JP3011483B2 - Power supply for microwave oven - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply for a microwave oven. More specifically, the present invention relates to a novel microwave power supply suitable for a commercial power supply having a relatively high voltage, for example, 200 V or 220 V.

[0002]

2. Description of the Related Art An example of an inverter circuit that can be used as a power supply device for a microwave oven is March 1, 1977.
Japanese Patent Application Laid-Open No. 52-35903 (H.
05B6 / 66) or JP-A-59-191290 (H05B6 / 68) published on Oct. 30, 1984. This prior art is a power supply device using a voltage resonance type inverter,
The resonance time constant is determined by the leakage inductance of the high-frequency transformer, the inductance of the primary winding and the secondary winding of the high-frequency transformer, and the resonance capacitor. In such a voltage resonance type inverter circuit, the fluctuation of the input voltage of the main circuit (the power supply voltage of the commercial power supply) appears directly as the fluctuation of the resonance voltage. Therefore, it is necessary to use a switching element having a large withstand voltage in anticipation of such a fluctuation of the resonance voltage as a switching element constituting the inverter. For example, assuming that the input voltage is 100 V, the peak voltage is about 141 V (= 100、2), and a resonance voltage of about 600 V is applied to the switching element at rated input. Then, since the resonance voltage fluctuates according to the fluctuation of the input voltage, in this case, for example, 900 V
A switching element having a rating of 50A is used.

On the other hand, recently, it has been proposed to use a 200 V commercial power supply for increasing the capacity and speed of a cooker such as a microwave oven. When the input voltage of the above-mentioned inverter circuit becomes 200 V, a switching element having a rating of about 1800 V 30 A is required. However, a switching element that operates in a high-frequency region and has such a large withstand voltage in a power supply device for a microwave oven has not yet been put to practical use. Therefore, Japanese Patent Application Laid-Open No.
The inverter circuit disclosed in 5903 or JP-A-59-191290 cannot be used as a power supply device for a microwave oven driven by a 200 V commercial power supply.

On the other hand, Japanese Patent Application Laid-Open No. 2-101962 (H02M) published on April 13, 1990 (1990)
3/28, 3/335) include a series circuit of two switching elements, a series circuit of two resonant capacitors, and 2
A series circuit of two feedback diodes is connected to a DC power supply,
A half-bridge converter is disclosed in which a series circuit of a primary winding of a transformer and a first resonance inductor is connected between a connection point of two switching elements and a connection point of two resonance capacitors. The half-bridge converter according to the prior art is mainly used for a stabilized power supply circuit, and the use of such a half-bridge converter as a power supply device for a microwave oven has not been performed at present.

[0005]

Problems to be Solved by the Invention
Although it is conceivable to use the half-bridge converter disclosed in Japanese Patent Publication No. 2 as a power supply for a microwave oven, the half-bridge converter of the related art cannot be used as it is as a power supply for a microwave oven. That is,
In the above-mentioned conventional technology, in order to control the output voltage of the half-bridge converter, the switching frequency of the two switching elements is controlled by the error voltage between the output voltage of the load and the reference value. Since the bridge converter has a constant voltage load, to control the magnetron output power,
What is necessary is just to detect the output current. In this case, a current transformer for detecting the output current of the magnetron is provided on the secondary side of the high-frequency transformer, and the output of the current transformer is inputted to the control circuit. In order to isolate the control circuit from the secondary side, a current transformer having a large creepage distance and a large space distance is required. Therefore, the power supply device becomes large and expensive. In addition, in this prior art half-bridge converter, a resonance inductor is connected in series to the primary winding. However, in order to obtain a power supply for a microwave oven having a resonance frequency of 50 kHz and an output power of about 800 W of a magnetron, an inductance of about 10 μH is required. It is necessary to use a large resonance inductor having the following. Therefore, when the half-bridge converter disclosed in Japanese Patent Application Laid-Open No. 2-101962 is used as it is, the power supply device for the microwave oven becomes large and expensive.

Therefore, a main object of the present invention is to provide a novel power supply for a microwave oven. Another object of the present invention is to provide a power supply device for a microwave oven that can be reduced in size and inexpensive by using a half-bridge converter.

