JP2008053070A - Induction heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating device capable of controlling a switching frequency so as not to be set not larger than a resonance frequency of a resonance circuit. <P>SOLUTION: 1 is an A.C. power source; 2 is a D.C. rectifier circuit rectifying A.C. power from the A.C. power source 1 to D.C. input power; 3 is an induction heating coil; 4a and 4b are switching elements; 5 is an input voltage detection means; 6 is an input current detection means; 7 is a resonance capacitor; 8 is an induction heating control circuit; and 9 is a signal input means for operating the induction heating control circuit 8 by arbitrary power. The induction heating control circuit 8 detects that the change of input power with respect to the switching frequency becomes moderate, and switches the switching elements 4a and 4b at a frequency higher than the resonance frequency f0, whereby the operation of the resonance circuit 10 comprising the induction heating coil 3 and the resonance capacitor 7 becomes inductive, and the switching element 4a can be prevented from failing. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an induction heating apparatus that performs heating by electromagnetic induction.

There exists an induction heating apparatus (for example, patent document 1) which detects an input, changes a switching frequency, and performs control.
JP 2004-022256 A

  When the switching frequency is lower than the resonance frequency, the resonance means changes from inductive to capacitive and there is a problem that an excessive short-circuit current flows through the switch means.

  In view of the above problems, an object of the present invention is to provide an induction heating apparatus capable of controlling the switching frequency so as not to be lower than the resonance frequency.

  In the induction heating apparatus according to the first aspect of the present invention, by detecting the resonance frequency relative to the switching frequency and operating the switch means at a frequency higher than the resonance frequency, the operation of the resonance means becomes inductive and the switch means fails. Can be prevented.

  According to the first aspect of the present invention, it is possible to provide an induction heating apparatus capable of controlling the switching frequency so as not to be lower than the resonance frequency.

  Hereinafter, preferred embodiments of the induction heating apparatus according to the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a configuration diagram of the present embodiment, and the electrical configuration of the present embodiment will be described based on this. In the figure, 1 is an AC power supply such as AC100V, for example, 2 is a DC rectifier circuit including a diode bridge, a smoothing capacitor, a choke coil, etc. for rectifying AC power from the AC power supply 1 into DC input power, 3 is a heating means Inductive heating coil, 4a and 4b include a reverse diode, for example, a switching element as a switch means including a transistor, 5 is an input voltage detection means, 6 is an input current detection means, 7 is a resonance capacitor, 8 is input power, for example An induction heating control circuit 9 as a control means including a microcomputer incorporating a detection means and a switching element driving means 9 is a signal input means for operating the induction heating control circuit 8 with arbitrary power.

  Both ends of the AC power source 1 are connected to the input side of the DC rectifier circuit 2, and an inverter circuit for converting the DC power into high-frequency power is configured on the output side of the DC rectifier circuit 2. The inverter circuit includes a series circuit of switching elements 4a and 4b and a resonance circuit 10 as a resonance means formed by connecting an induction heating coil 3 and a resonance capacitor 7 in series. The switching elements 4a and 4b are connected such that the conduction direction of both is directed from the positive electrode on the output side of the DC rectifier circuit 2 toward the negative electrode. On the other hand, the reverse direction diodes connected in reverse between these conduction terminals have their conduction directions from the output side negative electrode of the DC rectifier circuit 2 to the positive direction. The resonance circuit 10 is connected in parallel between conduction terminals of the switching element 4b, for example, between the collector and the emitter.

  On the other hand, the input side of the input voltage detection means 5 for detecting the input voltage supplied to the inverter circuit and the input current detection means 6 for detecting the input current are connected to the AC power source 1 on the input side of the DC rectifier circuit 2. The output side for outputting these detection results is connected to the induction heating control circuit 8. A signal input means 9 is connected to the induction heating control circuit 8.

