Induction heating cooking device

The present invention relates to a high resistance pot such as iron having a large resistivity (resistivity), a low resistance pot such as aluminum or copper having a small specific resistance, or an induction heating cooker capable of discriminating no load. ..

The conventional induction heating cooker detects an inverter that supplies high-frequency power to the resonance circuit of the induction heating coil and the resonance capacitor, an impedance detecting means that detects the input impedance of the induction heating coil, and the resonance frequency of the resonance circuit. With the input impedance detected by the impedance detecting means and the resonance frequency detected by the frequency detecting means, a high-resistance pot such as iron, a low-resistance pot such as copper or aluminum, and none are provided. Determine the load state (see, for example, Patent Document 1).

<patcit num="1"><text>Japanese Patent No. 2647079 (Page 7, Fig. 3)</text></patcit>

<p> However, in the case of a conventional induction heating cooker, since it operates at a resonance frequency at the time of discrimination, for example, a current of nearly 30 A flows when there is no load. In order to prevent the induction heating cooker from interfering with such an excessive inverter current, parts with a large current rating must be used, and a thyristor is used for the bridge rectifier circuit on the power supply side. Since the voltage is controlled by controlling the energization angle of the inverter, there is a problem that the cost is high and the size becomes large.</p><p> The present invention has been made in view of the above problems, and is an induction heating that does not require a high cost and enables load discrimination between a no-load, high-resistance pan, and low-resistance pan without passing an excessive current through the circuit. The purpose is to get a cooker.</p>

<p> The induction heating cooker according to the present invention includes a DC power supply circuit, a load circuit including a heating coil and a resonance capacitor connected in series with each other, a current detecting means for detecting a current flowing through the load circuit, and the DC power supply circuit. A first switching element connected between the positive side of the load circuit and one end of the load circuit, and a second switching element connected between the other end of the load circuit and the negative side of the DC power supply circuit. A third switching element connected between the positive side of the DC power supply circuit and the other end of the load circuit, and a fourth switching element connected between one end of the load circuit and the negative side of the DC power supply circuit. The switching element and a control circuit for controlling the heating output of the load circuit by driving each of the switching elements are provided, and the control circuit drives the second and fourth switching elements at the time of pot determination. The load current is detected when the drive frequency of the first and third switching elements is set to a frequency lower than the drive frequency while the frequency is gradually lowered, and the material of the pot and the no-load state are determined. It is designed to be discriminated.</p>

<p> In the induction cooking cooker of the present invention, the drive frequency of the second and fourth switching elements is gradually lowered, and the drive frequency of the first and third switching elements is set to a frequency lower than the drive frequency thereof. Since the load current at the time of is detected to determine the material of the pot and the no-load state, the circuit current is suppressed at the time of judgment, and the pot material such as a high resistance pot can be used without passing an excessive current. Alternatively, it is possible to discriminate between a low resistance pan and a no-load state.</p>

Embodiment 1. FIG. 1 is a circuit configuration diagram of an induction cooker showing Embodiment 1 of the present invention. As shown in FIG. 1, for example, the AC voltage of the commercial power supply 1 is full-wave rectified by the rectifier circuit 2 by the diode bridge, and the voltage full-wave rectified by the rectifier circuit 2 is smoothed by the smoothing capacitor 3. A DC power supply circuit is composed of the rectifier circuit 2 and the smoothing capacitor 3. Further, the first switching element 4 of the switching element composed of the full bridge is the top plate of the induction heating cooker main body on which the positive side of the smoothing capacitor 3 constituting the DC power supply circuit and the cooking pot 11 are placed. A second switching element 5 is connected in series with the heating coil 8 and is arranged below the 10 and is connected to a heating coil 8 for inducing and heating the cooking pot 11, and the heating coil 8 and the load of the resonance circuit are connected. It is connected between the resonance capacitor 9 constituting the circuit and the negative side of the smoothing capacitor 3 of the DC power supply circuit.

Further, the third switching element 6 is connected between the positive electrode side of the smoothing capacitor 3 and the connection point of the resonance capacitor 9 and the second switching element 5, and the fourth switching element 7 is the first. It is connected between the connection point of the switching element 4 of 1 and the heating coil 8 and the negative electrode side of the smoothing capacitor 3. The switching elements 4, 5, 6 and 7 are made of, for example, an insulated gate type bipolar transistor, and a diode for power regeneration is connected in antiparallel and built in.