[0007]

SUMMARY OF THE INVENTION Briefly stated, the present invention is driven by a commercial power supply and connected in parallel to a first series connection of first and second switching elements and a first series connection. A first series connection of the first and second resonance capacitors, and a primary winding and a primary winding connected between a connection point of the first series connection and a connection point of the second series connection. Magnetically coupled to supply voltage to magnetron 2
A half-bridge converter including a high-frequency transformer having a secondary winding, setting voltage output means for outputting a setting voltage for instructing a desired output power of the magnetron, current detecting means for detecting a current flowing through the primary winding, and current detecting means Averaging means for averaging the current detected by the averaging means, error voltage output means for outputting an error voltage corresponding to the difference between the voltage obtained by the averaging means and the set voltage, at a frequency proportional to the magnitude of the error voltage A switching signal generating means for generating first and second switching signals for alternately turning on the first and second switching elements, a voltage detecting means for detecting a voltage level of the commercial power supply, and an output from the voltage detecting means. Start-up detecting means for detecting whether the magnetron has started, and the magnetron has not yet been started by the start-up detecting means When it is detected, at least one of the first and second switching signals from the switching signal generator is stopped in response to the detection signal from the voltage detector that the voltage level of the commercial power supply exceeds a predetermined value. A power supply device for a microwave oven, comprising: a stop unit; and a release unit that releases a stop of at least one of the first and second switching signals by the stop unit in response to a detection signal indicating that the magnetron has been started from the start detection unit. is there.

[0008]

The commercial power is applied as the input voltage of the half-bridge converter through the diode bridge. For example, a bipolar transistor, a field effect transistor, or an IGBT (Insulated Gate Bipolar Trans
istor) is supplied with first and second switching signals having different phases from the switching control means, so that the first and second switching elements are alternately provided. Turned on or off, the half-bridge converter oscillates with a resonance time constant determined by the primary leakage inductance of the high-frequency transformer and the first and second resonance capacitors. The first and second switching elements are connected in series by a connection line, and a current transformer serving as current detection means is magnetically coupled to the connection line. The current transformer detects a current flowing through the first and second switching elements, that is, a current flowing through the primary winding. The output of the current transformer is input to an averaging means such as a band-pass filter or a low-pass filter via a diode bridge, for example. It has a magnitude corresponding to the difference between a voltage having a level corresponding to the average current voltage obtained from the averaging means and a set voltage for commanding a desired output voltage of the magnetron from a set voltage output means such as a microcomputer. An error voltage is output from the error voltage output means. The switching control means includes, for example, a V / F conversion means, and outputs first and second switching signals having a frequency proportional to the magnitude of the error voltage but having a constant on-time to output the first and second switching signals. Give to the element.

For example, voltage detecting means including a potential transformer and a comparator coupled to the commercial power supply detects whether the voltage level of the commercial power supply has exceeded a predetermined value.
The activation detection means detects whether the magnetron has been activated based on the output of the current detection means. For example, a flip-flop that constitutes a part of the stopping means and the releasing means enters the first state in response to the detection signal from the voltage detecting means, and enters the second state in response to the starting detecting signal from the starting detecting means. become. In response to a first output of the flip-flop, at least one of the first and second switching signals from the switching signal generating means is gated,
It is prohibited to apply it to at least one of the first and second switching elements. Therefore, the oscillation operation of the half-bridge converter is stopped when the voltage level of the commercial power supply exceeds a predetermined value in a state where the magnetron has not been started yet. Then, the gate is released in response to the second output of the flip-flop. Therefore, the half-bridge converter oscillates intermittently.

[0010]

According to the present invention, the output power of the half-bridge converter, that is, the magnetron, can be feedback-controlled based on the current detected by the current detecting means. Therefore, a smaller and less expensive power supply device for a microwave oven can be obtained as compared with the case where the half-bridge converter disclosed in Japanese Patent Application Laid-Open No. 2-101962 is used as it is. Further, when the voltage level of the commercial power supply is larger than the predetermined value when the magnetron has not been started yet, the oscillation operation of the half-bridge converter is stopped, so that the secondary voltage of the high-frequency transformer is suppressed. Therefore, since a high surge voltage is not applied to the magnetron, the high-frequency transformer, and the secondary circuit, components having a relatively low withstand voltage can be used as these components, and the size and cost are further reduced.

[0011] The above and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings.

[0012]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. The power supply device 10 for a microwave oven according to this embodiment includes a main circuit controlled by a control circuit 12, and this main circuit is mainly constituted by a half-bridge converter and receives a 200V AC power supply from a commercial power supply 14. 200V
Is subjected to full-wave rectification by a diode bridge 16, and a half-bridge converter is connected to an output of the diode bridge 16 via a normal mode choke coil 18.

That is, the output of the diode bridge 16 is connected to the series connection of the switching elements 20 a and 20 b via the normal mode choke coil 18. In this embodiment, IGBTs are used as the switching elements 20a and 20b. This 2
The two switching elements 20a and 20b
Are connected in series. In the series connection of the switching elements 20a and 20b, a series connection of two discharge resistors 24a and 24b and a series connection of two resonance capacitors 26a and 26b are respectively connected in parallel. At the same time, a ringing choke converter 28 for generating power for the control circuit 12 is connected to the output of the diode bridge 16. A diode 30a is connected in parallel to the switching element 20a, and a series connection of a diode 30b and a current detection resistor 32 is connected in parallel to the switching element 20b.

