JPH11265783A - Electromagnetic cooking device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To continuously heat by feeble input under the condition that a switching loss can be reduced. SOLUTION: A turn-on delay part 41c refers terminal voltage VCS of a snubber capacitor 15, and it extends turn-on delay time TD1' as TD1'=TD1+ TDX, when the terminal voltage VCS does not get to the direct-current power- supply voltage by reduction of the input setting value if the turn-on delay time remains at a fixed value TD1. Then, charging time of the snubber capacitor 15 is extended and the terminal voltage VCS is increased, thereby potential difference impressed between a collector and an emitter is reduced when IGBT 6 is turned on next time, thus a switching loss is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic cooker for heating a cooking vessel by supplying a high-frequency current to a heating coil.

[0002]

2. Description of the Related Art An electromagnetic cooker is safe and excellent in heat efficiency without using fire, and is becoming popular as a cooking heater incorporated in a system kitchen or the like. A plurality of electromagnetic cookers are often installed in a system kitchen. In order to prevent the generation of interference noise when the plurality of electromagnetic cookers are used at the same time, a half-heater that constantly controls heating at a constant frequency is used. A bridge-type inverter may be employed.

FIG. 10 shows an electrical configuration of a half-bridge type inverter employed in a conventional electromagnetic cooker. In FIG. 10, an AC input terminal of a rectifier circuit 1 composed of a diode bridge is connected to a commercial AC power supply 2, and a DC output terminal is connected to both ends of a smoothing capacitor 3.

[0004] Both ends of the smoothing capacitor 3 are connected to arms composed of positive and negative IGBTs 6 and 7 via DC buses 4 and 5, so that a half-bridge type inverter main circuit 8 is connected. Make up. Freewheel diodes 9 and 10 are connected between the collector and the emitter of the IGBTs 6 and 7, respectively.

The output terminal 8a of the inverter main circuit 8 has:
One end of the heating coil 11 is connected, and the heating coil 1
1 and the DC bus 5, a resonance capacitor 12
And a parallel circuit of a diode 13. still,
The heating coil 11 and the resonance capacitor 12
4.

Further, one end of a snubber capacitor 15 is connected to the output terminal 8a.
Is connected to the DC bus 5 via the collector-emitter of the IGBT 16. The diode 17 is connected between the collector and the emitter of the IGBT 16. These constitute a so-called snubber circuit 18 and are provided to reduce switching loss when the IGBTs 6 and 7 are off.

The oscillation signal of a predetermined frequency output from the oscillator 19 is supplied to a variable on-time setting unit 20 and a fixed on-time setting unit 21. A current transformer 22 is inserted on the AC input side of the rectifier circuit 1, and the current transformer 2
The output terminal 2 is connected to the input terminal of the input setting unit 24a via the input current detection unit 23. The input current detector 23 detects the input current value detected by the current transformer 22 as A /
D conversion, and the input setting unit 24a
Output.

Although not specifically shown, the operation section 25 has keys for the user to select various automatic cooking menus (control programs), and a key for setting the amount of heating by a power amount such as 1 KW or 2 KW. Keys are provided. Then, the input setting unit 24a performs feedback control based on the input current detection value Vin given from the input current detection unit 23 so that the input current value corresponds to the setting of the electric energy in the operation unit 25, and the variable ON time PW to setting unit 20
An M signal is provided.

Further, the heating stop unit 24b issues a heating stop command to the variable on-time setting unit 20 when a predetermined condition is satisfied.
And a fixed on-time setting unit 21. The input setting unit 24a and the heating stop unit 24b are blocks of the functions of the microcomputer 24.

The output signal of the variable on-time setting unit 20 is supplied to first and third driving units 26 and 27, and the output signal of the fixed on-time setting unit 21 is supplied to the second and third driving units 28 and 27. Has been given. And the first, second and third
The output terminals of the driving units 26, 28 and 27 are connected to the IGBT 6,
7 and 16 are respectively connected to the gates.

FIG. 13 shows a detailed electrical configuration of the first drive section 26. As shown in FIG. In FIG. 13, the output signal of the variable on-time setting unit 20 is given to a photocoupler 29, and one output terminal of the photocoupler 29 is connected to the gate of the IGBT 6 via a series circuit of resistors 30 and 31. ing. The resistor 30 has a diode 3
2 are connected in anti-parallel. Further, the other output terminal of the photocoupler 29 is connected to the emitter of the IGBT 6. The resistance values of the resistors 30 and 31 are, for example, 150Ω, 1
It is set to about 0Ω.

The operation of the electromagnetic cooker provided with the inverter configured as described above will be described below with reference to FIGS. The heating of the pot is performed by supplying a high-frequency current to the heating coil 11 by an inverter. FIG. 11 shows the signal waveform of each part in this case. As shown in FIGS. 11A and 11B, the IGBTs 6, 7
Is, for example, a control cycle T of the inverter of about 20 KHz.
Inv is turned on and off alternately.

