JP2008166107A - Induction heating cooker - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating cooker improved in reliability of a product having no occurrence of overvoltage in the output of a power supply circuit and giving no overvoltage and excess current to a switching element constituting an inverter. <P>SOLUTION: The induction heating cooker is provided with a resonance load circuit including a heating object, a power supply circuit having a step-up power supply means which is connected to a commercial power supply and converts into a DC variable power supply, an inverter which converts the DC voltage generated by the power supply circuit and supplies the power to the resonance load circuit, and a control means to control the power supply circuit and the inverter. The control means controls so that, at the time of heating start, the operation of the step-up power supply means of the power supply circuit is started after a prescribed time of driving the inverter, and at the time of heating stop, the driving of inverter is stopped after a prescribed time of stopping the operation of the step-up power supply means of the power supply circuit. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an induction heating cooker.

  An induction heating cooker efficiently heats a pan by generating eddy current in a pan or the like to be heated placed near a heating coil that passes a high-frequency current, and the pan itself self-heats due to its Joule heat. In recent years, the replacement of cooking utensils using gas stoves or electric heaters has been progressing due to their excellent safety and temperature controllability.

  The induction heating cooker connects the inductance of the heating coil including the pot (load) of the object to be heated and the resonant load circuit of the resonant capacitor as a power control circuit for supplying a high-frequency current to the heating coil, and the switching element is 20 to 20 A so-called resonant inverter that performs on / off control at a drive frequency of about 40 kHz is generally used.

  The resonance type inverter is classified into a voltage resonance type and a current resonance type. The former is often used for a 100V power source and the latter is used for a 200V power source.

  Initially, only a load of magnetic material such as iron could be heated, but in recent years, a load of nonmagnetic stainless steel or the like can be heated. The one that can be heated is also proposed.

  That is, a resonant load circuit including a load of a metal body to be heated, a power supply circuit that generates a DC voltage, and a DC voltage generated by the step-up power supply means and the step-down power supply means of the power supply circuit is converted into a high-frequency voltage to convert the resonant load And an inverter for supplying power to the circuit, the inverter having upper and lower arms composed of at least two switching elements connected in series, and the power supply circuit generates two or more different DC voltages In addition, by adopting a configuration in which the output voltage of the power supply circuit is switched according to the material of the load of the metal body to be heated, the induction heating cooker that can efficiently supply desired power to the load of different materials Proposals have been made (see, for example, Patent Document 1).

JP 2004-319296 A

  However, in the above prior art, when the booster power supply means of the power supply circuit is started and stopped, output voltage fluctuations and power supply current fluctuations are likely to occur due to the weight of the load. The power supply current fluctuates until a state is reached. Also, the output voltage rises when stopped.

  Therefore, when the inverter is stopped when heating is stopped, the load impedance to the power supply circuit suddenly increases and the output current of the power supply circuit decreases, so the output voltage of the power supply circuit rises rapidly and an overvoltage is applied to the switching element. There is a problem that the reliability of the switching element is impaired.

  The present invention has been made to solve the above-described problem, and includes a resonant load circuit including an object to be heated, a power supply circuit having a boost power supply means connected to a commercial power supply and converted to a DC variable power supply, and the power supply. In an induction heating cooker including an inverter that converts a DC voltage generated by a circuit into an AC voltage and supplies power to the resonant load circuit, and a control unit that controls the power supply circuit and the inverter. The operation of the boosting power supply means of the power supply circuit is started after a predetermined time after the inverter is driven, and when the heating is stopped, the inverter is stopped after a predetermined time after the operation of the boosting power supply means of the power supply circuit is stopped. It is controlled by the control means.

  The power supply circuit includes a heating availability determination unit that detects an input current of the power supply circuit and a current of the resonance load circuit to determine whether to heat the object to be heated. The heating availability determination unit includes the power supply at the start of heating. Whether or not heating is possible is determined before the operation of the boosting power source means of the circuit is started.

  Since the induction heating cooker of the present invention is configured as described above, there is no occurrence of overvoltage in the output of the power supply circuit, and therefore the overvoltage and overcurrent stress applied to the elements constituting the inverter can be made minute. The frequency of failure can be drastically reduced and the reliability of the product can be improved.