Two switching elements 20a and 20
b between the series connection point of the high frequency transformer 34 and the series connection point of the resonance capacitors 26a and 26b.
4a is connected. The two secondary windings 34b and 34c are magnetically coupled to the primary winding 34a so that the energy stored in the primary winding 34a is transmitted. The above-described diodes 30a and 30b function to return the energy stored in primary winding 34a but not transmitted to secondary windings 34b and 34c to commercial power supply 14. The secondary winding 34b supplies a high voltage between the heater or cathode of the magnetron 36 and the anode, and the secondary winding 34c supplies a heater voltage to the cathode of the magnetron 36. That is, in the secondary winding 34b,
A voltage doubler rectifier circuit including a diode 38 and a capacitor 40 is connected, and an output voltage of the voltage doubler rectifier circuit is applied between a cathode and an anode of the magnetron 36. The heater of the magnetron 36 is a magnetron 3
6 is connected to a secondary winding 34c through a high-frequency filter 36a built in.

A potential transformer 42 is connected to the input of the commercial power supply 14, ie, the diode bridge 16, and an output of the potential transformer 42 is supplied to the control circuit 12. Further, a current transformer 44 is magnetically coupled to the connection wire 22 wound once as shown in FIG. 1, and the output of the current transformer 44 is also supplied to the control circuit 12. By winding the connection line 22 as shown in FIG. 1, the current transformer 44 can individually detect the current flowing through the switching element 20a or the current flowing through the switching element 20b, and superimpose the current flowing through the switching elements 20a and 20b. The total current that has been applied can also be detected. That is, the current of the switching element 20a flowing through a part of the path of the connection line 22 is in the same direction as the current of the switching element 20b flowing between the primary winding 34a and the switching element 20b. When the two current paths are magnetically coupled in common, the current transformer 44 can detect the total current flowing through the switching elements 20a and 20b. Also, the switching element 20a
And 20b are alternately turned on, so that the current transformer 44 can detect an individual current during each on period.

In the embodiment shown in FIG. 1, a cooling fan 46 is disposed near the magnetron 36, and the cooling fan 46 is driven or stopped by a fan driver 48. The main circuit of the power supply device 10 for a microwave oven according to the embodiment shown in FIG.
It includes a half-bridge converter similar to that disclosed in US Pat. No. 101962. That is, switching elements 20a and 20b are alternately turned on or off by switching signals Q1 and Q2 from control circuit 12, so that primary winding 34a of high-frequency transformer 34
A high-frequency current as shown by a solid line in FIG. This high frequency current is multiplied by the high frequency transformer 34b, and a high frequency high voltage is induced in the secondary winding 34b. The high-frequency high voltage induced in the secondary winding 34b is converted into a DC high voltage by the voltage doubler rectifier circuit, and the DC high voltage is applied between the cathode and the anode of the magnetron 36, whereby the magnetron 36 oscillates. I do.

In a steady state, that is, in a normal oscillation state, the primary winding 34a of the high frequency transformer 34
Alternatively, a primary current flows as shown by a solid line in FIG. Further, a current flows as indicated by a one-dot chain line in FIG. 2 through the resonance capacitors 26a or 26b, and a voltage as indicated by a two-dot chain line in FIG. 2 is generated in the resonance capacitors 26a and 26b. Then, the voltage of the primary winding 34a of the high-frequency transformer 34 is indicated by a dotted line in FIG.

The control circuit 12 shown in FIG. 4 includes a microcomputer 50, which controls the cooking time, the output power of the magnetron 36, and the like.
As shown by the solid line in FIG. 2 or FIG.
The primary current flows through the primary winding 34a, and this primary current is detected by the current transformer 44 shown in FIG. The output of the current transformer 44 is provided to a diode bridge 52, and the output of the diode bridge 52 is provided to a (−) input of a comparator 56 through a band-pass filter 54. The bandpass filter 54 extracts a harmonic component of the current detected by the current transformer 44. In this embodiment, the pass band of the band-pass filter 54 is set to twice the frequency of the commercial power supply 14 shown in FIG. That is, the band-pass filter 54 includes the switching elements 20a and 20a.
The second harmonic component of the power supply frequency is extracted from the current b, that is, the current of the primary winding 34a. The output of the band-pass filter 54 is averaged by a smoothing circuit 55 composed of a resistor and a capacitor. The frequency spectrum of the output of the current transformer 44 is shown in FIG. 5, in which the 120 Hz component having the maximum amplitude is input from the commercial power supply 14 (FIG. 1) to the half bridge converter as shown in FIG. There is a correlation with the current. Therefore, the harmonic component of the output of the current transformer 44 is
4 and averaged by the smoothing circuit 55, the input power of the half-bridge converter can be equivalently detected. In this embodiment, the half-bridge converter is feedback-controlled according to the output of the current transformer 44.

