Induction heating apparatus capable of stably operating at least one switching element contained therein

An induction heating apparatus capable of stably operating at least one switching element contained therein is provided with a control circuit 5 which comprises a resonance waveform detector 6 for detecting a high frequency AC waveform supplied from an inverter circuit 3 to a heating coil 4 to produce a detection signal DS1 corresponding to high frequency AC power waveform; a phase comparator 8 for producing an adjusting signal PH corresponding to a phase difference between detection signal DS1 from resonance waveform detector 6 and drive signal D1 from drive circuit 7; and an addition circuit 13 for superimposing the drive signal D1 from drive circuit 7 on detection signal DS1 from resonance waveform detector 6 to supply to phase comparator 8the superimposed signal on a level same as or over operation threshold value VTH for the phase comparator 8.

TECHNICAL FIELD

This invention relates to an induction heating apparatus, in particular, of the type capable of stably operating a switching element provided therein even though resonance current flowing through an inverter circuit is lowered in controlling the oscillation frequency in drive signals to the switching element by detecting the resonance current flowing through the inverter circuit in the induction heating apparatus.

BACKGROUND OF THE INVENTION

A know induction heating apparatus shown inFIG. 6, comprises an AC power source1; a rectifier2for commutating AC power from AC power source1into DC power; an inverter circuit3having two insulated gate bipolar transistors (IGBTs)11and12as switching elements for converting DC power from rectifier2into a high frequency AC power; a heating coil4connected to output terminals of inverter circuit3; and a control circuit5for producing drive signals D1, D2to turn IGBTs11and12in inverter circuit3on and off, and thereby, supplies high frequency AC power to heating coil4.

AC power source1comprises a commercial AC power supply, and rectifier2comprises diodes24in bridge connection for commutating AC power from AC power source1, and a capacitor23for bypassing or smoothing switched current from diodes24. IGBTs11,12comprise first and second IGBTs11and12connected in series between positive and negative terminals of rectifier2, and reflux diodes21and22each connected to first and second IGBTs11and12in the adverse direction. A series circuit of a resonance capacitor25and heating coil4is connected in parallel to second IGBT12. Heating coil4is driven by high frequency AC power to produce high frequency magnetic flux in magnetic coupling with a heated object made of metal such as iron for induction heating of the heated object.

Control circuit5comprises a drive circuit7for producing drive signals D1and D2to IGBTs11and12, a resonance waveform detector6for detecting high frequency AC waveform such as electric current, voltage or power through heating coil4to produce detection signals DS1in response to high frequency AC waveform through heating coil4, a phase comparator8for comparing phases in detection signals DS1from resonance waveform detector6and in drive signals D1from drive circuit7to produce an adjusting signal PH of the level corresponding to the phase difference between detection signals DS1and drive signals D1, an integrating circuit57for converting adjusting signal PH from phase comparator8into an averaged DC voltage, and an impedance regulator40for producing an impedance corresponding to output level from integrating circuit57to vary oscillation frequency in drive signals D1from drive circuit7. Not shown but, drive circuit7comprises an oscillator which may produce oscillation outputs for driving IGBTs11and12. Otherwise, drive circuit7may comprise a driver or drivers for shaping output signals from oscillator into a waveform suitable for driving of IGBTs11and12. Accordingly, drive signals D1from drive circuit7represent output signals from oscillator or drivers. For example, oscillator may comprise a well-known variable frequency (VF) converter, and phase comparator8may comprise a well-known digital phase comparator.

Resonance waveform detector6comprises a detective transformer26for picking out resonance current flowing through heating coil4or resonance capacitor25, a resistor27connected in series to detective transformer26for converting resonance current picked out by detective transformer26into voltage of the level corresponding to resonance current, and a limiter61having a resistor28and diodes29and30. A junction of resistor28and diode29provides an output terminal of resonance waveform detector6connected to a first input terminal IN1of phase comparator8through a capacitor38for removing DC component from output signals of limiter61so that resonance waveform detector6produces detection signals DS1to phase comparator8. In this way, resonance waveform detector6detects resonance current of high frequency AC power supplied from inverter circuit3to heating coil4to produce detection signals DS1corresponding to high frequency AC waveform. Since inverter circuit3furnishes heating coil4with high frequency resonance current, detective transformer26produces detection signals of widely fluctuating level, however, limiter61serves to limit voltage value of detection signal DS1by resonance waveform detector6below a predetermined voltage level. Drive circuit7produces drive signals D1to a second input terminal IN2of phase comparator8through a resistor47.

