Induction heating cooker with phase difference control

An induction heating cooker comprises an inverter circuit. The inverter circuit has a heating coil and a resonance capacitor that resonates with the heating coil to generate high-frequency electric power with which an object to be heated is inductively heated. The cooker further includes a phase comparator for comparing the phase of a first signal that correlates to the phase of an output voltage of the inverter circuit with the phase of a second signal that correlates with the phase of a current flowing to the resonance capacitor; a phase difference setter for setting the phase difference of the first and second signals; and a frequency controller for controlling an oscillation frequency of the inverter circuit according to a signal from the phase difference comparator to establish the phase difference set by the phase difference setter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an induction heating cooker that employs 
an inverter circuit over inductively heating an object, and particularly 
to an induction heating cooker of large input power that causes no noise 
from its power source, achieves excellent efficiency and is capable of 
continuously changing its input power for a wide range. 
2. Description of the Prior Art 
An induction heating cooker produces no flame, and therefore, is safe and 
achieves excellent heating efficiency. 
FIG. 1 is a block circuit diagram showing a conventional induction heating 
cooker employing an inverter circuit 104 of the quasi-E class. An input 
setting circuit 118 sets an input value according to which a PWM 
oscillator 116 provides a pulse signal. According to the pulse signal, a 
driving circuit 114 sets an ON time TON for a transistor 112. The 
transistor 112 is turned on and off in response to pulse signals from the 
driving circuit 114 to put a heating coil 106 and a resonant capacitor 108 
in a series resonant state. Accordingly, the heating coil 106 generates 
magnetic flux, which causes an electromagnetic induction action to 
generate an eddy current in an object (not shown) such as a pan. As a 
result, the object is heated. An advantage of the inverter circuit 104 of 
quasi-E class is that high-frequency electric power can be generated with 
a single switching element (the transistor 112). 
If the input power is increased, a resonance voltage VCE is increased as 
shown in FIG. 2a. The high resonance voltage is critical to a withstand 
voltage of the switching element (transistor 112). To reduce the input 
power as shown in FIG. 2b, the ON time TON of the transistor 112 shall be 
shortened. In this case, the transistor 112 is usually turned on before 
the resonance voltage VCE reaches zero volts. If this happens, an 
excessive short-circuit current IS flows to the transistor 112 to destroy 
the transistor. 
Supposing the cooker is 200 V in power source voltage and 2 KW in maximum 
input power, the resonance voltage VCE will reach 1100 V for the maximum 
input power. When the ON time TON of the switching element is reduced to 
bring the input power to 1 KW, the magnitude of the short-circuit current 
will be 80 A. 
Supposing the cooker is 3 KW in maximum input power, the resonance voltage 
VCE will be 1800 V for the maximum input power. To bring the input power 
below 2 KW, the short-circuit current IS must be very large. To avoid 
this, it is necessary to repeatedly turn on and off the inverter circuit. 
This may, however, change the temperature of the cooker and deteriorate 
cooking efficiency. 
If the maximum input power is 3.5 KW to shorten the cooking time, the 
resonance voltage VCE may reach 2000 V or over. There is no such switching 
element that can withstand the resonance voltage of 2000 V and achieve a 
high-speed switching operation. The inverter circuit of quasi-E class is, 
therefore, not applicable for a large power induction heating cooker. 
For such a large power induction heating cooker, a bridge inverter circuit 
has been proposed. In this type of cooker, a voltage larger than a power 
source voltage is applied to its switching element so that input power of 
the cooker may easily be increased. In addition, the cooker can heat an 
object made of non-magnetic material such as aluminum and stainless steel. 
To control the input power of the cooker, the bridge inverter circuit is 
turned on and off. Alternatively, as shown in FIG. 3, an input controlling 
circuit 133 may provide a control signal based on which thyristors 107a 
and 107b are controlled, thereby continuously controlling the input power. 
This technique is called phase control. 
In FIG. 3, a half bridge inverter circuit 125 receives signals from an 
inverter driving circuit 113 to alternately turn transistors 115 and 117 
on and off, thereby applying high-frequency electric power to a heating 
coil 119. 
A conventional induction heating cooker employing the bridge inverter 
circuit that is turned on and off to control input power has a problem of 
generating a repulsive force in heating an aluminum pan. As shown in FIG. 
5, heating the aluminum pan with a cooker of 2000 W in input power 
generates a repulsive force of 920 g. If the aluminum pan weighs, for 
example, about 1 Kg, the pan may move over a top plate of the cooker. This 
is dangerous. If the bridge inverter circuit is turned on and off to 
decrease the input power from 2000 W, a replusive force of 920 g is 
intermittently generate whenever the inverter circuit is turned on, to 
gradually move the aluminum pan and generate unpleasant noise. 
In FIG. 3, the input power is continuously controlled, and an input current 
IIN from an AC power source 101 is intermittently supplied, as shown in 
FIGS. 4a and 4b. Due to this, the power source emits noise. 