Thus, the band-pass filter 54
Accordingly, harmonic components of the commercial power supply 14 are extracted and averaged by the smoothing circuit 55, and a voltage corresponding to the average current is supplied from the smoothing circuit 55 to the (-) input of the comparator 56. To the same (-) input of the comparator 56, a set voltage Vs corresponding to the output power of the magnetron 36 that matches the cooking conditions set by the user from the microcomputer 50 is given. ) Input is reference voltage 57
Is entered. Therefore, the comparator 56 outputs an error voltage corresponding to the difference between the output voltage from the smoothing circuit 55 and the set voltage Vs. This error voltage is supplied to the V / F conversion circuit 60 through the soft start circuit 58. In this embodiment, an integrated circuit “GP605” manufactured by GENNUM is used as the V / F conversion circuit 60. FIG. 7 shows a functional block diagram of this integrated circuit “GP605”.

The integrated circuit "GP605" is an integrated circuit developed for controlling the switching of the high-frequency power supply. When a voltage is applied to the 13th terminal in FIG. Two switching control signals having a frequency proportional to the magnitude of the applied voltage are obtained. Therefore, in this embodiment, the integrated circuit “GP6
The output voltage of the comparator 56 is applied to the thirteenth terminal of the terminal 05 ". Two switching control signals QA and QB output from the eighth and sixth terminals are shown in FIG. Switching control signals QA and QB
In either case, the ON time T _ON is constant and the frequency is proportional to the magnitude of the input voltage, that is, the output voltage of the comparator 56 as shown in the graph of FIG.

When the signal SEO applied to the tenth terminal is at a low level, this integrated circuit "GP605" outputs two switching control signals having a phase difference of 180 degrees from each other as shown in FIG. Is the signal SE
When O is at a high level, the two switching control signals are in phase. The switching control signal QA as shown in FIG. 8 from the V / F conversion circuit 60 is given to the driver 62 via the AND gate 61, and the switching control signal QA
B is given to the driver 62 as it is. Driver 62
Although the details are not shown, the switching control signal QA
And switching signals Q1 and Q2 of a predetermined voltage, for example, 16 V, in synchronization with QB and QB. As described above, switching signals Q1 and Q2 are applied to respective gates of switching elements 20a and 20b. As the driver 62, an IR (International
A high-side / low-side switch, such as the integrated circuit "IR2110" manufactured by Rectifier, Inc., can be used. However, a circuit configuration as shown in FIG. 10 may be used as the driver 62.

Driver 62 shown in FIG. 10 includes photocouplers 621 and 622 driven by switching control signals QA and QB, respectively, and outputs of photocouplers 621 and 622 are supplied to switching signal Q through inverting buffer amplifiers 623 and 624, respectively.
Output as 1 and Q2. That is, the photocoupler 621 or 622 is not driven when the switching control signal QA or QB is at the high level, that is, during the off period, and the photocoupler 621 is not driven when the switching control signal QA or QB is at the low level, that is, during the on period T _ON.
Or 622 is driven. When the photocoupler 621 or 622 is driven, the input of the inverting buffer amplifier 623 or 624 becomes low level, which is output as the high level switching signal Q1 or Q2 via the inverting buffer amplifier 623 or 624. The switching elements 20a and 20b are alternately turned on by the switching signals Q1 and Q2, and the half-bridge converter performs an oscillating operation. The detailed operation is well known, and the description thereof is omitted.

The soft start circuit 58 shown in FIG.
This is a circuit for gradually increasing the input voltage of the / F conversion circuit 60. Although the soft start circuit is also incorporated in the integrated circuit "GP605", the delay time is short and the operation is different from the operation described later.
The built-in soft start circuit is not used. After the magnetron 36 starts oscillating, the soft start circuit 58 gradually increases the oscillating frequency of the switching elements 20a and 20b.
Output power gradually increases. More specifically, as shown in FIG.
And a series connection of a resistor 582 and a diode 581
A transistor 583 is connected to the series connection point of
Is connected, a resistor 584 is connected to the emitter of the transistor 583, and a capacitor 585 is connected to the collector. Therefore, the error voltage from comparator 56 is applied to the emitter of transistor 583 through resistor 584. When the error voltage reaches a predetermined base voltage, the transistor 583 turns on, and the capacitor 585 is charged by the error voltage. Therefore, the error voltage from comparator 56 is delayed according to the time constant determined by resistor 584 and capacitor 585, and V / F
It is provided to conversion circuit 60.

A transistor 586 is connected in parallel to the capacitor 585, and a control signal from the start-up control circuit 66 (FIG. 2) is applied to the base of the transistor 586. Since this control signal goes high at the time of startup, the collector of the transistor 583 is grounded by the transistor 586. Therefore, at the time of startup, a voltage of almost 0 V is applied to V / F conversion circuit 60, and switching control signals QA and QB, that is, switching signal Q1
And the frequency of Q2 is low and therefore the primary winding 34a
The current shown in FIG. 1 is small, and the high-frequency voltage induced in the secondary winding 34b is minimized.