Integrating circuit57comprises first and second dividing resistors41and42connected between output terminal of phase comparator8and ground, and a capacitor43connected between a junction of first and second dividing resistors41and42and ground. An impedance regulator40comprises a field-effect transistor (FET)44as a variable impedance element, a resistor45connected between source terminal of FET44and ground, and third and fourth dividing resistors37and46connected between an input terminal of drive circuit7and ground. FET44has a control or gate terminal connected to a junction of first and second dividing resistors41and42and capacitor43, and a drain terminal connected to a junction of third and fourth dividing resistors37and46.

Resonance waveform detector6delivers detection signals DS1to a first input terminal IN1of phase comparator8, and drive circuit7provides drive signals D1for a second input terminal IN2of phase comparator8. As shown inFIG. 7, detection signals DS1from resonance waveform detector6are supplied to first terminal IN1of phase comparator8earlier than drive signals D1from drive circuit7, indicating that detection signals DS1from resonance waveform detector6precede in phase drive signals D1from drive circuit7. Under the preceding condition in phase of detection signals DS1, at the moment detection signals DS1of high voltage level from resonance waveform detector6reach first input terminal IN1of phase comparator8, drive signals D1of low voltage level from drive circuit7come to IN2of phase comparator8which therefore produces an adjusting signal PH of high voltage level shown inFIG. 7(c). Then, when phase comparator8receives detection signal DS1of high voltage level from resonance waveform detector6and drive signal D1of high voltage level from drive circuit7, it produces an adjusting signal PH of intermediate voltage level M. Thereafter, phase comparator8maintains to produce adjusting signal PH of intermediate level M, even though either or both of detection signal DS1from resonance waveform detector6and drive signal D1from drive circuit7are shifted to low voltage level.

To the contrary, drive signals D1from drive circuit7reach phase comparator8earlier than detection signals DS1from resonance waveform detector6under the preceding condition in phase of drive signals D1, indicating that drive signals D1from drive circuit7precede in phase detection signals DS1from resonance waveform detector6. Under the preceding condition in phase of drive signals D1, at the moment drive signals of high voltage level from drive circuit7reach second input terminal IN2of phase comparator8, detection signals DS1of low voltage level from resonance waveform detector6come to IN1of phase comparator8which therefore produces an adjusting signal PH of low voltage level L shown inFIG. 7(c). Subsequently, when both of resonance waveform detector6and drive circuit7produce detection signals DS1and drive signals of high voltage level to phase comparator8, it produces an adjusting signal PH of intermediate voltage level M. Next to this, phase comparator8keeps adjusting signal PH of intermediate voltage level M even though either or both of detection signal DS1from resonance waveform detector6and drive signal D1from drive circuit7are shifted to low voltage level.

Specifically, when phase of detection signal DS1from resonance waveform detector6to first input terminal IN1advances ahead of phase of drive signal D1from drive circuit7to second input terminal IN2, phase comparator8produces an adjusting signal PH of high voltage level H in intermediate voltage level M. Otherwise, when phase of detection signal DS1from resonance waveform detector6to first input terminal IN1lags behind phase of drive signal D1from drive circuit7to second input terminal IN2, phase comparator8produces an adjusting signal PH of low voltage level L in intermediate voltage level M. Further, phase comparator8continues to produce an adjusting signal PH of intermediate level M when detection signal DS1from resonance waveform detector6and drive signal D1from drive circuit7are simultaneously on the high or low voltage level.

Adjusting signal PH from phase comparator8causes electric current to flow through first dividing resistor41of integrating circuit57into capacitor43which serves to average adjusting signals PH from phase comparator8. Voltage in capacitor43of varied level by electrically charging or discharging is applied to gate terminal of FET44. When high level voltage in capacitor43by charging is applied to gate terminal of FET44, it is turned on to increase electric current through FET44, thus reducing impedance in impedance regulator40. Adversely, when low level voltage in capacitor43by discharging is applied to gate terminal of FET44, it diminishes electric current therethrough to increase impedance in impedance regulator40.

