JP3304200B2 - Induction heating cooker - Google Patents

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 for controlling power according to input power.

[0002]

2. Description of the Related Art A conventional induction heating cooker includes a rectifier circuit for converting an AC power supply to a DC power supply, a smoothing circuit for smoothing the DC power supply, a parallel resonance circuit including a heating coil, a capacitor and a load, and a heating coil. A switching element for supplying a high-frequency current; and a flywheel diode. The switching element generates a resonance current in a parallel resonance circuit. The high-frequency magnetic field generated in the heating coil causes an eddy current generated in a load placed on the heating coil. The power control method uses a voltage resonance type inverter that heats a load by a loss, and changes the current flowing through the heating coil according to the value of the time during which the switching element is turned on (ON time). This is so-called frequency control in which high-frequency current is controlled to vary power.

In this method, as shown in FIG. 4, in the case of a load (pan) having an appropriate impedance, the flyback voltage (VCE) due to resonance is completely reduced to 0 V (negative voltage VF), and the next energization starts. Then, a negative flywheel current flows through the flywheel diode as a load current (IC).

However, if the ON time is shortened to reduce the output, an improper load (pan) that increases the equivalent resistance of the heating coil and lowers the equivalent inductance (a part of a stainless steel pan) As shown in FIG. 5, since the flyback voltage (VCE) based on the resonance voltage waveform does not fall below 0 V, the flyback current is turned on at the next timing, so that the flywheel current decreases and the collector loss (P C) becomes an ON loss. ) Increased remarkably, sometimes leading to deterioration or destruction of the element.

[0005]

In the conventional induction heating cooker, as described above, when the power is reduced, for example, when the material of the pot is changed, when the switching element operates, the resonance voltage drops to 0 V or less. Before the switching element is turned on, the ON loss is significantly increased, and there is a disadvantage that the element is deteriorated or destroyed.

In order to solve these problems, frequency control is performed when the input power set value is equal to or greater than a certain value, and when the input power set value is equal to or less than a certain value, ON-OFF for turning ON / OFF the inverter energization in accordance with the input power set value. A control method has been proposed (for example, see Japanese Patent Application Laid-Open No. 63-40291). However, in this method, a beating sound at the time of ON-OFF is apt to be generated, and particularly in the case of a stainless steel pot, the phenomenon becomes remarkable. Further, since the load temperature is often waved by the ON-OFF heating, it is required to reduce the range of the ON-OFF control as much as possible. Therefore, it is necessary to set the ON-OFF control range in consideration of the worst case, assuming all kinds of loads, so that not only a huge amount of time is spent for the measurement work, but also for general well-used enamel pots and the like. However, since the control condition is adjusted to the worst load even in the range where the frequency control can be sufficiently performed, the condition is deteriorated.

Further, as a method of correcting the impedance to an appropriate value when the load changes, a method of switching the value of the resonance capacitor has been considered (for example, Japanese Patent Application Laid-Open No. Hei 2 (1994)).
162681). However, this method requires a relay for switching the resonance circuit, a capacitor, and the like, and has the drawback that the number of parts increases, the number of assembling steps increases, and the cost increases.

FIG. 6 shows a comparison between the ON time of the switching element and the power in different pots. As can be seen from FIG. 6, when the ON time of the switching element is t1, the electric power of the enamel pot is 900W, whereas the electric power of the stainless steel pan 1 is 1200W and that of the stainless steel pan 2 is 100W.
Even if the ON time of the switching element is the same as 0 W, the power differs depending on the material of the pot. The power correction is performed by varying the ON time of the switching element so that the power becomes a predetermined set power.

For example, when the set power is 1200 W, the time is t3 in the enamel pan, and t1 and t1 in the stainless steel pans 1 and 2, respectively.
Power control is performed so as to reach t2. Here, considering the enamel pan as a reference, the amount by which the ON time of the switching element is varied (the amount of power correction) is (t3-t) in the stainless steel pan 1.
1), (t3-t2) for the stainless steel pan 2 A pot with a larger power correction amount (stainless steel pan 2 than stainless pot, stainless steel pan 1 than stainless steel pot 2) is in the state of FIG.

