JP3966600B2 - Electromagnetic cooker - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic cooker that supplies a high-frequency current to a heating coil to heat a cooking vessel.
[0002]
[Prior art]
Electromagnetic cookers are safe and heat efficient without using fire, and are becoming popular as cooking heaters incorporated into system kitchens and the like. A plurality of electromagnetic cookers are often incorporated in a system kitchen, and in order to prevent interference sound from occurring when these multiple electromagnetic cookers are used at the same time, a half of which performs heating control at a constant frequency at all times. A bridge type inverter may be employed.
[0003]
FIG. 10 shows the electrical configuration of a half-bridge inverter used in a conventional electromagnetic cooker. In FIG. 10, the AC input terminal of the rectifier circuit 1 configured by a diode bridge is connected to the commercial AC power supply 2, and the DC output terminal is connected to both ends of the smoothing capacitor 3.
[0004]
Both ends of the smoothing capacitor 3 are connected to the arms composed of the positive and negative IGBTs 6 and 7 via the DC buses 4 and 5, thereby forming a half-bridge type inverter main circuit 8. Yes. Free wheel diodes 9 and 10 are connected between the collectors and emitters of the IGBTs 6 and 7, respectively.
[0005]
One end of the heating coil 11 is connected to the output terminal 8 a of the inverter main circuit 8, and a parallel circuit of a resonant capacitor 12 and a diode 13 is connected between the other end of the heating coil 11 and the DC bus 5. Has been. The heating coil 11 and the resonance capacitor 12 constitute a resonance circuit 14.
[0006]
Further, one end of the snubber capacitor 15 is connected to the output terminal 8 a, and the other end of the snubber capacitor 15 is connected to the DC bus 5 via the collector-emitter of the IGBT 16. A diode 17 is connected between the collector and emitter of the IGBT 16. These constitute a so-called snubber circuit 18 and are provided to reduce the switching loss when the IGBTs 6 and 7 are off.
[0007]
An oscillation signal having a predetermined frequency output from the oscillator 19 is supplied to the variable on-time setting unit 20 and the fixed on-time setting unit 21. A current transformer 22 is inserted on the AC input side of the rectifier circuit 1, and an output terminal of the current transformer 22 is connected to an input terminal of the input setting unit 24 a via an input current detection unit 23. The input current detection unit 23 A / D converts the input current value detected by the current transformer 22 and outputs the input current value to the input setting unit 24a as the input current detection value Vin.
[0008]
Although not specifically illustrated, the operation unit 25 includes a key for a user to select various automatic cooking menus (control programs), a key for setting a heating amount with an electric energy such as 1 KW, 2 KW, and the like. Is 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 as to obtain an input current value according to the setting of the electric energy in the operation unit 25, and a variable on-time. A PWM signal is supplied to the setting unit 20.
[0009]
Further, the heating stop unit 24b outputs a heating stop command to the variable on-time setting unit 20 and the fixed on-time setting unit 21 when a predetermined condition is satisfied. The input setting unit 24a and the heating stop unit 24b are blocks of the function of the microcomputer 24 (hereinafter referred to as a microcomputer).
[0010]
The output signal of the variable on-time setting unit 20 is supplied to the 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. Yes. The output terminals of the first, second, and third driving units 26, 28, and 27 are connected to the gates of the IGBTs 6, 7, and 16, respectively.
[0011]
FIG. 13 shows a detailed electrical configuration of the first drive unit 26. In FIG. 13, the output signal of the variable on-time setting unit 20 is given to the photocoupler 29, and one output terminal of the photocoupler 29 is connected to the gate of the IGBT 6 through a series circuit of resistors 30 and 31. ing. A diode 32 is connected in antiparallel to the resistor 30. 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 set to about 150Ω and 10Ω, for example.
[0012]
The operation of the electromagnetic cooker including the inverter configured as described above will be described below with reference to FIGS. 11 to 14. The pot is heated by supplying a high-frequency current to the heating coil 11 by an inverter. FIG. 11 shows signal waveforms at various parts in this case. As shown in FIGS. 11A and 11B, the IGBTs 6 and 7 are alternately turned on and off in an inverter control cycle Tinv of about 20 KHz, for example.
