JPS5939869B2 - induction heating device - Google Patents

Description

[Detailed description of the invention] The present invention relates to an induction heating device using an inverter.

Conventionally, when electromagnetic induction heating is applied to household cooking appliances, load fluctuations become large due to differences in the materials, shapes, etc. of objects to be heated, such as cooking pots.

Therefore, the oscillation mode of the inverter that drives the induction heating coil may change, or commutation may fail and oscillation may not be possible. Conventionally, to deal with this problem, a self-excited oscillation method has been adopted that detects the current phase of the heating coil, detects the load condition, and changes the oscillation frequency. In a no-load state where the coil is removed from the coil, the inductance LH increases and the resistance molecule H decreases as shown in FIG. In the figure, the horizontal axis represents the distance d between the heating coil and the heated object. In the above-mentioned no-load state or a state close to this, the oscillation frequency of the inverter drops to near the audible range, producing noise. In FIG. 2, 1 is a commercial AC power supply, and 2 is a rectifier circuit, which outputs an unsmoothed pulsating current.

3 is an inverter circuit, which includes a current limiting coil 4, a commutating capacitor 5, a commutating coil 6, and a semiconductor switching element (hereinafter referred to as SC).
(abbreviated as R) and a diode 8, converts the pulsating current into an ultra-audible frequency signal, and supplies it to a load circuit 11 consisting of a filter capacitor 9 and a heating coil 10.

The output of a common collector amplifier circuit consisting of a trigger transistor 12 and a resistor 13 is applied to the gate of the SCRT of the inverter circuit 3, and the inverter 3 oscillates.

Load circuit 11 i.e. filter capacitors 9 and 9
The series circuit of the heating coil 10 and the heating coil 10 forms a series resonant circuit, and the output current of the inverter 3 generates an alternating current magnetic field to inductively heat the cooking pot 14 disposed close to the heating coil 10. Reference numeral 15 denotes a current transformer that detects the current flowing through the heating coil 10, and obtains an AC voltage waveform across the resistor 16 that is approximately in phase with the current.

IT is a phase advance circuit consisting of capacitors 18 and 19 and resistors 20 and 21, and it is a delay until the AC voltage of the current transformer 15 output becomes zero voltage with a negative slope with respect to the time when the SCRT is turned on by a trigger pulse. This is to correct the phase. 22 is a phase detection circuit to which the AC voltage signal transmitted via this phase advance circuit IT is applied, and a voltage comparison circuit can be used, and the input signal is a positive voltage at the power supply voltage level (hereinafter referred to as H level), and a negative voltage. at zero level (
This produces an output of L level (hereinafter referred to as L level).

The falling slope at which the output of the phase detection circuit 22 switches from the H level to the L level is determined by the capacitor 23,
It is differentially shaped by a resistor 24 and a diode 25 and output as a negative polarity pulse. The transistor 27 to which this negative polarity pulse is applied is normally biased to the base by the trigger circuit power supply 28 via the resistors 29 and 30 and is in a saturated state, so the collector level is at L level. Due to the input, it is turned off during that time, and an H level rectangular pulse is obtained from the collector. This rectangular pulse turns on the transistor 12 and triggers the gate of the SCR 7. In this way, the phase of the load circuit 11 current is detected and the SCR is adjusted accordingly.
A self-excited oscillation circuit that obtains a trigger signal No. 7 is constructed, and oscillates at a series resonance frequency determined by the capacitance of the filter capacitor 9 and the inductance of the heating coil 10. Also, 31 is
Zero volt switching (Z
VS) circuit, the output of the rectifier circuit 2 obtains an H level output when the voltage is zero and an L level output when the voltage is positive, and similarly to the phase detection circuit 22, the falling slope of the rectifier circuit 2 output is detected by the capacitor 32.
, resistor 33 and diode 34, the pulse cuts off the transistor 27, and the SCR7
trigger the gate.

