JP5071012B2 - Induction heating cooker - Google Patents

Description

  The present invention relates to an induction heating cooker used for general household and business use, and in particular, even when a drive signal in which an excessive current flows is generated in a switching element constituting the cooker, the current is instantaneously stopped. The present invention relates to an induction heating cooker that has an overcurrent protection function to prevent destruction of the cooker.

  Conventionally, in the case of cooking by induction heating, the induction heating cooker of Conventional Example 1 in which the switching element is reliably protected by detecting the current of the switching element and stopping at a predetermined value or more is patented. It is disclosed in Document 1.

  In addition, when a current of a predetermined value or more flows through the switching element, the switching of the switching element is stopped, the stop operation is maintained, and the microcomputer is restarted by a microcomputer (hereinafter referred to as a microcomputer). A heating cooker is disclosed in Patent Document 2.

Hereinafter, the induction heating cooker of Conventional Example 1 will be described with reference to FIG. FIG. 3 shows a circuit configuration of a conventional induction heating cooker. In FIG. 3, the DC power supply 31 specifically obtains the commercial AC power supply 31a via the rectifier 31b. The heating coil 34 is connected in series to the DC power source 31, and an object to be heated 36 such as a pan is placed on the heating coil 34 via a plate 35. The resonance capacitor 33 c is connected in parallel with the heating coil 34. The choke coil 33 f is connected in series with the heating coil 34. The switching element 33a is, in this prior art example uses IGBT breakdown voltage 900 V. The reverse conducting diode 33b is connected in parallel with the switching element 33a. By the high frequency switching of the switching element 33a, the high frequency power is supplied to the object to be heated 36 through the heating coil 34, so that the high frequency power conversion means 33 is configured. Further, with this circuit configuration, when a normal oscillation signal is applied to the switching element 33a in normal operation, the current flowing through the switching element 33a becomes a resonance current and when the current is zero (current flows through the reverse conducting diode). (Including when you are).

  The control circuit 37 includes an oscillation circuit and controls the switching element 33a. In the self-clamping means 33e, both ends of the switching element 33a have a predetermined value (specifically, a voltage higher than the voltage generated during normal operation and lower than the withstand voltage of the switching element 33a. Since the voltage at the time is about 400V and the withstand voltage of the switching element 33a is 900V, it is set to 600V.) When the voltage becomes higher than this, the switching element 33a is self-clamped. In the case of this conventional example, this is realized by inserting a Zener diode having a withstand voltage of 600 V and a reverse blocking diode connected in series between the collector side of the switching element 33a and the gate.

  The overcurrent detection means 38 detects the reverse conduction current flowing through the reverse conduction diode 33b, and when the detected current is a predetermined value or more, operates the stop means 37 inside the control means 37 to oscillate the switching element 33a. Stop. In the case of this conventional example, the current detection unit 33d is configured to detect only the reverse conduction current using a current transformer.

  The power control of this conventional example is performed by detecting the input current input from the commercial power supply 31a to the rectifier 31b by the current transformer 31c.

  FIG. 4 shows the drive signal and current (Ic) voltage (Vce) waveform of the switching element 33a during normal operation. In the figure, the switching element is turned on when the drive signal is HIGH and turned off when the drive signal is LOW. The oscillation frequency in the case of this conventional example is about 45 kHz. As shown in FIG. 2 (a), the current waveform becomes a resonance waveform by adopting this inverter circuit configuration, and the current flowing through the switching element 13 is zero or turned off while the current is flowing through the reverse conducting diode 14. Thus, the conventional turn-off loss does not occur, and the loss can be greatly reduced.

  FIG. 5 shows a switching element at the time of no-load start-up (for example, when a heated object 36 (a pan or the like) that should normally be placed on the heating coil 34 has not been placed because the user forgot to place it). The waveform of each part 33a is shown. In this conventional example, the oscillation start timing is set when the input voltage of the commercial power supply 31a is detected and the voltage is sufficiently small (that is, the valley of the power supply voltage envelope with a frequency of 50 Hz / 60 Hz). In the case of no load In the inverter constant of this conventional example (inductance value of the heating coil 34, etc.), as shown in (c), an extremely large current flows through the reverse conducting diode 33b as compared with the normal operation. When a large current flows through the reverse conducting diode 33b, the recovery current flowing through the reverse conducting diode 33b also increases, and a surge voltage is generated at the voltage across the switching element 33a as shown in the result (d).

