JPH01227390A - High frequency heating equipment - Google Patents

Abstract

PURPOSE:To shorten the time necessary to start the oscillation of a magnetron and to prevent the generation of smoke and fire at a short circuit by providing a detector winding to detect the output voltage of a high voltage transformer and a fuse in the said winding. CONSTITUTION:The rectification direction of a rectifier circuit 44 for the output voltage signal of an inverter circuit 25 is set so as that the input current Iin from a commercial electric source 20 can be detected with an input current detector and the voltage signal corresponding to a high voltage VAK can be detected with a detector winding 37 of a high voltage transformer 30. The current reference signal 45 is set low in the on-time of a transistor 48 in the current reference signal switching circuit 47 and in the off-time of that, and the voltage reference signal 38 is set between the allowable applied voltage and the high voltage at the rating output of a magnetron 34. Then the input current is suppressed low before the start of the oscillation of the magnetron 34, and it is controlled high after the start of the oscillation. And a fuse 49 is connected to the winding 37. Thereby at a short circuit of the winding 37 the fuse 49 can be disconnected to prevent the generation of smoke and fire.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a high-frequency heating device using a magnetron, and more particularly to a power supply section of a high-frequency heating device equipped with an inverter circuit.

BACKGROUND OF THE INVENTION The main circuit configuration of a conventional high-frequency heating device is shown in FIG. A unidirectional power source 3 obtained by rectifying a commercial power source 1 with a diode bridge 2 is converted to a high frequency by an inverter circuit 4.

5 is an inductor, and 6 is a capacitor, which serves as a high frequency filter for the inverter circuit. Reference numeral 7 denotes a common capacitor, which forms a resonant circuit with the primary winding 9 of the high-voltage transformer 8.

10 is a power transistor for switching, 11 is a diode for commutation, and synchronizes the power transistor 10 with a resonant circuit to control on/off.
This reduces losses. 12 is an inverter control circuit including a power transistor drive circuit. The power source that has become high frequency in the inverter circuit 4 is boosted by the transformer 8 and then 2
A high voltage and high frequency voltage is generated in the next winding 13. The secondary voltage of the transformer is voltage-doubled and rectified by a high-voltage capacitor 14 and a high-voltage diode 15, and then applied to the magnetron 16.

On the other hand, the cathode 17 of the magnetron is supplied with power by the heater winding 18 of the transformer 8, becomes heated, and begins to emit electrons. In this way, the magnetron 16 oscillates and generates radio waves. Reference numeral 20 denotes an input current detection section which is controlled by an inverter control circuit 12 to keep this output constant, thereby stabilizing the output of the high frequency heating device.

Problems to be Solved by the Invention In the high-frequency heating device shown in FIG. 6, the inverter control circuit 12 receives the signal from the input current detection section 19,
is adjusted to a predetermined value. (Adjust the conduction time ratio of the switching element 1o) On the other hand, in the magnetron, until the cathode temperature rises sufficiently to the desired temperature and starts oscillation, current flows only through the heater section and no current flows between the anode and cathode. If the input current detection section is controlled to a value during oscillation operation of the magnetron, an excessive voltage will be generated between the anode and the cathode, resulting in breakdown voltage failure. Further, an inconvenience may occur in that an excessive current flows through the heater, shortening the life of the magnetron.

Therefore, the configuration is such that the input current is initially controlled to a value smaller than a predetermined value in the steady state, and a time control mechanism is provided in the control section to switch the input current to the steady value after a sufficient time has elapsed to start the oscillation. 12 to solve the above problem.

However, the time required to start the oscillation is, for example, 4! when the magnetron is cold! a: When the temperature is warm, there is a difference of 2 seconds, but in such a case, with the input current control method using the time control mechanism, the time required to limit the input current to a small value is the maximum time required to start oscillation. (4 sec), for example, it must be set to about 5 SOC.

With this setting, even when the magnetron is warmed up, there is always a time period of 5 seconds in which the input current is controlled to a small value. Therefore, in a conventional high-frequency heating device using such a control method, although this time difference (s-2 = ssec) can be controlled to a predetermined input current, the input current is controlled to be small. As a high-speed cooker, it was a complete waste of time, which was a major drawback.

Figure 7 shows the input current rin until the magnetron starts oscillating.
, magnetron P0 and anode-cathode voltage ■A
A timing diagram of K is shown. During the periods a and b, the input current is limited to a small value, so that the magnetron does not generate the predetermined output P0, and during the period C, the output of the magnetron reaches the predetermined value.

Although the time required for the magnetron to start oscillating is the period a, the current is also limited during the period b, and this period is wasted. To reduce this wasted time,
There is a method in which heater power is supplied by a heater transformer separate from the voltage transformer 8 and the cathode of the magnetron 7 is preheated in advance, but this method has the disadvantage that the device becomes larger and the cost increases.