  Moreover, since it can be determined in a stable power supply state whether or not the object to be heated can be heated at the start of heating, erroneous determination can be prevented.

  An embodiment of the present invention will be described below with reference to FIGS.

  FIG. 1 is a circuit block diagram of the induction heating cooker of the present invention. After being connected to the commercial power source 1 and converted into direct current by the rectifying means 2, the boost power supply means 3 and the step-down power supply means 4 are connected to the output terminal of the power supply circuit 9 which can vary the output DC voltage. An inverter 5 is connected to the coil 6 for flowing a high-frequency current.

  The control means 7 performs start-up and stop control of the step-up power supply means 3, setting of the output voltage of the step-down power supply means 4, setting of driving conditions (switching frequency and duty) of a switching element (not shown) of the inverter 5. A load performed by a temperature sensor (not shown) for detecting the heating power of the load, such as a metal pan, which is an object to be heated, which is disposed in the vicinity of the heating coil 6 through which a high-frequency current flows, by the operation of the operating means 8 performed by the user Set conditions such as temperature control.

  Further, the control means 7 generates an inverter from the current value detected by the input current detection element 10 that detects the input current of the power supply circuit 9 and the inverter current detection element 11 that detects the current flowing in the resonance load circuit of the heating coil 6. 5 is used to determine whether the state of the load 5 is high impedance or low impedance, or whether the resonance frequency of the load of the inverter 5 is high or low. You are typing. The material of the load (metal pan) is estimated based on the determination result of the heating enable / disable determining unit 12, and the driving of the switching element of the inverter 5 is controlled by the control unit 7 so that the load is not heated if the load is not suitable for heating. To do.

  FIG. 2 is a diagram for explaining the circuit configuration of the boost power supply means 3. One end of the choke coil 31 is connected to the positive output terminal of the rectifying means 2, the switching element 32 is connected to the output end side of the other end of the choke coil 31, and the negative output terminal (reference potential) of the rectifying means 2 is connected. ) To form a closed loop. A diode 33 is connected to the output end of the choke coil 31 and a smoothing capacitor 34 is connected. Reference numeral 35 denotes a load of the boost power source means 3.

  In the step-up power supply means 3, the DC voltage of the rectifying means 2 is applied to the choke coil 31 during the ON period of the switching element 32 to accumulate energy, and the energy is released to the capacitor 34 via the diode 33 during the OFF period of the switching element 32. Then, the voltage is applied to the load 35 of the boost power supply means 3.

  With such a circuit configuration, by changing the on / off duty of the switching element 32, the voltage can be boosted to an arbitrary voltage higher than the input voltage from the rectifier 2 and output to the load circuit 35 of the boost power source 3. .

  Further, by controlling the on / off duty of the switching element 32 by the control means 7 and increasing the capacity of the smoothing capacitor 34 sufficiently, a sufficiently smoothed DC voltage can be output.

FIG. 3 is a diagram for explaining the control timing of the boost power supply means 3. When the switching element 32 is on, the choke coil 31 is a current I _L flowing voltage of the rectifier means 2 is applied, energy is accumulated in the choke coil 31.

Next, when the switching element 32 is turned off, the energy accumulated in the choke coil 31 is converted into a voltage, a voltage is generated is higher than the input voltage V _in, it is charged in the capacitor 34 through the diode 33 by the voltage The At this time, the voltage V _O1 output to the load circuit 35 of the step-up power supply means 3 is a voltage higher than the input voltage V _in.

Boosted power supply unit 3, by the envelope waveform of the input current I _L and the input voltage V _in the waveform is controlled by the control means 7 on and off timing of the switching element 32 so that the similar waveforms, including phase, induction cooking The power factor of the vessel can be made high.

FIG. 4 is a diagram for explaining the circuit configuration of the step-down power supply means 4. A switching element 41 is connected to the output terminal of the step-up power supply means 3, a choke coil 42 is connected to the output side of the switching element 41, and a diode 43 is connected in antiparallel with the output of the step-down power supply means 4. A capacitor 44 is connected to the output side of the choke coil 42. 45 represents the load of the step-down power supply means 4.