[0013]
The on-period Ton1 of the IGBT 6 changes based on the output signal given from the variable on-time setting unit 20 with Tinv / 2 as the upper limit. On the other hand, the ON period Ton2 of the IGBT 7 is fixed to approximately Tinv / 2 based on the output signal given from the fixed on-time setting unit 21. However, in order to prevent a short circuit between the IGBTs 6 and 7, turn-on delay times TD1 and TD2 are secured for switching between the ON periods of the two.
[0014]
As shown in FIG. 11C, the IGBT 16 of the snubber circuit 18 is turned on after a certain time Tα has elapsed after the IGBT 7 is turned on by the third driving unit 27, and a certain time Tα has elapsed after the IGBT 6 is turned off. Then it is turned off. As a result, when the IGBTs 6 and 7 shift from the on state to the off state, the voltage change between the collector and the emitter is moderated to prevent the occurrence of switching loss, and a short-circuit current flows through the snubber capacitor 15 when the IGBT 7 is on. It also prevents that.
[0015]
Here, the fixed time Tα is set so that the voltage change at the turn-off time of the IGBTs 6 and 7 converges within the time regardless of any load or setting input within the appropriate range. Further, the IGBT 16 is controlled to be turned on / off so that the switching loss at the time of turning off the IGBTs 6, 7 is reduced and the snubber capacitor 15 is not charged during the period from when the IGBT 6 is turned off until the IGBT 7 is turned on.
[0016]
The control cycle consists of the following four cycles. FIG. 11 (d) shows the waveform of the current IL flowing through the heating coil 11 at this time, and FIG. 11 (e) shows the waveform of the collector-emitter voltage Vtr2 of the IGBT 7.
(1) IGBT6: ON / IGBT7: OFF
A current is supplied to the heating coil 11 and the resonance capacitor 12 is charged through the paths of the smoothing capacitor 3, IGBT 6, heating coil 11, resonance capacitor 12 and smoothing capacitor 3 (see FIG. 11 (d) and A).
[0017]
(2) IGBT6: Off / IGBT7: Off
The resonance capacitor 12 is further charged by the delay current of the heating coil 11 through the path of the heating coil 11, the resonance capacitor 12, the freewheel diode 10, and the heating coil 11 (see FIGS. 11D and 11B).
[0018]
(3) IGBT6: Off / IGBT7: On
The resonant capacitor 12 is discharged through the path of the resonant capacitor 12, the heating coil 11, the IGBT 7, and the resonant capacitor 12, and a current in the reverse direction is passed through the heating coil 11 (see FIGS. 11D and 11C). When the resonant capacitor 12 is completely discharged, current flows through the diode 13 connected in parallel (see FIG. 11 (d), C ′).
[0019]
(4) IGBT6: Off / IGBT7: Off
In the path of the heating coil 11, the free wheel diode 9, the smoothing capacitor 3, the diode 13, and the heating coil 11, the delayed current of the heating coil 11 is regenerated to the power source side via the free wheel diode 9 (FIG. 11 (d), D).
[0020]
By repeating the above cycle, a high-frequency current is supplied to the heating coil 11, and cooking is performed by inducing eddy current in the pan 34 (see FIG. 10) placed on the top plate 33. . The input current control is performed by changing the on period Ton1 of the IGBT 6. If the on period Ton1 is lengthened, the input current increases and the heating amount of the pan 34 increases.
[0021]
[Problems to be solved by the invention]
However, in such a conventional electromagnetic cooker, if the on period Ton1 of the IGBT 6 is shortened in order to perform weak input heating, the following problems occur. FIG. 12 shows signal waveforms of the respective parts at this time. That is, as shown in FIG. 12A, when the ON period Ton1 of the IGBT 6 becomes less than a certain time, the amount of current supplied to the heating coil 11 decreases (see FIG. 12D and A). In the periods C and C 'and the cycle (4), 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 (4). The capacitor 15 will continue to be charged.
[0022]
In this state, the IGBT 6 is turned on in the next cycle {circle around (1)}. Therefore, a short circuit occurs in the path of the DC bus 4, IGBT 6, snubber capacitor 15, IGBT 16 and DC bus 5 due to the potential difference between the DC power supply voltage and the voltage Vtr2. Current flows. Here, FIG. 12 (f) shows a current waveform Itr1 flowing through the IGBT 6, and a short-circuit current flows at a point P shown in FIG. 12 (f).
