JPH09306660A - Induction heating cooker - Google Patents

Abstract

(57) Abstract: An induction heating cooker that does not use a current transformer and does not reduce the detection accuracy of a no-load or a small load even when there is a change in inverter power or power supply voltage. A control unit that converts a DC power supply into a high-frequency current by a switching element and supplies the heating coil to a heating circuit to control an inverter circuit, and a voltage holding circuit that holds a minimum voltage of a smoothing circuit and a heating circuit. A no-load level setting circuit 11 for setting a disabled level and a voltage holding circuit 10
Of the non-load level setting circuit 11, a drive control circuit 13 that controls the drive of the switching element 8, and a voltage detection circuit 14 that detects the input voltage of the inverter circuit. Load level setting circuit 11
Output is changed according to the inverter power setting and the inverter input voltage. When the output of the voltage holding circuit 10 is larger than the output of the no-load level setting circuit 11, the comparison circuit 12 judges that heating is impossible and stops heating. I decided to do it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating cooker which detects no load and small load and stops heating.

[0002]

2. Description of the Related Art An induction heating cooker in which a high frequency current is applied to a heating coil to generate a high frequency magnetic field, an eddy current is applied to a load magnetically coupled to the heating coil, and Joule heat generated by the eddy current causes the load to generate heat It is used in a microwave oven, a tabletop stove, a rice cooker, etc., and is widely used as an alternative to gas and electric heat. However, since the resonance of the heating coil and the resonance capacitor is used to generate the high-frequency current, it is necessary to continue to use it especially when an improper load is used in the electromagnetic cooker etc. As a result, the switching element or the like may be damaged.

For example, if a no load or a small load (knife, spoon, etc.) is placed and heated, an overcurrent may flow into the switching element and damage may occur due to the small resistance of the resonance circuit. As a measure against this, the conventional induction heating cooker uses a current transformer that detects the input current to compare its output with the resonance voltage of the resonance circuit or to compare the heating coil current and stop heating when a small object is detected. (Bokusho Sho 6
JP-A-1-29117 and JP-A-3-105891), a current transformer for detecting an input current or a resistor is used to compare the output with the voltage of the heating coil.
When small objects are detected, heating stops (Japanese Patent Publication No. 59-3287).
No. 8 publication).

[0004]

In the above conventional induction heating cooker, it is possible to detect a small load and reduce damage to switching elements. However, since load detection uses a current transformer or the like, the number of parts is reduced. It is difficult to reduce the size and size of the printed circuit board, and the manufacturing cost is high. In addition, even when the load is appropriate, when the ON time of the switching element is short and the power is small, the effective power consumed by the load is reduced, and the resonance voltage of the resonance circuit and the voltage of the heating coil are increased. It was sometimes falsely detected.

As a method for improving the above-mentioned drawback of not being able to reduce the number of parts, a load detecting method which does not use a current transformer is disclosed in Japanese Patent Application Laid-Open No. 8-31560. There is a problem that the detection accuracy becomes low.

[0006]

In order to solve the above-mentioned problems, the detection accuracy of no load or small load is not lowered even if the power supply voltage fluctuates without using a current transformer.
If the load is appropriate, the induction heating cooker will not be erroneously detected even when the power is small.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to an induction heating cooker comprising an inverter circuit for supplying a high-frequency current to a resonance circuit composed of a heating coil, a capacitor and a load to heat the load, and a control section for controlling the inverter circuit. In, the control unit includes a voltage holding circuit that holds the minimum voltage of the smoothing circuit, a no-load level setting circuit that sets an unheatable level, and a comparison circuit that compares the output of the voltage holding circuit with the output of the no-load level setting circuit. Has a drive control circuit for controlling the drive of the switching element and a voltage detection circuit for detecting the input voltage of the inverter circuit, and the output of the no-load level setting circuit detects the energization power and the voltage detection circuit set by the drive control circuit. The output voltage of the voltage holding circuit is larger than that of the no-load level setting circuit. Was assumed to output a signal for stopping the heating drive control circuit determines to a heating impossible.