In this configuration, the output voltage V _OUT can be changed to 0 to 100% by changing the on / off duty of the switching element 41. However, the output voltage V _OUT varies depending on the impedance of the load 45 of the step-down power supply means 4 even if the on / off duty of the switching element 41 is the same.

FIG. 5 is a diagram for explaining the control timing of the step-down power supply means 4. The relationship between the on / off timing of the switching element 41, the current I _L2 flowing through the choke coil 42, the output voltage V _OUT , and the voltage V _O1 at the output terminal of the boosting power source means 3 as the input voltage is shown.

When the switching element 41 is on, the voltage V _O1 applied to the step-down power supply means 4 is charged to the capacitor 44 via the choke coil 42. At this time, the voltage V _O1 is applied to the choke coil 42 to cause a current to flow, and energy is accumulated in the choke coil 42.

Next, when the switching element 41 is turned off, the energy accumulated in the choke coil 42 is converted into a voltage and supplied to the load 45 of the step-down power supply means 4 via a closed circuit including the diode 43. When the choke coil 42 is depleted of energy, it is supplied from the charging energy of the capacitor 44 at the output terminal. The energy supplied to the load 45 is determined by the on / off duty of the switching element 41, whereby the output voltage _VOUT is determined.

  FIG. 6 is a diagram illustrating the circuit configuration of the inverter 5. The circuit has a general single-ended push-pull circuit configuration. The upper and lower arms of a series body of switching elements 51 and 52 are connected to the output of the power supply circuit 9, and a resonance load circuit composed of the heating coil 6, an object to be heated (not shown), and a resonance capacitor 55 is connected to the middle point. To do. Damper diodes 53 and 54 are connected in antiparallel to the series bodies of the switching elements 51 and 52, respectively, and further snubber capacitors 56 and 57 are connected to suppress the occurrence of overvoltage and loss in the switching elements 51 and 52.

  A high frequency current flows through the resonant load circuit including the object to be heated exclusively by turning on and off the switching elements 51 and 52, and a load (pot or the like) placed near the heating coil 6 by the current flowing through the heating coil 6. An eddy current is generated and the load itself generates heat. With such a circuit configuration, the electric power input to the load (pan etc.) by the current flowing through the heating coil 6 can be controlled by the output set voltage of the step-down power supply means 4 and the switching frequency of the inverter 5.

  In this embodiment, since the DC voltage sufficiently smoothed by the operation of the boosting power source means 3 can be input to the inverter 5, the heating that flows when the switching elements 51 and 52 of the inverter 5 are exclusively turned on and off. The envelope of the high frequency current of the coil 6 becomes flat. In the case of a DC power source that is not sufficiently smoothed when there is no boosting power source means 3, the envelope of the high frequency current flowing through the heating coil 6 fluctuates at twice the frequency of the commercial power source, so ) Vibration may occur in itself, and abnormal noise due to commercial frequency may occur. Therefore, by using the boosting power source means 3, it is possible to suppress the generation of abnormal noise due to the vibration of the load (eg, pan) caused by the commercial frequency.

  In general, in order for the output of the boost power supply means 3 to be a smoothed voltage, the capacity of the capacitor 34 connected to the output of the boost power supply means 3 must be sufficiently large. For this reason, if the power factor correction operation of the step-up power supply means 3 is suddenly performed at the start of heating, an overcurrent may flow through the switching element 32 or the power supply current may become excessive. Therefore, at the start of heating, it is necessary to perform a soft start operation so that the operation of the boosting power source means 3 gradually appears. However, the output voltage and power supply current during the soft start operation are not stable because they include transient fluctuations. Similarly, a soft stop operation is required when heating is stopped.

  Furthermore, the output voltage of the boosting power source means 3 changes rapidly due to a change in load current. Specifically, when the load current is reduced to zero during the step-up operation, the output voltage rises rapidly and an overvoltage is applied to the smoothing capacitor 34, or an overvoltage is applied to the switching element 41 of the step-down power supply means 4 connected to the output. Since it is applied, it causes a failure.

The step-up operation of the output voltage V _{O1 in} the step-up power supply means 3 is after the switching element 32 is turned off, as shown in FIG. Therefore, when the load current decreases while the switching element 32 is on, an overvoltage is generated in the output voltage V _O1 even if an operation for stopping the boosting operation at that time is performed (even if the switching element 32 is turned off).