Further, the circuit current detection resistor 12 is connected to the emitter side of the second and fourth switching elements 5 and 7, and detects the current flowing through the load circuit of the heating coil 8 and the resonance capacitor 9. In detecting the current, for example, the current is detected based on the voltage drop generated by the current flowing through the circuit current detection resistor 12.

The switch 13 is a switching means for switching the constants of the heating coil 8 and the resonance capacitor 9, and when the switch 13 is on, for example, the winding BC and the resonance capacitor 9 in the middle between the winding AC in the heating coil 8. A resonant capacitor 14 is connected in parallel between them. Here, since the capacitance of the resonance capacitor 14 is sufficiently larger than the capacitance of the resonance capacitor 9, when the switch 13 is turned on, it flows between the windings AB of the heating coil 8 and in the loop of the resonance capacitor 14. Since the current is dominant, the winding between the BCs of the heating coil 8 is apparently omitted.

The inductance between the winding AC in the heating coil 8 and the capacitance of the resonance capacitor 9 can be optimally heated in a low resistance pan, and the resonance frequency can be set to a constant of, for example, 60kHz to 80kHz, and the heating coil can be used. The inductance between the windings AB of 8 and the capacitance of the resonance capacitor 14 can be optimally heated in a high resistance pan, and the resonance frequency is a constant of, for example, 20kHz to 30kHz.

The control circuit 15 controls the on / off drive of each of the switching elements 4, 5, 6 and 7, detects the circuit current based on the voltage drop information of the circuit current detection resistor 12, and controls the on / off of the switch 13. And so on.

In the induction cooking cooker configured as described above, the cooking pot 11 mainly placed on the top plate 10 is, for example, a high resistance pot such as iron having a large resistivity (resistivity) or a low specific resistance. The load determination operation of a low resistivity pan made of copper, aluminum, etc., or a no-load state will be described with reference to FIGS. 1 and 2. FIG. 2 is an operation waveform diagram showing the switching operation of the switching elements 4, 5, 6 and 7 at the time of load determination. When the power is turned on, the control circuit 15 first controls to turn off the switch 13 for load determination. Then, as shown in FIG. 2, the first switching element 4 and the second switching element 5 are turned on in a predetermined period T, respectively, and the heating coil is connected to the DC power supply circuit including the rectifier circuit 2 and the smoothing capacitor 3. A current is passed through the load circuit of 8 and the resonance capacitor 9. After that, the fourth switching element 7 and the second switching element 5 are alternately turned on and off to drive the fourth switching element 7, the heating coil 8, the resonance capacitor 9, and the second switching element 5. And the vibration current is passed through the closed loop of the circuit current detection resistor 12.

Subsequently, the control circuit 15 turns on the third switching element 6 and the fourth switching element 7, respectively, and allows a current to flow from the DC power supply circuit to the load circuit of the heating coil 8 and the resonance capacitor 9. After that, as described above, the second switching element 5 and the fourth switching element 7 are alternately turned on and off to drive the fourth switching element 7, the heating coil 8, the resonance capacitor 9, and the second switching element 5. , And a vibration current is passed through the closed loop of the circuit current detection resistor 12. In this way, the set of the first and second switching elements 4 and 5 and the set of the third and fourth switching elements 6 and 7 are alternately switched and driven in a predetermined period T, and at the time of the switching, in series. The second and fourth switching elements 5 and 7 are driven and controlled so that the connected heating coil 8 and the resonance capacitor 9 form a closed loop. Then, the above series of operations is repeated. In the above operation, when the first switching element 4 and the second switching element 5 or the third switching element 6 and the fourth switching element 7 are turned on, the current flowing from the DC power supply circuit to the heating coil 8 Is increased, and then a vibration current that decays in the closed loop flows.