[0023]
In order to suppress the occurrence of such a short-circuit current as much as possible, as shown in FIG. 13, the gate resistance value at turn-on is set to be large by connecting a series circuit of resistors 30 and 31 to the gate of the IGBT 6. As shown in FIG. 14, the rise of the gate signal VG1 is moderated to delay the timing at which the IGBT 6 is turned on.
[0024]
However, since the rise of the collector-emitter voltage of the IGBT 6 becomes gentle by making the rise of the gate signal VG1 in this way, a switching loss (turn-on loss) that occurs when the IGBT 6 is turned on occurs. The switching loss at the time of turn-on becomes larger as the setting input is lower. If continuous heating is performed with the turn-on loss being increased, the temperature of the IGBT 6 rises, and in the worst case, thermal destruction occurs.
[0025]
Therefore, in the conventional electromagnetic cooker, for example, when performing weak input heating corresponding to cooking with a long period of cooking with low heat, a low input that does not cause the turn-on loss of the IGBT 6 is set as the lower limit. For example, periodic heating such as heating for 3 seconds and then stopping heating for 3 seconds had to be performed.
[0026]
And in such a heating system, when a to-be-cooked object is a small amount, it will be in a boiling state suddenly, and the in-cooked object will burnt when stewed cooking was performed.
This invention is made | formed in view of the said situation, The objective is to provide the electromagnetic cooker which can perform continuous heating by a weak input in the state which can reduce a switching loss.
[0027]
[Means for Solving the Problems]
  In order to achieve the above object, an electromagnetic cooker according to claim 1, a rectifier circuit that rectifies an AC power source to generate a DC power source;
  Positive and negative DC buses supplied with DC power generated by the rectifier circuit;
  First and second switching elements connected in series between the positive and negative DC buses;
  in frontNo.2 switching elementsBothA resonance circuit connected between the terminals and configured by a heating coil and a resonance capacitor for inductively heating the cooking vessel;
  SaidSecondA snubber circuit connected between both terminals of the switching element and configured by a snubber capacitor and a third switching element;
  SaidSecondAfter turning off the switching elementThe firstIn the turn-on delay time until the switching element is turned on,The firstDelay time control means for adding an additional delay time so as to reduce the voltage applied to the switching element,
  The first and second switching elements are alternately turned on at a constant frequency with a period during which both are turned off at the same time,
  The third switching element is controlled to be turned on after the second switching element is turned on, and to be turned off after the first switching element is turned off.It is characterized by that.
[0028]
  If constituted in this way, for example, saidFirstEven when the on-time of the switching element becomes shorter and the amount of current supplied to the heating coil decreases, the additional delay time is added to the turn-on delay time by the delay time control means, and the snubber capacitor is charged during that time. The time will be longer. 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 is reduced.FirstWhen the switching element is turned on, the voltage applied to the switching element is reduced. Therefore,FirstA high-frequency current can be continuously supplied to the heating coil in a state where switching loss in the switching element can be reduced, and continuous heating can be performed with a weak input.
[0029]
In this case, as described in claim 2 or 3, the terminal voltage detecting means for detecting the terminal voltage of the snubber capacitor is provided,
The delay time control means may be configured to change the additional delay time according to the terminal voltage of the snubber capacitor detected by the terminal voltage detection means (claim 2). Specifically, the delay time control means is connected to the terminal It is preferable that the additional delay time is changed so that the terminal voltage of the snubber capacitor detected by the voltage detecting means is maximized.
[0030]
  By configuring in this way, the delay time control means changes the additional delay time so that the terminal voltage of the snubber capacitor rises, and charges the snubber capacitor until the terminal voltage becomes maximum.FirstWhen the switching element is turned on, the voltage applied to the switching element can be minimized.
[0031]
  Further, as described in claim 4, it is preferable to provide input setting means for setting a lower limit for the input current value so that the maximum terminal voltage of the snubber capacitor detected by the terminal voltage detection means does not fall below a predetermined value. With such a configuration, for example, when the setting of the input current value becomes very low,FirstSince the on-time of the switching element is also shortened, the amount of current supplied to the heating coil is significantly reduced, and the maximum terminal voltage level of the snubber capacitor is relatively lowered. Therefore, if the input setting means sets a lower limit for the input current value,FirstWhen the switching element is turned on, the voltage level applied to the switching element can be suppressed.
