JP2845483B2 - Cooking device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a cooker such as an electromagnetic cooker or a microwave oven for heating an object to be heated using an inverter circuit.

(Related Art) In recent years, cookers such as an electromagnetic cooker using induction heating and a microwave oven using microwave heating have come into widespread use. In these cookers, an inverter circuit is used to generate high-frequency power.

Hereinafter, an electromagnetic cooker will be described as an example. An electromagnetic cooker that heats an object to be heated by electromagnetic induction is safe because it does not generate flames, and because the top plate for placing the object to be heated can be made of crystallized glass, it is clean and heat efficient. Various electromagnetic cookers have been developed, having advantages such as high cost.

In the conventional electromagnetic cooker shown in FIG.
Is supplied to the inverter circuit 103. The heating coil 107 and the resonance capacitor 109 are set in a series resonance state by the drive circuit 115 turning the transistor 113 on and off, that is, a switching operation, and a pan or the like (not shown) is formed by an electromagnetic induction effect of a magnetic flux generated from the heating coil 107. An eddy current is generated in the object to be heated.

In addition, in the conventional example shown in FIG. 8, when the power is turned on, the initial circuit 131 operates, and the oscillation stop circuit 135 is operated for a predetermined time set by the oscillation stop timer 133 to stop the oscillation operation of the inverter circuit 103. . Thereafter, when the oscillation stop circuit 135 recovers, the voltage V _TON set by the on-time setting circuit 123 is _supplied to the pulse width modulation circuit 119. When the pulse width modulation circuit 119 outputs a pulse signal having a pulse width corresponding to the voltage V _TON , the driving circuit 115 outputs the transistor 113 for a time corresponding to the pulse width of the pulse signal.
Turn on. Therefore, depending on the value of the voltage V _TON , the transistor
The on time of the switch 113 is set, and the transistor 113 is turned on and off based on the pulse signal, so that a high-frequency current flows through the heating coil 107 to heat the object to be heated.

Here monitors whether the load detection circuit 125 is a proper load, it if the voltage V _I corresponding to the input current from the AC power supply unit is higher than the voltage V _TON is a proper load Is determined, and the heating operation is continued.

Reverse voltage V _I is judged when below the voltage V _TON is improper loading of such a no-load state or aluminum pan. In this case, the oscillation stop timer 133 is operated to stop the oscillation operation of the inverter circuit 103.

The input power is controlled by the input control circuit 121 by changing the ON time of the transistor. Transistor 11
If the on-time of transistor 3 is long, as shown in FIG. 9 (A), a voltage between the collector and the emitter of transistor 113, that is, a resonance voltage _VCE is obtained in a sinusoidal waveform within the off-period of transistor 113. the inside of the on-time T _oN collector current I _C of the transistor 113 increases linearly.
When the on-time of the transistor 113 is short, as shown in FIG. 9B, the resonance voltage V _CE does not drop to OV and has a predetermined potential immediately before the transistor 113 is turned on. Transistor 113 is short-circuited by this potential, short-circuit current I _S power loss flow is increased.

(Problems to be Solved by the Invention) FIG. 10 shows a short-circuit current with respect to an on-time of a transistor in a conventional electromagnetic cooker having a maximum input power of 1.2 kW at a voltage of 100 V and an input corresponding to the on-time at this time. The power is also shown.

At the start of the oscillation of the inverter circuit 103, the capacitor C11
Since 0 is gradually charged and the value of the voltage V _TON gradually increases in response to this, a short-circuit current of 80 A flows as shown in FIG. 10, and the loss in the transistor 113 becomes extremely large.

FIG. 11 shows the short-circuit current with respect to the on-time of the transistor in a conventional electromagnetic cooker with a maximum input power of 2 kW at a voltage of 200 V, and also shows the on-time and the corresponding input power at this time.

