JPH074791Y2 - High frequency heating device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a high-frequency heating device such as a microwave oven, and more particularly to a device for converting a commercial power supply into a high-frequency power supply by an inverter.

[Conventional technology]

Conventionally, in a high frequency heating device, a unidirectional power source obtained from a commercial power source is converted into a high frequency power higher than the commercial frequency by an inverter, and this high frequency output is boosted by a high voltage transformer and supplied to a magnetron. Therefore, not only can the size of the high-voltage transformer be reduced, but also by controlling the switching operation of the inverter according to the input power of the inverter, it is possible to minimize the change in the input power when the power supply voltage changes.

FIG. 5 shows such a high-frequency heating device, in which a commercial power source (1) is rectified by a diode bridge rectifier circuit (2) and then smoothed by a smoothing reactor (3) and a smoothing capacitor (4). The unidirectional power supply thus obtained is intermittently made high in frequency by an inverter (8) composed of a power transistor (5) as a switching element, a freewheeling diode (6) and a resonance capacitor (7). , A high-voltage transformer (9) boosts the high-frequency high-voltage from the high-voltage transformer (9) into a DC high-voltage by a half-wave voltage doubler rectifier circuit consisting of a capacitor (10), diodes (11) and (12). It converts and supplies high voltage power to the magnetron (13).

When detecting the input power of the inverter (8), for example, a hall element (14) is used, and the power supply voltage of the positive output terminal (+) of the rectifier circuit (2) is divided by a resistor (R ₁ ) to generate a hall voltage. In addition to applying to the element (14), a magnetic concentrator (15) made of ferrite as shown in FIG. 6 is used to input the input current of the power supply line (16) of the negative output terminal (-) of the rectifier circuit (2) to the magnetic field. It is converted to and added to the Hall element (14).

Here, assuming that the applied voltage of the power supply voltage to the Hall element (14) is V _C and the magnetic flux density is B, the output voltage V _H of the Hall element (14) is expressed by V _H = K · V _C · B + V _O Since K and V _O are element-specific constants, the output voltage V _H is proportional to V _C · B, that is, the input power,
The detected power value of the input power is obtained by the power detection circuit (17) as shown in FIG.

In FIG. 5, (18) is a control signal circuit that outputs an analog signal for controlling the operating state of the inverter (8). For example, the input power is determined based on the detected power value from the power detection circuit (17). It outputs an analog signal of an appropriate level to keep it constant. (19) is a control signal circuit (18)
Pulse width modulation circuit (PWM circuit) that outputs a rectangular wave signal with a pulse width according to the level of the analog signal from (20)
Is a drive circuit that amplifies the rectangular wave signal input from the PWM circuit (19) and drives the transistor (5).

Therefore, when the power supply voltage fluctuates, the Hall element (14)
The control signal circuit (18) changes the detected power value due to
Outputs a control signal based on this power value, adjusts the on-time of the transistor (5) to change the input current, controls the input power to the original value, and minimizes the fluctuation of the input power.

By the way, the output characteristic as the power supply circuit for driving the magnetron (13) is shown by the thick line in FIG. 8 when the input power is constantly controlled against the fluctuation of the power supply voltage as described above. It will be something like.

Therefore, when the voltage-current characteristic changes from A to B, for example, due to changes in the load (object to be heated) of the magnetron (13) or the temperature of the magnetron (13) itself, the inverter power supply circuit keeps the anode At the current I _P , the input power Eb · I _{P of the} magnetron (13) decreases, so the operating frequency is decreased to increase the anode current I _P from I _A to I _B.

As a result, the oscillation mode of the magnetron (13) becomes abnormal, and stable microwave oscillation cannot be expected.

In order to solve this problem, in the past, for example, Jikho Sho 61-4
As disclosed in Japanese Patent No. 2315 (H05B 6/68), when the anode current of the magnetron exceeds a predetermined current based on the output characteristics of the inverter power supply circuit, the primary side current of the high voltage transformer is suppressed. .

However, in such control, as shown in FIG. 9, the output characteristic shown in FIG. 8 is followed until the anode current I _P reaches the limit value set in advance in the control circuit. because after reaching are those to be suppress the anode current I _P to a constant value, after the anode current I _P in the process of magnetron characteristics change from C to D reaches the limit value, the input power of the magnetron It will fall sharply.

The alternate long and short dash line in FIG. 8 shows the characteristics of a power supply using a normal low frequency leakage transformer.

[Problems to be solved by the device]

The present invention has been made in view of such problems of the conventional technology, and the purpose thereof is to rapidly reduce the input of the magnetron even if the output characteristics of the magnetron change due to a high temperature environment or the like. The present invention is intended to provide a high-frequency heating device having an inverter power supply circuit that does not cause noise.

[Means for Solving the Problems]

In order to achieve the above-mentioned object, the high-frequency heating apparatus of the present invention is provided with a current detecting means for detecting the anode current of the magnetron, and the control signal circuit is provided with a detected power value of the power detecting means and a reference value for constant control. In comparison, a constant power control circuit that adjusts the control signal level of the switching operation of the inverter so that the input power of the inverter becomes constant, and a control suppression signal is formed according to the detected current value of the current detection means, and a high temperature environment is generated. A power suppression circuit that variably corrects the control signal level based on the control suppression signal when the output characteristics of the magnetron change due to such reasons.