3A and 3B respectively show the anode voltage and gate trigger pulse waveform of the SCR 7, and the solid line portion indicates that the distance d between the heating coil and the heated object is 1077! This is a steady state of U, in which it oscillates with a period T1 (μSec). 3
5 is a frequency low limit circuit, which includes a relaxation oscillator composed of a programmable union transistor (PUT) 36, PUT gate bias resistors 37, 38, a charging resistor 39, and a capacitor 40, and a capacitor for each SCR7 gate pulse output. It consists of a transistor 41 and resistors 42 and 43 for discharging the charge of 40.

FIG. 3C shows the charging/discharging voltage waveform of the capacitor 40. When the SCR 7 is triggered at cycle T1 in the steady state, when the charging voltage reaches V1, it is discharged by the transistor 41 and the PUT 36 is turned off. Retained.
However, when the oscillation period becomes longer and reaches period T2 (shown by the broken line in FIG. 3), the charging voltage of the capacitor 40 at time T2 becomes V.
2, the PUT 36 is turned on, discharging the charge in the capacitor 40, and obtaining an L-level rectangular pulse at the PUT 36 gate.

Here, the relaxation oscillation period T2 of the PUT 36 is set within a range in which the oscillation frequency of the inverter 3 does not decrease to cause noise or commutation failure.

The rectangular pulse is differentially shaped through a capacitor 43, a resistor 44 and a diode 45, the transistor 27 is cut off, and the output of the transistor 12 is applied to the gate of the SCR 7.

Therefore, even if the load fluctuates and the SCR 7 becomes unloaded, it will operate at the lowest frequency determined by the oscillation period T2 of the PUT 36. Here, the load circuit 11 current has a delayed phase with respect to the oscillation period T2, and a signal for generating an SCR gate pulse is output from the phase detection circuit 22 at time T3 slightly delayed from time T2. , are blocked by a monostable multivibrator 47. That is, normally one transistor 48 of this monostable multivibrator 47 is off, and the other transistor 49 is on. When PUT36 is not oscillating, it does not operate and the collector of transistor 48, which is its output, is at the H level, but when a pulse is generated from the gate of PUT36, transistor 48 is turned on, and transistor 48 is turned on.
9 is turned off, the collector of transistor 48 changes to L level, and the output of the phase detection circuit is locked to L level. The operating period of this monostable multivibrator 47 can be selected, for example, from time T2 to time T4.

As explained above, the induction force device of the present invention detects the phase of the current or voltage of the heating coil, and sets the minimum oscillation frequency of the inverter, which varies depending on the load, to an ultra-audible frequency. The oscillation frequency drops to the audible range and no longer produces noise.

Further, according to the present invention, the above-mentioned problems at no-load can be solved while maintaining the features of the self-excited oscillation circuit.
It can also be applied to non-magnetic materials such as aluminum pots.

Furthermore, since a heated object detection circuit using a magnetic switch or the like is not required, the number of mechanical components can be reduced and reliability can be further improved.

[Brief explanation of drawings]

Fig. 1 is a characteristic diagram showing changes in inductance LH and resistance RH with respect to the distance d between the heating coil and the heated object, Fig. 2 is a circuit diagram of an embodiment of the present invention, and Fig. 3 explains the operation of the same embodiment. FIG. 1... AC power supply, 2... Rectifier circuit, 3
...Inverter circuit, 7...SCRl
lO...Heating coil, 11...Load circuit, 14...Cooking pot, 17...Phase advance circuit, 22...Phase detection Circuit, 28...
Power supply for trigger circuit, 31... ZVS circuit, 35
...Frequency low limit circuit, 36...PUT
l47... Monostable multivibrator.

Claims (1)

[Claims] 1. An inverter driven by a DC power source, an induction heating coil driven by the output of the inverter, a phase detection circuit that detects the phase of the current of the heating coil, and a phase detection circuit that detects the phase of the current of the heating coil. an oscillation circuit that variably controls the oscillation frequency of an inverter; and a frequency low limit circuit that detects when the output from the oscillation circuit drops below a certain frequency and maintains oscillation at a predetermined frequency. heating device.