To deal with this, a large capacity reactor 103 is inserted between the AC 
power source 101 and the bridge circuit 105. The reactor or a thyristor, 
however, has a loss that lowers efficiency. 
A thyristor, if employed, requires a radiating plate, which raises another 
problem of increasing the size of the cooker. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an induction heating 
cooker that allows large input power, causes no noise from its electric 
power source, has excellent efficiency and is capable of continuously 
changing its input power over a wide range. 
According to a first aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; and frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means. 
According to a second aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means; input setting means for setting a heating force 
for heating the object; and first phase-difference changing means for 
changing the set phase difference in response to a value set by the input 
setting means. 
According to a third aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means; material information detecting means for 
detecting information relating to material of the object; and second 
phase-difference changing means for changing the set phase difference 
according to the material information detected by the material information 
detecting means. 
According to a fourth aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means; and phase difference restricting means for 
restricting the set phase difference so that the heating coil and 
resonance capacitor may form an inductive resonance circuit. 
According to a fifth aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means; and frequency restricting means for restricting 
the frequency controlled by the frequency controlling means not to be 
decreased lower than a predetermined value. 
According to a sixth aspect of the present invention, there is provided an 
induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means; and current restricting means for restricting 
the current flowing to the resonance capacitor not to be decreased lower 
than a predetermined value. 
According to a seventh aspect of the present invention, there is provided 
an induction heating cooker comprising an inverter circuit involving a 
heating coil and a resonance capacitor that resonates with the heating 
coil to generate high-frequency electric power for inductively heating an 
object to be heated; phase comparing means for comparing the phase of a 
first signal that correlates to the phase of an output voltage of the 
inverter circuit with the phase of a second signal that correlates to the 
phase of a current flowing to the resonance capacitor; phase difference 
setting means for setting a phase difference of the first and second 
signals; and frequency controlling means for controlling, according to a 
signal from the phase comparing means, an oscillation frequency of the 
inverter circuit to establish the phase difference set by the phase 
difference setting means, the frequency controlling means gradually 
lowering the oscillation frequency of the inverter circuit from high to 
low at the start of operation of the frequency controlling means. 
The induction heating cooker according to the first aspect of the present 
invention has the phase difference setting means for setting the phase 
difference between the phase of the first signal correlating to the phase 
of the output voltage of the inverter circuit and the second signal 
correlating to the phase of the current flowing to the resonance 
capacitor. The phases of the first and second signals are compared with 
each other, and the oscillation frequency of the inverter circuit is 
controlled to establish the set phase difference. With this arrangement, 
input power of the cooker can continuously be changed in a wide range, and 
noise from a power source of the cooker is eliminated. 
The induction heating cooker according to the second aspect of the present 
invention has the input setting means in addition to the features of the 
first aspect. The phase difference set by the phase difference setting 
means is changed in response to an input set by the input setting means. 
With this arrangement, the same input power may be secured by the same 
setting for heated objects of different materials and different shapes. 
The induction heating cooker according to the third aspect of the present 
invention has the material information detecting means in addition to the 
features of the first aspect. The detecting means detects information 
relating to material of an object to be heated, and the phase difference 
is changed according to the detected information. With this arrangement, 
input power can be stabilized irrespective of the material of the object. 
The induction heating cooker according to the fourth aspect of the present 
invention has all the features of the cooker of the first aspect, and in 
addition, restricts the phase difference of the first and second signals 
to make the heating coil and resonance capacitor from an inductive 
resonance circuit. With this arrangement, an oscillation frequency of the 
inverter is set larger than a resonance frequency of the resonance 
circuit, thereby preventing a switching element from sustaining an 
excessive short-circuit current. 
The induction heating cooker according to the fifth aspect of the present 
invention has all the features of the cooker of the first aspect, and in 
addition, restricts a frequency controlled by the frequency controlling 
means not to be lowered below a predetermined value. With this 
arrangement, the inverter circuit can be securely driven even when the 
oscillating operation of the frequency controlling means is unstable. 
The induction heating cooker according to the sixth aspect of the present 
invention has all the features of the cooker of the first aspect, and in 
addition, restricts a current flowing to the resonance capacitor not to be 
lowered below a predetermined value. With this arrangement, even an object 
having low impedance can be heated with the inverter circuit being 
securely driven and with no excessive current that may destroy the 
switching element. 
The induction heating cooker according to the sixth aspect of the present 
invention has all the features of the cooker of the first aspect, and in 
addition, gradually reduces the oscillation frequency of the inverter 
circuit from high to low at the start of operation of the frequency 
controlling means. With this arrangement, the inverter circuit can be 
securely driven even at the start of the cooker operation wherein the 
circuit operation is unstable. 
These and other objects, features and advantages of the present invention 
will be more apparent from the following detailed description of preferred 
embodiments in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
A basic arrangement of an induction heating cooker according to the present 
invention will be explained with reference to FIG. 7. 
An AC power source 1 is connected to a DC power source circuit 3. The DC 
power source circuit 3 comprises a bridge circuit 5 for rectifying DC 
power, and a capacitor 7 for smoothing a pulsating rectified current. 