  As shown in (a), this surge voltage is small immediately after the start of starting, but as the power supply voltage increases, the surge voltage also increases, and when the current flowing through the reverse conducting diode 33b reaches a predetermined value, the stopping means Oscillation is stopped by 37d. When the oscillation is not stopped (and the self-clamping means 33e is not present), a surge voltage exceeding the withstand voltage of the switching element 33a is generated near the power supply voltage peak, and the switching element 33a is destroyed. Moreover, although it depends on the allowable current of the reverse conducting diode 33b, the reverse conducting diode 33b may be thermally destroyed by an excessive current.

  FIG. 6 shows the waveforms of the respective parts of the switching element 33a when the oscillation of the control means 37 due to external noise or the like is erroneously started near the peak of the power supply voltage and there is no load. In this case, the surge voltage generated at the voltage across the switching element 33a is extremely large from the first time (exceeding the breakdown voltage when there is no self-clamping means 33e), but the self-clamping means 33e causes the switching element 33a to self- Clamps and does not destroy the device. Further, since the stopping means 37d detects the reverse conducting diode current at this time and the subsequent oscillation stops, there is no possibility of continuous self-clamping. In general, the self-clamping of the switching element 33a generates a very large loss at the time of clamping, and the element generates a large amount of heat. Therefore, when continuous self-clamping is performed, there is a possibility of thermal destruction. Further, the breakdown by the first surge voltage cannot be avoided only by the stopping means 37d.

  As is clear from the above description, according to Conventional Example 1, since the loss of the switching element 33a can be reduced with a simple configuration, a small and low-cost induction heating cooker with little necessary cooling can be obtained. Further, it is possible to detect the load abnormality with the stopping means 37d, and further, when the abnormality cannot be met with the stopping means 37d, the self-clamping means 33e operates to prevent the switching element 33a from being destroyed. Further, the means for detecting the input voltage in the protection circuit as in the conventional induction heating cooker is unnecessary, and the cost can be reduced.

  On the other hand, in the configuration of the conventional example 1, the stopping unit 37d stops the oscillation when the current flowing through the switching element 33a and the reverse conducting diode 33b reaches a predetermined value, and continues the subsequent oscillation stop. Although the effect is recognized, it is necessary to separately provide a stop cancellation function for canceling the oscillation stop when the oscillation is recovered and restarted.

In addition, the conventional example 2 is provided with a control means for outputting a signal for resetting the overcurrent stop circuit. However, noise resistance may be deteriorated due to an increase in wiring, and the reset function malfunctions due to overcurrent. The stop operation is not reset, or the reset operation continues and the heating stop due to overcurrent does not operate.
JP 2000-113973 A JP-A-5-47465

  However, in the configuration in which the operation of the switching means by the conventional overcurrent detection is stopped, the overcurrent detection means detects the overcurrent, instantaneously cuts off the drive signal to the switching means, and stops the heating operation. Since the overcurrent stop operation is continued even if the overcurrent is no longer detected due to the interruption of the switching means, an element for canceling the overcurrent stop operation is separately provided to perform the heating operation again. It had the problem of being necessary.

  In addition, when an element with a function to reset the overcurrent stop operation is provided, the wiring to the reset element increases, so malfunction due to noise occurs, the reset operation becomes inoperative, or the continuous reset operation continues. Therefore, there is a problem that there is a possibility of not stopping when an overcurrent occurs.

  The present invention solves the above-described conventional problems, and includes a heating coil that induction-heats an object to be heated, a resonance capacitor that forms a resonance circuit with the heating coil, and a switching element that generates a resonance current in the resonance circuit And current detection means for detecting the current of the switching element, and when the overcurrent of a predetermined value or more flows through the switching element by the current detection means, the overcurrent detection is forcibly turned off. And a time constant latch means for holding the output of the overcurrent detection means for a predetermined time, and after the overcurrent is generated and the switching element is forcibly turned off, the switching element is turned off for a predetermined time determined by the time constant. An object of the present invention is to provide an induction heating cooker that can be continued.

In order to solve the conventional problems, an induction heating cooker according to the present invention includes a heating coil that induction-heats an object to be heated, a resonance capacitor that forms a resonance circuit with the heating coil, and a resonance current that is generated in the resonance circuit. A switching element for detecting the current of the switching element, and a current detection means for detecting a current of the switching element. When an overcurrent of a first predetermined value or more flows through the switching element by the current detection means, After overcurrent detection means for forcibly turning off the drive signal and time constant latch means for holding the output of the overcurrent detection means for a first predetermined time, after the overcurrent occurs and the switching element is forcibly turned off said first predetermined time is assumed to continue off, the overcurrent detecting unit is configured by a voltage comparator, said time constant latch means, transistors and U After the voltage comparator is operated by detecting an overcurrent, the reference voltage of the voltage comparator is changed and the overcurrent stops and returns to the detection voltage at the normal operation level. However, it is possible to maintain the overcurrent state by operating the transistor for a time determined by the time constant of the capacitor and the resistor .