The on-period Ton1 of the IGBT 6 is based on an output signal given from the variable on-time setting unit 20,
It changes with inv / 2 as the upper limit. On the other hand, the ON period Ton2 of the IGBT 7 is substantially equal to Tinv / Ton2 based on the output signal given from the fixed ON time setting unit 21.
It is fixed to 2. However, in order to prevent a short circuit between the IGBTs 6 and 7, the turn-on delay times TD1 and TD2 are secured when the ON periods of both are switched.

The IGBT 16 of the snubber circuit 18 is shown in FIG.
As shown in (c), the IGBT is driven by the third drive unit 27.
7 is turned on after a lapse of a predetermined time Tα after turning on, and the IGBT 6 is turned off after a lapse of a certain time Tα after turning off. With this, IGB
When T6 and 7 shift from the on state to the off state,
The change in the voltage between the collector and the emitter is moderated to prevent the occurrence of switching loss, and also to prevent the short-circuit current from flowing through the snubber capacitor 15 when the IGBT 7 is turned on.

Here, the fixed time Tα is set to be constant regardless of the load or setting input within an appropriate range.
And 7 are set such that the voltage changes at the time of turn-off converge within the time. The IGBT 16 is an IGBT
The switching loss at the time of turning off the BTs 6 and 7 is reduced, and the on / off control is performed so that the snubber capacitor 15 is not charged during a period from when the IGBT 6 is turned off to when the IGBT 7 is turned on.

The control cycle consists of the following four cycles. FIG. 11D shows the waveform of the current IL flowing through the heating coil 11 at this time, and FIG. 11E shows the IGBT
7 is a waveform of the collector-emitter voltage Vtr2. IGBT6: ON / IGBT7: OFF A current is supplied to the heating coil 11 and the resonance capacitor 1 is supplied through the path of the smoothing capacitor 3, the IGBT6, the heating coil 11, the resonance capacitor 12, and the smoothing capacitor 3.
2 is charged (see FIG. 11D, A).

IGBT6: OFF / IGBT7: OFF In the path of the heating coil 11, the resonance capacitor 12, the freewheel diode 10, and the heating coil 11, the resonance capacitor 12 is further charged by the delay current of the heating coil 11 (FIG. 11D). , B).

IGBT 6: OFF / IGBT 7: ON The resonance capacitor 12, the heating coil 11, the IGBT 7 and the resonance capacitor 12
Is discharged to cause a current in the opposite direction to flow through the heating coil 11 (see FIGS. 11D and 11C). When the resonance capacitor 12 is completely discharged, the current flows through the diode 13 connected in parallel (see FIG. 11 (d), C ').

IGBT6: OFF / IGBT7: OFF In the path of the heating coil 11, the freewheel diode 9, the smoothing capacitor 3, the diode 13, and the heating coil 11, the delay current of the heating coil 11 is supplied to the power supply through the freewheel diode 9. (Fig. 11 (d),
D).

A high frequency current is supplied to the heating coil 11 by repeating the above cycle,
An eddy current is induced in a pot 34 (see FIG. 10) placed on the pan to perform heating cooking. The input current control is performed by changing the on-period Ton1 of the IGBT 6, and if the on-period Ton1 is lengthened, the input current increases and the amount of heating of the pot 34 increases.

[0021]

However, in such a conventional electromagnetic cooker, since a weak input heating is performed, an I-type heater is required.
As the on-period Ton1 of the GBT 6 is shortened, the following problem occurs. FIG. 12 shows the signal waveform of each part at this time. That is, as shown in FIG. 12A, when the ON period Ton1 of the IGBT 6 becomes equal to or less than a certain time, the amount of current supplied to the heating coil 11 decreases (see FIG. 12D, A). And C 'and the cycle, the snubber capacitor 15 cannot be fully charged until the terminal voltage Vtr2 of the IGBT 7 becomes equal to the DC power supply voltage. Therefore, no regenerative current flows in the cycle and the snubber capacitor 15 continues to be charged. .

Since the IGBT 6 is turned on in the next cycle in that state, the DC power supply voltage and the voltage Vtr
A short-circuit current flows in the path of the DC bus 4, the IGBT 6, the snubber capacitor 15, the IGBT 16 and the DC bus 5 due to the potential difference from the DC bus 4. Here, FIG.
FIG. 12 shows a current waveform Itr1 flowing in T6.
A short-circuit current flows at a point P shown in FIG.

In order to suppress such a short-circuit current as much as possible, as shown in FIG. 13, the gate resistance of the IGBT 6 at the time of turn-on is increased by passing the gate of the IGBT 6 through a series circuit of the resistors 30 and 31. And set it to
As shown in (1), the rise of the gate signal VG1 is made gentle to delay the timing at which the IGBT 6 is turned on.

However, since the rise of the gate signal VG1 is made gentler in this way, the rise of the collector-emitter voltage of the IGBT 6 becomes gentler.
Switching loss (turn-on loss) that occurs when the GBT 6 is turned on occurs. The switching loss at the time of turn-on increases as the set input decreases, and if continuous heating is performed while the turn-on loss is large, the temperature of the IGBT 6 rises, leading to thermal destruction in the worst case.