Thereafter, when the magnetron 36 is sufficiently operatively started, the start-up control circuit 66 outputs the transistor 58.
6, the control signal applied to the base becomes low level,
Since the transistor 586 is turned off, the error voltage from the comparator 56 is directly applied to the V
/ F conversion circuit 60. Note that the diode 587 in FIG. 11 forms a discharge path for the capacitor 585.

In such a steady state, the primary current of the primary winding 34a is detected by the current transformer 44, and the harmonic component is extracted by the band-pass filter 54 and averaged by the smoothing circuit 55. From the output power of the magnetron 36 commanded by the set voltage Vs from the microcomputer 50, the current magnetron 36
Is larger than the set voltage Vs, the error voltage from the comparator 56 becomes smaller. Therefore, switching control signals QA and QB from V / F conversion circuit 60 have relatively low frequencies as shown in FIG. 9, and the frequencies of switching signals Q1 and Q2 also decrease. As a result, the off time of the switching elements 20a and 20b becomes longer and the oscillation frequency of the half-bridge converter becomes lower, so that the primary current also becomes smaller and the output power of the magnetron 36 becomes smaller. Conversely, when the current output power of the magnetron 36 is smaller than the output power of the magnetron 36 commanded by the set voltage Vs from the microcomputer 50, the output voltage of the smoothing circuit 55 becomes smaller than the set voltage Vs. , The error voltage from the comparator 56 increases. For this purpose, the V / F conversion circuit 60 outputs switching control signals QA and QB having a relatively high frequency. Therefore, the oscillation frequency of the half-bridge converter increases, and the primary current increases. As described above, in this embodiment, the magnitude of the primary current of the primary winding 34a of the high-frequency transformer 34 is detected by the current transformer 44, and a current feedback loop is formed to perform power control. Therefore, it is possible to perform power control which was difficult in the circuit disclosed in Japanese Patent Application Laid-Open No. 2-101962.

Referring to FIG. 4, the output voltage of potential transformer 42 shown in FIG. The outputs of the two diode bridges 52 and 64 are both supplied to a start control circuit 66. The start control circuit 66 performs the intermittent oscillation operation by the switching element 20a until the magnetron 36 starts the oscillation operation, activates the soft start circuit 58 after the magnetron 36 oscillates, and performs the normal oscillation operation from the above-described intermittent oscillation operation. Move to Specifically, as shown in FIG.
Is applied to one input of the comparator 661, and the other input of the comparator 661 is supplied with a reference voltage 662. Then, the output of the comparator 661 is inverted and applied to the other input of the AND gate 61. Comparator 661
The transistor 663 is connected between the output of the transistor 66 and the ground. The output of the diode bridge 52 is
64, and a reference voltage 665 is provided to the other input of the comparator 664. The output of the comparator 664 is provided as a trigger input of the monostable multivibrator 666, and the output of the monostable multivibrator 666 is provided to the reset input of the RS flip-flop 667. A start command signal from the microcomputer 50 (FIG. 4) is given to the set input of the RS flip-flop 667. An inverted output of the RS flip-flop 667 is provided to the base of the transistor 663 described above.
This inverted output is also supplied to the fan driver 48 shown in FIG.

The comparator 661 includes the potential transformer 42
, Ie, whether the input voltage of the main circuit has exceeded a certain value. Comparator 664 determines whether the output of current transformer 44, ie, the primary current, has exceeded a certain value. That is, at the time of startup, since the RS flip-flop 667 is set by the command signal from the microcomputer 50, its inverted output becomes low level. Therefore, when the transistor 663 is turned off and the output voltage from the diode bridge 64, that is, the voltage of the commercial power supply 14 exceeds a certain value, the output of the comparator 661, that is, the input of the AND gate 61 becomes low level,
Therefore, switching control signal QA from V / F conversion circuit 60 is not provided to driver 62. Therefore,
At the time of startup, the switching signal Q1 from the driver 62 is not output, and only the switching signal Q2 is output.
Therefore, one switching element 20a constituting the half-bridge converter is turned off when the input voltage of the main circuit becomes equal to or higher than a predetermined value at the time of startup, and the resonance operation by this switching element 20a is stopped. Therefore, the secondary windings 34b and 34 of the high-frequency transformer 34
As shown in FIG. 13, a high frequency high voltage is not induced in c while the input voltage of the main circuit exceeds a certain level. Therefore, at the time of startup, the secondary winding 34 of the high-frequency transformer 34
The voltage appearing at b can be reduced.