[0032]
According to a fifth aspect of the present invention, the delay time control means may be configured to change the additional delay time in accordance with the level of the input current value. With this configuration, the delay time control means is configured to change the input current value. The terminal voltage of the snubber capacitor can be increased by changing the additional delay time according to the level decrease.
[0040]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described below with reference to FIGS. Note that the same parts as those in FIG. FIG. 1 shows an electrical configuration. In this embodiment, instead of the microcomputer 24 shown in FIG. 10, a microcomputer (control means) 41 having an input setting unit (input setting unit) 41a, a heating stop unit 41b, and a turn-on delay unit (delay time control unit) 41c is arranged. Has been. The turn-on delay unit (delay time control means) 41c changes the IGBT 6 (first switching means) turn-on delay time TD1 as described later.
[0041]
A snubber capacitor voltage detection unit (terminal voltage detection means, hereinafter referred to as a voltage detection unit) 42 detects the terminal voltage VCS of the snubber capacitor 15, and its input terminal is connected to the output terminal 8a of the inverter main circuit 8. It is connected. The voltage detection unit 42 is supplied with output signals from the variable on-time setting unit 20 and the fixed on-time setting unit 21 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.
[0042]
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 at 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. In the periods C and C 'of the cycle (3) and the cycle (4), the snubber capacitor 15 is charged until the inter-terminal voltage Vtr2 of the IGBT 7 becomes equal to the DC power supply voltage.
[0043]
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 in the period D, and when the terminal voltage VCS is substantially equal to the DC power supply voltage, the case shown in FIG. Control is performed in the same manner. 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 from when the IGBT 6 is turned off with an ON period of about 50% duty until the IGBT 7 is turned on. The turn-on delay time is controlled with a fixed value TD2.
[0044]
The turn-on delay unit 41c reduces the turn-on delay time to the fixed delay time TD1 when the input set value set by the operation unit 25 becomes small and the terminal voltage VCS of the snubber capacitor 15 in the period D does not reach the DC power supply voltage. The additional delay time TDX is added to the above, and control is performed so as to extend as TD1 ′ = TD1 + TDX. In this case, the additional delay time TDX is variable, and the turn-on delay unit 41c gradually extends the additional delay time TDX until the terminal voltage VCS reaches the maximum level while referring to the terminal voltage VCS.
[0045]
Here, FIG. 3 is an enlarged view of the period D shown in FIG. 2E, and shows the 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 until the terminal voltage VCS reaches the level (2). Therefore, as shown by the broken line in FIG. 3, when the IGBT 6 is turned on, the voltage applied between its collector and emitter, that is, the potential difference (1)-(2) between the DC power supply voltage and the terminal voltage VCS is very high. It was a factor that greatly increased switching loss.
[0046]
On the other hand, in this embodiment, the turn-on delay time is extended as TD1 '= TD1 + TDX, so that the snubber capacitor 15 is charged during that time and the terminal voltage VCS reaches the level (2)'. The potential difference at is reduced to (1)-(2) '. Therefore, as shown at point Q in FIG. 2 (f), the short-circuit current flowing through the IGBT 6 is reduced by the amount that the potential difference is reduced from (1)-(2) to (1)-(2) '. Thus, the switching loss is reduced as compared with the conventional case.
[0047]
FIG. 4 shows an example of measurement performed by the inventor of the present invention, in which the temperature change of the IGBT 6 (vertical axis) when the iron pan (cooking vessel) 34 is heated by changing the input set value (horizontal axis). ). As the input set value decreases, the temperature of the IGBT 6 also decreases. However, when the turn-on delay time is always set to the fixed value TD1 as in the prior art, as shown by the broken line in FIG. The temperature rises rapidly.
[0048]
On the other hand, when the turn-on delay time is changed in a region where the input power amount is low as in the present embodiment, the temperature rise degree of the IGBT 6 in the low input region is relatively slow as shown by the solid line in FIG. become.
[0049]
Further, 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 amount of input power decreases, the amount of current supplied to the heating coil 11 gradually decreases, and accordingly, the maximum level of the terminal voltage VCS of the snubber capacitor 15 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 41a provides a restriction in advance so that the input electric energy does not become WL or less, so that low input heating below a certain level is not performed.
