JP3085503B2 - High frequency heating equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency heating device for driving a magnetron by an inverter circuit.

[0002]

2. Description of the Related Art In recent years, a magnetron drive circuit of a high-frequency heating device has been reduced in size and weight by converting it to an inverter, and the content of the control circuit has been changed from analog to LS.
Digital circuits using I are being switched. As a known example of this kind, the invention of Japanese Patent Application Laid-Open No. 64-52396 can be seen. FIG. 2 shows a circuit example of a high-frequency heating device having a magnetron inverter circuit with a digitized control circuit.

In FIG. 2, a commercial AC power supply 1 is rectified and smoothed by a rectifying and smoothing circuit including a diode bridge 2, a choke coil 3, and a smoothing capacitor 4. The inverter circuit is the primary winding 5 of the magnetron driving transformer 5
a, a resonance capacitor 6 connected in parallel to the primary winding 5a, a semiconductor switching element 7 and a diode 8 also connected in series to the primary winding 5a, and a semiconductor switching element in the drive circuit 9 according to a drive signal from the flip-flop 103 of the control circuit. 7 is turned on and off to generate a high-frequency output on the secondary side of the transformer 5. The high-frequency voltage generated in the secondary winding 5b of the transformer 5 includes a capacitor 10 and a diode 11
Is rectified and boosted by the voltage doubler rectifier circuit constituted by
Applied to the magnetron 12. The output of the tertiary winding 5c of the transformer 5 is applied to the filament of the magnetron 12. Thereby, the magnetron 12 oscillates a microwave.

Here, the smoothing capacitor 4 of the rectifying and smoothing circuit is used.
Is generally small, the voltage E after rectification and smoothing has a pulsating flow as shown in FIG.

On the other hand, when the semiconductor switching element 7 is turned on and off, currents as shown in FIGS. 4A to 4D flow through the primary winding 5a and the magnetron 12 of the transformer 5, respectively, and a voltage is generated. Assuming that the ON time of the semiconductor switching element 7 is TON and the inductance of the primary winding 5a of the transformer 5 is L, the peak value of the primary winding current i1 is given by approximately E / L × TON. Therefore, if the ON time TON is kept constant, the peak value of the primary winding current i1 will increase or decrease according to the change in the rectified smoothing voltage E, and an excessive current will flow at the peak.

Since the magnetron current iMG also substantially increases with the primary winding current i1 during the ON period, the peak value similarly becomes large, causing a problem that the magnetron 12 causes a moding phenomenon. Occurs.

A large primary winding voltage v 1 is generated due to resonance between the primary winding 5a of the transformer 5 and the resonance capacitor 6 when the semiconductor switching element 7 is turned off. Is determined by the energy 1 / 2Li 2 stored in the transformer, a large peak voltage is also generated when the primary winding current i 1 is excessively large, and the transformer 5 and the semiconductor switching element 7 must use a large withstand voltage. Must.

If the primary winding current i 1 is large, the core of the transformer 5 is likely to saturate. To prevent this, the core area must be increased, resulting in an increase in the size of the transformer. I will.

From the above points, it is necessary to suppress these voltages and currents, and this has been conventionally performed using the following method.

A count number based on the output set value is set in a register (B) 109 from an output setting unit 110 for setting a high frequency output of the magnetron 12. Further, the counted number is input from the register (B) 109 to the register (A) 107. The edge detection circuit 104 detects the fall of the primary winding voltage v1 based on the output of the voltage detection unit 13, and generates a synchronization pulse as shown in FIG. The increase / decrease circuit 112 increases / decreases the count number of the register (A) 107 based on the synchronization pulse as follows.

The voltage detecting section 13 includes a detecting winding 5 of the transformer 5.
The primary winding voltage v1 is detected by d, and this value is compared with the reference value vs1 by the comparator 114. The current detector 14 detects the magnetron current iMG, and the comparator 115 compares this value with the reference value vs2. NO if both output values are less than the reference value
The output of R113 becomes H, and the up / down counter of the register (A) 107 is set to up.

The increase / decrease circuit 112 outputs one clock pulse to the register (A) 107 in response to the synchronization pulse, thereby increasing the count number by one. Conversely, if one of the output values is equal to or greater than the reference value, the output of the NOR 113 becomes L, and the up / down counter of the register (A) 107 is set to down. Decrease the number by one. After that, the counter 106 inputs the count number of the register (A) 107 by the LOAD signal of the increase / decrease circuit 112 and counts down the count by the reference clock pulse of the reference clock generation circuit 108 to control the ON time of the semiconductor switching element 7. Is going. Therefore, when the voltage / current exceeds the reference value in a range where the rectified smoothed voltage E is large, the count number is reduced to thereby reduce the count number as shown in FIG.
As shown in (1), the ON time is shortened in accordance with the rectified smoothed voltage E, and the voltage and current are controlled to be less than a predetermined value.