  In an inverter of an induction heating cooker using a power supply circuit that does not have a conventional boosting function, the commercial power supply is converted to a DC voltage by a rectifying means, and the output is connected to the inverter to stop heating. In this case, the drive signal of the inverter is stopped immediately. In this configuration, even if the load current of the inverter rapidly decreases, there is no possibility that a transient high voltage is generated because there is no large inductance in the path of the power supply circuit through which the power supply current flows. Therefore, no special measures are required for the overvoltage prevention element for protecting the circuit element from the transient high voltage and the drive timing of the inverter.

  However, in the circuit configuration of the power supply circuit 9 having the boosting power supply means 3, as described above, sudden load fluctuations, particularly a decrease in load current, lead to generation of an overvoltage and must be avoided. Therefore, when heating is stopped, the output power setting of the inverter 5 may be gradually decreased so that the load current of the inverter 5 does not rapidly decrease. However, this method requires a predetermined time. Since energization is continued for a while from when the heating should be stopped, unnecessary power consumption, cooking failure due to an increase in load temperature, and the like occur.

  Further, when the control of the inverter 5 performed by the control means 7 is performed by software of a microcomputer, there arises a problem that the heating end process becomes complicated.

  For this reason, in this embodiment, when the heating of the load is stopped by the setting of the user or at the end of the predetermined automatic heating operation, the operation of the boosting power supply means 3 of the power supply circuit 9 is stopped first. The operation of the inverter 5 is stopped after a predetermined time (short time) has elapsed. By performing such an operation, an abrupt decrease in the load current that occurs when the inverter 5 is stopped does not cause an overvoltage because the operation of the boosting power source means 3 has already stopped. In addition, since only a short time is required until the inverter 5 stops, it is possible to minimize the consumption of unnecessary power and to make the heating amount negligible for the temperature rise of the load. Does not lead to failure.

7 and 8 are diagrams for explaining the relationship between the operation timing of the inverter 5, the step-up power supply means 3 and the step-down power supply means 4 at the end of heating, and the output voltage V _O1 of the step-up power supply means 3. FIG. FIG. 6 is a diagram for explaining an operation waveform at the end of heating when complying with a conventional operation timing using a power supply circuit that does not include the step-up power supply means 3 and the step-down power supply means 4. FIG. 8 is a diagram for explaining the relationship between the operation timing of the inverter 5, the step-up power supply means 3 and the step-down power supply means 4 at the end of heating and the output voltage V _O1 of the step-up power supply means 3 in the embodiment of the present invention.

In FIG. 7, when the operation of the step-up power supply means 3, the operation of the step-down power supply means 4 and the operation of the inverter 5 of the power supply circuit 9 are stopped at the same time when the heating is finished, the output voltage of the step-up power supply means 3 A high voltage V _P is generated immediately after the stop from the stable voltage V ₁ , and then the output voltage V _O1 gradually decreases due to a leakage current or the like due to the circuit portion of the step-down power supply means 4 or the inverter 5 and approaches the stop voltage V _3. Go. At this time, when the high voltage V _P exceeds the breakdown voltage of the element used in the step-down power supply means 4 and the inverter 5, the element breaks down and breaks down in the short-circuit mode, thereby being connected to the subsequent stage. An overvoltage is applied to the inverter 5 and the switching elements 51 and 52 of the inverter 5 are damaged due to the overvoltage, which may result in a failure to use.

In FIG. 8, at the end of heating, the step-up power supply means 3 is stopped first, and then the operation of the step-down power supply means 4 and the operation of the inverter 5 are stopped after a predetermined time T _{1 has} elapsed. As a result, a load having a predetermined impedance is connected to the output of the boosting power supply means 3 even at the timing when the operation of the boosting power supply means 3 is stopped. Therefore, the transient high voltage V _P as shown in FIG. Does not occur.

  Furthermore, since the smoothing capacitor 34 of the boost power supply means 3 is discharged, the output voltage of the boost power supply means 3 can be quickly reduced.

That is, while the inverter 5 is energized, it is stable at the voltage V ₁ according to the power control of the inverter 5, and at the end of heating, the output voltage of the step-up power supply means 3 decreases to V ₂ , and the step-down power supply means 4 and the inverter 5 After stopping, the battery is charged again up to the stop voltage V ₃ .