  The induction heating control circuit 8 variably controls the switching operation, that is, the ON time of the switching elements 4a and 4b based on the detection results obtained by the input voltage detection means 5 and the input current detection means 6. Therefore, in the induction heating control circuit 8, output terminals for outputting pulse drive signals are connected to drive terminals such as bases of the switching elements 4a and 4b, respectively. Then, the switching elements 4a and 4b are alternately switched by the pulse drive signals supplied to the drive terminals of the switching elements 4a and 4b to cause resonance between the induction heating coil 3 and the resonance capacitor 7, thereby inducing the induction. A high frequency current is passed through the heating coil 3.

  Next, the effect | action is demonstrated about the said structure. The power input from the AC power source 1 is rectified to DC by the DC rectifier circuit 2 and supplied to the inverter circuit at the subsequent stage. Based on the information input from the signal input means 9, the target power is instructed to the induction heating control circuit 8. The induction heating control circuit 8 calculates input power based on the voltage / current information input from the input voltage detection means 5 and the input current detection means 6. The on / off frequency (switching frequency) of the switching elements 4a and 4b is controlled from the switching element driving means included in the induction heating control circuit 8 so that the input power becomes the target power, and the inverter current is increased or decreased. As a result, a high-frequency current is supplied to the induction heating coil 3, an alternating magnetic field is generated from the induction heating coil 3, and an object to be heated placed near the induction heating coil 3 is electromagnetically heated.

  FIG. 3 shows the inverter current and the waveform of each part depending on on / off of the switching elements 4a and 4b. From the figure, it can be seen that the switching elements 4a and 4b are alternately turned on at the on / off frequency with a slight dead time (a time during which both are turned off simultaneously). When the switching element 4a is turned on, the DC power from the DC rectifier circuit 2 is supplied to the resonance circuit 10 including the induction heating coil 3 and the resonance capacitor 7 through the switching element 4a, and the inverter current Iinv is forward (in FIG. 1). Flows to the right). Electromagnetic energy is stored in the induction heating coil 3 by the positive inverter current Iinv, and charge energy is stored in the resonant capacitor 7. When the switching element 4a is turned off, that is, when the dead time is reached, the electromagnetic energy of the induction heating coil 3 is released through the reverse diode of the switching element 4b, and the forward inverter current Iinv continues to flow, and the electromagnetic energy of the induction heating coil 3 Moves to the resonant capacitor 7 as charge energy.

  Subsequently, when the switching element 4b is turned on and the induction heating coil 3 finishes releasing electromagnetic energy, the resonant capacitor 7 releases charge energy through the switching element 4b, and the inverter current Iinv is in the negative direction (leftward in FIG. 1). Flowing into. Similarly, electromagnetic energy is stored in the induction heating coil 3 by the inverter current Iinv in the negative direction. When the switching element 4b is turned off, that is, when the dead time is reached, the electromagnetic energy of the induction heating coil 3 is released through the reverse diode of the switching element 4a and the smoothing capacitor of the DC rectifier circuit 2, and the negative inverter current Iinv continues to flow. The electromagnetic energy of the induction heating coil 3 moves to the smoothing capacitor as charge energy.

  By repeating the switching operation as described above, the inverter current Iinv flows through the induction heating coil 3 and thus the resonance circuit 10 as an alternating current having a frequency substantially equal to the switching frequency.

  Here, frequency control in the induction heating control circuit 8 will be described.

  FIG. 2 shows a change in impedance with respect to the frequency of the resonance circuit 10. The impedance is minimized at the resonance frequency f0, inductive when higher than the resonance frequency f0, and capacitive when lower. Accordingly, when the inverter current Iinv applied to the resonance circuit 10 becomes the resonance frequency f0 or less, the inverter becomes capacitive, and there is a fear that the short-circuit current flows through the switching element 4a, so that the switching frequency needs to be set to the resonance frequency f0 or more. is there. In addition to this, in order to reduce the inrush current at the start of the operation, the induction heating operation starts at a high frequency away from the resonance frequency f0. Further, since the impedance of the resonance circuit 10 increases as the distance from the resonance frequency f0 increases, the input power at the start starts from a low power and the switching frequency is changed so as to be the target input power.