[0013]

However, according to this method, the output of the synchronization pulse as a reference for controlling the on-time is once per one cycle of the on-off of the semiconductor switching element 7, so that the on-time is increased or decreased. Is step by step, and as shown in part A of FIG. 3A, when the rectified smoothing voltage E suddenly increases due to some disturbance, the on-time cannot be increased or decreased in steps. It is expected that the voltage and current will suddenly increase and exceed the reference value. As a result, a large current will flow through the semiconductor switching element 7 and stress will be applied, possibly leading to destruction.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 3-172587, as a high-frequency heating device for quickly detecting a sudden change in the primary winding current of the transformer and turning off the semiconductor switching element, the primary winding current of the transformer is detected. Value A
A / D converter converts the voltage to digital and feeds it back to the control circuit. When an excessive current flows due to a sudden increase in the current voltage due to some disturbance as shown in FIG. It can be followed by detecting a sudden change in current immediately and turning off the semiconductor switching element.However, if the power supply is momentarily interrupted for any reason, it will start operating immediately after the power is restored, with the ON time immediately before the instantaneous interruption. Similarly, there is a possibility that the semiconductor switching element may be stressed by an overcurrent.

According to the present invention, the voltage and current of each part of the inverter circuit are reliably suppressed to a set reference value or less even in the case of disturbance such as a sharp change in the voltage of the rectifying / smoothing circuit, thereby improving reliability and safety. With the goal.

[0016]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and includes a rectifying and smoothing circuit for rectifying and smoothing a commercial power supply to form a DC power supply, a transformer, a capacitor, a diode, a switching element, and the like. In a high-frequency heating device composed of an inverter circuit composed of: a magnetron driven by this inverter circuit, and a digital control circuit that controls the on-time of a switching element of the inverter circuit, the rectified smoothing voltage changes sharply. To cope with this, it is only necessary that the on-time can be instantaneously changed according to the change in the rectified smoothing voltage.

Therefore, a register for storing the ON time of the switching element in the digital control circuit, and the amount of change in the ON time added to or subtracted from the stored value of this register for each switching cycle of the switching element and stored in the register again. An addition / subtraction circuit, a high-speed A / D converter operable at several tens of MHz, taking in the voltage after the rectification and smoothing, and converting this into a digital value for each switching cycle of the switching element; An on-time change amount calculating circuit for calculating a change in the digital value for each switching cycle of the switching element and calculating the on-time change amount based on the calculated digital value. It changes the rate of change.

[0018]

According to the present invention, the on-time of the switching element is stored in the register, and the on-time change amount is added to or subtracted from the value stored in the register for each switching cycle of the switching element by the addition / subtraction circuit. The increase / decrease of the on-time is added to the value stored in the register for each cycle, and the amount of increase / decrease is added, and the result is set as a new value as a new on-time.

The high-speed A / D converter takes in the rectified and smoothed voltage and converts it into a digital value for each switching cycle of the switching element at a conversion speed of several tens of MHz. The change of the converted digital value for each switching cycle of the switching element is calculated, and the on-time change amount is calculated based on the change.If the change of the converted digital value is large, the change amount of the on-time is increased and the reverse is performed. If the change is small, the change amount is reduced. As a result, it is possible to change the on-time change step time width, which has been fixed because it was conventionally performed step by step, according to the change in the voltage after rectification and smoothing.

[0020]

An embodiment of the present invention will be described below.

FIG. 1 is a circuit diagram of a high-frequency heating device according to an embodiment of the present invention. In FIG. 1, FIG.
The same reference numerals are given to the same portions as those of the conventional circuit shown in FIG. 1 and their operations are the same, and the on-time control of the semiconductor switching element 7 is performed in the same manner as in FIG.

In FIG. 1, a register (A) 107 for storing the ON time of the switching element 7 in a digital control circuit and a switching element 7 in order to cope with a steep change of the rectified and smoothed voltage E after the rectification and smoothing. The on-time variation is added to or subtracted from the value stored in the register (A) 107 every switching cycle of
7, a high-speed A / D converter 101 operable at several tens of MHz, taking in a rectified and smoothed voltage E and converting it into a digital value for each switching cycle of the switching element 7, and a high-speed A / D converter 101. An on-time change amount calculation circuit 102 is provided for calculating a change of the digital value converted by the / D converter 101 for each switching cycle of the switching element 7 and calculating the on-time change amount based on the change.

As in FIG. 2, a count number based on the output set value from the output setting unit 110 is set in the register (B) 109. The comparison circuit 108 compares the value of the register (B) 109 with the addition / subtraction circuit 105 described below.
The value of the register (A) 107 which is increased or decreased by the above operation is compared, and when the latter value becomes equal to the former value, the addition inhibition signal INH is sent to the addition / subtraction circuit 105 to inhibit the addition, and the register (B) ) Register (A) 1 from the value of 109
07 is prevented from increasing.