As is clear from FIG. 11, for example, the input power is 750 W
In this case, a short circuit current of 90 A flows, and when the ON time of the transistor is 3 μsec, a short circuit current of 130 A flows, resulting in a large power loss. In addition, heat is generated in accordance with the power loss. If the cooling capacity of the cooling fan at this time is not sufficient, the transistor may be thermally damaged.

Therefore, if the on-time of the transistor is set to about 12 μsec, it is necessary to control the cycle of continuation and stop of the oscillation of the inverter circuit in seconds.

If the cycle of the continuation and the stop of the oscillation of the inverter circuit is controlled in seconds, the oscillation stop period of the inverter circuit becomes long, causing uneven heating.

In recent years, when boiling vegetables, etc., without boiling for reasons such as vitamins are not damaged, 90 ℃
It is desired to be able to perform so-called warming and heating by heating to a certain degree.

Therefore, in order to solve the above-described generation of the uneven heating and perform the heat retention heating, it is necessary to control the cycle of continuation and stop of the oscillation of the inverter circuit as short as possible, for example, at the power supply frequency level of the commercial power supply.

However, in the conventional device in which the on-time of the transistor 113 is gradually increased at the start of the oscillation of the inverter circuit 103, in order to obtain the input voltage of 750W, as shown in FIG. 1
Since the current flows through 13, the loss in the transistor 113 may increase and thermal destruction may occur.

The present invention has been made in view of the above-described problems, and it is an object of the present invention to provide a cooker capable of preventing the occurrence of uneven heating while keeping the short-circuit current of a transistor as low as possible, and performing heat retention and heating together. I do.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides an inverter circuit having switching means and high-frequency power generating means for generating high-frequency power in accordance with ON / OFF of the switching means. In a cooker that heats an object to be heated by using high-frequency power obtained by oscillating and stopping oscillation of the inverter circuit, the on-time of the switching means at the start of oscillation of the inverter circuit is short-circuited. The gist of the present invention is to include an on-time setting unit that sets the on-time when the current is maximized to be equal to or longer than a predetermined minimum ion time in order to prevent the switching unit from being destroyed.

(Operation) In the present invention, the inverter circuit having the switching means and the high-frequency power generating means is caused to oscillate and stop oscillating to obtain the high-frequency power required for heating the object to be heated. The high-frequency power generation means generates high-frequency power according to the ON time of the switching means. Also, there is provided an on-time setting means for setting the on-time, and when the oscillation operation of the inverter circuit starts, the on-time is set to be equal to or longer than a minimum on-time set in advance to prevent destruction of the switching means. The setting is made to operate the switching means. As a result, the short-circuit current of the switching means at the start of the oscillation operation of the inverter circuit can be significantly reduced.

(Example) Hereinafter, an example according to the present invention will be described in detail with reference to the drawings.

First, referring to Fig. 1, the input power is the maximum with the 200V voltage specification.
The configuration of the 2KW electromagnetic cooker will be described. A commercial power supply PW, which is an AC power supply, is connected to the DC power supply circuit 1 via a triac TS, which is a two-way three-terminal thyristor. The DC power supply circuit 1 has four diodes D1, D2,
D3, D4 and a smoothing capacitor C1.
Converts AC power from commercial power supply PW to DC power. This DC power supply circuit 1 is connected to an inverter circuit 3,
A predetermined DC power is supplied to the inverter circuit 3.

In the inverter circuit 3, the heating coil 7 and the resonance capacitor 9 are connected in series, and the transistor 13 is connected in parallel with the resonance capacitor 9. Transistor
The base of 13 is connected to the drive circuit 15, and the drive circuit 15
The heating coil 7 and the resonance capacitor 9 are set in a series resonance state by turning on and off the transistor 13 based on the signal from An eddy current is generated in the object to be heated to heat the object to be heated.