When the second switching element 5 and the fourth switching element 7 are alternately turned on and off in the series of operations at the time of load determination, the control circuit 15 sets the drive frequency to a predetermined frequency, for example. The first predetermined frequency is gradually lowered from 80 kHz to the second predetermined frequency of 60 kHz. At this time, the drive frequencies of the first switching element 4 and the third switching element 6 are lower than the frequency changing from the first predetermined frequency 80 kHz to the second predetermined frequency 60 kHz, for example, 1/3. It is operated at a frequency, and the ratio of the drive time (on time) to a predetermined period T of the frequency is, for example, 10% or less. By doing so, the current flowing through the load circuit of the heating coil 8 and the resonance capacitor 9 can be reduced.

Then, the control circuit 15 is a load circuit when the above-mentioned series of operations at the time of load determination are performed while gradually lowering the drive frequencies of the second switching element 5 and the fourth switching element 7 described above from 80 kHz to 60 kHz. The circuit current flowing through the circuit current is detected based on the voltage drop information obtained by the circuit current detection resistor 12. The circuit current increases as the drive frequency approaches the resonance frequency of the heating coil 8 and the resonance capacitor 9 of the load circuit because the load impedance decreases.

Here, a method of determining whether the cooking pot 11 is a high resistance pot such as iron, a low resistance pot such as copper or aluminum, or a no-load state will be described. Depending on the material of the high-resistance pot or the low-resistance pot, the magnetic coupling state between the cooking pot 11 and the heating coil 8 changes, and the characteristics of the heating coil 8 change accordingly. FIG. 3 shows an example of a change in the characteristics of the heating coil 8 in a pan made of the high resistance material, a pan made of a low resistance material, or a no-load state.

In FIG. 3, when there is no load, that is, when the cooking pot 11 is not placed on the top plate 10, the inductance L of the heating coil 8 is 440 μH and the resistance value is 0.5 Ω. This is an example in which the magnetic coupling state changes when a resistance pot or a low resistance pot is placed, and the characteristics of the heating coil 8 change as shown in FIG.

When there is no load, the inductance value L of the heating coil 8 is the largest and the resistance value R is the smallest. Moreover, since the inductance value L is the largest, the resonance frequency is the lowest. In the case of a low resistance pan, the inductance value L is the smallest and the resistance value R is also small. Moreover, since the inductance value L is the smallest, the resonance frequency is the highest. In the case of a high resistance pan, the inductance value L is larger than that of the low resistance pan and is between the no-load and the low resistance pan, and the resistance value R is the largest. Moreover, since the inductance value L is between the no-load and the low resistance pan, the resonance frequency also exists between the two.

When the control circuit 15 performs a series of operations at the time of load determination described above, when the cooking pot 11 is the low resistance pot, the circuit current flowing through the load circuit becomes maximum at the resonance frequency of 75.9 kHz, and the cooking pot 11 In the case of a high resistance pan, the maximum is at the resonance frequency of 67.3kHz. In the case of no load, the circuit current becomes maximum at 60 kHz, which is the closest to the resonance frequency of 53.7 kHz.

For each impedance Z in the high resistance pot, low resistance pot, or no load state, the resonance frequency is f, the inductance of the heating coil is L, the capacitance of the resonance capacitor is C, and the resistance of the heating coil. If R, then Z = ((2πfL-1 / 2πfC))²+ R²)^1/2It is represented by. Substituting the data of Fig. 3 into the above equation, in the case of the low resistance pot and the high resistance pot, the current becomes maximum at the resonance frequency, and at this time, only the resistance component exists as the impedance, so the impedance in the case of the low resistance pot. Is 1Ω, and the impedance in the case of a high resistance pan is 20Ω. In the case of no load, the impedance at 60 kHz, which is the closest to the resonance frequency f (53.7 kHz), is about 33 Ω.

That is, as can be seen from the respective impedance values, the maximum current value is the smallest when there is no load at 60 kHz, which is close to the resonance frequency, followed by the resistance value at the resonance frequency (67.3 kHz). The high resistance pot is small, and the low resistance pot with the smallest resistance value at the resonance frequency (75.9kHz) has the largest current.