Therefore, it is very difficult to shorten the rise time of the cathode temperature when the magnetron's cathode is cold, so such conventional configurations have their own limits in shortening the dead time.

Furthermore, during the dead time period, the input current is limited to a small value, so that insufficient power is supplied to the magnetron heater, and the magnetron oscillates when the cathode temperature is lower than the desired temperature. are doing. Therefore, oscillation tends to become unstable when cathode emission is insufficient, and so-called emission modeing tends to occur. Therefore, there was a serious drawback that the life of the magnetron was likely to be shortened.

Means for Solving the Problems: A unidirectional power source, an inverter circuit including a switching element that converts the unidirectional power source into a high frequency signal, and a high voltage transformer that boosts the output of the inverter circuit to a high voltage and high frequency signal.
A magnetron energized by the high-voltage transformer and a detection winding for detecting the output voltage of the high-voltage transformer are provided, and the detection winding is made of a material that has a characteristic of melting when a certain predetermined current is exceeded. It has the following configuration.

Function The present invention uses the above-described configuration to detect the output voltage of the high voltage transformer, detects the start of oscillation of the magnetron based on a change in the output voltage, and limits the output even though the magnetron is oscillating. This eliminates the above-mentioned wasted time. Furthermore, since the detection winding has the characteristic of melting when a certain current is exceeded, smoke will be emitted when the winding is short-circuited due to a mistake in soldering the control board.

This makes it possible to create a highly reliable high-frequency heating device without the risk of ignition.

Embodiments Hereinafter, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1 is a circuit diagram of a high-voltage power generation section of a high-frequency heating device according to the present invention. In FIG. 1, power from a commercial power source 20 is rectified by a diode bridge 21 to form a unidirectional power source 22. In FIG.

23 is an inductor, and 24 is a capacitor, which serves as a filter for the high frequency switching operation of the inverter circuit 25.

The inverter circuit 825 is connected to the resonant capacitor 26. It is composed of a switching power transistor 27, a diode 28, and a drive circuit 29.

The power transistor 27 has a predetermined period and duty (i.e.,
Switching operation is performed based on the on/off time ratio). The resulting high-frequency power is supplied to the primary winding 31 of the transformer 30, appears as a high-frequency high-voltage output in the secondary winding 32, is rectified by the voltage doubler rectifier circuit 33, and is supplied between the cathode 35 and anode 36 of the magnetron 34. and trance 3
Low voltage high frequency power is generated in the tertiary winding 37 of the magnetron 34, heating the cathode 35 of the magnetron 34 and operating the magnetron.

The input current detector 36 detects the input current Ii from the commercial power supply 2o.
The difference between the signal whose output is rectified by the input current signal rectifier circuit 37 and the current reference signal 38 is amplified by the current error amplification circuit 39 and inputted to the comparator 4o. The comparator 4o generates an on/off pulse 42 for the power transistor 27 using this input signal and the sawtooth wave generated by the sawtooth wave generating circuit 41. This input current detector 36
The components up to the comparator 40 constitute the input current control section 43, and when the input current "in" decreases, the current error amplifier circuit 39
The output of the input current i increases, and the on-time of the on-off pulse 42 becomes longer. act in the direction of increasing Conversely, when the input current Ii increases, it operates to reduce the input current. In this way, the input current control section 43 performs control so that the input current Ii becomes a predetermined value.

In addition, since the detection winding 31 provided in the transformer 3o has high coupling with the secondary winding, it detects the high voltage AK, and the output is rectified by the output voltage signal rectifier circuit 44. Compare with voltage reference signal 45 by comparator 46,
Input the output logic to the current reference signal switching circuit section 47,
The current reference signal 38 is switched according to the high voltage vAK.

That is, until the magnetron 34 starts oscillating, the high voltage vAK increases with a small amount of input current, so the current reference signal 38 is kept low, and when the magnetron 34 starts oscillating and the high voltage vAK decreases, the input current " The voltage reference signal 38 is set so that the current reference signal 38 is switched by the transistor 48 so as to increase in.In other words, when the output voltage of the detection winding 37 decreases, it is detected that the magnetron 34 has started oscillation. Detects and inputs current
It will be necessary to adjust the unit to the rating.

FIG. 2 shows the high voltage power supply vAK voltage waveforms when the magnetron 34 is oscillating and when it is not, and the difference between the two is obvious. This voltage in the negative direction is the forward voltage that causes the magnetron 34 to oscillate. When the relationship between the two is determined, the operating principle diagram as shown in FIG. 3 is obtained.

In Fig. 2, 1 is the allowable applied voltage of the magnetron 34, 11 is the input current from the commercial power supply 20 when the magnetron is not oscillating and the high voltage vAKP is equal to vl, and v2 is These are the high voltage vAKP at the rated output of the magnetron 34 and the input current I at that time. Input current 1 when magnetron 34 is not oscillating
1 is the output voltage v when it is small and oscillates.
2 is smaller than ■1.