  As a result, for example, even when an overcurrent occurs in the switching element due to a malfunction caused by noise during cooking, the heating operation can be stopped for a predetermined time, and the reset operation control of the overcurrent detection means by an external circuit is possible. By eliminating the need for malfunction, malfunction due to noise associated with an increase in wiring to the overcurrent detection means can be prevented.

  The induction heating cooker of the present invention prevents, for example, a situation in which the protection operation continues and the heating operation does not restart when the protection operation of the switching element is performed with the overcurrent detection means, Therefore, even when an element with a reset function is provided, overcurrent detection is prevented by a simple time constant circuit consisting of a resistor and a capacitor. It is possible to continue the heating stop operation when overcurrent is detected for a predetermined time, and stop / restart the heating operation even if there is no set / reset command for the overcurrent detection means from other circuit blocks. This makes it possible to obtain an advantageous effect of preventing malfunction caused by an increase in wiring.

  According to a first aspect of the present invention, there is provided a heating coil that induction-heats an object to be heated, a resonance capacitor that forms a resonance circuit with the heating coil, a switching element that generates a resonance current in the resonance circuit, and a current of the switching element Current detecting means for detecting, and when the overcurrent of a first predetermined value or more flows through the switching element by the current detecting means, overcurrent detecting means for forcibly turning off the drive signal of the switching element, And a time constant latch means for holding the output of the overcurrent detection means for a first predetermined time, and continues to be turned off for a first predetermined time after the switching element is forcibly turned off due to the occurrence of an overcurrent. Thus, for example during heating cooking, even if an overcurrent occurs in the switching element due to a malfunction due to noise, the heating operation is stopped for the first predetermined time. Things are possible, by the reset operation control of the overcurrent detection means according to an external circuit not required, it is possible to prevent malfunction due to noise caused by the wiring increases to the overcurrent detecting means.

The overcurrent detection means is constituted by a voltage comparator, and the time constant latch means is composed of a series circuit of a transistor, a capacitor and a resistor, and the voltage comparator detects the overcurrent and operates when the voltage is detected. Even after the reference voltage of the comparator is changed and the overcurrent stops and returns to the detection voltage at the normal operation level, the transistor operates for the time determined by the time constant of the capacitor and the resistor and the overcurrent By maintaining the state, it is possible to prevent a malfunction due to an increase in wiring when a separate reset means is provided and to protect the switching element stably.

According to a second aspect of the invention, in particular, in the first aspect of the invention, the switching element is driven by a control signal output from a control microcomputer, and an interrupt signal for stopping the control signal is sent to the microcomputer by an output signal from the overcurrent detection means. The time constant setting time can be as short as a few hundred microseconds until interrupt processing is performed, and the constant can be set small. Therefore, it is possible to achieve miniaturization and simplification of circuit components.

According to a third aspect of the invention, in particular, in the second aspect of the invention, the microcomputer is provided with a timer counter, and after the output signal of the overcurrent detecting means is interrupted, the microcomputer waits for a second predetermined time to confirm the stop state of the device. Even if the temperature of the electronic component or the printed wiring board rises due to the flow of a large current at a level at which overcurrent detection is activated by starting heating again later, a second predetermined time is waited for. After confirming the operation of each part such as that the signal from the current detection has stopped reliably, and further confirming that the temperature of each part is operable by the temperature detection element provided inside the equipment, By starting the heating again, it is possible to prevent an unstable operation state or a restart of the heating operation while the temperature of each part is abnormal. Also, it is that the equipment due to temperature rise is to prevent a situation that leads to destruction.

In particular, the fourth invention is the smoothing capacitor for holding the charge obtained by converting the commercial AC power source into the DC voltage source, and the smoothing current detection for detecting the current of the smoothing capacitor in any one of the first to third inventions. And the overcurrent detection means determines that an overcurrent has occurred in the switching element when the current detected by the smoothing current detection means exceeds a second predetermined value, and is driven by the overcurrent detection means. When an overcurrent occurs in the path that short-circuits the smoothing capacitor to the large current terminal of the switching element by turning off the signal, the switching element is detected by the smoothing current detecting means in the same manner as when the current of the switching element is detected. The heat generated by the continuous overcurrent of the switching element is stopped by forcibly stopping the driving of the switching element and continuing the forced stop for the first predetermined time. Destruction can be prevented.