With the above-mentioned configuration, the voltage holding circuit detects the minimum voltage of the DC power supply smoothing circuit, and the no-load level setting circuit outputs the heating impossible level, and the comparison circuit outputs the minimum voltage detection value and the heating voltage. Whether the heating is possible or not is determined by comparing the impossible levels. The drive control circuit controls the drive of the switching element by determining whether heating is possible by this comparison circuit to determine the set input power of the induction heating cooker, and the voltage detection circuit detects the input voltage of the inverter circuit and sets the no-load level. The circuit sets the non-heating level according to the set input power of these induction cookers and the input voltage of the inverter circuit.

Therefore, in the case of a small load or no load, the power consumed by the resonance circuit becomes small, the minimum voltage detection value of the DC power supply smoothing circuit rises, the voltage holding circuit output becomes higher than the non-heating level, and the comparison circuit becomes It is determined that heating is not possible, and a signal for stopping heating is output to the drive control circuit. When the load is appropriate, the power consumption of the resonance circuit is large, the minimum voltage detection value of the DC power supply smoothing circuit decreases, and when it is lower than the heating non-allowable level set by the no-load level setting circuit, it is determined that heating is possible and heating can be continued. .

Further, since the no-load level setting circuit changes the heating impossible level in accordance with the energization power set by the drive control circuit and the input voltage detected by the voltage detection circuit, it is possible to prevent the inverter power and the input power supply voltage from varying. Performs highly accurate load detection determination.

[0011]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block circuit diagram of an induction heating cooker to which an embodiment of the present invention is applied, FIG. 2 is an equivalent circuit diagram of an inverter circuit portion, and FIG. 3 shows a change in rectified voltage due to load impedance. It is a waveform diagram showing
FIG. 4 is also an example of a voltage holding circuit diagram, and FIG. 5 is an example of an output waveform of the voltage holding circuit due to fluctuations in the power supply voltage.
FIG. 6 is a setting example of the no-load level by the power supply voltage, and FIG. 7 is a signal waveform diagram of each part.

In FIG. 1, an inverter circuit first rectifies an AC power source 1 into a direct current by a rectifying circuit 2 composed of a rectifying stack, smooths its output by a smoothing circuit composed of capacitors 3 and 4 and a choke coil 5, and heats it. It is input to the parallel resonance circuit configured by the coil 6 and the resonance capacitor 7. A high frequency current is caused to flow in the parallel resonance circuit by a switching element 8 composed of high frequency switching and a damper diode 9 connected in anti-parallel, and an eddy current is generated and heated in a load itself arranged near the heating coil 6.

Next, the control unit includes a voltage holding circuit 10 for substantially holding the lowest voltage point of the voltage obtained by smoothing the DC power supply, a no-load level setting circuit 11 for setting a detection voltage level under no load and small object load, and the voltage. A comparison circuit 12 that compares the outputs of the holding circuit 10 and the no-load level setting circuit 11, a drive control circuit 13 that controls the drive of the switching element 8 by the output of this comparison circuit 12, and a voltage detection circuit that detects the input voltage of the inverter circuit. It is composed of 14. Further, the output of the no-load level setting circuit 11 is changed according to the energized power level of the switching element 8 set by the drive control circuit 13 and the output of the voltage detection circuit 14.

In FIG. 2, the load impedance ZL is composed of a heating coil 6, a resonance capacitor 7 and a heating load, and the state of the heating load and ON / OF of the switching element 8 are set.
It changes depending on the F cycle. When the ON time of the switching element 8 is constant, the active power consumed by the load impedance ZL is large if the heating load is appropriate, and the active power consumed is small when there is no load or a small load. Become. Similarly, when the ON time of the switching element 8 becomes long, the effective power consumed by the load impedance ZL becomes large.

FIG. 3 shows changes in the smoothed voltage due to changes in the load impedance ZL. When the active power consumed by the load impedance ZL is large, the active power consumed in the direction of arrow A in the figure is small. Arrow B in the figure
The detected waveform changes in the direction. Therefore, if the lowest voltage of the smoothed voltage is detected, it is possible to detect the no load or the small object load. Contrary to the above, the shorter the ON time of the switching element 8 is, the smaller the active power consumed by the load impedance ZL becomes. Therefore, in this case, the smoothed voltage waveform does not change in the detected waveform in the arrow B direction in the figure. it is obvious.

Further, the minimum voltage of the smoothed voltage at the limit of the no-load direction that the inverter circuit section can tolerate may be the same as the output of the no-load level setting circuit 11. For example, if L1 and L2 in FIG. 3 are the outputs of the no-load level setting circuit 11, respectively, L2 can allow and heat a load closer to the no-load state than L1.