When the magnetron 36 starts oscillating, the primary current increases, and when the output voltage of the diode bridge 52 exceeds the reference voltage 665, the output of the comparator 664 goes high, thereby causing the monostable multivibrator 6 to operate.
66 is triggered and accordingly the RS flip-flop 667
Is reset, and its inverted output becomes high level. Therefore, the transistor 663 is turned on, and the input of the AND gate 61 is fixed at the high level. As a result, the switching control signal QA is
And the switching elements 20a and 20b perform a normal oscillation operation.

If the reference voltage 662 of the comparator 661 is set to a voltage corresponding to 220 V of the commercial power supply 14,
The output voltage of the next winding 34b is about 8800 V (= 220 × 2
0 × 2: where “20” is the turns ratio of the high-frequency transformer 34). Therefore, the surge voltage generated when the magnetron 36 (FIG. 1) does not oscillate can be reduced, and the breakdown voltage of the high-frequency transformer 34, the diode 38, and the capacitor 40 can be reduced.

If such a start control circuit 66 is not used, when the magnetron 36 is not oscillating, 1
The voltage of 1200 V (= 280 × 20 × 2) is applied to the secondary winding 3
4b. This voltage is rated for magnetron 36 (1
0 kV). Therefore, the start control circuit 66
If not used, the rating of the magnetron 36 must be increased, and the dielectric strength of other components must be increased. However,
Since the high voltage at the time of startup can be suppressed by the startup control circuit 66, the withstand voltage of the high-frequency transformer 34 and the like can be reduced, so that a more inexpensive microwave power supply device can be obtained.

When the embodiment of FIG. 12 is used, the secondary winding 3
Since an intermittent voltage as shown in FIG. 13 is also induced in the magnetron 3c, the heating time of the heater heated by the voltage of the secondary winding 34c is prolonged.
6 has a longer startup time. In order to address this new problem, in this embodiment, the fan driver 48 (FIG. 1) is controlled by the output of the RS flip-flop. That is, a start command signal from the microcomputer 50 is applied to the set input of the RS flip-flop 667, and the inverted output of the RS flip-flop 667 goes low in response to this signal. The fan driver 48 stops the cooling fan 46 while the inverted output of the RS flip-flop 667 is at the low level, that is, at the time of startup. Therefore, even if the heater voltage of the magnetron 36 is small, the magnetron 36 is sufficiently heated relatively quickly. Therefore, it is possible to prevent the startup time from being lengthened by intermittently operating the half-bridge converter at startup.

In the embodiment shown in FIG. 14, the switching control signal QA from the V / F conversion circuit 60 is also supplied to the toggle flip-flop 76, so that the output of the toggle flip-flop 76 is switched as shown in FIG. It goes high or low for each control signal QA. To toggle flip-flop 76, for example, the output of comparator 661 shown in FIG. 12 (or its inversion) is applied as an enable signal. Therefore, toggle flip-flop 76 is enabled when the input voltage of the half-bridge converter is lower than a predetermined value. You. The output of the toggle flip-flop 76 is given as a switching signal to the tenth terminal of the integrated circuit "GP605" shown in detail in FIG.

While the switching signal is at a high level, V / F
The conversion circuit 60, that is, the integrated circuit "GP605" operates in a single mode instead of a dual mode. Therefore, during this high level period, switching control signals QA and QB are in-phase signals as shown in FIG. Therefore, switching elements 20a and 20b
Are turned on at the same time, and the diode bridge 16, the common mode choke coil 18 and the switching element 20a
The current flows through the paths 20b and 20b, and energy is stored in the common mode choke coil 18. Then, in the subsequent low level period, the switching control signal QA
The switching element 20a is turned on in synchronization with the switching control signal QB, and the switching element 2a is synchronized in synchronization with the switching control signal QB.
Since 0b is turned off, the output of the diode bridge 16 and the energy stored in the common mode choke coil 18 are given as the input voltage of the half bridge converter. Even if an input voltage is applied such that the voltage applied to the magnetron 36 is equal to or lower than a predetermined value (for example, 3.8 kV to 4.0 kV), no current flows through the magnetron 36, and the primary side of the high frequency transformer 34 No energy is transmitted to the side. However, as described above, the energy stored in the common mode choke coil 18 increases the input voltage, and the start and end of the oscillation of the magnetron 36 advance and delay as shown by the dotted lines in FIG. Therefore, the primary current increases, and according to the embodiment shown in FIG. 14, an improvement in the power factor can be expected.

FIG. 17 shows a detection circuit 78 for detecting that the diode 38 constituting the voltage doubler rectifier circuit connected to the secondary winding 34b of the high frequency transformer 34 has been destroyed.
Is shown. The detection circuit 78 includes a comparator 80, and the output from the diode bridge 52 shown in FIG.
0, and a reference voltage is applied to the other input of the comparator 80. Therefore, the comparator 80 detects whether the output of the diode bridge 52 has exceeded a certain level. The output of the comparator 80 is provided to a trigger input (terminal No. 5) of the retriggerable monostable multivibrator 82. The retriggerable monostable multivibrator 82 outputs a high-level or low-level continuous signal when a subsequent trigger input is given within a predetermined time T1 by the operation of the AND gate 84. The above-mentioned fixed time T1 is determined by the resistor R and the capacitor C shown in FIG.