According to a fifth aspect of the invention, in particular, in the fourth aspect of the invention, the smoothing capacitor comprises a parallel connection of a plurality of capacitors, and includes a smoothed shunt detection means for detecting a current shunted to the plurality of smoothing capacitors. By detecting the current of the smoothing capacitor, it is possible to set the capacity of the current detection means low, preventing destruction of the current detection means when overcurrent flows, and reducing size and cost. Can be realized.

In particular, the sixth invention includes any one of the first to fifth inventions, further comprising temperature detecting means for detecting a temperature of the switching element, wherein the temperature detecting means causes the switching element to have a high temperature equal to or higher than a third predetermined value. If the first predetermined time or the second predetermined time, which is the duration of the OFF after the forced-off by the overcurrent detection, is set longer than the low temperature, the overcurrent detection is performed. Considering the heat generated by the switching element during operation of the device, if the switching element temperature is close to the upper limit of the operating temperature range, set a longer OFF duration to ensure a margin for the upper limit of the operating temperature. I can do it.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, this invention is not limited by the form of this invention.

(Embodiment 1)
FIG. 1 is a circuit block diagram of an induction heating cooker according to the first embodiment of the present invention.

  In FIG. 1, a DC power source 1 includes a commercial power source 1a, a rectifier 1b, and input detection means 1c. The smoothing circuit 2 includes a choke coil 2a and a first smoothing capacitor 2b. In parallel with the first smoothing capacitor 2b, a second smoothing capacitor 2c and a series circuit 2d of smoothed shunt detection means are connected. is doing. The inverter unit 3 includes a first switching element 3a, a second switching element 3b, a resonant capacitor 3c, a current detection unit 3d, a self-clamping unit 3e, and a temperature detection unit 3f. High frequency current is supplied. The plate 5 is made of tempered glass, and the object to be heated 6 is placed on the plate 5 so as to face the heating coil 4 and is made of a metal material having a large specific resistance value such as iron or stainless steel. .

  The control means 7 includes a first drive means 7a, a second drive means 7b, a microcomputer (hereinafter referred to as a microcomputer) 7c, and a stop means 7d. A timer counter function 7e is provided inside the microcomputer 7c. It is prepared and configured.

  The overcurrent detection means 8 includes a comparator 8a, a current detection output 8b, and a time constant latch means 8c.

  About the induction heating cooking appliance comprised as mentioned above, the operation | movement and an effect | action are demonstrated below.

  First, the DC power source 1 inputs the output of the commercial power source 1 a to the rectifier 1 b via the input detection unit 1 c and outputs the DC output of the rectifier 1 b to the smoothing circuit 2. At this time, the input detection means 1c detects the amount of current injected from the commercial power supply 1a to the rectifier 1b, and the output of the input detection means 1c is input to the microcomputer 7c inside the control means 7, and the microcomputer 7c performs an inverter section. 3 is controlled to adjust the heating power of induction heating.

  The smoothing circuit 2 smoothes the output of the DC power supply 1 by the choke coil 2 a and the LC of the first smoothing capacitor 2 b and the second smoothing capacitor 2 c and supplies power to the inverter unit 3. At this time, the smoothed shunt detection means 2d has a small value of about several tens of milliohms, and is set to a level that does not hinder the smoothing operation even when connected in series with the second smoothing capacitor 2c. In the present embodiment, the capacities of the first smoothing capacitor 2b and the second smoothing capacitor 2c are set to substantially the same value, so that when an excessive current flows to the inverter unit 3, the current is divided into about half. The converted current is converted into a voltage by the smoothed shunt detecting means 2d, and the voltage at the positive input terminal of the comparator 8a in the overcurrent detecting means 8 is changed.