Therefore, in the conventional electromagnetic cooker, when performing low-input heating corresponding to, for example, cooking in which cooking is performed for a long time with low heat, a low input that does not cause a turn-on loss of the IGBT 6 is set as a lower limit. Set, for example, 3
Periodic heating had to be performed, for example, heating was stopped for 3 seconds after heating for 2 seconds.

[0026] In such a heating method, there are problems such as sudden boiling when a small amount of food is cooked, and burning of the food when stew cooking is performed. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electromagnetic cooker capable of performing continuous heating with a weak input in a state where switching loss can be reduced.

[0027]

According to one aspect of the present invention, there is provided an electromagnetic cooker comprising: a rectifier circuit for rectifying an AC power supply to generate a DC power supply; and a DC power supply generated by the rectifier circuit. , A first and second switching element connected in series between the positive and negative DC buses, and a first and a second switching element.
A resonance circuit, which is connected between either one of the two terminals of the switching element and is constituted by a heating coil and a resonance capacitor for inductively heating the cooking vessel, and is connected between both terminals of the one switching element, A snubber circuit including a capacitor and a third switching element;
An additional delay is added to the turn-on delay time from turning off the switching element to which the resonance circuit is connected to turning on the other switching element so as to reduce the voltage applied to the other switching element at the time of turning on. And delay time control means for adding time.

With this configuration, even if the on-time of the other switching element is shortened and the amount of current supplied to the heating coil is reduced, the delay time control means adds an additional delay to the turn-on delay time. By adding the time, the charging time of the snubber capacitor during that time increases. Then, as the terminal voltage of the snubber capacitor rises, the potential difference between the terminal voltage and the DC power supply voltage of the rectifier circuit decreases, so that the voltage applied to the switching element when the other switching element is turned on next time is reduced. Reduced. Therefore, a high-frequency current can be continuously supplied to the heating coil in a state where the switching loss in the other switching element can be reduced, and continuous heating can be performed with a weak input.

In this case, a terminal voltage detecting means for detecting a terminal voltage of the snubber capacitor is provided, and a delay time controlling means is provided for the terminal of the snubber capacitor detected by the terminal voltage detecting means. It is preferable that the additional delay time is changed according to the voltage (claim 2). Specifically, the delay time control means is added so that the terminal voltage of the snubber capacitor detected by the terminal voltage detection means becomes maximum. It is preferable that the delay time is changed (claim 3).

According to this structure, the delay time control means includes:
The additional delay time is changed so that the terminal voltage of the snubber capacitor rises, and the snubber capacitor is charged until the terminal voltage of the snubber capacitor becomes maximum, so that when the other switching element is turned on, the voltage applied to the switching element is reduced. Can be minimized.

According to a fourth aspect of the present invention, there is provided an input setting means for setting a lower limit of the input current value so that the maximum terminal voltage of the snubber capacitor detected by the terminal voltage detecting means does not fall below a predetermined value. good. With such a configuration, for example, when the setting of the input current value becomes very low,
Accordingly, the ON time of the other switching element is also shortened, so that the amount of current supplied to the heating coil is significantly reduced, and the maximum terminal voltage level of the snubber capacitor is relatively reduced. Therefore, if the input setting means sets the lower limit for the input current value, the voltage level applied to the other switching element when the other switching element is turned on can be suppressed.

As described in claim 5, the delay time control means may be configured to change the additional delay time in accordance with the level of the input current value. The terminal voltage of the snubber capacitor can be increased by changing the additional delay time according to the decrease in the input current value level.

According to a sixth aspect of the present invention, there is provided an electromagnetic cooker comprising: a rectifier circuit for rectifying an AC power supply to generate a DC power supply; a positive and negative DC bus to which the DC power generated by the rectifier circuit is supplied; First and second switching elements connected in series between the positive and negative DC buses;
And a resonance circuit, which is connected between either one of the two terminals of the second switching element and comprises a heating coil and a resonance capacitor for inductively heating the cooking vessel, and is connected between both terminals of the one switching element. And a snubber circuit including a snubber capacitor and a third switching element, and when the switching element to which the resonance circuit is not connected is turned on, the voltage applied to the switching element is reduced. Capacitance switching means for switching the capacitance of the snubber capacitor.

With this configuration, even when the on-time of the other switching element is shortened and the amount of current supplied to the heating coil is reduced, the capacitance of the snubber capacitor is switched by the capacitance switching means. By doing so, the rise of the terminal voltage of the snubber capacitor during that time can be accelerated. Then, since the potential difference between the terminal voltage of the snubber capacitor and the DC power supply voltage of the rectifier circuit is reduced, the voltage applied to the switching element when the other switching element is turned on next time is reduced.
Therefore, a high-frequency current can be continuously supplied to the heating coil in a state where the switching loss in the other switching element can be reduced, and continuous heating can be performed with a weak input.