An object of the present invention is to provide an induction heating cooker capable of coping with a difference in load material or the like by switching from frequency control to ON-OFF control when a power correction amount detects a certain value or more. I do.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and comprises a rectifier circuit for converting an AC power supply to a DC power supply, a smoothing circuit for smoothing the DC power supply, a heating coil, A resonance circuit comprising a capacitor and a load, a switching element for supplying a high-frequency current to the heating coil, and a control element for controlling the whole; the switching element generates a resonance current in the resonance circuit, and a high-frequency wave generated in the heating coil. In an induction heating cooker for heating a load by eddy current loss generated in a load placed on a heating coil by a magnetic field, an input voltage detection circuit for detecting an input voltage to a control element, and an input current detection circuit for detecting an input current The control element calculates and detects the input power from the detected input voltage and input current during the heating operation, and detects and sets the input power. The power correction is performed by changing the variable of the ON time of the switching element so as to eliminate the difference with the power. When the power correction value is equal to or more than a predetermined value and the set power level is a predetermined low level, ON- OF
The power is controlled by the F control, and the power is controlled by the frequency control in other normal times, and a plurality of variables for the ON time of the switching element are set for each power setting level.

[0012]

With the above arrangement, the input voltage detection circuit detects the input voltage, and the input current detection circuit detects the input current. The control element calculates the input power from the detected input voltage and input current, and changes the variable of the ON time of the switching element so as to eliminate the difference between the input power and the set power during the heating operation, and performs power correction. When the power correction value is equal to or more than a predetermined value and the set power level is a predetermined low level, for example, when the power is reduced by an improper load, the power is controlled by the ON-OFF control to reduce the loss of the switching element. In addition, the power is controlled by smooth frequency control when a load having a normal appropriate impedance or when the set power level is a predetermined high level.

[0013]

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 according to an embodiment of the present invention, FIG. 2 is a diagram showing a relationship between a power level, a set power, and a power control method of the induction heating cooker, and FIG. It is a chart of the power correction processing of the control element of the induction heating cooker.

In FIG. 1, reference numeral 1 denotes a heating coil, and the heating coil 1 and the resonance capacitor 2 constitute a parallel resonance circuit. Reference numeral 3 denotes a pot as a load, which is arranged near the heating coil 1. A switching element 4 supplies a high-frequency current to the parallel resonance circuit by a flywheel diode 5. Reference numeral 6 denotes a rectifying element for rectifying the AC power. The power rectified by the rectifying element 6 is smoothed by a smoothing circuit composed of a choke coil 7 and a smoothing capacitor 8, and is supplied to the heating coil 1 and the switching element 4. Supplied.

VCE and I C are switching elements 4 respectively.
3 shows the flyback voltage and load current.

Reference numeral 9 denotes a control element (microcomputer) for controlling the entire induction heating cooker.
-1, 10-2 is an input voltage detection circuit that detects an input voltage via a current transformer 11-1, and 12 is an ON time of the switching element 4. Is a switching element drive circuit that turns on and off the switching element 4 at high speed by a signal of the duty ratio setting circuit 12, and 14 supplies DC stabilized power to the entire control circuit. Power supply circuit.

The control element 9 calculates the input power of the inverter from the input voltage obtained by the input voltage detection circuit 10 and the input current obtained by the input current detection circuit 11, and eliminates the difference between the set power and the input power. Thus, the output to the duty ratio setting circuit 12 is varied. The switching element drive circuit 13 turns on and off the switching element 4 at a high speed according to the signal of the duty ratio setting circuit 12, thereby generating a resonance current in the parallel resonance circuit and generating a high-frequency magnetic field on the surface of the pan 3, and as a result, The heating is performed by the eddy current generated in the pan 3.

The power levels of the induction heating cooker are six levels from 0 to 5, and the variables for setting the set power of each level and the ON time of the switching element 4 for obtaining the set power in the porcelain pot are shown in FIG. As shown in the figure, the power control method is usually (power correction amount less than A) frequency control for power levels 1 to 5. When the power correction amount is A or more, power levels 3, 4, and 5 are frequency control, and power levels 1 and 2
At the power level 3 (set power 600 W), set power 100 W or 300 W is obtained by ON-OFF control of energization for 1 second or 3 seconds, respectively, in 6 seconds.

The details of the power correction processing will be described below with reference to FIG.

In step 101, the currently input power is calculated from the product of the input voltage obtained by the input voltage detection circuit 10 and the input current obtained by the input current detection circuit 11. At 102, if the power level is 0, the power supply is stopped at 103,
If the power level is other than 0, the switching element 4
(Hereinafter referred to as “OUT”, which sets the ON time of
The power is increased when the UT is large, and the power is decreased when OUT is small. Each power level is set in accordance with FIG. 2 (for example, if the power level is 3, OUT = c).