  From the relationship between the impedance and the frequency in FIG. 2, the slope of the impedance change with respect to the frequency change becomes gentle near the resonance frequency f0, and the slope becomes zero at the resonance frequency f0. When the input power is low relative to the target power, the switching frequency is lowered. At this time, it is detected that the change width of the input power is moderate relative to the change width of the switching frequency, and the resonance frequency is detected. The operation of the built-in switching element driving means is limited so that the frequency is higher than f0.

  Specifically, for example, the switching frequency as the inverter operating frequency is lowered by about 1 kHz every 20 ms, the input voltage / current is measured every 20 ms, the difference from the measured value is calculated every time, and the power value is calculated. What is necessary is just to comprise so that it may be judged that a change width is about 50 W or less and it is loose, and frequency reduction is stopped. When monitoring only the input current, measure the input current at regular time intervals, calculate the difference from the previous current measurement value, and if the calculated value is less than the predetermined value and the input current change is determined to be gradual, Good.

  As described above, it is detected that the change in the input power with respect to the switching frequency has become gradual, and the operation of the resonance circuit 10 becomes inductive by operating at a frequency higher than the resonance frequency f0. It is possible to prevent failure. Even when the input power is insufficient with respect to the target power, the control can be performed with the maximum power that can be output with respect to the target power in a safe frequency region higher than the resonance frequency f0.

  As described above, the induction heating apparatus of this embodiment includes the resonance circuit 10 as the resonance means, the switching elements 4a and 4b as the switch means for intermittently supplying the input power to the resonance circuit 10 by the switching operation, and the resonance circuit. The input voltage detection means 5 and the input current detection means 6 as detection means for detecting the input to the input terminal 10 and the switching frequency of the switching elements 4a and 4b based on the detection results of the input voltage detection means 5 and the input current detection means 6 An induction heating apparatus having an induction heating control circuit 8 as a control means for changing the induction heating control circuit 8 is configured such that when the switching frequency is reduced, the change width of the detection result is predetermined. Detecting the switching frequency when the value is lower than the value as the resonance frequency f0, the switching frequency should not be lower than the resonance frequency f0 Characterized in that it is intended to control.

  In this way, since the change in input power with respect to the switching frequency has become gradual, the resonance frequency f0 of the resonance circuit 10 is detected, and the switching elements 4a and 4b are operated at a frequency higher than the resonance frequency f0. The operation of the circuit 10 becomes inductive, and the switching elements 4a and 4b can be prevented from failing. Further, even when the input power is insufficient with respect to the target power, the control can be performed with the maximum power that can be output with respect to the target power in a safe frequency region higher than the resonance frequency f0. As described above, it is possible to provide an induction heating apparatus capable of controlling the switching frequency so as not to be lower than the resonance frequency f0 of the resonance circuit 10.

  In addition, this invention is not limited to the said Example, It can change in the range which does not deviate from the meaning of this invention. The inverter circuit or the like is not limited to the circuit configuration described above, and can be applied to any type of circuit.

It is a circuit block diagram which shows the electric constitution of the induction heating apparatus in the Example of this invention. It is the characteristic figure which showed the impedance change with respect to the frequency of a resonant circuit. It is the wave form diagram which showed each waveform of the drive signal of a switching element, and the inverter current which flows through a resonance circuit.

Explanation of symbols

4a, 4b Switching element (switch means)
5 Input voltage detection means 6 Input current detection means 8 Induction heating control circuit (control means)
10 Resonant circuit (resonant means)

Claims (1)

An induction heating apparatus comprising: a resonance unit; a switch unit that supplies power; a detection unit that detects an input; and a control unit that controls the switch unit based on a detection result of the detection unit. The means detects the switching frequency when the detection result is equal to or lower than a predetermined value when the switching frequency is changed as a resonance frequency, and controls the switching frequency not to be lower than the resonance frequency. An induction heating device characterized by that.

Images