In this case, a terminal voltage detecting means for detecting the terminal voltage of the snubber capacitor is provided, and the capacitance switching means is adapted to respond to the terminal voltage of the snubber capacitor detected by the terminal voltage detecting means. Alternatively, a configuration in which the capacitance of the snubber capacitor is switched may be adopted.

According to this structure, the capacitance switching means can increase the terminal voltage of the snubber capacitor by, for example, switching the terminal voltage of the snubber capacitor so that the capacitance decreases when the terminal voltage of the snubber capacitor decreases to some extent.

In addition, the capacitance switching means may be configured to switch the capacitance of the snubber capacitor when the input current value falls below a predetermined value. Becomes very low, the on-time of the other switching element is correspondingly shortened, and the amount of current supplied to the heating coil is significantly reduced.If the capacity switching means is switched to reduce the capacity, As a result, the rise of the terminal voltage of the snubber capacitor can be accelerated.

Further, as described in claim 9, when the capacitance switching means switches the capacitance of the snubber capacitor, the capacitance switching means may switch in a state where the conduction control for the first and second switching elements is temporarily stopped. good. With such a configuration, the first and second capacitors are switched when the capacity is switched.
By stopping the conduction control of the switching element, it is possible to prevent a short-circuit current from flowing through the third switching element.

In addition, it is preferable to connect a resistor in parallel with the resonance capacitor. According to this configuration, even if the conduction control for the first and second switching elements is temporarily stopped while the capacitance switching unit switches the capacitance of the snubber capacitor, the electric charge charged in the resonance capacitor is transferred to the resistor. Can be quickly discharged via the.

[0040]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. It is to be noted that the same parts as those in FIG. 10 are denoted by the same reference numerals, and the description thereof will be omitted. FIG. 1 shows an electrical configuration. In this embodiment, the microcomputer 24 shown in FIG.
Instead of the above, an input setting unit (input setting unit) 41a, a heating stop unit 41b, and a turn-on delay unit (delay time control unit)
A microcomputer (control means) 41 having 41c is arranged. Turn-on delay section (delay time control means) 41c
Is adapted to change the IGBT 6 (first switching means) turn-on delay time TD1 as described later.

The snubber capacitor voltage detecting section (terminal voltage detecting means, hereinafter referred to as a voltage detecting section) 42 detects the terminal voltage VCS of the snubber capacitor 15 and has an input terminal connected to the output of the inverter main circuit 8. It is connected to terminal 8a. Further, the output signals of the variable on-time setting unit 20 and the fixed on-time setting unit 21 are provided to the voltage detection unit 42 in order to obtain the measurement timing of the terminal voltage VCS. The output terminal of the voltage detection unit 42 is connected to the input terminal of the turn-on delay unit 41c. Other configurations are the same as those shown in FIG.

Next, the operation of the present embodiment will be described with reference to FIGS. As described above, when the input set value set by the user on the operation unit 25 is relatively large and the ON time of the IGBT 6 is relatively long, the amount of current supplied to the heating coil 11 is large. , During periods C and C ′ of the cycle and in the cycle, IGB
The snubber capacitor 15 is charged until the terminal voltage Vtr2 of T7 becomes equal to the DC power supply voltage.

Therefore, the turn-on delay unit 41c refers to the terminal voltage VCS of the snubber capacitor 15 detected by the voltage detection unit 42 during the period D, and when the terminal voltage VCS is substantially equal to the DC power supply voltage, The control is performed as in the case shown. That is, the turn-on delay time from when the IGBT 7 (second switching element) is turned off to when the IGBT 6 is turned on is a fixed value TD1, and when the IGBT 6 is turned off during the ON period of approximately 50% duty, the IGB is turned off.
The turn-on delay time until T7 turns on is a fixed value T
Controlled with D2 set.

The turn-on delay section 41c fixes the turn-on delay time when the input set value set by the operation section 25 becomes small and the terminal voltage VCS of the snubber capacitor 15 during the period D does not reach the DC power supply voltage. The additional delay time TDX is added to the delay time TD1, and TD1 '= TD1 + T
Control to extend as DX. In this case, the additional delay time TDX is variable, and the turn-on delay unit 41c gradually extends the additional delay time TDX while referring to the terminal voltage VCS until the terminal voltage VCS reaches the maximum level.

Here, FIG. 3 shows the period D shown in FIG.
Is an enlarged portion, and shows a change in the terminal voltage VCS of the snubber capacitor 15 in this case. Conventionally, since the turn-on delay time is always a fixed value TD1, the snubber capacitor 15 at the time of low input is charged only up to the level of the terminal voltage VCS. Therefore, as shown by a broken line in FIG. 3, when the IGBT 6 is turned on, the voltage applied between the collector and the emitter, that is, the potential difference between the DC power supply voltage and the terminal voltage VCS (
−) Is a very large factor causing an increase in switching loss.

