency conversion circuit 3 is prevented from becoming excessive, so that excessive thermal stress is not applied to semiconductor elements such as the switching transistor 15, and the primary Coil 12a
The system will not overheat, and furthermore, problems such as activation of safety devices such as indoor wiring circuit breakers can be prevented.

In the above embodiment, the usable range of commercial power supply voltage is 8.
Although it is set to 0V or more and 120V or less, it can be set to 8V so that it can be used with either 100V or 200V commercial power supply.
The magnetron 21 may be configured to be able to be driven within a range of 0V to 260V.

In addition, the present invention is not limited to the maximum on-time width being set to correspond to an input voltage of 95V,
Various modifications are possible, for example, it may be changed as appropriate depending on the rating of each component such as the switching transistor 15 to be used.

[Effects of the Invention] As is clear from the above description, the high-frequency cooking device of the present invention controls the on-time width of the switching element of the frequency conversion circuit in such a way that the anode current of the magnetron is maintained at a constant value. , when the on-time width of the switching element becomes larger than a preset maximum on-time width, it is configured to be limited to the maximum on-time width, so that the high-frequency output of the magnetron is kept as constant as possible. It is possible to prevent the input current from becoming excessive while maintaining the input current, thereby preventing excessive thermal stress from being applied to semiconductor elements such as switching elements, and preventing the primary coil of the step-up transformer from becoming overheated. Moreover, it is possible to prevent problems such as activation of safety devices such as indoor wiring circuit breakers.

[Brief explanation of drawings]

The drawings show an embodiment of the present invention. Fig. 1 is an electric circuit diagram of a high-frequency heating cooking device, Fig. 2 is a block diagram showing details of the control circuit, and Fig. 3 is a diagram of the primary coil of a step-up transformer. Figure 4 shows the relationship between high frequency voltage and high frequency current. Figure 4 shows the relationship between commercial power supply voltage (input voltage) and input current when the anode current is constant. Figure 5 shows the relationship between the input voltage and input voltage. FIG. 6 is a diagram showing the relationship between the input voltage and the input current when the on-time width of the switching transistor is kept constant. In the drawing, 3 is a frequency conversion circuit, 12 is a step-up transformer, 1
3 is a resonance capacitor, 15 is a switching transistor (switching element), 16 is a control circuit, 21 is a magnetron, 22 is a current transformer (anode current detection means), 25 is an on-timing detection circuit, 28 is a power supply voltage detection circuit, 2
9 is a current averaging circuit, 30 is an error amplifier circuit, 32 is an on-time width determining circuit, 34 is a voltage averaging circuit, 35 is a voltage range comparison circuit, 33 is an AND circuit, 36 is a maximum on-time width setting circuit (limiting means).

Claims (1)

[Claims] 1. A frequency conversion circuit that converts commercial power into high-frequency power by controlling the on and off periods of switching elements, a step-up transformer that receives the AC output from this frequency conversion circuit on its primary side and boosts the voltage, and a second step of this step-up transformer. A magnetron to which the output of the next side is given, an anode current detection means for detecting the anode current of this magnetron, and the frequency conversion circuit that adjusts the anode current to a constant value based on the detection signal from the anode current detection means. a control circuit for controlling the on-time width of the switching element, and the control circuit is configured such that when the on-time width of the switching element exceeds a set maximum on-time width, the on-time width remains constant at the maximum on-time width. A high-frequency heating cooking device characterized by having a limiting means for limiting.