  The inverter unit 3 alternately turns on the first switching element 3a and the second switching element 3b at about 23 kHz, so that a high-frequency current flows through the series LC circuit of the resonance capacitor 3c and the heating coil 4, and the heating coil A heated object 6 made of a metal material facing 4 is heated via a plate 5. The current detection unit 3d detects a current flowing through the second switching element 3b, and in a normal heating operation, a current flowing through a loop formed by the second switching element 3b, the resonant capacitor 3c, and the heating coil 4 However, when the first switching element 3a is erroneously turned on due to external noise when the second switching element 3b is on, the output of the smoothing circuit 2 is output to the first switching element 3a. The second switching element 3b is short-circuited. At this time, an excessive current flows through the first switching element 3a, the second switching element 3b, and the current detection means 3d, and the current is converted into a voltage by the current detection output 8b and is in the overcurrent detection means 8. The voltage at the positive input terminal of the comparator 8a is varied. Further, as described in the prior art, the self-clamping means 3e prevents voltage breakdown by turning on the second switching element 3b when the collector voltage rises instantaneously. The temperature detection means 3f is disposed in the vicinity of the second switching element 3b, detects the temperature, and transmits temperature information to the microcomputer 7c inside the control means 7.

  In the control means 7, the first drive means 7a and the second drive means 7b amplify the inverter control signal output from the microcomputer 7c, and the first switching element 3a and the second switching element in the inverter unit 3 are amplified. 3b is on / off controlled respectively. At this time, the microcomputer 7c receives the output of the input detection means 1c, outputs it while changing the pulse width of the inverter control signal so as to obtain a preset input current, and outputs the first switching element 3a and the second switching element. The on / off time of the element 3b is controlled. Further, the stopping means 7d plays a role of shifting from the ON state to the OFF state when the second switching element 3b is controlled to be turned on / off during the normal heating operation, but even when the overcurrent detection means 8 is operated, The stop signal is received via the diode D1 inside the overcurrent detection means 8, and the second switching element is turned off in preference to the inverter control signal of the microcomputer 7c.

In the overcurrent detection unit 8, the voltage at the positive input terminal of the comparator 8a varies depending on the output value obtained by converting the overcurrent detected by the smoothed shunt detection unit 2d or the current detection unit 3d by the current detection output 8b. When the voltage at the negative input terminal determined by R5 and resistor R6 becomes lower, the output terminal of the comparator 8a operates the stopping means 7d inside the control means 7 via the diode D1, and further the microcomputer via the diode D2. An overcurrent detection signal is transmitted to 7c. When the microcomputer 7c receives the overcurrent detection signal, the microcomputer 7c stops the inverter control signal to turn off the first switching element 3a and the second switching element 3b, and a time longer than the stop time by the time constant latch means 8c has elapsed. When this is detected by the timer counter function 7e, the inverter control signal is gradually increased from a small pulse width to increase the output of the input detection means 1c from a small value so that the pulse width is substantially equal to a predetermined value. Perform soft start control to increase or decrease. The time counted by the timer counter function 7e is a value determined for each temperature information sent from the temperature detecting means 3f. If the second switching element 3b is at a high temperature, the counting time is long. It is set to be.

  At the same time when the stopping means 7d inside the control means 7 is operated via the diode D1, the output terminal of the comparator 8a turns on the transistor Q4 via the capacitor C5 inside the time constant latch means 8c, so that the negative input of the comparator 8a The resistor R5 that has determined the voltage level of the terminal is short-circuited, and the voltage at the negative input terminal rises. The operation of each element at the time of overcurrent detection will be described in detail using the operation waveform of FIG.

  FIG. 2 shows the waveform of each part during the operation of the overcurrent detection means 8. 2, (a) is the current waveform of the smoothed shunt detection means 2d, (b) is the current waveform of the current detection means 3d, (c) is the waveform of the positive input terminal of the comparator 8a, and (d) is the comparator 8a. (E) shows the voltage across C5 in the time constant latch means 8c. The current of the smoothed shunt detecting means 2d shown in (a) flows from the first switching element 3a → the heating coil 4 → the resonance capacitor 3c → the second smoothing capacitor 2c. On the other hand, the current flowing through the current detection means 3d shown in (b) flows through the path of the resonance capacitor 3c → the heating coil 4 → the second switching element 3b. The waveform obtained by voltage conversion by combining (a) and (b) is (c), and the first switching element 3a and the second switching element 3b are simultaneously turned on at time t1, and as a result, the smoothed shunt detection means The output of 2d and the current detection output 8b overlap, and the voltage at the positive input terminal of the comparator 8a falls below the overcurrent determination value at time t2. This overcurrent determination value is biased to the negative input terminal of the comparator 8a by the divided voltage of R5 and R6. When the positive input terminal of the comparator 8a drops to a voltage that is about an overdrive voltage lower than the voltage of the negative input terminal at the time t3, the output terminal of the comparator 8a is turned on and drops to approximately the common potential. The output terminal of the comparator 8a is turned on to short-circuit the diode D1 and the cathode terminals of the diode D2 and the low potential side of the capacitor C5 to the common potential. At the start of operation of the overcurrent detecting means 8, the voltage across the capacitor C5 is discharged by the series circuit of the resistors R7, R8, R11. When the comparator 8a operates at the time point t3, it is charged via the series circuit of the resistors R7 and R8, and the voltage across the capacitor C5 rises. At time t4, when the voltage between the terminals of the resistor R7 becomes smaller than the voltage between the base and emitter of the transistor Q4 (hereinafter referred to as Vbe), the transistor Q4 is turned off, so that the voltage at the negative input terminal of the comparator 8a is The voltage gradually returns while discharging C6 to the divided voltage value of the resistors R5 and R6, and the positive input terminal at time t5 becomes higher than the voltage of the negative input terminal, so that the output terminal of the comparator 8a also returns to OFF. .