On the other hand, in this embodiment, the turn-on delay time is extended as TD1 '= TD1 + TDX, and during this time, the snubber capacitor 15 is charged and the terminal voltage VCS reaches the level'. Is reduced to (-'). Therefore,
As shown at the point Q in FIG. 2F, the short-circuit current flowing through the IGBT 6 is reduced by the amount by which the potential difference is reduced from (−) to (− ′), and the switching loss is reduced as compared with the conventional case. .

FIG. 4 is a measurement example performed by the inventor of the present invention, and shows the temperature change of the IGBT 6 when the iron set (cooking vessel) 34 is heated by changing the input set value (horizontal axis). (Vertical axis) is shown. IGBT6 according to the decrease of the input set value
However, when the turn-on delay time is always set to a fixed value TD1 as in the conventional case, as shown by the broken line in FIG. 4, when the input power amount decreases to some extent, the temperature of the IGBT 6 rapidly rises.

On the other hand, when the turn-on delay time is changed in a region where the input power is low as in the present embodiment, as shown by a solid line in FIG.
The degree of temperature rise of the BT 6 becomes relatively slow.

The temperature Tth shown in FIG. 4 is a temperature rise limit value of the IGBT 6, and the input electric energy corresponding to the limit value Tth is WL. As the input power drops,
Since the amount of current supplied to the heating coil 11 gradually decreases, the terminal voltage VCS of the snubber capacitor 15
The maximum level also decreases. Therefore, the potential difference applied when the IGBT 6 is turned on also increases, and the temperature eventually reaches the limit value Tth. Therefore, the input setting unit 41
In the case of a, the input power amount is limited in advance so as not to be equal to or less than WL, and low input heating at a certain level or less is not performed.

As described above, according to the present embodiment, the turn-on delay section 41c is provided with the terminal voltage V of the snubber capacitor 15.
When the terminal voltage VCS does not reach the DC power supply voltage while the turn-on delay time remains at the fixed value TD1 due to the decrease of the input set value with reference to CS, the turn-on delay time is reduced to T.
D1 '= TD1 + TDX.

Then, the charging period of snubber capacitor 15 is extended and the terminal voltage rises, so that IGBT
6 turns on, the potential difference applied between the collector and the emitter is reduced, and the switching loss is reduced.
It is possible to perform continuous heating by a weak input, and unlike the conventional art, for example, it is possible to satisfactorily perform cooking for a long time without burning or suddenly boiling the food.

When the input set value is small, for example, a method of keeping the IGBT 16 in an off state to prevent the snubber circuit 18 itself from operating can be considered. In such a case, the IGBT 7 is turned on and off. Switching losses are expected to increase. On the other hand, according to the present embodiment, it is possible to reduce the switching loss of the IGBT 6 without increasing the switching loss generated in the IGBT 7.

Further, according to the present embodiment, the input setting unit 41
In the case of a, a limit is provided in advance so that the input electric energy does not become equal to or less than WL, so that low input heating at a certain level or less is not performed. Therefore, the temperature rise of the IGBT 6 does not reach the limit value Tth. The device can be used within a safe operating range.

FIG. 5 shows a second embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted. Only the different parts will be described below. The electromagnetic cooker according to the second embodiment includes a microcomputer 43 having a snubber capacitor switching unit (hereinafter, referred to as a switching unit) 43c instead of the turn-on delay unit 41c of the microcomputer 41.

The snubber capacitor 15 is replaced by a first snubber capacitor 44 having a small capacity.
A second snubber capacitor 45 having one end connected to the output terminal 8a of the inverter main circuit 8 is connected in parallel to the first snubber capacitor 44 via a normally-on relay 46. . The capacities of the first and second snubber capacitors 44 and 45 are the same as those of the first embodiment with the snubber capacitors 44 and 45 connected in parallel.
5 is set to be equal to the capacity.

The first and second snubber capacitors 44 and 45, the relay 46, the IGBT 16 and the diode 1
Reference numeral 7 denotes a snubber circuit 47. The switching section 43c of the microcomputer 43 controls the opening and closing of the relay 46 by giving a control signal Vs.

A resistor 48 is connected in parallel to the resonance capacitor 12 and the diode 13. The resistance value of the resistor 48 is set to a value sufficiently larger than the impedance of the resonance capacitor 12 when the inverter main circuit 8 is operating. Other configurations are the same as those of the first embodiment. The switching unit 43c and the relay 46 constitute a capacity switching unit.

Next, the operation of the second embodiment will be described with reference to FIGS. The switching unit 43c has a relatively high input set value, and when the terminal voltage VCS detected by the voltage detection unit 42 during the period D is substantially equal to the DC power supply voltage, the switching unit 43c keeps the relay 46 closed while performing the second switching operation. Control is performed in the same manner as in the first embodiment with the first and second snubber capacitors 44 and 45 connected in parallel.