Therefore, in FIG. 1, the rectification direction of the output voltage signal rectifier circuit 44 is adjusted so that the input current detector 36 can detect the input current in and the detection winding 37 can detect the voltage signal corresponding to the high voltage voltage AK. The current reference signal 46 is set so that when the transistor 48 of the current reference signal switching circuit section 47 is on, it is set to 1, and when it is turned off, it is set to 2, and the voltage reference signal 38 is set so that it corresponds to between vl and v2. According to the above-mentioned moving cloth principle, the input current is suppressed to I2 until the magnetron 34 starts oscillating, and when it starts oscillating, I2
controlled by.

A fuse 49 is connected to the detection winding 37. Since the detection winding 37 is connected to the drive circuit 29 on the printed circuit board, the possibility of a short circuit due to solder defects is much greater than that of other windings. Due to the current flowing through the wires, the wires may emit smoke and catch fire, which is dangerous. Therefore, even if the windings are short-circuited, the fuse 49 is disconnected for safety. Taking such measures is very important and will greatly improve reliability.

FIG. 4 shows an example of the transformer of the high frequency heating device of the present invention. The cores 50 and 51 are fixed to the transformer stand 53 with a fastener 54 with a gap spacer 52 in between. Each winding is wound around the bobbin 55. Reference numeral 56 is the primary winding, which is made of Lindt wire and has housings 57 and 58 attached to its ends in order to pass a large high-frequency current. The high voltage side lead SO of the secondary winding 59 is a high voltage lead with housings at both ends. The low voltage side is connected to a transformer stand 53. The tertiary winding 61 is made of a high-voltage coated wire, and has grouper-shaped terminals 62 and 63 at the tip. The detection winding 64 for detecting the output voltage is provided with a fuse resistor 65 for safety in the event of a short circuit, and the lead of the detection winding is connected to the transformer base 53 through a protective tube 66 with a claw. This protective tube prevents the leads from breaking, and a two-terminal housing is attached to the tip of the detection winding 64.

The fuse resistor 65 is mounted at a position where it can be easily seen from the side of the transformer, so that it can be easily seen from the side of the transformer, and is installed during manufacturing to prevent forgetting, and to visually inspect if the fuse is activated and burnt out.

It goes without saying that in order to prevent smoke generation 9 from ignition in the event of a short circuit, a fusing wire may be used in the detection winding in addition to a fuse or a fuse resistor.

Figure 5 shows vAKP after the circuit in Figure 1 starts operating.
FIG. 3 is a starting characteristic diagram showing changes in values of and Iiyu. When the magnetron starts oscillating, vAKP decreases. Then, the set value of the predetermined voltage V, and when it goes below
Control to switch to 2xn
1 has been done.

Effects of the Invention As described above, according to the high frequency heating device of the present invention, the following effects can be obtained.

(1) This method uses a detection winding to detect the start of oscillation of the magnetron and control the input current, which reduces the time required for the magnetron to start oscillating, and improves the reliability of the magnetron because the unstable oscillation state of the magnetron is short. improves.

(2) Since the detection winding is provided with a fuse, smoke generation 9 can be prevented from ignition in the event of a short circuit, and a highly safe high-frequency heating device can be provided.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the high-frequency heating device of the present invention, Fig. 2 is a waveform diagram between the anode and cathode of the magnetron of the same device, and Fig. 3 is a diagram of the voltage peak value and input current applied to the magnetron of the same device. 4 is a perspective view of the voltage transformer of the same device, FIG. 5 is a timing chart when the magnetron of the device starts oscillating, FIG. 6 is a circuit diagram of a conventional high-frequency heating device, and FIG. 7 is a diagram of the conventional high-frequency heating device. FIG. 2 is a timing chart when the magnetron of a conventional device starts oscillating. 22...Unidirectional power supply, 25...Inverter circuit, 2T...Switching element, 3o...
... High voltage transformer, 34 ... Magnetron, 37 ... Detection winding, 49 ... Fuse. Name of agent: Patent attorney Toshio Nakao and 1 other person22
--- 17 cylinders (25-- Inverter Dro, bowl --- Mabuo 1-D Ji 37-# Maru 14 group 49 --- and knee 2nd figure 3rd figure 4th figure 5th figure

Claims (3)

[Claims] (1) An inverter circuit including a unidirectional power source, a switching element that converts the unidirectional power source into high frequency, a high voltage transformer that boosts the output of the inverter circuit to high voltage and high frequency, and a magnetron energized by the high voltage transformer. and a high-frequency heating device comprising a detection winding for detecting the output voltage of the high-voltage transformer, the detection winding having a characteristic of melting when a certain predetermined current is exceeded. (2) The high-frequency heating device according to claim 1, wherein the detection winding is provided with a fuse resistor. (3) The high-frequency heating device according to claim 1, wherein part or all of the detection winding is made of a conductive material that has a characteristic of melting due to self-heating caused by electric current.