In operation, two drive signals D1and D2from drive circuit7are alternately applied to each base terminal of a pair of IGBTs11and12to alternately turn IGBTs11and12on and off. Drive signals D1and D2forwarded from drive circuit7do not simultaneously turn IGBTs11and12on, however, do turn one of IGBTs11and12on, while turning the other off. Moreover, a dead time is provided for simultaneously turning IGBTs11and12off after turning one off and before turning the other on. When first IGBT11is turned on while second IGBT12is kept off, electric current from AC power source1through rectifier2, first IGBT11, heating coil4and resonance capacitor25to rectifier2to activate heating coil4and electrically charge resonance capacitor25. Adversely, when second IGBT12is turned on while first IGBT11is kept off, resonance current flows from resonance capacitor25through heating coil4and IGBT12to resonance capacitor25, electrically discharging resonance capacitor25. In this way, IGBTs11and12are alternately turned on and off to perform high frequency induction heating of heating coil4.

During the operation of heating coil4, detective transformer26detects resonance current passing between heating coil4and resonance capacitor25to cause limiter61to produce detection signal DS1to first input terminal IN1of phase comparator8. Concurrently, drive circuit7produces a drive signal D1to second input terminal IN2of phase comparator8through resistor47. As mentioned in connection withFIG. 7, when phase of detection signal DS1moves forward faster than phase of drive signal D1moves late so that detection signal DS1is on high voltage level and drive signal D1is on low voltage level, phase comparator8generates an adjusting signal PH of high voltage level H. To the contrary, when phase of drive signal D1advances faster than phase of detection signal DS1moves late so that drive signal D1is on high voltage level, and detection signal DS1is on low voltage level, phase comparator8generates an adjusting signal PH of low voltage level L. When both of drive signal D1from drive circuit7and detection signal DS1from limiter61have high or low voltage level or when one of drive signal D1and detection signal DS1has high voltage level and the other has low voltage level, phase comparator8generates an adjusting signal PH of intermediate level M.

Integrating circuit57averages outputs from phase comparator8to provide impedance regulator40with the averaged output. Accordingly, with faster phase of detection signal DS1, phase comparator8generates adjusting signal PH of high voltage level H to lower impedance of FET44in impedance regulator40. Then, a large amount of electric current flows through FET44and resistor45to ground to elevate voltage on resistor37so that drive circuit7reduces the oscillation frequency to diminish drive frequency of IGBTs11and12. To the contrary, with faster phase of drive signal D1, phase comparator8generates adjusting signal PH of low voltage level L to increase impedance of FET44in impedance regulator40. Then, a small amount of electric current flows through FET44and resistor45to ground to reduce voltage on resistor37so that drive circuit7increases the oscillation frequency to augment drive frequency of IGBTs11and12.

In this way, upper and lower limits of oscillation frequency in drive circuit7and oscillation pulses issued from drive circuit7are determined dependent on the value of voltage on resistors37,45and46in impedance regulator40. Drive circuit7varies oscillation frequency of drive signals D1and D2in response to level of adjusting signal PH from phase comparator8and produces drive signals D1and D2of varied oscillation frequency to IGBTs11and12.

When AC power source1produces the output around zero voltage, input power to inverter circuit3comes to zero voltage accordingly, and simultaneously, high frequency AC power from inverter circuit3to heating coil4approaches zero voltage. This causes resonance waveform detector6to produce to phase comparator8detection signal DS1of lowered voltage level below operation threshold value VTHfor phase comparator8which therefore may fail to perform normal operation accompanied by abnormal oscillation in drive circuit7.

FIG. 8indicates waveforms of electric current and voltage at selected positions in induction heating apparatus shown inFIG. 6. During the period T of time shown inFIG. 8(a), approximately zero voltage of AC power source1, results in reduction in amplitude of resonance current ILflowing through heating coil4, and as shown inFIG. 8(b), resonance waveform detector6provides first input terminal IN1of phase comparator8with detection signals DS1of reduced voltage. Accordingly, when resonance waveform detector6generates detection signal DS1of lowered voltage below operation threshold value VTHof phase comparator8, it cannot produce adjusting signal PH in response to phase difference between detection signal DS1from resonance waveform detector6and drive signal (oscillation pulse) D1from drive circuit7.