  At the end of the operation of the overcurrent detection means 8 (t5), the voltage across the capacitor C5 has risen to the control power supply level. By reducing the time constant level discharged by the resistors R7, R8, R11, the transistor Q4 A reverse bias was applied to the Vbe voltage to enable discharge before reaching the breakdown level. It is also possible to simplify the circuit by determining the time until restart by counting down by the microcomputer 7c.

  However, even if the output of the microcomputer 7c is stopped, if the always-on state continues due to external noise or the like, the first switching element 3a and the second switching element 3b are simultaneously turned on again at time t5, and the comparator 8a The voltage at the positive input terminal falls below the overcurrent determination value at time t6, the comparator 8a operates from time t7 to charge the capacitor C5, and the operation from t3 to t7 is repeated.

  At this time, as a method of protecting the first switching element 3a and the second switching element 3b by suppressing the frequency of occurrence of overcurrent, there is a means for increasing the capacity of the capacitor C5 to widen the interval between t3 and t4. However, when the capacitor C5 is large, the capacitor 8 cannot be discharged immediately after the output terminal of the comparator 8a is turned off, and the base voltage of the transistor Q4 becomes higher than the emitter voltage and may be destroyed. As a countermeasure, a series circuit of a transistor Q5 and a resistor R10 is provided, and when the high potential side of the capacitor C5 becomes higher than the control power supply by the charge pump operation, the transistor Q5 is turned on by the base resistor R9 and the capacitor R5 is discharged by the resistor R10. As a result, the voltage across the both ends sharply decreases. Therefore, even when it is difficult to secure the heating stop time by the microcomputer 7c, such as when the driving means malfunctions due to external noise or when the microcomputer 7c goes out of control, the time constant latch operation at the same level can be achieved by increasing the capacity of the capacitor C5. Is possible.

  In the first embodiment, the time constant latch means 8c includes as constituent elements a time constant composed of a series circuit of resistors R7 and R8 and a capacitor C5, and a predetermined time (first predetermined time) determined by the time constant for charging the capacitor C5. The ON state of the output terminal of the comparator 8a can be held for only (time).

  In addition, in order to pull up the negative input terminal of the comparator 8a to a control power supply voltage for a predetermined time and perform a latch operation, a series circuit of a transistor Q4, a resistor R8, and a capacitor C5 is included as a component, and the capacitor C5 is charged. The transistor Q4 is driven based on the current, and the operation of the overcurrent detection means is maintained for a predetermined time (first predetermined time).

  Accordingly, the operation and release of the latch can be realized by the same circuit without requiring a complicated circuit configuration such as separately providing a reset circuit for the overcurrent detection means (claim 2).

  The first switching element 3a and the second switching element 3b are on / off controlled by an inverter control signal output from the microcomputer 7c in the control means 7, and the output terminal of the comparator 8a is controlled via the diode D1. The stop means 7d inside the means 7 is operated, and at the same time, an overcurrent detection signal is transmitted to the microcomputer 7c via the diode D2.

  More specifically, the stop unit 7d in the control unit 7 is operated as soon as the output terminal of the comparator 8a operates without waiting for the stop processing time of the microcomputer 7c according to the signal from the overcurrent detection unit 8, and the inverter unit 3 And the interrupt process of the microcomputer 7c is executed while the output terminal of the comparator 8a is continuously operated by the time constant latch means 8c, the inverter control signal is stopped, and the first switching element 3a, 2 switching element 3b is turned off.