At 105, the current control method is determined.
If the control method is 1, the set power set for each power level is compared at 106 with the power calculated at 101, and if the calculated power is equal to or less than the set power, OUT is increased at 107 and the power is increased.
If the calculated power is greater than the set power at 106, OUT is reduced at 108 to reduce the power. Then 109-117
Then, the difference between OUT set at 105 and OUT lowered at 108 for each power level is taken, and the difference is used as a correction amount. 11
At 8, it is determined how much the correction amount is, and when it is A or more, it is determined that the load is an inappropriate load like a stainless steel pan, and the control method is changed to 2 at 119.

On the other hand, when the control method is 2 at 105, the power level is determined at 120, and when the control level is 1 or 2, the power supply is turned on at 121 to 123 for 1 second for 6 seconds or 3 seconds for 6 seconds. OFF control is performed. Further, when the power is on at 124, power correction is performed at 125 to 128 in the same manner as when the control method is 1. At 120, the power level is 3,
In the cases of 4 and 5, the power correction is performed in the same manner as in the case where the control method is 1 in 129 to 136. When the control method is 2 and the process of reducing the power is performed as a result of the power correction, the control method is returned to 1 in 137.

In such a flow, the control method is set to 1 (frequency control for all power levels 1 to 5) or 2 according to the power correction amount.
(Power levels 1 and 2 are ON-OFF control, power level 3
5 to frequency control). For example, as shown in FIG. 6 in the above-mentioned conventional example, the power correction is performed by changing the ON time of the switching element 4 so that the power becomes a predetermined set power. Since the amount by which the ON time of the switching element 4 is varied (the amount of power correction) increases, when the power is reduced in the case of switching to a stainless steel pan, the frequency control is changed to O.
The control is switched to the N-OFF control, so that an increase in the loss of the switching element 4 can be prevented.

[0024]

According to the present invention, an input voltage detection circuit for detecting an input voltage and an input current detection circuit for detecting an input current are connected to a control element, and the control element detects the input voltage during a heating operation. The input power is calculated and detected from the input current, the power is corrected by changing the variable of the ON time of the switching element so as to eliminate the difference between the input power and the set power, and the power correction value is determined. ON when the set power level is a predetermined low level with a value higher than
-Power control is performed by OFF control, and power control is performed by frequency control in other normal times. Since a plurality of variables of the ON time of the switching element are set for each power setting level, the worst load (pan) of the worst load (pan) is set in advance. By grasping the characteristics, without setting the minimum power, the ON-OFF control does not cause abnormal loss of the switching element due to load fluctuation such as load (pan) with inappropriate impedance or low power level. The reliability is significantly improved.
In addition, it becomes possible to appropriately change the power control range for the difference in load (pan), and the frequency control range is expanded,
The range in which smoother frequency control characteristics can be obtained is widened.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a relationship between a power level of an induction heating cooker, a set power, and a power control method.

FIG. 3 is a chart of power correction processing of a control element of the induction heating cooker.

FIG. 4 is a voltage and current waveform diagram at a proper load in a general induction heating cooker.

FIG. 5 is a diagram showing voltage and current waveforms at an improper load.

FIG. 6 is a diagram showing the relation between the switching element ON time and the power for each type of pot.

[Explanation of symbols]

 4 switching element 9 control element (microcomputer) 10 input voltage detection circuit 11 input current detection circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H05B 6/12

Claims (1)

(57) [Claims] A rectifier circuit for converting an AC power supply to a DC power supply, a smoothing circuit for smoothing the DC power supply, a heating coil,
A resonance circuit comprising a capacitor and a load, a switching element for supplying a high-frequency current to the heating coil, and a control element for controlling the whole; the switching element generates a resonance current in the resonance circuit, and a high-frequency wave generated in the heating coil. In an induction heating cooker for heating a load by an eddy current loss generated in a load placed on a heating coil by a magnetic field, an input voltage detection circuit (10) for detecting an input voltage to the control element (9); The control element (9) calculates and detects the input power from the detected input voltage and input current during the heating operation and detects the difference between the input power and the set power during the heating operation. The power correction is performed by changing the variable of the time during which the switching element (4) is turned on so that the power correction value disappears. (A) When the set power level is a predetermined low level, power control is performed by ON-OFF control, and power control is normally performed by frequency control. The variable of the time during which the switching element (4) is turned on is power. An induction heating cooker wherein a plurality of settings are made for each setting level.