In this view, Japanese Patent Disclosure No. 6-176862 discloses an induction heating cooker which comprises a self-excitation oscillator for producing oscillation pulses as drive signals to a switching element, a comparative voltage detector for producing detection signals in response to electric power supplied from a rectifying circuit to an inverter circuit, a resonance voltage detector for producing detection signals in response to resonance voltage applied from inverter circuit to a heating coil, and a comparator for producing to self-excitation oscillator output signals in response to differential voltage between detection signals from comparative voltage detector and resonance voltage detector. As induction heating cooker of this reference adds voltage from a circuit power source to detection signal from comparative voltage detector through a waveform shaper, comparative voltage detector produces to comparator detection signals which are not lowered below operation threshold value of comparator even when AC power source produces approximately zero voltage to prevent comparator from producing abnormal trigger pulses to self-excitation oscillator. In this case, comparator does not produce also normal trigger pulses, however, self-excitation oscillator oscillates with the natural frequency to prevent abnormal oscillation of self-excitation oscillator which may produce abnormal drive signals to switching element.

Induction heating cooker of the reference, however, has a defect of performing abnormal operation. Specifically, while a control circuit promptly responds to existence or absence of or alteration in a heated object, self-excitation oscillator oscillates with the natural frequency, and when the natural frequency by self-excitation oscillator is rapidly and increasingly deviated from oscillation frequency by self-excitation oscillator driven by trigger pulses of comparator, drive circuit may disadvantageously supply control terminal of switching element with abnormal drive signals.

Therefore, an object of the present invention is to provide an induction heating apparatus capable of always stably turning a switching element of an inverter circuit on and off even during the period at which electric power produces the output of lowered voltage level. Another object of the present invention is to provide an induction heating apparatus capable of preventing rapid change in oscillation frequency of a drive circuit even when a control circuit promptly responds to change in a load.

SUMMARY OF THE INVENTION

The induction heating apparatus according to the present invention comprises a power source (60); an inverter circuit (3) having at least one switching element (11,12) for converting power from power source (60) into a high frequency AC power; a heating coil (4) connected to output terminals of inverter circuit (3); and a control circuit (5) having a drive circuit (7) for producing drive signals (D1, D2) to turn switching element (11,12) on and off and thereby supplying the high frequency AC power to heating coil (4). Control circuit (5) comprises a resonance waveform detector (6) for detecting a high frequency AC waveform supplied from inverter circuit (3) to heating coil (4) to produce a detection signal (DS1) corresponding to high frequency AC power waveform; a phase comparator (8) for producing an adjusting signal (PH) corresponding to a phase difference between detection signal (DS1) from resonance waveform detector (6) and drive signal (D1) from drive circuit (7); and an addition circuit (13) for superimposing the drive signal (D1) from drive circuit (7) on the detection signal (DS1) from resonance waveform detector (6) to supply the superimposed signal to phase comparator (8). Drive circuit (7) determines the oscillation frequency of drive signals (D1, D2) to switching element (11,12) in response to adjusting signal (PH) from phase comparator (8).

When power source (60) produces the output of low voltage level, resonance waveform detector (6) generates to phase comparator (8) a detection signal (DS1) of lowered voltage level below the operation threshold value (VTH) for phase comparator (8). However, addition circuit (13) superimposes the drive signal (D1) from drive circuit (7) on the detection signal (DS1) from resonance waveform detector (6) to prepare the superimposed signal of the level at least a part of which reaches or exceeds the operation threshold value (VTH) for phase comparator (8). Specifically, drive signals (D1, D2) are biased, amplified or adjusted to a certain high voltage level in drive circuit (7) and originally generated with a constant frequency before the modulation, and detection signals (DS1) are generated with generally constant phase difference and varied with generally same frequency relative to drive signals (D1, D2). Accordingly, even though power source (60) generates the output of lowered voltage level, at least a part of the superimposed signal of detection signal (DS1) and drive signal (D1) can be maintained on a level same as or over the operation threshold value (VTH) for phase comparator (8), while keeping normal operation of phase comparator (8). For that reason, phase comparator (8) supplies drive circuit (7) with a correct adjusting signal (PH) corresponding to phase difference between detection signal (DS1) and drive signal (D1) so that drive circuit (7) provides switching element (11,12) with drive signals (D1, D2) with the oscillation frequency corresponding to the level of adjusting signal (PH) from phase comparator (8). Consequently, even though control circuit (5) rapidly responds to change in load, the apparatus can prevent rapid change in oscillation frequency of drive circuit (7) to stably and reliably turn switching element (11,12) in inverter circuit (3) on and off.