The high-speed A / D converter 101 detects the rectified and smoothed voltage E, converts this voltage value to a digital value at a high speed of several tens of MHz, and outputs the digital converted value to the on-time change amount calculating circuit 102. I have. The on-time change amount calculation circuit 102 reads the digital conversion value of the A / D converter 101 for each switching cycle of the switching element 7,
The on-time change amounts QS0 to QSn are calculated from the difference between the stored read value and the previous read value, and are calculated and added to the addition / subtraction circuit 105.
, And whether the ON time is to be subtracted or added is determined depending on whether the converted value has increased or decreased, and the result is similarly output to the addition / subtraction circuit 105 as an addition / subtraction signal.
However, when the digital conversion value from the A / D converter 101 is less than the specified value, the on-time change amount QS0 to QS
Outputs zero as n. This is because a specified output cannot be obtained if the above-described on-time control is performed in all voltage phases.

The addition and subtraction circuit 105 is configured such that the edge detection circuit 104 outputs the primary winding voltage v 1 based on the output of the voltage detection unit 13.
The addition / subtraction of the outputs Q0 to Qn of the register (A) 107 and the variation data QS0 to QSn from the on-time variation calculation circuit 102 is performed based on an addition / subtraction signal using a synchronization pulse output by detecting the fall of Do. The adjusted data is stored again in the register (A) 107 and transmitted to the counter 106 as the ON time.

With such a circuit configuration, the on-time changes as follows. First, when there is no disturbance, the rectified smoothed voltage E has a waveform obtained by full-wave rectifying a sine wave as shown on the left side of FIG. Until the voltage E reaches the specified value, A /
Since the output data of the D converter 101 is less than the specified value, the on-time is constant. When the voltage E exceeds a certain specified value, the voltage increases in each switching cycle. Therefore, the on-time change amount calculation circuit 102 outputs L to the addition / subtraction signal and instructs the addition / subtraction circuit 105 to perform the subtraction.

In the portion where the voltage is rising, dv / dt gradually decreases over time, and the amount of change to be subtracted also gradually decreases. When the voltage reaches the peak, dv / d
t becomes zero and the change amount becomes zero. When the voltage starts to decrease further, the on-time change amount calculating circuit 1
02 outputs H to the addition / subtraction signal, and the addition / subtraction circuit 105
Is added, and the added amount gradually increases. When the voltage value drops to the specified value, the change of the on-time stops and becomes constant. Therefore, in this case, the on-time changes as in FIG. 3B.

When the rectified smoothed voltage E suddenly increases, the on-time change amount calculation circuit 102 detects this from the change in the output data of the high-speed A / D converter 101, and instantaneously increases the on-time change amount. And the addition / subtraction circuit 10
The on-time is shortened by the subtraction operation of No. 5. For this reason, an excessive current can be prevented from flowing through the semiconductor switching element 7.

[0029]

As described above, according to the present invention, the register for storing the ON time of the switching element in the digital control circuit, and the ON time change amount in the stored value of this register for each switching cycle of the switching element. Is added to or subtracted from the register and stored again in the register. To increase or decrease the on-time, the amount of increase or decrease is added to the value stored in the register every cycle, and the result is set as a new ON / OFF value. Time and tens of MHz
A high-speed A / D converter that takes in the rectified and smoothed voltage and converts it into a digital value for each switching cycle of the switching element;
An on-time change amount calculation circuit for calculating a change in the digital value converted by the converter for each switching cycle of the switching element and calculating the on-time change amount based on the calculated change is provided according to the voltage change rate of the rectifying and smoothing circuit. Since the rate of change of the on-time is changed, the amount of change in the on-time is increased when the change in the converted digital value is large, and the amount of change is reduced when the change is small. As a result, the step width of the change of the on-time, which has been fixed because it was conventionally performed in steps of one step, can be changed according to the change in the voltage after rectification and smoothing. Overcurrent can be reliably suppressed, and improvement in reliability and safety can be expected.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a high-frequency heating device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional high-frequency heating device.

FIG. 3 is a characteristic diagram of a rectifying / smoothing circuit of a conventional example, and FIG.
FIG. 2B is a voltage characteristic diagram, and FIG. 2B is an on-time characteristic diagram of a switching element controlled according to a rectified smoothed voltage.

FIG. 4 is a voltage current signal waveform diagram of each part of a conventional inverter circuit.

[Explanation of symbols]

 7 Switching element 101 A / D converter 102 On-time change amount calculation circuit 105 Addition / subtraction circuit 107 Register

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H05B 6/68

Claims (1)

(57) [Claims] A rectifying and smoothing circuit for rectifying and smoothing a commercial power supply to produce a DC power supply, a transformer, a capacitor, a diode,
An inverter circuit composed of switching elements and the like;
A magnetron driven by this inverter circuit,
In a high-frequency heating device comprising a digital control circuit for turning on and off a switching element of an inverter circuit, a register (107) for storing the on time of the switching element (7) in the digital control circuit.
And an addition / subtraction circuit (105) for adding or subtracting the on-time change amount to or from the value stored in the register (107) for each switching cycle of the switching element (7) and storing the result in the register (107) again; A high-speed A / D converter (1) that takes in voltage E and converts it into a digital value for each switching cycle of switching element (7)
01), and an on-time change amount for calculating a change of the digital value converted by the high-speed A / D converter (101) for each switching cycle of the switching element (7) and calculating the on-time change amount based on the calculated change. Calculation circuit (10
2. A high-frequency heating device provided with 2).