Since the current relationship is as described above when the high resistance pan, the low resistance pan, or no load is applied, the control circuit 15 performs the above-mentioned series of operations at the time of load determination, and the circuit current. The average circuit current of the current flowing through the load circuit detected by the detection resistor 12 in a predetermined period T is calculated, and when the average circuit current is equal to or more than a preset first predetermined value (for example, 2A), the resistance is low. It is judged as a pot, and if it is less than the second predetermined value (for example, 0.5A), it is judged as no load. If it is equal to or less than the first predetermined value and equal to or more than the second predetermined value, it is determined to be a high resistance pan.

When the control circuit 15 determines that there is no load, the drive frequency of the second switching element 5 and the fourth switching element 7 described above is changed again from 80 kHz to 60 kHz, and the user can use the low resistance pan. , Or the above-mentioned load determination operation is repeated until the high resistance pan is placed on the top plate 10. If it is determined that the pan has a low resistance, the switch 13 is kept off and the heating operation of the load by the pan is started. If the pan is determined to be a high resistance pan, the switch 13 is controlled to be turned on. Then, the heating operation of the load by the high resistance pan is started.

FIG. 4 shows a switching operation waveform diagram of the switching elements 4, 5, 6 and 7 during heating. The heating control circuit 15 drives the first switching element 4 and the second switching element 5, and drives the third switching element 6 and the fourth switching element 7 with a delay of half a cycle to supply a DC power supply. A current is passed from the circuit to the heating coil 8 to perform a heating operation.

When it is determined that the high resistance pan is used, the switch 13 is turned on. Therefore, the resonance capacitor 14 is connected in parallel between the winding BC of the heating coil 8 and the resonance capacitor 9, and the heating coil 8 is connected. The current between the windings AB and in the loop of the resonant capacitor 14 is dominant. Further, the resonance frequency in this case drops to 20kHz to 30kHz as described above. The control circuit 15 adjusts the period T to set the drive frequency of the switching elements 4, 5, 6 and 7 to, for example, 30 kHz, and sets the first and second switching elements 4 and 5 and the third and fourth sets. The pair of switching elements 6 and 7 is alternately switched and driven, and the drive time (on time) T1 of the first switching element 4 and the third switching element 6 in the period T, the second switching element 5 and Heating control is performed by controlling the current flowing through the heating coil 8 by changing the ratio of the drive time (on time) T2 of the fourth switching element 7.

When it is determined that the pan has a low resistance, the resistance value of the pan is small and it is difficult to heat the pan. Therefore, the pan is heated by increasing the frequency of the eddy current generated at the bottom of the pan to increase the eddy current loss. Therefore, the control circuit 15 keeps the switch 13 off, sets the drive frequencies of the switching elements 4, 5, 6, and 7 to the resonance frequency of the low resistance pan of 75.9 kHz, and sets the first and second switches in the same manner as described above. The set of switching elements 4 and 5 and the set of third and fourth switching elements 6 and 7 are alternately switched and driven, and the first switching element 4 and the third switching element 6 within the period T are driven. Heating control is performed by controlling the current flowing through the heating coil 8 by changing the ratio of the time (on time) T1 and the drive time (on time) T2 of the second switching element 5 and the fourth switching element 7.

Then, during heating of the high-resistance or low-resistance cooking pot 11, the control circuit 15 calculates the amount of change in the circuit current flowing through the load circuit detected by the circuit current detection resistor 12 for each time. When the amount of change per hour is a predetermined value A (for example, 5A) or more, the heating portion (not shown) on the top plate 10 in which the cooking pot 11 having high resistance or low resistance faces the heating coil 8 depending on the user. ) Is determined to have been removed. That is, when the high-resistance or low-resistance cooking pot 11 is removed from the heating portion on the top plate 10, the input impedance of the heating coil 8 sharply decreases, so that the circuit current sharply increases and the amount of current change increases. When the control circuit 15 determines that the high-resistance or low-resistance cooking pot 11 has been removed from the top plate 10, the control circuit 15 controls the switch 13 to be turned off, and again the second switching element 5 described above. And the drive frequency of the fourth switching element 7 is changed from 80kHz to 60kHz to perform the load determination operation.