The present invention can provide a highly reliable induction heating apparatus that can correctly turn a switching element in inverter circuit on and off.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the induction heating apparatus according to the present invention will be described hereinafter in connection withFIGS. 1 to 5of the drawings. Same reference symbols as those shown inFIGS. 6 to 8are applied to similar portions in these drawings, omitting explanation therefor.

Unlike the prior art induction heating apparatus shown inFIG. 5, the induction heating apparatus of an embodiment shown inFIG. 1, is characterized in that control circuit5comprises an addition circuit13for superimposing drive signal D1from drive circuit7on detection signal DS1from resonance waveform detector6to supply the superimposed signal to phase comparator8, a heat controller33for producing an output signal EC in response to the amount of electric power supplied from power source60, and a phase shifter14for changing timing of inputting drive signal D1to phase comparator8. Power source60comprises an AC power supply1, and a rectifier2connected to AC power supply1for rectifying and converting AC power supplied from AC power supply1into DC power.

Addition circuit13is connected between a junction of capacitor38and resistor23in limiter61and one output terminal of drive circuit7for producing drive signals D1, and it comprises a resistor35and a capacitor36connected in series to each other. Accordingly, furnished to first input terminal IN1of phase comparator8are detection signals DS1from resonance waveform detector6through capacitor38and also drive signals D1from drive circuit7through capacitors36and38for removing DC component from drive signals D1. Therefore, DC component-free drive signals D1from drive circuit7and detection signals DS1from resonance waveform detector6are superimposed or joined into a merged current supplied to capacitor38so that phase comparator8can compare phases with accuracy and high sensitivity.

FIG. 2is a waveform diagram indicating electric current and voltage at selected positions of induction heating apparatus shown inFIG. 1. During the period other than the term T of AC power source1producing the output of approximately zero voltage, resonance waveform detector6keeps detection voltage of signals DS1on or above operation threshold value VTHto first input terminal IN1of phase comparator8as shown inFIG. 2(b). Unlike this, during the period T wherein AC power source1produces outputs of approximately zero voltage, inverter circuit3produces resonance current ILof smaller amplitude to heating coil4so that resonance waveform detector6produces detection signals DS1of lower voltage level to first input terminal IN1of phase comparator8as shown inFIG. 2(b). Under the circumstances, the induction heating apparatus of this embodiment causes addition circuit13to add and superimpose detection signals DS1from resonance waveform detector6on drive signal D1from drive circuit7so that at least a part of the superimposed signal of detection signal DS1and drive signal D1can be maintained on a level same as or over the operation threshold value VTHfor phase comparator8, even though resonance waveform detector6produces detection signal DS1of lower voltage level than operation threshold value VTHof phase comparator8.

In detail, drive circuit7originally generates drive signals D1, D2with a constant frequency, and previously biases, amplifies or adjusts them to a certain high voltage level before the modulation, and detection signals DS1are generated with generally constant phase difference and varied with generally same frequency relative to drive signals D1and D2. Accordingly, addition circuit13combines detection signals DS1from resonance waveform detector6and drive signals D1from drive circuit7to form the merged signals thereof so that at least a part of merged signals can be retained on a level same as or above operation threshold value VTHfor phase comparator8although power source60produces the output of reduced voltage level. Thus, under the lowered output voltage from power source60, phase comparator8can keep the normal operation to prepare adjusting signals PH corresponding to phase difference between detection signals DS1from resonance waveform detector6and drive signals D1from drive circuit7, and forward adjusting signals PH to drive circuit7through integrating circuit57and impedance regulator40so that drive circuit7can correctly produce drive signals D1and D2responsive to level of adjusting signals PH. Therefore, as shown inFIG. 2(d), during the period T, phase comparator8assuredly prepares adjusting signals PH in relation to phase difference between detection signals DS1from resonance waveform detector6to first input terminal IN1and drive signals D1from drive circuit7to second input terminal IN2, and certainly develops adjusting signals PH to drive circuit7without lack or deficiency of signals PH. Accordingly, drive circuit7oscillates with a given frequency determined by adjusting signals PH from phase comparator8to produce drive pulses or signals D1and D2oscillated with changed oscillation frequency from outputs.