[Action]

In the high-frequency heating device configured as described above, the constant power control circuit provided in the control signal circuit that controls the switching operation of the inverter makes the control signal level of the switching operation of the inverter constant at the input power of the inverter. In addition to the above control, the power suppression circuit provided in the control signal circuit variably corrects the control signal level according to the detection of the magnetron anode current, and the output is increased or decreased according to the increase or decrease of the input power of the inverter. In order to control the anode current of the magnetron so that the inverter is controlled at the equilibrium point between the constant control of its input power and the control of the anode current of the magnetron, the inverter can be controlled when the output characteristics of the magnetron change in a high temperature environment. It is possible to prevent the input power of the power supply from being rapidly reduced.

〔Example〕

An embodiment of the present invention will be described with reference to FIGS. 1 to 4.

In FIG. 1, the same symbols as those used above indicate the same or corresponding ones, (21) is a current transformer as a current detecting means for detecting the anode current of the magnetron (13), and (18) 'is an inverter (8). 2) is a control signal circuit for outputting an analog signal of an appropriate level for controlling the operation state of the above), and has a circuit configuration as shown in FIG.

That is, the control signal circuit (18) 'is a constant power control circuit (22), a rectifying / smoothing circuit (23), a power suppressing circuit (24).
And an upper limiter circuit (25).

The constant power control circuit (22) includes an open collector type comparator (26), a reference voltage setting resistor (R ₂ ),
(R ₃ ), (R ₄ ), the output resistance (R ₅ ), the current limiting resistance (R ₆ ), and the control signal level potential forming capacitor (C ₁ ). ), The detected power value of the input power of the inverter (8) output from the power detection circuit (17) is input to the negative-phase input terminal of the comparator (26), and the comparator (26) has a constant positive-phase input. The capacitor (C ₁ ) is turned on and off by comparing with the control reference value.
The potential of, that is, the control signal level is adjusted.

In addition, the rectifying and smoothing circuit (23) is a current transistor (21).
It is composed of a diode (D ₁ ) for rectifying the output voltage of the detected current value output from the device and a capacitor (C ₂ ) for smoothing the rectified output.

Further, the power suppression circuit (24) includes a voltage dividing resistor (R ₇ ), an input resistor (R ₈ ), an operational amplifier (27) forming an inverting amplifier, a voltage dividing resistor (R ₉ ), (R ₉ ) for setting a reference potential. ₁₀ ), resistor for gain adjustment (R
₁₁ ) and a limiter diode (D ₂ ) to form a control suppression signal according to the detected current value and suppress the level of the capacitor (C ₁ ).

The upper limiter circuit (25) is for setting the upper limit of the anode current of the magnetron (13) to its absolute maximum rated value, and the voltage divider for level setting of the capacitor (C ₁ ) corresponding to the rated value. It consists of resistors (R ₁₂ ) and (R ₁₃ ) and a limiter diode (D ₃ ).

Next, the operation of the embodiment will be described.

The input power (hereinafter referred to as the inverter input) of the inverter (8) obtained by rectifying and smoothing the commercial power source is converted into a high frequency by the switching operation of the transistor (5), and is boosted by the high voltage transformer (9). It is supplied to the magnetron (13) as a high DC voltage by a wave doubler rectifier circuit.

Now, when the power supply voltage rises and the inverter input tries to increase, the output voltage of the Hall element (14) increases,
The negative phase input voltage of the comparator (26) rises and the comparator (26) turns on.

PW was but connexion, capacitor (C ₁₎ is by discharging through the comparator (26), the potential of the capacitor (C _1), that is, the control signal level drops, which has received the control signal
The M circuit (19) outputs a rectangular wave signal having a pulse width that shortens the on time of the transistor (5), which controls the switching operation of the transistor (5) to reduce the input current and reduce the inverter input. The input is controlled to be constant by lowering the control signal level until the original value becomes.

Here, the input power (hereinafter referred to as magnetron input) to the magnetron (13) is the product Eb of the anode voltage Eb and the anode current I _P.
・ The anode voltage Eb is almost constant when the magnetron (13), which is given by I _P and is proportional to the inverter input, is still cold immediately after the magnetron (13) starts driving. , magnetron input will be approximately proportional to the anode current I _P, by the inverter input is constant control, the anode current I _P does not change much.

At this time, the average value of the current detected by the current transformer (21) does not change and the control suppression amount of the power suppression circuit (24) becomes constant, so the output potential of the operational amplifier (27) affects the control signal level. Never give.

Also, when the power supply voltage drops and the inverter input tries to decrease, the output voltage of the Hall element (14) drops, so the comparator (26) turns off and the capacitor ( ₁ )
Is gradually charged, the control signal level rises, and control is performed such that the on time of the transistor (5) is lengthened.
The input current rises and the control signal level rises until the inverter input returns to its original value.