  As a result, the setting time of the time constant latch means 8c is required only for a short time of several hundred microseconds until interrupt processing is performed, and it becomes possible to set the CR constant constituting the time constant to be small, and the circuit components can be made compact. Can be realized. (Claim 3)

  The microcomputer 7c has a timer counter function 7e, and after the output signal of the overcurrent detection means 8 is interrupted, the microcomputer 7c waits for a predetermined time (second predetermined time) longer than the stop time by the time constant latch means 8c. After confirming the stopped state, heating is started again.

  As a result, even when the temperature of the electronic component or the printed wiring board rises due to the flow of a large current at which the overcurrent detection means 8 operates, the operation is stopped for a predetermined time (second predetermined time). Check that the temperature of each part is at an operable level by the temperature detection element provided inside the device, and that the operation of each part is normal, such as that the signal from the overcurrent detection has stopped reliably. In order to start heating again after confirming this, it is possible to prevent an unstable operation state or restarting the heating operation while the temperature of each part is abnormal. Even when an electric current is generated, it is possible to prevent the device from being destroyed due to a temperature rise or the like (claim 4).

  Further, the smoothing current flowing through the first smoothing capacitor 2b, the second smoothing capacitor 2c, and the second smoothing capacitor 2c for holding the electric charge from the DC power source 1 obtained by converting the commercial power source 1a into the DC voltage source is reduced. The output of the smoothed / divided flow detecting means 2d that detects the divided current in half is input to the overcurrent detecting means 8.

  As a result, the first smoothing capacitor 2b and the second smoothing capacitor 2c are short-circuited when the first switching element 3a and the second switching element 3b are simultaneously turned on due to malfunction due to external noise or the like. Discharge current is generated in the same path as the smooth current. Therefore, when the smoothing current flowing through the first smoothing capacitor 2b or the second smoothing capacitor 2c becomes equal to or greater than a predetermined value (second predetermined value) due to a discharge current due to a short circuit, the first switching element 3a and the second It is determined that an overcurrent has occurred due to simultaneous conduction of the switching elements 3b, and the overcurrent detection means 8 turns off the drive signal.

  Therefore, even when an overcurrent occurs in a path that short-circuits the smoothing capacitor, the driving of the switching element is forcibly stopped, and the switching element is continuously overrun for a predetermined time (first predetermined time). Thermal destruction due to current generation can be prevented (claim 5).

  Further, like the first smoothing capacitor 2b and the second smoothing capacitor 2c, the smoothing capacitor is composed of a plurality of capacitors connected in parallel, and further, the smoothing current divided into the plurality of smoothing capacitors by the smoothed shunt detection means 2d is obtained. Detected.

  As a result, the current capacity of the smoothed shunt detection means 2d can be halved, and the overcurrent detection means can be reduced in size and cost (claim 6).

  Further, the temperature of the temperature detecting means 3f arranged to detect the temperature of the second switching element 3b is detected, and the second switching element is recognized as having a high temperature equal to or higher than a predetermined value (third predetermined value). In such a case, the OFF duration time (first predetermined time or second predetermined time) after the forced OFF by the overcurrent detection is set longer than that at low temperature.

  Thus, even when an overcurrent flows through the second switching element 3b and generates heat, when the temperature of the switching element is higher than a predetermined value (third predetermined value) and close to the upper limit of the operating temperature range The off duration (the first predetermined time or the second predetermined time) is set longer to ensure a margin for the upper limit of the operating temperature, and the off duration during cold when there is a margin for the upper limit. The cooking stop time can be shortened by shortening (the first predetermined time or the second predetermined time).

  In the first embodiment, the sum of the output of the smoothed shunt detection means 2d and the output of the current detection output 8b is taken. However, even if only one of the functions is provided, it stops sufficiently when an overcurrent occurs. Needless to say, it has a function and can detect an overcurrent at a small size and at a low cost.

  Further, the smooth shunt detection means 2d and the current detection unit 3d are current detection units having substantially the same function, and the connection position is not limited, and the operation of the inverter unit 3 is stopped by detecting an overcurrent. Needless to say, the same effect can be obtained by providing a current detection means considering the size reduction and cost reduction according to the connection location to be provided and inputting a signal to the overcurrent detection means 8.

  Further, although the temperature of the second switching element is detected by the temperature detecting means 3f, the cooling is performed regardless of whether it is configured to detect the temperature of the first switching element or both. Needless to say, appropriate effects can be obtained by setting conditions as required.