When the terminal voltage VCS in the period D does not reach the DC power supply voltage due to the input set value being set relatively low, the switching unit 43c sets the control signal Vs
And the relay 46 is opened to disconnect the second snubber capacitor 45, and the snubber circuit 47 is operated only by the first snubber capacitor 44. At this time, FIG.
As shown in FIG. 7E and FIG. 7, even if the ON time of the IGBT 6 is shortened and the amount of current supplied to the resonance circuit 14 is reduced, the first snubber capacitor 44 having a small capacitance remains in the period D.
(Ie, within the turn-on delay time TD1),
The terminal voltage reaches the DC power supply voltage. Therefore, as shown at point R in FIG. 6F, the next time the IGBT 6 is turned on, no short-circuit current flows.

At this time, the input setting section 41a stores the input set value when the control signal Vs is output from the switching section 43c, and is set by the operation section 25 in the subsequent control. When the input set value coincides with the stored value, a command may be given to the switching unit 43c to output the control signal Vs.

FIG. 8 is a diagram corresponding to FIG. When the snubber circuit 47 is operated only by the first snubber capacitor 44 in the region where the input electric energy is low as in the second embodiment, as shown by the solid line in FIG.
Is decreased in proportion to the decrease of the input set value, and greatly decreases from Ta to Tb. Therefore, approximately 0W
Until the low input heating can be performed continuously.

FIG. 9 shows a control state when the switching of the relay 46 is performed in accordance with the input set value.
And the control signal Vs (b). In FIG. 9, when switching the relay 46 from the closed state to the open state, the switching unit 43c
By giving a control signal to the input setting unit 41a, the heating stop unit 41
The conduction control of the IGBTs 6 and 7 is stopped via b (see FIG. 9A, time point A).

Then, the electric charge remaining in the resonance capacitor 12 is discharged via the resistor 48, so that the voltage Vtr2 gradually decreases from the DC power supply voltage and becomes approximately 0 V after the lapse of the estimated time Ta (FIG. 9 ( a), time point B). Furthermore,
After the waiting voltage Vtr2 has reached 0 V for the allowance time Tb, the switching unit 43c outputs the control signal Vs to the relay 46 and disconnects it to disconnect the second snubber capacitor 45 (FIG. 9B, time point). C). Next, after the elapse of the control system switching waiting time Tc, the conduction control of the IGBTs 6 and 7 is started, and the continuous heating is performed with a weak input (see FIG. 9A, time point D).

Also, when returning from the weak input heating to the normal heating control, the switching is similarly performed. That is, the switching unit 43
c gives a control signal to the heating stop unit 41b via the input setting unit 41a to stop the conduction control of the IGBTs 6 and 7 (see FIG. 9A, time point E). Wait for the discharge of the residual charges (see FIG. 9A, time point F).

Further, after waiting for the allowance time Tb, the switching unit 43c stops outputting the control signal Vs to the relay 46 and connects the second snubber capacitor 45 to the first snubber capacitor 44 in parallel (FIG. 9 ( b), time point G).
Then, after waiting for the control system switching waiting time Tc to elapse, normal heating control is started (see FIG. 9A, time point H).

As described above, according to the second embodiment, when the input set value decreases and the terminal voltage VCS of the first and second snubber capacitors 45 and 45 does not reach the DC power supply voltage, the switching unit 43c switches the relay. The second snubber capacitor 45 is cut off, and the snubber circuit 47 is operated only by the first snubber capacitor 44.

Therefore, even if the amount of current supplied to the resonance circuit 14 is reduced, the first snubber capacitor 44 having a small capacitance is used.
Is charged within a short time, the turn-on delay time TD1
Even if the IGBT 6 is turned on after the elapse of the period, no short-circuit current flows due to insufficient charging capacity, and a high-frequency current can be continuously supplied to the heating coil 11 at a low input set value. Thus, it is possible to perform continuous heating with a weak input while suppressing the switching loss of the IGBT 6, and
Control for reducing switching loss can be performed more easily than in the first embodiment.

Further, according to the second embodiment, the resistor 48 is connected in parallel to the resonance capacitor 12, and the switching unit 43c
When the second snubber capacitor 45 is disconnected or connected, the conduction control for the IGBTs 6 and 7 is temporarily stopped, and the transition between the disconnection and the connection is performed during that time. At that time, a short-circuit current flows through the IGBT 16 Can be prevented, and even if the conduction control for the IGBTs 6 and 7 is stopped, the charge charged in the resonance capacitor 12 can be quickly discharged via the resistor 48.

The present invention is not limited to the embodiment described above and shown in the drawings, and the following modifications or extensions are possible. Additional delay time T in the first embodiment
X does not necessarily need to be changed according to the terminal voltage VCS of the snubber capacitor 15 and may be added to the turn-on delay time TD1 as a constant value when the terminal voltage VCS does not reach the DC power supply voltage. . For example, when the user turns on a “dent” key (low output setting key), for example, among the keys for selecting a cooking (control) program provided on the operation unit 25, the input setting unit 41a is turned on. Alternatively, the turn-on delay time may be automatically extended in accordance with a control program in which heating is first performed at a high input to boil the object to be cooked, and thereafter weak input heating is continuously performed. Further, if the "heat keeping key" is turned on during the high input heating, the control may be switched so that the weak input heating is continuously performed from that point.