In this case, the control signal level cannot rise above the limit value of the operational amplifier (27), that is, its output potential, and even if the potential of the capacitor (C ₁ ) reaches the limit value of the operational amplifier (27), the inverter input remains If the value does not return to, the anode current I _P will decrease due to the decrease in magnetron input.

Due to this decrease in the anode current I _P , the control suppression amount of the power suppression circuit (24) decreases and the limiter value by the operational amplifier (27) increases, so that the potential of the capacitor (C ₁ ) can be increased and the control signal level The rise can increase the inverter input, and accordingly, the magnetron input also increases and the anode current I _P also increases, so the limiter value by the operational amplifier (27) decreases, and thus the inverter input becomes constant. To balance.

The increase in the control signal level due to the action of the power suppression circuit (24) cannot exceed the set value by the upper limiter circuit (25).

Now, the inverter input is controlled to be constant as described above. However, due to load fluctuations and temperature changes of the magnetron (13), as shown in FIG.
If the voltage-current characteristic of is changing from F to G, the inverter input decreases due to the decrease of the anode voltage Eb, so the comparator (26) turns off and the control signal level rises, and the anode current It works by increasing I _P to make the input constant.

That is, the anode current I _P rises according to the output characteristics described with reference to FIG.

But as the anode current I _P rises, the power suppression circuit (24)
Since the control suppression amount of is increased and the limiter value by the operational amplifier (27) is decreased, the control signal level can be suppressed by the output potential of the operational amplifier (27) at a certain value.
This is point P in FIG.

Then, when the anode voltage Eb further decreases from the point P,
As shown in FIG. 4, the anode current I _P rises to the point P ₁ , and accordingly, the limiter value is reduced by the value determined by the gain of the inverting amplifier by the power suppression circuit (24), and the control signal level is lowered. Is reduced. Then, since the inverter input is lowered due to the reduction of the control signal level, the magnetron input is also reduced and the anode current I _P is reduced to point P ₂ . Furthermore, the limiter value by the operational amplifier (27) rises due to the decrease in the anode current I _P , the control signal level rises, and the operating point of the inverter (8) becomes the constant control of the input voltage and the control of the anode current I _P. It comes to converge on the equilibrium point.

When the anode voltage Eb further decreases due to the change in magnetron characteristics, the above-mentioned operation is repeated, and the operating point changes on the output characteristics as shown in FIG. 3, and the anode current I _P increases. Inverter input decreases, and it will function as an inverter power supply with output impedance similar to the power supply using low frequency leakage transformer.

Therefore, if the constant of the inverting amplifier by the operational amplifier (27) is selected to an appropriate value, the output characteristics of the inverter power supply can be freely set to the characteristics most suitable for the magnetron input. It is possible to continue stable microwave oscillation of the magnetron (13) without causing a sharp input decrease as shown in the publication.

[Effect of device]

Since the present invention is configured as described above, it has the following effects.

The constant power control circuit provided in the control signal circuit that controls the switching operation of the inverter controls the control signal level of the switching operation of the inverter so that the input power of the inverter becomes constant, and is also provided in the control signal circuit. The power suppression circuit variably corrects the control signal level according to the detection of the magnetron's anode current, and controls the magnetron's anode current to increase or decrease its output according to the increase or decrease of the input power of the inverter. Since the control is performed at the equilibrium point between the constant control of the input power and the control of the anode current of the magnetron, it is possible to prevent the input power of the magnetron from dropping sharply when the output characteristics of the magnetron change in a high temperature environment. It is possible to continue the stable oscillation of the magnetron.

[Brief description of drawings]

1 to 4 show a high frequency heating apparatus 1 according to the present invention.
FIG. 1 shows an overall circuit diagram, FIG. 2 shows a detailed connection diagram of a control signal circuit, FIGS. 3 and 4 show inverter power supply output characteristic diagrams, and FIG. FIG. 5 is an overall circuit diagram, FIG. 6 is a perspective view of the power detecting means, FIG. 7 is a connection diagram of the power detecting circuit, and FIGS. 8 and 9 are inverter power supply output characteristic diagrams, respectively. (8) …… Inverter, (9) …… High-voltage transformer, (1
3) …… Magnetron, (14) …… Hall element, (21)
... Current transformer, (18) '... Control signal circuit, (22)
...... Constant power control circuit, (24) …… Power suppression circuit.

Claims (1)

[Scope of utility model registration request] 1. An inverter for increasing the frequency of a primary side power source of a high voltage transformer having a magnetron connected to a secondary side, a power detecting means for detecting an input power of the inverter, and a power value detected by the power detecting means. A control signal circuit for controlling the switching operation of the inverter to stabilize the input power, and a high-frequency heating device, comprising a current detection means for detecting the anode current of the magnetron, the control signal circuit, the detection A power constant control circuit that compares the power value with a reference value for constant control and adjusts the control signal level of the switching operation so that the input power becomes constant, and according to the detected current value of the current detection means. The control signal level is generated based on the control signal when the control characteristic is generated and the output characteristics of the magnetron are changed due to a high temperature environment or the like. High-frequency heating device provided with a power suppression circuit that variably corrects.