When such presence / absence of load is detected, the active power consumed by the load impedance ZL should be small. Therefore, when the control unit performs load detection, by setting the set level of the no-load level setting circuit 11 to be smaller than the rated input power of the induction heating cooker,
More reliable detection is possible.

FIG. 4 shows a circuit example of the voltage holding circuit 10. The smoothed DC power supply output is divided by the resistors 101 and 102, and the cathode of the diode 103 is connected to the voltage dividing point.
The resistor 104 has one end connected to the control circuit power supply and the other end connected to the anode of the diode 103 and the capacitor 105. The other end of the capacitor 105 is connected to the reference point of the control circuit power supply. When the resistance value of the resistor 101 is large, the diode 106 is connected to the resistor 101 in antiparallel.

According to this circuit example, the voltage of the voltage holding circuit output V2 in the figure becomes almost the same potential by the diode 103 (and 106) at the time when it becomes the lowest voltage of the DC power supply smoothing voltage, and the DC power supply smoothing voltage becomes When the voltage becomes higher, the charging waveform is such that the capacitor 105 is charged from the control circuit power supply through the resistor 104. By using a combination of a resistor and a capacitor having a time constant longer than the AC power supply cycle, the minimum DC power supply smoothing voltage can be detected, and the value can be almost maintained.

FIG. 5 is an example of a waveform near the zero cross showing the output change of the voltage holding circuit 10 due to the fluctuation of the power supply voltage.
VACH and VACL are power supply voltage waveforms, VOUTH and VO
UTL is the output voltage of the voltage holding circuit 10, and the horizontal axis is time. The minimum voltage at the zero-cross point is VACH, VAC
It is assumed that L has the same potential and that VOUTH and VOUTL have the same potential up to t1.

When the power supply voltage is low, at t1, VA
Since CL <VOUTL, the voltage holding circuit 10 is in a discharge state, VOUTL decreases, and VACL> VOU.
It continues until t4, which is TL, and then becomes a charged state. On the other hand, when the power supply voltage is high, VACH <V at t2
Since it becomes OUTH, the voltage holding circuit 10 is in a discharging state, VOUTH decreases, continues until t4 where VACH> VOUTH, and then becomes a charging state. here,
Since (t4-t1)> (t3-t2), VOUT
L <VOUTH. Therefore, even if the power supply voltage smoothed output at the zero cross point is the same, the output of the voltage holding circuit 10 also rises and falls depending on the level of the power supply voltage itself.

FIG. 6 shows an output example of the no-load level setting circuit 11 which sets the no-load level by the power supply voltage based on the above phenomenon. The horizontal axis shows the input power supply voltage and the vertical axis shows the no-load level. The setting according to the embodiment of the present invention is shown by a solid line, and the setting according to the conventional example is shown by a dotted line. Here, in the conventional setting, the range B
Can be detected normally, but range A may be erroneously detected. With the setting of this example, both ranges A and B can be detected normally.

FIG. 7 shows the signal waveforms of the respective parts, the voltage of the AC power supply 1 in the case where the proper load is gradually moved away from the heating coil 6, and in this example, the load condition has five stages from section 1 to 5, DC power supply smoothing output, voltage holding circuit 10 output V
2, the set voltage of the no-load level setting circuit 11, the comparison circuit 1
2 shows the waveform change of the output of No. 2.

In section 1, since sufficient active power is supplied to the load, the voltage holding circuit output V2 drops to almost 0V. When the load is gradually moved away from the heating coil 6 from the section 2, the active power consumed by the load becomes smaller, so that the voltage holding circuit output V2 becomes higher.

In section 3, since the no-load level setting voltage is higher than the voltage holding circuit output V2 at first, the output result of the comparison circuit 12 is suitable for heating, but this state continues, and the voltage holding circuit output V2 sets the no-load level. When the voltage becomes higher than the voltage, the output of the comparison circuit 12 is partially inverted, and an inappropriate heating condition occurs.

In the section 4, since the voltage holding circuit output V2 is higher than the no-load level setting voltage in many portions, the output result of the comparison circuit 12 is almost unsuitable for heating. In the section 5, the voltage holding circuit output V2 becomes higher than the no-load level setting voltage, so that the output result of the comparison circuit 12 becomes unsuitable for heating.