Alternatively, a temperature sensor (temperature detecting means) for detecting the temperature of the pot 34 is provided on the top plate 33, and when the temperature detected by the temperature sensor reaches a predetermined value (predetermined temperature), The control may be switched so that the weak input heating is continuously performed from the time point. Resonance circuit 14
May be connected to the IGBT 6 side. The switching element is not limited to the IGBT, but may be a power transistor or a power MOSFET.

In the second embodiment, when removing the resistor 48 and shifting from high-input heating to low-input heating, the input setting unit 41a first supplies a control signal to the heating stop unit 41b to turn on only the IGBT 6. Stop the control, IG
The conduction control of the BT 7 may be continued for a short time and then stopped. Then, the charge remaining in the resonance capacitor 12 is extremely short-time (for example, 3, 4) by the switching operation of the IGBT 7 having a frequency of 10 kHz.
Within about one cycle). Thereafter, similarly to the second embodiment, the low input heating is started after waiting for the allowance time Tb. The same applies to the case of shifting from low input heating to high input heating. With this configuration, the charge charged in the resonance capacitor 12 can be discharged and consumed more quickly by the switching operation of the one IGBT 7 by the conduction control (on / off), and the time required for switching the control state can be reduced. can do. The resistance 4
8, the number of parts can be reduced.
The cooking container is not limited to a pot, but may be a frying pan or an iron plate.

[0072]

Since the present invention is as described above,
The following effects are obtained. According to the electromagnetic cooker of the first aspect, even when the ON time of the switching element is shortened and the amount of current supplied to the heating coil is reduced, the delay time control means adds the additional delay time to the turn-on delay time. By adding, the charging time of the snubber capacitor during that time becomes longer, and the terminal voltage of the snubber capacitor rises, so that the potential difference between the terminal voltage and the DC power supply voltage of the rectifier circuit is reduced. Is turned on, the voltage applied to the switching element is reduced. Therefore, a high-frequency current can be continuously supplied to the heating coil in a state where the switching loss in the switching element can be reduced, and continuous heating can be performed with a weak input.

According to the electromagnetic cooker according to the second or third aspect, the delay time control means changes the additional delay time so that the terminal voltage of the snubber capacitor increases (claim 2), and the terminal voltage is reduced. Since the snubber capacitor is charged up to the maximum (claim 3), the voltage applied to the other switching element when the other switching element is turned on can be minimized, and the switching loss can be further reduced.

According to the fourth aspect of the present invention, the input setting means sets the lower limit of the input current value so that the maximum terminal voltage of the snubber capacitor detected by the terminal voltage detecting means does not fall below a predetermined value. Therefore, the voltage level applied to the switching element when the switching element is turned on can be suppressed.

According to the electromagnetic cooker, the delay time control means changes the additional delay time according to the level of the input current value. By changing the voltage, the terminal voltage of the snubber capacitor can be increased.

According to the electromagnetic cooker of the present invention, even when the ON time of the switching element is short and the amount of current supplied to the heating coil is reduced, the capacity of the snubber capacitor is reduced by the capacity switching means. It is possible to accelerate the rise of the terminal voltage of the snubber capacitor during the switching, and the potential difference between the terminal voltage of the snubber capacitor and the DC power supply voltage of the rectifier circuit is reduced. The voltage applied to the switching element is reduced. Therefore, a high-frequency current can be continuously supplied to the heating coil in a state where the switching loss in the other switching element can be reduced, and continuous heating can be performed with a weak input.

According to the electromagnetic cooker of the present invention, the capacity switching means speeds up the rise of the terminal voltage of the snubber capacitor by, for example, switching the terminal voltage of the snubber capacitor so as to reduce the capacity when the terminal voltage is reduced to some extent. be able to.

According to the electromagnetic cooker of the present invention, when the input current value falls below a predetermined value, the capacity switching means switches the capacity of the snubber capacitor to be reduced, thereby shortening the on-time of the switching element. Even if the amount of current supplied to the heating coil is significantly reduced, the rise of the terminal voltage of the snubber capacitor can be accelerated.

According to the electromagnetic cooking device of the ninth aspect, when the capacity switching means switches the capacity of the snubber capacitor, the switching is performed while the conduction control for the first and second switching elements is temporarily stopped. At the time of capacitance switching, it is possible to prevent a short-circuit current from flowing through the third switching element.

According to the electromagnetic cooker of the present invention, even if the conduction control for the first and second switching elements is temporarily stopped while the capacitance switching means switches the capacitance of the snubber capacitor, the resonance capacitor is charged. The discharged charge can be quickly discharged via the resistor.

[Brief description of the drawings]

FIG. 1 is a functional block diagram showing an electrical configuration of a first embodiment of the present invention.