  Further, the temperature information detected by the temperature detecting means 3f is sent to the microcomputer 7c, and the forced off time is changed according to the OFF duration determined for each detected temperature in the microcomputer 7c. For example, the temperature detecting means 3f Is constituted by a PTC thermistor whose resistance value increases as the temperature rises, and is replaced with a resistor R8 inside the time constant latch means 8c, so that the OFF duration can be changed longer in a high temperature state.

  As described above, when the induction heating cooker according to the present invention determines that an overcurrent has flowed through the switching element and stops the operation of the inverter, it continues the stop state for a predetermined time and resets from another block. Since it is possible to restart cooking without receiving a command and perform cooking and heating, it can also be applied to uses such as a cooking device having an inverter used for general households and business use.

The circuit block diagram of the induction heating cooking appliance in the 1st Embodiment of this invention The figure which shows the operation | movement waveform of the overcurrent detection means of the induction heating cooking appliance in the 1st Embodiment of this invention. Circuit block diagram of induction heating cooker in the conventional example The figure which shows the operation | movement waveform at the time of normal operation | movement of the induction heating cooking appliance in a prior art example. The figure which shows the operation | movement waveform at the time of abnormal load of the induction heating cooking appliance in a prior art example. The figure which shows the operation | movement waveform at the time of the control means malfunctioning of the induction heating cooking appliance in a prior art example.

Explanation of symbols

1 DC power supply 1a Commercial power supply 1b Rectifier 1c Input detection means (current transformer)
2 smoothing circuit 2a choke coil 2b first smoothing capacitor 2c second smoothing capacitor 2d smoothing shunt detection means 3 inverter part 3a first switching element 3b second switching element 3c resonance capacitor 3d current detection part 3e self-clamping means 3f Temperature detection means 4 Heating coil 5 Plate 6 Object to be heated 7 Control means 7a (First) driving means 7b Second driving means 7c Microcomputer (microcomputer)
7d Stop means 7e Timer counter function 8 Overcurrent detection means 8a Comparator 8b Current detection output 8c Time constant latch means

Claims (6)

A heating coil for inductively heating an object to be heated;
A resonant capacitor that forms a resonant circuit with the heating coil;
A switching element for generating a resonance current in the resonance circuit;
Current detection means for detecting the current of the switching element,
Overcurrent detection means for forcibly turning off the drive signal of the switching element when an overcurrent of a first predetermined value or more flows through the switching element by the current detection means;
Time constant latch means for holding the output of the overcurrent detection means for a first predetermined time;
Have
The first predetermined time after the overcurrent has occurred and the switching element is forcibly turned off is to be kept off .
The overcurrent detection means is constituted by a voltage comparator,
The time constant latch means is composed of a transistor, a capacitor, and a resistor. When the voltage comparator operates by detecting an overcurrent, the reference voltage of the voltage comparator is changed, and the overcurrent stops and is normally An induction heating cooker characterized in that the transistor operates and maintains an overcurrent state only for a time determined by a time constant of the capacitor and the resistor even after returning to an operation level detection voltage . The switching element is driven by the control signal output from the control microcomputer, and the microcomputer interrupts the microcomputer to generate a control signal stop interrupt by the output signal of the overcurrent detection means to stop the driving of the switching element. The induction heating cooker according to claim 1, wherein The microcomputer includes a timer counter and waits for a second predetermined time after the output signal of the overcurrent detection means is interrupted, and after confirming the stable state of the device, starts heating again. 2. The induction heating cooker according to 2 . A smoothing capacitor that holds a charge obtained by converting a commercial AC power source into a DC voltage source; and a smoothing current detection unit that detects a current of the smoothing capacitor. The overcurrent detection unit is detected by the smoothing current detection unit. current is determined that the overcurrent is generated in the switching element becomes the second predetermined value or more, the in any one of claims 1 to 3, characterized in that turns off the drive signal by an overcurrent detection means The induction heating cooker described. The induction heating cooker according to claim 4 , wherein the smoothing capacitor includes a plurality of capacitors, and includes a smoothed shunt detection means for detecting a current shunted to the plurality of smoothing capacitors. Temperature detecting means for detecting the temperature of the switching element, and when the temperature detecting means recognizes that the switching element is at a high temperature equal to or higher than a third predetermined value, it is forcibly turned off by the overcurrent detecting means. The induction heating cooker according to any one of claims 1 to 5 , wherein the first predetermined time or the second predetermined time which is an OFF duration is set longer than that at a low temperature.

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