With pulse width modulation circuit 19 is connected to the driving circuit 15 is connected to the on-time setting circuit 23, when the input voltage V _TON for on time setting from the on-time setting circuit 23, the voltage V _TON A pulse signal having a corresponding pulse width is output to the drive circuit 15. When the drive circuit 15 receives the pulse signal from the pulse width modulation circuit 19, it turns on the transistor 13 for a time corresponding to the pulse width of the pulse signal. Therefore, the ON time T _{ON of the} transistor 13 changes according to the value of the voltage V _TON from the ON time setting circuit 23. Ie voltage V
_{When the TON} changes, the pulse width of the pulse signal from the pulse modulation circuit 19 changes, the high-frequency power generated by changing the _ON time T _ON of the transistor 13 changes, and the heating output by the inverter circuit 3 changes. ing. The connection point between the heating coil 7 and the capacitor 9 is connected to the feedback input terminal of the pulse width modulation circuit 19 by feedback.
The resonance voltage between the heating coil 7 and the capacitor 9 is supplied to a pulse width modulation circuit 19.

 Next, the internal configuration of the on-time setting circuit 23 will be described.

The resistors R1 and R2 are connected in series, a predetermined DC voltage Vcc is applied to one end of the resistor R1, and one end of the resistor R2 is connected to the ground. Capacitor C3 in parallel with this resistor R2
Is connected. The connection point between the resistors R1 and R2 is connected to the input terminal P1 of the pulse width modulation circuit 19. The input terminal P1 is connected to the collector of the transistor Tr9 via the resistor R3, and the base of the transistor Tr9 is connected to the resistor R4.
To the output terminal of the comparator CON2.

The oscillation stop circuit 35 includes a transistor Tr12 and a resistor R8.The collector of the transistor Tr12 is connected to the input terminal P1, and the base of the transistor Tr12 is connected to the oscillation stop timer 33 and the on / off control circuit 43 via the resistor R8. Is connected to The oscillation stop timer 33 is
Is connected to

Next, the load detection circuit 25 and its peripheral circuits, that is, the input control circuit 21 and the input current monitoring circuit 27 will be described. A current transformer CT is provided on a power supply line between the AC power supply PW and the DC power supply circuit 1, and outputs a detection signal having a value proportional to the input current i _IN from the AC current PW. A resistor R15 is connected in parallel with the current transformer CT. A bridge circuit in which four diodes D6, D7, D8, D9 are bridge-connected is connected to the resistor R15, and a resistor R16 is connected to this bridge circuit. Capacitor C4 is connected in parallel with resistor R16,
The time constant determined by the resistor R16 and the capacitor C4 is set to a time corresponding to a half cycle of the AC power supply, that is, the commercial power supply PW, for example, a value larger than 10 ms. Further, resistors R13 and R14 are connected in series between a DC power supply that outputs a predetermined voltage Vcc and ground, and a connection point between the resistors R13 and R14 is connected to the cathode side of the diode D8. The cathode of the diode D6 is connected to the non-inverting input terminal of the comparator CON1, and is connected to the emitter of the transistor Tr15 via the resistor R18. The collector of the transistor Tr15 is connected to the cathode of the diode D8, and the base of the transistor Tr15 is connected to the on / off control circuit 43 via the resistor R17. Thus the input current monitoring circuit 27 outputs a voltage V _I proportional to the input i _IN. In the load detection circuit 25, the resistors R21 and R22 are connected in series, and this connection point is connected to the inverting input terminal of the comparator CON1. The inverting input terminal of the comparator CON1 is connected to the input terminal P1 of the pulse width modulation circuit 19 via the resistor R21. The output terminal of the comparator CON1 is connected to the oscillation stop timer 33 via the resistor R25.

In the input control circuit 21, a variable resistor R23 and a resistor R24 are connected in series between a DC power source that outputs a predetermined voltage Vcc and ground, and a variable terminal of the variable resistor R23 is connected to a comparator C.
Connected to non-inverting input terminal of ON2. The non-inverting input terminal of the comparator CON2 is connected to the on / off control circuit 43. The inverting input terminal of the comparator CON2 is connected to have the same potential as the non-inverting terminal of the comparator CON1.