Next, the operation of the boost power supply means 3 at the start of heating will be described. FIG. 9 and FIG. 10 are diagrams for explaining the relationship between the operation timing of the inverter 5, the step-up power supply means 3 and the step-down power supply means 4 at the start of heating, and the output voltage of the step-up power supply means 3. FIG. It is a figure explaining the operation | movement waveform at the time of the heating start at the time of complying with the conventional operation timing using the power supply circuit which does not have the power supply means 3 and the pressure | voltage fall power supply means 4. FIG. FIG. 10 is a diagram for explaining the relationship between the operation timing of the inverter 5, the step-up power supply means 3 and the step-down power supply means 4 at the start of heating and the output voltage V _O1 of the step-up power supply means 3 in the embodiment of the present invention.

  In FIG. 9, since the boosting power supply means 3 starts operating simultaneously with the operation of the inverter 5, the boosting power supply means 3 reaches a predetermined voltage regardless of whether the load of the inverter 5 is high impedance or low impedance. Works. In this operation example, the heating availability determination unit 12 of the control unit 7 determines that there is no load or a very low impedance load at the start of heating, and the load is not suitable for heating for the time being. Thus, the ON / OFF operation of the step-up power supply means 3, the step-down power supply means 4 and the inverter 5 is repeated, and the heating availability determination means 12 confirms whether or not the load can be heated several times. In such an operation, since a large amount of charge / discharge current flows for boosting by the boosting power supply means 3, when it is determined that the load cannot be finally heated, a sudden power supply circuit is used when the operation of the inverter 5 is stopped. 9 may cause a transient high voltage to be generated at the output of the power supply circuit 9. If the voltage reaches a voltage exceeding the breakdown voltage of the circuit element, the element will be destroyed and break down. .

In FIG. 10, since the step-up power supply means 3 is not operated even in the same operation, even if the step-up power supply means 3, the step-down power supply means 4 and the inverter 5 are turned on and off repeatedly at a predetermined cycle, the operation is transient. High voltage is not generated. At the start of heating for driving the inverter 5, there is a load, and the heating availability determination means 12 determines that the load is not a very low impedance load, and in the case of a load that is suitable for heating, the operation of the inverter 5 is performed. The operation of the boosting power source means 3 is started after a predetermined time T _{2 has} elapsed.

  Next, the effect | action by the above structure is demonstrated.

  When heating an object to be heated, such as a pan, by induction heating, the heating availability determination means 12 first determines whether the object to be heated can be heated, and determines whether heating is possible.

  The operation of determining whether or not the object to be heated can be heated is performed by heating the object to be heated at a low power level, so that no load or small load is obtained from the detected value of the input current of the power supply circuit 9 and the current of the resonant load circuit flowing in the heating coil 6. Or, a load having an extremely low load impedance is identified, and in such a case, heating is stopped.

  When it is determined that heating is possible, the power supply circuit 9 and the inverter 5 are controlled by the control means 7 so that the target electric power (heating power set by the user with the operation means 8) is continued.

When the booster power supply means 3 is provided in the power supply circuit 9 as in the configuration of the present embodiment, the charging current resulting from the operation of the booster power supply means 3 is superimposed on the input current immediately after the start of heating. Therefore, immediately after the start of power-on energization, after the charging of the boosting capacitor 44 of the boosting power supply unit 3 is completed, immediately after the operation of the boosting power supply unit 3 is started, or when the boosting operation of the boosting power supply unit 3 is stable, etc. In the operating state, the output current of the inverter 5 may be the same, but the input current may be different. That is, there are the following cases.
(1) The total value of the large charging current flowing through the boosting capacitor 34 immediately after power-on and the current flowing through the resonant load circuit of the inverter 5.
(2) The total value of the charging current after completion of charging of the boosting capacitor 34 of the boosting power source means 3 and the current flowing through the resonant load circuit of the inverter 5.
(3) The total value of the charging current of the boosting capacitor 34 immediately after the boosting operation of the boosting power supply means 3 and the current flowing through the resonant load circuit of the inverter 5 are performed.
(4) A total value of the charging current during the boosting operation that flows through the boosting capacitor 34 after the boosting operation of the boosting power source means 3 is stabilized and the current that flows through the resonant load circuit of the inverter 5.