[0050]
As described above, according to the present embodiment, the turn-on delay unit 41c refers to the terminal voltage VCS of the snubber capacitor 15, and the input set value decreases, so that the terminal voltage VCS remains unchanged when the turn-on delay time remains the fixed value TD1. When no DC power supply voltage is reached, the turn-on delay time is extended as TD1 '= TD1 + TDX.
[0051]
Then, the charging period of the snubber capacitor 15 is extended to increase the terminal voltage, so that the potential difference applied between the collector and the emitter is reduced when the IGBT 6 is turned on and the switching loss is reduced. Unlike conventional ones, heating can be performed, and for example, long-time stewed cooking can be performed well without burning the cooked food or suddenly boiling it.
[0052]
In addition, when the input set value is small, for example, a method may be considered in which the snubber circuit 18 itself is not operated by maintaining the IGBT 16 in the off state. In such a case, however, the switching loss due to the on / off of the IGBT 7 is considered. It is 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.
[0053]
Furthermore, according to the present embodiment, the input setting unit 41a is provided with a restriction in advance so that the input power amount does not become WL or less, so that low input heating below a certain level is not performed. The temperature rise does not reach the limit value Tth and can be used within a range where the element operates safely.
[0054]
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 is omitted. Only 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.
[0055]
Further, the snubber capacitor 15 is replaced with a first snubber capacitor 44 having a small capacity. A second snubber capacitor having one end connected to the output terminal 8 a of the inverter main circuit 8 with respect to the first snubber capacitor 44. 45 are connected in parallel via a normally-on type relay 46. The capacities of the first and second snubber capacitors 44 and 45 are set to be equal to the capacity of the snubber capacitor 15 of the first embodiment when they are connected in parallel.
[0056]
The first and second snubber capacitors 44 and 45, the relay 46, the IGBT 16, and the diode 17 constitute a snubber circuit 47. The switching unit 43c of the microcomputer 43 controls the opening / closing of the relay 46 by giving a control signal Vs.
[0057]
A resistor 48 is connected in parallel to the resonant capacitor 12 and the diode 13. The resistance value of the resistor 48 is set to a sufficiently large value with respect to the impedance of the resonant 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 capacity switching means.
[0058]
Next, the operation of the second embodiment will be described with reference to FIGS. When the input set value is set to be relatively high and the terminal voltage VCS detected by the voltage detection unit 42 in the period D is substantially equal to the DC power supply voltage, the switching unit 43c keeps the relay 46 closed. 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.
[0059]
When the input set value is set to be relatively low and the terminal voltage VCS in the period D does not reach the DC power supply voltage, the switching unit 43c gives the control signal Vs to open the relay 46, thereby causing the second The snubber capacitor 45 is disconnected, and the snubber circuit 47 is operated only by the first snubber capacitor 44. At this time, as shown in FIGS. 6E and 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 is Charged within D (that is, 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 short-circuit current does not flow when the IGBT 6 is turned on next time.
[0060]
At this time, the input setting unit 41a stores the input setting value when the control signal Vs is output by the switching unit 43c, and the input setting value set in the operation unit 25 in the subsequent control. When the value matches the stored value, a command may be given to cause the switching unit 43c to output the control signal Vs.
[0061]
FIG. 8 corresponds to FIG. When the snubber circuit 47 is operated only by the first snubber capacitor 44 in the low input power region as in the second embodiment, the temperature of the IGBT 6 in the low input region is the input set value as shown by the solid line in FIG. It decreases in proportion to the decrease of T and decreases greatly from Ta to Tb. Therefore, low input heating can be continuously performed up to about 0 W.
[0062]
FIG. 9 shows a control state when the relay 46 is switched according to the input set value by the inter-terminal voltage Vtr2 (a) of the IGBT 7 and the control signal Vs (b). In FIG. 9, when the relay 46 is switched from the closed state to the open state, the switching unit 43c gives a control signal to the input setting unit 41a and stops the conduction control of the IGBTs 6 and 7 via the heating stop unit 41b. (See FIG. 9A, time point A).