The triac trigger circuit 45 is connected to the gates of the triacs T and S and is also connected to the on / off control circuit 43, and causes the triac TS to perform an on / off operation, that is, a switching operation, based on a signal from the on / off control circuit 43. When the on / off control circuit 43 determines that the input power is set low by the variable resistor R23, the on / off control circuit 43 turns on / off the triac TS via the triac trigger circuit 45. Therefore, when the input power is set to a low value, for example, 1 KW or less, the supply of power to the inverter circuit 3 is controlled by the ON / OFF operation of the triac TS. On the other hand, when the input voltage is set to a high input voltage, for example, 1 KW or more, the time of the transistor 13 is changed so that the inverter circuit 3
To control the input power to the

Next, with reference to FIGS. 2 (A), (B) and (C), an operation for heating an appropriate object to be heated such as an iron pot will be described.

The input power set by the variable resistor R23 is a low value,
For example, when 1KW or less, the on-off control circuit 43 turns on the triac TS at time t ₁ as shown in FIG. 2 (C) through the triac trigger circuit 45. Further, the on / off control circuit 43 sets the triac TS to ON from time t _{1 at} time t _1.
turning off the transistor TR12 in m sec prior to the time t _0. Thereby, the capacitor C3 is charged, and FIG. 2 (B)
The voltage V _TON gradually increases as shown in FIG. This capacitor
The voltage V at time t ₁ is obtained by charging C3 for 10 ms.
_TON rises to a voltage that can set the on-time of transistor 13 to, for example, 12 μsec.

Together with the time t ₁ the triac TS inverter circuit 3 starts the oscillation operation is turned on, the transistor 13 performs switching operation at a sufficiently long on-time 12 .mu.sec. At this time, the peak value of the current flowing through the transistor 13 is 60 A as shown in FIG. 2 (A), which can be suppressed to a sufficiently small value.

Next, when the elapsed time t _1, the voltage V _I corresponding to the input current i _IN is applied to the non-inverting input terminal of the comparator CON1. And the divided voltage of the comparator CON1 resistors R21 and R22 to input to the inverting input terminal as a reference voltage, by comparing the reference voltage and the voltage V _I, to determine the magnitude relationship between the voltage V _TON and the voltage V _I . As shown in FIG. 2 (B), the voltage V _I
If the voltage exceeds the voltage V _TON , it is determined that the load is appropriate, and the heating operation by the inverter circuit 3 is continued. Turns on the triac TS between the example 30m seconds from time t ₁ to t ₃ for operating the inverter circuit 3 as shown in FIG. 2 (C). Also from time t ₃ to t _4, for example,
The triac TS is turned off for only 40 msec, and the transistor Tr12 is turned on based on the signal from the on / off control circuit 43 to stop the operation of the inverter circuit 3. In the same manner, the heating operation is continued by repeating the on / off operation of the triac TS.

At this time, when the oscillation of the inverter circuit 3 starts,
As described above, the ON time of the transistor 13 is 12 μm, which is sufficiently long.
Since it is set to sec, short-circuit current can be suppressed.

Further, during a period when the operation of the inverter circuit 3 is stopped, the transistor Tr15 is turned on based on a signal from the on / off control circuit 43 to discharge the charge of the capacitor C4, thereby setting the capacitor C4 in an initial state.

Next, an operation of detecting a no-load state will be described with reference to FIGS.

Triac When TS is turned on at time t _1, then the comparator CON1 is compared with the voltage V _I and reference voltage. Thus to determine the magnitude relationship between the voltage V _I and the voltage V _TON, voltage
If V _I is less than the voltage V _TON but equal, it is determined that the pot is not placed on the heating coil 7, that is, no load, and the comparator CON 1 outputs a low-level signal. Is output to the oscillation stop timer 33. The oscillation operation of the inverter circuit 3 is stopped, for example, for 3 seconds based on the signal from the oscillation stop timer 33.