When the diode 38 is normal, a primary current shown in FIG. 18A flows through the primary winding 34a of the high-frequency transformer 34. Therefore, the diode bridge 5
The output of No. 2 is as shown in FIG. 18B, and exceeds the reference voltage only when the primary current is a positive half wave. Therefore,
The output of the comparator 80 is as shown in FIG. 18C, and the high-level cycle is longer than the certain time T1. Therefore, the ninth retriggerable monostable multivibrator 80
The output from the No. terminal becomes a high-level continuous signal as shown in FIG. Note that FIG. 18E and FIG.
8 (F) shows one input and output of the AND gate 84, respectively.

When the diode 38 is broken, a primary current shown in FIG. 19A flows through the primary winding 34a of the high-frequency transformer 34. Therefore, the output of the diode bridge 52 is as shown in FIG. 19B, and exceeds the reference voltage every half-wave of the primary current. Therefore, the comparator 8
The output of 0 is as shown in FIG. 19C, and the high-level cycle is shorter than the fixed time T1. Therefore, the output from the ninth terminal of the retriggerable monostable multivibrator 80 is a low-level continuous signal as shown in FIG. Note that FIGS. 19E and 19F
Denotes one input and output of the AND gate 84, respectively. A low-level continuous signal of the retriggerable monostable multivibrator 80 is supplied to, for example, the microcomputer 50 (FIG. 4) as an abnormality detection signal.

Instead of the detection circuit 78 shown in FIG.
0 may be used. In the embodiment shown in FIG. 20, the terminal voltage of the detection resistor 32 shown in FIG.
To one input. A reference voltage 88 is applied to the other input of the comparator 86.
, The current level of the negative half wave of the primary current is detected. Therefore, as shown in FIG. 19A, when the current level of the negative half-wave of the primary current exceeds a certain value due to the failure of the diode 38, a detection signal is output from the comparator 86.

In the embodiment shown in FIG. 21, the magnetron 3
6 is heated by the output of the ringing choke converter 28 provided through the ring core 90 as well as the output of the secondary winding 34c of the high frequency transformer 34. That is, the output line of ringing choke converter 28 shown in detail in FIG. 22 is wound around ring core 90. Another winding is applied to the ring core 90, and the other winding is connected to the cathode of the magnetron 36 through the diode 92. Therefore, the output of the ringing choke converter 28 induced through the ring core 90 is applied to the heater together with the output of the secondary winding 34c. However, the heater of the magnetron 36 may be supplied with the heater current only from the ringing choke converter 28. Note that the capacitor 94 is a smoothing capacitor. Ringing choke converter 2
The other output of the control circuit 12 is, as shown in FIG.
(FIG. 1 or FIG. 4) as the power source and the driver 62
Each is used as a voltage source (FIG. 4).

According to the embodiment shown in FIG. 21, the following effects can be expected. That is, as shown in FIG. 1 or FIG. 21, the magnetron 36 has a built-in high frequency filter 36a, and the impedance of the high frequency filter 36a is 2πfL (L is the inductance of the high frequency filter).
To change. Therefore, in order to control the output power of the magnetron 36, the oscillation frequency of the half-bridge converter is changed, so that it is difficult to always supply a constant heater current. Therefore, if a heater current is also supplied from another power supply indispensable to the half-bridge converter, for example, the ringing choke converter 28, a constant current can be supplied to the heater of the magnetron 36 regardless of the operating frequency, and the magnetron 36 can be stabilized. Expected behavior.

The overcurrent detection circuit 68 shown in FIG. 4 outputs a high-level signal when an excessively large primary current is detected, and the abnormal voltage detection circuit 70 outputs a high-level signal when an excessively large or small power supply voltage is detected. The signal of is output. Outputs of the overcurrent detection circuit 68 and the abnormal voltage detection circuit 70 are supplied to a flip-flop 74 through an OR gate 72.
Therefore, when an abnormal state occurs in the half-bridge converter, a signal from flip-flop 74 is supplied to the first terminal of integrated circuit "GP605" (see FIG. 7).
Therefore, the operation of V / F conversion circuit 60 is stopped, switching control signals QA and QB are both kept at the high level, and the operation of the half-bridge converter is stopped.

The band-pass filter 54 as the harmonic component extracting means and the smoothing circuit 55 as the averaging means shown in FIG. 4 may be replaced with a low-pass filter.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a waveform diagram showing current or voltage of each part in the embodiment of FIG.

FIG. 3 is a waveform diagram showing current or voltage of each part in the embodiment of FIG.