In other words, addition circuit13can serve to always stably turn IGBTs11and12in inverter circuit3on and off, preventing drastic fluctuation in oscillation frequency of drive circuit7although control circuit5rapidly responds to change in load. Thus, this embodiment can provide a highly reliable induction heating apparatus that can reliably turn IGBTs11and12in inverter circuit3on and off.

As shown inFIG. 1, heat controller33comprises an input power detector31for producing a detection signal DS2of voltage level corresponding to amount of electric power supplied from power source60and consumed in inverter circuit3such as the amount of electric current value or product of electric current and voltage values, a normal power supply34for producing a variable reference voltage, and a comparator32for comparing detection signal DS2from input power detector31and reference voltage from normal power supply34to produce an output signal EC corresponding to potential difference between detection signal DS2and reference value. Input power detector31may comprise for example a current detecting resistor connected in series to rectifier2and capacitor23, and an output terminal of input power detector31is connected to a non-inverted input terminal of comparator32. Normal power supply34has a function for a user of induction heating apparatus to optionally adjust desired level of voltage, current and power generated from normal power supply34to inverted input terminal of comparator32. Comparator32compares voltage level of detection signal DS2from input power detector31with reference voltage from normal power supply34to produce output voltage EC corresponding to an error voltage between voltage levels of detection signal DS2and reference voltage.

In case of the light load, relatively small amount of electric current flows through inverter circuit3, and current detecting resistor picks out relatively low voltage in input power detector31, and in case of the rated load, relatively large amount of electric current flows through inverter circuit3, and current detecting resistor perceives relatively high voltage in input power detector31. Accordingly, comparator32compares detection signal DS2from input power detector31with reference voltage from normal power supply34to produce output signal EC of high and low voltage levels respectively in case of the light and rated loads.

Phase shifter14comprises a switch or FET51which has one main or drain terminal connected to one output terminal of drive circuit7through a resistor47, a control or gate terminal connected to output terminal of comparator32through a resistor48and the other main or source terminal connected to ground through a resistor54; a resistor52and a capacitor53connected in parallel to each other between resistor48and gate terminal of FET51; and a resistor50and a capacitor55connected in parallel to each other between source terminal of FET51and second input terminal IN2of phase comparator8. Phase shifter14serves to remove noise from output signals EC from heat control circuit33through resistor52and capacitor53, and switch FET51to on or off in view of level of output signals EC from heat control circuit33to delay timing for supplying drive signals D1from drive circuit7to second input terminal IN2of phase comparator8.

FIGS. 3 and 4are graphs indicating electric current and voltage at selected positions of the induction heating apparatus shown inFIG. 1respectively during the rated and light load periods other than the period T.

As comparator32produces output signals EC of low voltage level during the rated load period to turn FET51in phase shifter14off to accelerate charging rate of electric charge to capacitor55. Accordingly, as shown inFIG. 3(f), drive signals D1from drive circuit7is forwarded to second input terminal IN2of phase comparator8with the slightly late phase. On the other hand, as comparator32produces output signals EC of high voltage level during the light load period to turn FET51on so that a large amount of electric current flowing through drain and source terminals of FET51to ground decreases accumulating rate of electric charge to capacitor55. Consequently, as shown inFIG. 4(f), drive signals D1from drive circuit7is forwarded to second input terminals IN2of phase comparator8with the much later phase than that during the rated load period. Thus, during the rated load period, FET51is turned off to deliver drive signals D1from drive circuit7to second input terminal IN2of phase comparator8with the short delay time, whereas during the light load period, FET51is turned on to supply drive signals D1from drive circuit7to second input terminal IN2of phase comparator8with the longer delay time.

In other words, phase comparator8receives drive signals D1from drive circuit7at second input terminal IN2with short delay time to produce an adjusting signal PH of long on-pulse width shown inFIG. 3(g).FIG. 3(g) indicates a time chart in the same condition as that inFIG. 7(c), however,FIG. 3(g) shows an adjusting signal PH of instantaneous or very short low voltage level or off-pulse width since drive signals D1from drive circuit7reach second input terminal IN2with almost no delay phase with phase of detection signals DS1supplied from resonance waveform detector6to first input terminal IN1. Specifically, during the rated load period, delay time is shortened of drive signals D1from drive circuit7to second input terminal IN2relative to detection signals DS1from resonance waveform detector6to first input terminal IN1to bring oscillation frequency of drive circuit7close to resonance frequency of resonance capacitor25and heating coil4. Thus, drive circuit7produces drive signals D1and D2of oscillation frequency close to resonance frequency of resonance capacitor25and heating coil4to turn IGBTs11and12on and off to lower impedance in resonance circuit of resonance capacitor25and heating coil4.