  Therefore, in the method of referring to the input current in the operation for determining whether the object to be heated can be heated at the start of heating, even if the load current flowing in the resonant load circuit of the inverter 5 is the same, the input current is different, (2), Since an accurate input current value is not detected except for (4), it is difficult to accurately determine whether the object to be heated can be heated, and there is a risk of erroneous determination.

  For this reason, in this embodiment, the operation for determining whether heating is possible or not is performed in a state where the operation of the boosting power source means 3 is stopped at the start of heating, that is, in a state where fluctuations in the input current related to the boosting operation are not generated. The state reflected only in electric power and the determination accuracy of heating availability are improved, and the input current is reflected only in electric power applied to the load of the inverter 5. This prevents false detections such that no load or small load can be heated, and misidentification of magnetic and non-magnetic materials. Temperature rises due to incorrect heating, or inverters that turn on power to loads that cannot be heated. Since the overload state to 5 can be prevented in advance, safety and reliability can be improved.

Incidentally, after the load determination end, it starts the operation of the boosted power supply unit 3 after a predetermined time T ₂ of driving the inverter 5 when continuing the heating. At this time, the charging / discharging current to the boosting capacitor 34 is superimposed on the input current. However, since the determination as to whether heating is possible has already been completed, it is not necessary to perform load determination temporarily at this timing. By restarting the load determination after the boost operation current starts to flow stably after a predetermined time, it is possible to determine the presence or material of the load and prevent erroneous heating.

  Thus, there is no occurrence of overvoltage at the output of the power supply circuit 9, so that overvoltage and overcurrent stress applied to the elements constituting the inverter 5 can be made minute, the frequency of occurrence of failure is drastically reduced, and the product Reliability can be increased.

  Moreover, since it can be determined in a stable power supply state whether or not the object to be heated can be heated at the start of heating, erroneous determination can be prevented.

It is a circuit block diagram of the induction heating cooking appliance of this invention. It is a figure explaining the circuit structure of a step-up power supply means. It is a figure explaining the control timing of a pressure | voltage rise power supply means. It is a figure explaining the circuit structure of a step-down power supply means. It is a figure explaining the control timing of a step-down power supply means. It is a figure explaining the circuit structure of an inverter. It is a figure explaining the operation | movement waveform at the time of completion | finish of a heating at the time of complying with the conventional operation timing using the power supply circuit which does not have a pressure | voltage rise power supply means and a pressure | voltage fall power supply means. It is a figure explaining the relationship between the operation timing of the inverter at the time of completion | finish of heating of the Example of this invention, a step-up power supply means, a step-down power supply means, and the output voltage _VO1 of a step-up power supply means. It is a figure explaining the operation | movement waveform at the time of the heating start at the time of complying with the conventional operation timing using the power supply circuit which does not have a step-up power supply means and a step-down power supply means. It is a figure explaining the relationship between the operation timing of the inverter at the time of the heating start of the Example of this invention, a step-up power supply means, a step-down power supply means, and the output voltage _VO1 of a step-up power supply means.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Commercial power supply 3 Boosting power supply means 5 Inverter 7 Control means 9 Power supply circuit 12 Heating availability determination means

Claims (2)

A resonance load circuit, a power supply circuit having a boost power supply means connected to a commercial power supply and converting to a DC variable power supply, and an inverter that converts a DC voltage generated by the power supply circuit into an AC voltage and supplies power to the resonance load circuit And an induction heating cooker provided with a control means for controlling the power supply circuit and the inverter,
At the start of heating, start the operation of the boost power supply means of the power supply circuit after a predetermined time of driving the inverter,
An induction heating cooker characterized in that when the heating is stopped, the control means controls the inverter to stop driving after a predetermined time after the operation of the boosting power supply means of the power supply circuit is stopped.   A heating availability determination unit is provided that detects the input current of the power supply circuit and the current of the resonant load circuit to determine whether the object to be heated can be heated, and the heating availability determination unit includes the heating circuit. 2. The induction heating cooker according to claim 1, wherein whether or not heating is possible is determined before the operation of the boosting power source means is started.

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