As described above, when the input power is set to a low value, the inverter circuit is controlled by the on / off operation of the triac, so that the oscillation cycle of the inverter circuit can be set short without causing uneven heating or the like.
The object to be heated can be appropriately heated or kept warm.

Next, a second embodiment according to the present invention will be described with reference to FIGS.

In the present embodiment, the load detection timer 41 is provided, and the triac TS is turned on by setting the ON time to a sufficiently long and constant value only during the predetermined timer time, and the load is detected after the elapse of the timer time. It is characterized by having done.

More specifically, the load detection timer 41 is connected to the base of the transistor Tr13 via the resistor R9, the emitter of the transistor Tr13 is connected to a predetermined DC power supply, and a predetermined voltage Vcc is applied to the emitter of the transistor Tr13. Have been. The collector of the transistor Tr13 is connected to the oscillation stop timer 33 and to the output terminal of the comparator CON1 via the resistor R25.

The load detection timer 41 is connected to the base of the transistor Tr10 via the resistor R6, and is connected to the transistor Tr10.
The collector of 10 is connected to the terminal P1 of the pulse width modulation circuit 19 via the resistor R10. Similarly, the load detection timer 41 is connected to the base of the transistor Tr11 via the resistor R7, and the collector of the transistor Tr11 is connected to the terminal P1 of the pulse width modulation circuit 19 via the resistor R11.

Fourth on-off control circuit at a time t ₁ as shown in FIG.
43 activates the load detection timer 41 and turns on the triac TS via the triac trigger circuit 45.

Load detecting timer 41 in between t ₂ from time t ₁ is a predetermined time set by the timer, for example, turning on only the transistor Tr10, Tr11 and Tr13 during the 10m sec. When the transistors Tr10 and Tr11 turn on, the pulse width modulation circuit uses the _divided voltage of the resistors R10 and R11 as the voltage V _TON for setting the on-time.
Give to 19. As a result, the transistor 13 is turned on for an ON time corresponding to the pulse width of the pulse signal from the pulse width modulation circuit 19, for example, 12 μs, and the inverter circuit 3 oscillates. At this time, the peak value of the current flowing through the transistor 13 is suppressed to a sufficiently small value as shown in FIG.

The current transformer CT detects the input current i _IN, and outputs a detection signal corresponding to the input current i _IN, after the detection signal is rectified by the bridge circuit composed of a diode D6, D7, D8, D9 , And a smoothing circuit including a resistor R16 and a capacitor C4. Since the time constant of this smoothing circuit is set longer than 10 ms, the time constant is set to 10 ms during which the load detection timer 41 is operating, that is, from the time t ₁ to t ₂ shown in FIG. 4 (B). In, the detection operation of the load detection circuit is prohibited. Specifically, forcibly H level output terminal of the comparator CON1 conducting during the 10m seconds from the transistor Tr13 is the time t ₁ based on a signal from the load detecting timer 41 as described above until t ₂ To be raised. As a result, the activation of the oscillation stop timer 33 is prohibited.

As described above, in the embodiment shown in FIGS. 3 and 4, the on-time of the transistor 13 is set to a sufficiently long constant value to turn on the triac TS.
The short circuit current flowing through 13 can be reduced.

Next, a third embodiment according to the present invention will be described with reference to FIG.

This embodiment is characterized in that the resistance values of the resistors R10 and R11 shown in FIG. 3 are set to small values, and the triac TS and the triac trigger circuit 45 are not required.

Specifically, in the resistor R1, R2 and time t ₁ as shown in Figure 5 when the time constant is set to be sufficiently small values of the resistors R10, R11 and capacitor C3 as compared with the time constant of the capacitor C3 Instantly, that is, at the time of the zero crossing of the commercial power supply, the transistors Tr10 and Tr11 are turned on, and the voltage V _TON is set to a constant value determined by the voltage dividing ratio of the resistors R10 and R11. For example, 12
This is set to μ seconds to operate the inverter circuit 3.