FIG. 4 is a block diagram showing a control circuit of the embodiment of FIG. 1 in detail.

FIG. 5 is a graph showing a frequency spectrum representing a harmonic component of a primary current of the high-frequency transformer of the embodiment of FIG. 1;

FIG. 6 is a graph showing a relationship between a 120 Hz component and an input current of a half-bridge converter.

FIG. 7 is a functional block diagram illustrating an example of a V / F conversion circuit according to the embodiment of FIG. 4;

8 is a waveform diagram showing a switching control signal from the V / F conversion circuit shown in FIG.

9 is a graph showing the relationship between the input voltage of the V / F conversion circuit shown in FIG. 7 and the frequency of the switching control signal.

FIG. 10 is a circuit diagram illustrating an example of the driver of FIG. 4;

FIG. 11 is a circuit diagram illustrating an example of a soft start circuit in FIG. 4;

FIG. 12 is a circuit diagram illustrating an example of a startup control circuit of FIG. 4;

13 is a primary current waveform diagram showing an intermittent oscillation operation achieved by the startup control circuit of FIG.

FIG. 14 is a circuit diagram showing a modification of the V / F conversion circuit of FIG. 4;

FIG. 15 is a waveform chart showing the operation of the modification of FIG.

FIG. 16 is a waveform chart showing the operation of the modification of FIG.

FIG. 17 is a circuit diagram illustrating an example of a diode breakdown detection circuit.

18 is a waveform chart showing an operation of the diode breakdown detection circuit of FIG. 17 in a normal state.

19 is a waveform chart showing an operation of the diode breakdown detection circuit of FIG. 17 at the time of abnormality.

FIG. 20 is a circuit diagram showing another example of the diode breakdown detection circuit.

FIG. 21 is a circuit diagram showing a modification of the heater circuit of the magnetron.

FIG. 22 is a circuit diagram showing a ringing choke converter according to a modification of FIG. 21;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Microwave oven power supply device 12 ... Control circuit 14 ... Commercial power supply 16 ... Diode bridge 18 ... Common mode choke coil 20a, 20b ... Switching element 22 ... Connection line 24a, 24b ... Discharge resistance 26a, 26b ... Resonant capacitor 28 ... Ringing Choke converters 30a, 30b Diode 32 Detection resistor 34 High frequency transformer 34a Primary winding 34b, 34c Secondary winding 36 Magnetron 38 High voltage diode 40 High voltage capacitor 42 Potential transformer 44 Current transformer 46 Cooling fan 48 ... Fan driver 50 ... Microcomputer 52,64 ... Diode bridge 54 ... Band pass filter 56,661,664,80,86 ... Comparator 58 ... Soft start circuit 0 ... V / F conversion circuit 62 ... driver 66 ... start control circuit 68 ... overcurrent detection circuit 70 ... abnormal voltage detection circuit

────────────────────────────────────────────────── ─── Continuation of front page (72) Katsuhiko Ito 2-18-18 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-58-57286 (JP, A) JP-A Sho JP-A-53-24649 (JP, A) JP-A-3-102742 (JP, A) JP-A-62-139293 (JP, A) JP-A-63-19790 (JP, A) A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H05B 6/66-6/68 H02M 7/48 H02M 7/5387

Claims (2)

(57) [Claims] 1. A first series connection of first and second switching elements driven by a commercial power supply, and a second series connection of first and second resonance capacitors connected in parallel to the first series connection. And a primary winding connected between the connection point of the first series connection and the connection point of the second series connection, and magnetically coupled to the primary winding to supply a voltage to the magnetron. Half-bridge converter including a high-frequency transformer having a secondary winding, a set voltage output unit for outputting a set voltage for instructing a desired output power of the magnetron, and a current detection unit for detecting a current flowing through the primary winding Averaging means for averaging the current detected by the current detecting means, and an error voltage output for outputting an error voltage corresponding to a difference between the voltage obtained by the averaging means and the set voltage. Means, switching signal generating means for generating first and second switching signals for alternately turning on the first and second switching elements at a frequency proportional to the magnitude of the error voltage, and a voltage level of the commercial power supply Voltage detecting means for detecting whether the magnetron has been started based on the output of the current detecting means, and when the magnetron has not yet been started by the starting detecting means. Responding to a detection signal indicating that the voltage level of the commercial power supply has exceeded a predetermined value from the voltage detection means, the first and second signals from the switching signal generation means.
Stopping means for stopping at least one of the switching signals, and stopping the at least one of the first and second switching signals by the stopping means in response to a detection signal from the activation detection means that the magnetron has been activated. A power supply device for a microwave oven, comprising a release unit for releasing. 2. A cooling fan disposed close to the magnetron, and fan stopping means for stopping the cooling fan until the start detecting means detects that the magnetron has been started. The power supply device for a microwave oven according to claim 1.