Meanwhile, phase comparator8receives at second input terminal IN2drive signals D1from drive circuit7with longer delay time during the light load period to produce adjusting signals PH of short on-pulse width as shown inFIG. 4(g). Since drive signals D1from drive circuit7reach second input terminal IN2with later phase than that of detection signals DS1from resonance waveform detector6to first input terminal IN1,FIG. 4(g) represents adjusting signals PH of longer low voltage level or off-pulse width similarly toFIG. 7(c). Thus, delay time is extended of drive signals D1from drive circuit7to second input terminal IN2relative to detection signals DS1from resonance waveform detector6to first input terminal IN1during the light load period to settle oscillation frequency of drive circuit7on a level sufficiently higher than resonance frequency of resonance capacitor25and heating coil4. Under the circumstances, drive circuit7produces drive signals D1and D2of oscillation frequency sufficiently higher than resonance frequency of resonance capacitor25and heating coil4to turn IGBTs11and12on and off with the oscillation frequency to increase impedance in resonance circuit of resonance capacitor25and heating coil4.

Like prior art induction heating apparatus shown inFIG. 6, adjusting signals PH from phase comparator8are averaged through integrating circuit57. Therefore, during the rated load period, adjusting signals PH of longer on-pulse width from phase comparator8cause capacitor43to accumulate electric charge to high voltage level to gate terminal of FET44. Therefore, impedance in FET44of impedance regulator40is lowered, and a large current passes through FET44and resistor45to ground to reduce oscillation frequency in drive circuit7. During the light load period, adjusting signals PH of shorter on-pulse width from phase comparator8cause capacitor43to charge to low voltage level to gate terminal of FET44. In this view, impedance in FET44of impedance regulator40is elevated to increase oscillation frequency in drive circuit7. Thus, with the increase and decrease in impedance of impedance regulator40, drive circuit7can respectively raise and lower oscillation frequency to adjust and determine oscillation frequency in response to amount of impedance in FET44of impedance regulator40.

During the rated and light load periods, control circuit5can supply drive signals D1from drive circuit7to phase comparator8with different or varied phase modulated through phase shifter14to control on-pulse width of adjusting signals PH from phase comparator8. Also, heat control circuit33serves to control or regulate electric power to heating coil4in response to amount of electric power supplied from power source60.

Embodiments of the present invention may be altered in various ways without limitation to the foregoing embodiments. In the embodiments, control circuit5superimposes drive signals D1from drive circuit7on detection signals DS1from resonance waveform detector6, otherwise, the other drive signals D2from drive circuit7may be superimposed on detection signals DS1from resonance waveform detector6after inversion of drive signals D2through an inverter58. Drive circuit7may comprise oscillator and driver not shown and may be formed of control IC for switching power source. Also, oscillator in drive circuit may comprise an analog IC or ICs or a digital IC or ICs for microcomputer.

As shown inFIG. 7, phase comparator8produces adjusting signals PH of high voltage level H when detection signals DS1are supplied to first input terminal IN1with earlier phase than that of adjusting signals PH to second input terminal IN2, and it produces adjusting signals of low voltage level L when adjusting signals PH are supplied to second input terminal IN2with earlier phase than that of detection signals DS1to first input terminal IN1. However, in agreement with operation of drive circuit7or if required, high and low voltage levels H and L of adjusting signals PH may be replaced with each other. Phase comparator8produces adjusting signals PH of three different high, intermediate and low voltage levels H, M and L, and drive circuit7serves to control oscillation frequency of drive signals D1and D2. Instead, control circuit5may employ a phase comparator for producing pulse signals simply indicating the phase, and oscillator oscillated in synchronization with pulse signals from phase comparator.

The present invention is applicable to induction heating apparatus for producing high frequency magnetic flux in heating coil in magnetic coupling with an object such as metallic pots and pans to heat the object.