[0063]
Then, since the electric charge remaining in the resonant capacitor 12 is discharged through the resistor 48, the voltage Vtr2 gradually decreases from the DC power supply voltage and becomes approximately 0 V after the lapse of the assumed time Ta (FIG. 9A, See time point B). Further, after the waiting voltage Vtr2 is reliably set to 0 V for the surplus time Tb, the switching unit 43c outputs the control signal Vs to the relay 46 and disconnects the second snubber capacitor 45 (FIG. 9B). , See time point C). Next, after waiting for the control system switching waiting time Tc to elapse, the conduction control of the IGBTs 6 and 7 is started, and continuous heating with weak input is performed (see FIG. 9A, time point D).
[0064]
The same switching is performed when returning from the weak input heating to the normal heating control. That is, the switching unit 43c 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), and during the assumed time Ta. Waiting for discharge of the residual charge of the resonant capacitor 12 (see FIG. 9A, time point F).
[0065]
Further, after waiting for a margin time Tb, the switching unit 43c stops outputting the control signal Vs to the relay 46, and the second snubber capacitor 45 is connected in parallel to the first snubber capacitor 44 (FIG. 9B, See 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).
[0066]
As described above, according to the second embodiment, the switching unit 43c opens the relay 46 when the input set value becomes low and the terminal voltage VCS of the first and second snubber capacitors 45 and 45 does not reach the DC power supply voltage. The second snubber capacitor 45 is disconnected, and the snubber circuit 47 is operated only by the first snubber capacitor 44.
[0067]
Therefore, even if the amount of current supplied to the resonance circuit 14 is reduced, the first snubber capacitor 44 having a small capacity is charged within a short time, so that even if the IGBT 6 is turned on after the turn-on delay time TD1 has elapsed, the charging is performed. A short-circuit current due to insufficient capacity does not flow, and a high-frequency current can be continuously supplied to the heating coil 11 with a low input set value. Thus, it is possible to perform continuous heating with weak input after suppressing the switching loss of the IGBT 6, and it is possible to perform control for reducing the switching loss more easily than the first embodiment.
[0068]
Further, according to the second embodiment, when the resistor 48 is connected in parallel to the resonant capacitor 12, and the switching unit 43c disconnects or connects the second snubber capacitor 45, the conduction control to the IGBTs 6 and 7 is performed. Since it temporarily stops and transitions between disconnection and connection in the meantime, it is possible to prevent a short-circuit current from flowing through the IGBT 16 at that time, and even if the conduction control to the IGBTs 6 and 7 is stopped, the resonance occurs. The electric charge charged in the capacitor 12 can be quickly discharged through the resistor 48.
[0069]
The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following modifications or expansions are possible.
The additional delay time TX in the first embodiment is not necessarily changed according to the terminal voltage VCS of the snubber capacitor 15, and when the terminal voltage VCS does not reach the DC power supply voltage, the turn-on delay is always a constant value. It may be added to the time TD1.
For example, the input setting unit 41a, when the user selects a cooking (control) program provided in the operation unit 25, for example, a “Nikomi” key (low output setting key) is turned on. Alternatively, the turn-on delay time may be automatically extended in accordance with a control program in which heating is initially performed at a high input to boil the food to be cooked and thereafter weak input heating is continuously performed. Further, when the “heat retention 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.
[0070]
  Alternatively, when a temperature sensor (temperature detecting means) for detecting the temperature of the pan 34 is provided on the top plate 33 and the temperature detected by the temperature sensor reaches a predetermined value (predetermined temperature), the temperature is weak from that point. The control may be switched so that the input heating is continuously performed.
  TheThe switching element is not limited to the IGBT but may be a power transistor, a power MOSFET, or the like.
[0071]
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 gives a control signal to the heating stop unit 41b to stop the conduction control of only the IGBT 6. The continuity control of the IGBT 7 may be continued for a short time and then stopped. Then, the electric charge remaining in the resonant capacitor 12 is discharged and consumed within a very short time (for example, about 3 or 4 cycles) by the switching operation of the IGBT 7 having a frequency of several 10 kHz. Thereafter, as in the second embodiment, the low input heating is started after the allowance time Tb has elapsed. The same applies to the transition from low input heating to high input heating. With this configuration, the charge charged in the resonant capacitor 12 can be discharged and consumed more quickly by the switching operation based on the conduction control (ON / OFF) of one IGBT 7, and the time required for switching the control state can be shortened. can do. In addition, since the resistor 48 can be eliminated, the number of parts can be reduced.