When the amount of change in the circuit current per hour is equal to or less than the predetermined value A and greater than or equal to the predetermined value B (for example, 1A), the cooking pot 11 having high resistance or low resistance is heated on the top plate 10 for some reason. It is determined that the part is displaced, and the heating output is lowered to continue heating. Further, when the amount of change in the circuit current per hour is equal to or less than the predetermined value B, heating is continued at the current heating output.

As described above, the drive frequencies of the second switching element 5 and the fourth switching element 7 configured by the full bridge at the time of load determination are set from, for example, the first predetermined frequency 80 kHz to the second predetermined frequency 60 kHZ. Gradually lower the drive frequency of the first switching element 4 and the third switching element 6 to be lower than the drive frequency that changes from the first predetermined frequency 80 kHz to the second predetermined frequency 60 kHz, for example, 1 /. Since it is operated at a frequency of 3, the circuit current of the load circuit is suppressed at the time of judgment, and it is possible to discriminate between a high resistance pan, a low resistance pan, and a no-load state without passing an excessive current. .. Further, since the circuit current can be suppressed only by switching control by the switching element, a high resistance pan, which does not require a voltage control means such as a thyristor or a component having a large current rating as in the conventional case, that is, does not require a high cost. Alternatively, a low resistance pot or an induction heating cooker that enables discrimination of a no-load state can be obtained.

Further, the drive frequency change range of the second switching element 5 and the fourth switching element 7 at the time of determination includes the resonance frequency of the low resistance pan and the resonance frequency of the high resistance pan. Since the impedance of the load circuit including the resonance frequency, that is, the inductance of the heating coil 8 and the capacitance of the resonance capacitor 9, does not resonate in a no-load state, an excessive current is passed through the load circuit. It is possible to provide an induction heating cooker capable of performing no-load determination without any load.

If it is determined to be a high resistance pot or a low resistance pot, the switch 13 can appropriately switch the heating coil constant and resonance capacitor constant so that optimum heating can be performed for each pot. Therefore, it is possible to provide an induction heating cooker capable of efficiently heating each pot.

In addition, when it is determined that there is no load, the determination operation is repeatedly executed. Therefore, after a while after the user turns on the power of the induction cooking cooker, the cooking pot 11 having high resistance or low resistance is used. Even if it is placed, it can be determined whether the cooking pot 11 on which it is placed is a high-resistance pot or a low-resistance pot, and heating can be started. A cooker can be provided.

Further, from the amount of change in the circuit current flowing through the load circuit during heating per hour, the high-resistance or low-resistance cooking pot 11 can be removed from the heating portion on the top plate 10 or deviated from the heating portion. If it is determined that it has been removed from the heated portion, the above-mentioned determination operation is performed again, and if it is determined that the portion is displaced from the heated portion, the heating output is controlled to be lowered. A vessel can be provided.

In the above embodiment, the circuit current detection resistor 12 is used to detect the circuit current flowing through the load circuit from the voltage drop information generated by the current flow, but the circuit current detection resistor 12 is used instead. A current transformer may be used to detect the circuit current.

Further, in the above embodiment, a diode is connected in antiparallel as a switching element and described as an insulated gate type bipolar transistor, but the present invention is not limited to this, and the present invention is not limited to this, for example, a semiconductor of MOS-FET or a semiconductor of MOS-FET. A diode may be externally connected to the semiconductor of the transistor in antiparallel.

<figref num="1">It is a circuit block diagram of the induction heating cooker in Embodiment 1 of this invention.</figref><figref num="2">It is operation waveform diagram which shows the switching operation of the switching element at the time of load determination which concerns on Embodiment 1. FIG.</figref><figref num="3">It is a figure which showed an example of the characteristic change of the pot by the high resistance material or the low resistance material, or the heating coil in a no-load state which concerns on Embodiment 1. FIG.</figref><figref num="4">It is an operation waveform diagram which shows the switching operation of the switching element at the time of heating which concerns on Embodiment 1. FIG.</figref>

Code description

1 Commercial power supply, 2 rectifying circuit, 3 smoothing capacitor, 4 1st switching element, 5 2nd switching element, 6 3rd switching element, 7 4th switching element, 8 heating coil, 9 resonance capacitor, 10 top Plate, 11 cooking pot, 12 circuit current detection resistor, 13 switch, 14 resonant capacitor, 15 control circuit.