The other circuit units are the same as those in FIG. 3, and the detailed description is omitted.

As described above, the embodiment shown in FIG. 5 can eliminate the need for the triac and the triac trigger circuit, and can reduce the cost with a simple circuit configuration.

Next, a fourth embodiment according to the present invention will be described with reference to FIG.

In this embodiment, the counter 55 and the reference oscillator 57 are constituted by digital circuits, and these circuit parts are provided by a microcomputer.
It is characterized by being controlled by 53.

The counter 55 counts the reference pulse from the reference oscillator 57, and outputs a pulse signal having a pulse width corresponding to the count number based on a command from the microcomputer 53 to the drive signal 15. The microcomputer 53 is an input setting circuit.
The counter 55 is controlled according to the value set by 51.
Here, the microcomputer 53 controls the pulse width of the pulse signal output from the counter 55 to be equal to or larger than a predetermined value.
Is set to a sufficiently long time, for example, 12 μsec or more. Thereby, the short-circuit current of the transistor 13 at the time when the operation of the inverter circuit 3 starts can be suppressed to a small value.

Next, a fifth embodiment according to the present invention will be described with reference to FIG.

This embodiment is characterized in that it is applied to a microwave oven.
That is, the input setting circuit 51 controls the pulse width modulation circuit 19 so that the ON time of the transistor 13 is equal to or longer than a predetermined value, for example, 12 μs or longer. An inverter circuit is formed by the primary winding L1 of the step-up transformer LT, the transistor 13, the capacitor 9 and the diode 11, and the ON time of the transistor 13 at the start of the operation of the inverter circuit is determined.
By setting it to 12 μs or more, the short-circuit current of the transistor 13 is suppressed to a small value. When high-frequency power is induced in the secondary winding L2 of the step-up transformer LT by the operation of the inverter circuit, the DC voltage boosted by the voltage doubler rectifier circuit including the diode D61 and the capacitor C61 is applied to the magnetron 61. The object to be heated is heated by irradiating the object with the high frequency (microwave heating) emitted from the magnetron 61.

The switching means constituting the inverter circuit is not limited to a transistor, and may be, for example, an FET, an I
It can be configured by using an appropriate switching means such as GBT or SIT.

[Effects of the Invention] As described above, according to the present invention, the ON time of the switching means at the start of the oscillating operation of the inverter circuit is longer than the ON time when the short-circuit current is maximized, and the switching means is prevented from being broken. Therefore, the short-circuit current of the switching means can be suppressed, the occurrence of uneven heating can be prevented, and the heat retention can be performed at the same time.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment according to the present invention, FIG. 2 is a signal waveform diagram of each part in FIG. 1, FIG. 3 is a block diagram showing a second embodiment according to the present invention, FIG. 4 is a signal waveform diagram of each part in FIG. 3, FIG. 5 is a signal waveform diagram of each part in the third embodiment according to the present invention, and FIG. 6 is a block diagram of a fourth embodiment according to the present invention. FIG. 7 is a block diagram of a fifth embodiment according to the present invention, FIG. 8 is a block diagram showing a conventional example, and FIG.
FIG. 12 to FIG. 12 are signal waveform diagrams of a conventional example. 3 Inverter circuit 7 Heating coil 13 Transistor 23 On-time setting circuit

Claims (1)

(57) [Claims] An inverter circuit having switching means and high-frequency power generation means for generating high-frequency power in accordance with the on / off of the switching means, wherein high-frequency power obtained by oscillating and stopping oscillation of the inverter circuit is provided. In the cooking device for heating an object to be heated by using the same, the ON time of the switching means at the start of oscillation of the inverter circuit is equal to or longer than the ON time when the short-circuit current is maximized, and the switching means is prevented from being broken. A cooking device having an on-time setting means for setting the on-time to be equal to or longer than a preset minimum on-time.