FIG. 2 is a diagram showing a signal waveform of each part in a region where an input set value is low.

FIG. 3 is an enlarged view showing a part of period D → A in FIG. 2 (e).

FIG. 4 is a diagram showing a temperature change (vertical axis) of the IGBT when an iron pot is heated while changing an input set value (horizontal axis).

FIG. 5 is a view corresponding to FIG. 1 showing a second embodiment of the present invention.

FIG. 6 is a diagram corresponding to FIG. 2;

FIG. 7 is a diagram corresponding to FIG. 3;

FIG. 8 is a diagram corresponding to FIG. 4;

FIG. 9 is a diagram showing a control state when switching between disconnection and connection of a second snubber capacitor;

FIG. 10 is a diagram corresponding to FIG. 1 showing a conventional technique.

FIG. 11 is a diagram corresponding to FIG. 2 in a region where the input set value is relatively high.

FIG. 12 is a diagram corresponding to FIG. 2;

FIG. 13 is a diagram showing an electrical configuration of a gate drive unit of the IGBT.

FIG. 14 is a diagram showing a voltage waveform of a gate signal.

[Explanation of symbols]

1 is a rectifier circuit, 2 is an AC power supply, 4 and 5 are positive and negative DC buses, 6 and 7 are IGBTs (first and second switching means), 8 is an inverter main circuit, 11 is a heating coil, 12 Is a resonance capacitor, 14 is a resonance circuit, 15 is a snubber capacitor, 16 is an IGBT (third switching element), 18 is a snubber circuit, 34 is a pan (cooking vessel), 4
1a is an input setting unit (input setting unit), 41c is a turn-on delay unit (delay time control unit), 42 is a snubber capacitor voltage detection unit (terminal voltage detection unit), 43c is a snubber capacitor switching unit (capacity switching unit), 44 And 45 are snubber capacitors, 46 is a relay (capacity switching means), 4
7, a snubber circuit; and 48, a resistor.

Claims (10)

[Claims] 1. A rectifier circuit for rectifying an AC power supply to generate a DC power supply, a positive and negative DC bus to which the DC power generated by the rectifier circuit is supplied, and a positive and negative DC bus. A first and a second switching element connected in series between the first and second switching elements; a heating coil connected between both terminals of the first and the second switching element for induction heating the cooking vessel; A resonance circuit formed of a capacitor, a snubber circuit connected between both terminals of the one switching element and formed of a snubber capacitor and a third switching element, and a switching element to which the resonance circuit is connected The voltage applied to the other switching element at the time of turning on is reduced during the turn-on delay time from turning off the other switching element to turning on the other switching element. Electromagnetic cooker, characterized by comprising a delay time control means for adding an additional delay time. 2. A terminal voltage detecting means for detecting a terminal voltage of a snubber capacitor, wherein the delay time control means changes an additional delay time according to a terminal voltage of the snubber capacitor detected by the terminal voltage detecting means. The electromagnetic cooker according to claim 1, wherein 3. The electromagnetic cooker according to claim 2, wherein the delay time control means changes the additional delay time so that the terminal voltage of the snubber capacitor detected by the terminal voltage detection means becomes maximum. 4. An input setting means for setting a lower limit of an input current value such that a maximum terminal voltage of the snubber capacitor detected by the terminal voltage detecting means does not fall below a predetermined value. Induction cooker. 5. The electromagnetic cooker according to claim 1, wherein the delay time control means changes the additional delay time according to the level of the input current value. 6. A rectifier circuit for rectifying an AC power supply to generate a DC power supply, a positive and a negative DC bus to which the DC power generated by the rectifier circuit is supplied, and a positive and a negative DC bus. A first and a second switching element connected in series between the first and second switching elements; a heating coil connected between both terminals of one of the first and the second switching element for inductively heating the cooking vessel; A resonance circuit formed of a capacitor, a snubber circuit connected between both terminals of the one switching element and formed of a snubber capacitor and a third switching element, and a switching element to which the resonance circuit is not connected And a capacitance switching means for switching the capacitance of the snubber capacitor so as to reduce the voltage applied to the switching element when the switch is turned on. Electromagnetic cooker, characterized in that. 7. A terminal voltage detecting means for detecting a terminal voltage of the snubber capacitor, wherein the capacitance switching means switches the capacitance of the snubber capacitor according to the terminal voltage of the snubber capacitor detected by the terminal voltage detecting means. The electromagnetic cooker according to claim 6, wherein 8. The electromagnetic cooker according to claim 6, wherein the capacity switching means switches the capacity of the snubber capacitor when the input current value falls below a predetermined value. 9. The capacitance switching means according to claim 6, wherein, when switching the capacitance of the snubber capacitor, the switching is performed in a state where the conduction control for the first and second switching elements is temporarily stopped. An electromagnetic cooker as described in Crab. 10. The electromagnetic cooker according to claim 6, wherein a resistor is connected in parallel with the resonance capacitor.