Therefore, when the output result of the comparison circuit 12 becomes unsuitable for heating, the drive control circuit 13 causes the switching element 8 to operate.
It is sufficient to stop the driving of No. 1 and the load heating. Further, if the energized power set by the drive control circuit 13, that is, the ON time of the switching element 8 becomes short, the effective power consumed by the load becomes small. Therefore, even if the load is appropriate, the minimum voltage of the DC power supply smoothing output is To rise. Therefore, the output voltage of the no-load level setting circuit 11 is changed in accordance with the energized power set by the drive control circuit 13, and for example, if the power level is low, the heating disabled level is set to be high, so that proper load detection is performed. You can Further, by changing the output of the no-load level setting circuit 11 according to the inverter input voltage detected by the voltage detection circuit 14, more accurate detection can be performed.

In this embodiment, the voltage holding circuit 1
0 can be configured by a simple resistance voltage dividing circuit and a CR charging / discharging circuit, etc., and can be greatly simplified as compared with an equivalent circuit that has conventionally performed load detection using a current transformer.

[0029]

According to the present invention, the control section has a voltage holding circuit for holding the minimum voltage of the smoothing circuit, a no-load level setting circuit for setting a heating disabled level, an output of the voltage holding circuit and a no-load level setting. It has a comparison circuit for comparing the outputs of the circuits, a drive control circuit for controlling the drive of the switching element, and a voltage detection circuit for detecting the input voltage of the inverter circuit,
If the output of the voltage holding circuit is larger than the output of the no-load level setting circuit, the comparison circuit judges that heating is not possible and outputs a signal to stop heating to the drive control circuit, so it is not necessary to use a current transformer. Also, no load or small load can be detected to stop heating, the printed circuit board for the control unit can be downsized, and the cost can be reduced.

Further, since the no-load level setting circuit changes the non-heating level according to the energization power set by the drive control circuit and the input voltage detected by the voltage detection circuit, the fluctuation of the inverter power is For example, when the ON time of the switching element is short and the power is small, the effective power consumed by the load is small, and the minimum voltage of the DC power supply smoothing output is increased even with an appropriate load. When the level is lower than the unheatable level set high according to the level, it is determined that the heating is possible, the heating can be continued, and the load detection determination can be performed with high accuracy even when the input power supply voltage fluctuates.

[Brief description of drawings]

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

FIG. 2 is an equivalent circuit diagram of an inverter circuit portion of the same.

FIG. 3 is a waveform diagram showing a change in rectified voltage due to load impedance.

FIG. 4 is also an example of a voltage holding circuit diagram.

FIG. 5 is an example of an output waveform diagram of the voltage holding circuit according to the fluctuation of the power supply voltage.

FIG. 6 is a diagram showing an example of setting a no-load level according to a power supply voltage.

FIG. 7 is a signal waveform diagram of each part of the same.

[Explanation of symbols]

 8 switching element 10 voltage holding circuit 11 no-load level setting circuit 12 comparison circuit 13 drive control circuit 14 voltage detection circuit

Claims (1)

[Claims] 1. A rectifier circuit for converting an AC power supply into a DC power supply, a smoothing circuit for smoothing the DC power supply, and a smoothing circuit for converting the smoothed DC power supply into a high frequency current by a switching element, which comprises a heating coil, a capacitor and a load. In an induction heating cooker including an inverter circuit that supplies a load to a resonance circuit to heat a load, and a control unit that controls the inverter circuit, the control unit includes a voltage holding circuit (10) that holds a minimum voltage of a smoothing circuit, and a heating unit. The no-load level setting circuit (11) for setting the disabled level, the comparison circuit (12) for comparing the output of the voltage holding circuit (10) with the output of the no-load level setting circuit (11), and the switching element (8). A drive control circuit (13) for controlling the drive and a voltage detection circuit (14) for detecting the input voltage of the inverter circuit are provided, and the no-load level setting circuit (1 The output of 1) is the drive control circuit (1
It is assumed that the output voltage of the voltage holding circuit (10) of the comparison circuit (12) is set to the no-load level setting circuit ( The induction heating cooker is characterized in that when it is larger than the output of 11), it is determined that heating is not possible and a signal for stopping heating is output to the drive control circuit (13).