The cooking container is not limited to a pan but may be a frying pan or an iron plate.
[0072]
【The invention's effect】
  Since this invention is as having demonstrated above, there exist the following effects.
  According to the electromagnetic cooking device of claim 1,FirstEven when the on-time of the switching element is shortened and the amount of current supplied to the heating coil is reduced, the additional delay time is added to the turn-on delay time by the delay time control means, so that the charging time of the snubber capacitor during that time Since the potential difference between the terminal voltage and the DC power supply voltage of the rectifier circuit decreases as the terminal voltage of the snubber capacitor rises, the next timeFirstWhen the switching element 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 switching loss in the switching element can be reduced, and continuous heating can be performed with a weak input.
[0073]
  According to the electromagnetic cooker of claim 2 or 3, 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 becomes maximum. Since the snubber capacitor is charged up to (claim 3),FirstWhen the switching element is turned on, the voltage applied to the switching element can be minimized to further reduce the switching loss.
[0074]
According to the electromagnetic cooker of claim 4, the input setting means sets the lower limit for the input current value so that the maximum terminal voltage of the snubber capacitor detected by the terminal voltage detection means does not fall below a predetermined value. The voltage level applied to the switching element when the element is turned on can be suppressed.
[0075]
According to the electromagnetic cooker of claim 5, since the delay time control means changes the additional delay time according to the level of the input current value, the delay time control means changes the additional delay time according to a decrease in the input current value level. Thus, the terminal voltage of the snubber capacitor can be increased.
[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 signal waveforms at various parts in a region where the input set value is low.
FIG. 3 is an enlarged view showing a portion of period D → A in FIG.
FIG. 4 is a view showing a temperature change (vertical axis) of an IGBT when an iron pan is heated by changing an input set value (horizontal axis).
FIG. 5 is a view corresponding to FIG. 1 showing a second embodiment of the present invention.
6 is a view corresponding to FIG.
7 is a view corresponding to FIG.
FIG. 8 is a view corresponding to FIG.
FIG. 9 is a diagram showing a control state when the second snubber capacitor is disconnected and the connection is switched.
FIG. 10 is a view corresponding to FIG.
11 is a view corresponding to FIG. 2 in a region where the input set value is relatively high.
FIG. 12 is a view corresponding to FIG.
FIG. 13 is a diagram showing an electrical configuration of an IGBT gate driver.
FIG. 14 is a diagram illustrating a voltage waveform of a gate signal.
[Explanation of symbols]
1 is a rectifier circuit, 2 is an AC power source, 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), 41 a is an input setting unit (input setting means), 41 c Is a turn-on delay section (delay time control means), 42 is a snubber capacitor voltage detection section (terminal voltage detection means), 43 c is a snubber capacitor switching section (capacitance switching means), 44 and 45 are snubber capacitors, and 46 is a relay (capacitance switching). Means), 47 is a snubber circuit, and 48 is a resistor.

Claims (5)

A rectifier circuit that rectifies an AC power source to generate a DC power source;
Positive and negative DC buses supplied with DC power generated by the rectifier circuit;
First and second switching elements connected in series between the positive and negative DC buses;
Is connected between both terminals of the pre-Symbol second switching element, a resonant circuit composed of the heating coil and the resonance capacitor for induction heating of the cooking container,
A snubber circuit connected between both terminals of the second switching element and configured by a snubber capacitor and a third switching element;
The turn-on delay before turning on the first switching element after turning off the second switching element, the additional delay time to reduce the voltage applied to the first switching element during the on A delay time control means for adding ,
The first and second switching elements are alternately turned on at a constant frequency with a period during which both are turned off at the same time,
It said third switching element is turned on after the second switching element is turned on, induction cooker, wherein Rukoto is controlled to be turned off after the first switching element is turned off. A terminal voltage detecting means for detecting the terminal voltage of the snubber capacitor;
The electromagnetic cooker according to claim 1, wherein the delay time control means changes the additional delay time according to the terminal voltage of the snubber capacitor detected by the terminal voltage detection means.   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.   The electromagnetic cooker according to claim 3, further comprising input setting means for setting a lower limit for the input current value so that the maximum terminal voltage of the snubber capacitor detected by the terminal voltage detection means does not fall below a predetermined value.   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.

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