A half-bridge inverter circuit 9 comprises two transistors 11 and 13, 
diodes 15 and 17 disposed between the collectors and emitters of the 
transistors 11 and 13, a heating coil 19, and a resonance capacitor 21 
connected to the heating coil 19 in series. 
A phase comparing circuit 23 receives an inverter voltage VIN as a first 
signal and a terminal voltage VCl of the capacitor 21 as a second signal. 
The phase of the second signal correlates to the phase of an inverter 
current IIN flowing to the capacitor 21. The phase comparing circuit 23 
compares the phases of the first and second signals with each other and 
provides a signal representative of the phase difference of both the 
signals to a low-pass filter 25. 
A phase difference setting circuit 27 sets the phase difference of the 
first and second signals. 
A voltage-controlled oscillator (VCO) 29 is a frequency controlling means 
for controlling the oscillation frequency of the invertor circuit 9 to 
establish the phase difference set by the phase difference setting circuit 
27. The VCO 29 changes the oscillation frequency in response to a signal 
voltage from the low-pass filter 25. 
A driving circuit 31 alternately turns the transistors 11 and 13 on and off 
according to signals from the VCO 29. 
The operation of the arrangement of FIG. 7 will be explained with reference 
to FIGS. 8a to 8d. 
When the transistors 11 and 13 are alternately turned on and off according 
to the signals from the driving circuit 31, the heating coil 19 and 
capacitor 21 are put under a series resonant state, and the heating coil 
19 generates high-frequency electric power with which an object such as a 
pan is heated. 
If the oscillation frequency of the inverter circuit 9 is equal to a 
resonance frequency fO of the series resonance circuit composed of the 
heating coil 19 and resonance capacitor 21, the series resonance circuit 
will have only resistance load, and load impedance Z will be expressed by 
the following equation (1): 
EQU Z=RL+RC (1) 
where RL is the load resistance and RC the resistance of the heating coil 
19. 
The equation (1) tells that the load impedance Z has only resistance 
components. Under this state, a load current takes its maximum value. 
During a period Ta shown in FIGS. 8a and, 8b, effective electric power is 
supplied to the series resonant circuit. At this time electrical energy 
output is maximum. 
To control input power, the phase difference setting circuit 27 sets the 
phase difference of the first and second signals VIN and VCl greater than 
90.degree. according to an external instruction signal SIN. By setting the 
phase difference greater than 90.degree., an inductive load state is 
established, and the phase of the inverter current IIN delays behind that 
of the inverter voltage VIN as shown in FIGS. 8c and 8d. At this time, the 
load impedance Z is expressed by the following equation (2): 
##EQU1## 
As shown in FIG. 8d, electric power is supplied to the series resonance 
circuit during a short period T2. In this way, the set phase difference 
greater than 90.degree. increases the load impedance Z and reduces a 
current flowing to the inverter circuit 9 to make the input power 
continuously low. 
FIG. 9 shows an induction heating cooker according to an another embodiment 
of the present invention. 
A material detecting circuit 33 detects information about the material of 
an object (pot) to be heated by the cooker. According to the material 
information, a phase difference set by a phase difference setting circuit 
27 is changed, thereby stabilizing input power irrespective of the 
material of the object. 
An inverter voltage phase detecting circuit 20 detects an inverter voltage 
VIN (FIG. 10a) and provides the same to a phase comparing circuit 23. A 
capacitor voltage phase detecting circuit 22 detects a terminal voltage 
VCl (FIG. 10c) of a resonance capacitor 21 and provides the same to the 
phase comparing circuit 23. An inverter current IIN (FIG. 10b) is in 
synchronization with the inverter voltage VIN, and the phase of the 
voltage VCl is delayed by 90.degree. behind that of the inverter current 
IIN. 
The phase comparing circuit 23 comprises an exclusive OR circuit, etc. The 
phase comparing circuit 23 receives the inverter voltage VIN and the 
voltage VCl, and provides a signal VPl (FIG. 10d) to a low-pass filter 25. 
The low-pass filter 25 receives a signal from the phase difference setting 
circuit 27 as well as the signal VPl and provides a signal VP2 indicated 
with a dotted line in FIG. 10d to a voltage-controlled oscillator (VCO) 
29. 
The signal VP2 from the low-pass filter 25 changes in response to a duty 
ratio of the signal VPl. When a series resonance circuit, formed by a 
heating coil 19 and a resonance capacitor 21 is inductive, the phase of 
the inverter current IIN is delayed behind the phase of the inverter 
voltage VIN to lower the signal VP2. An oscillation frequency of the VCO 
29 changes in response to its input voltage, i.e., the signal VP2 as shown 
in FIG. 10e. A driving circuit 31 drives an inverter circuit 9 according 
to a signal from the VCO 29. 
The inverter voltage phase detecting circuit 20, capacitor voltage phase 
detecting circuit 22, phase comparing circuit 23, low-pass filter 25, VCO 
29 and driving circuit 31 form a phase-locked loop (PLL). The PLL control 
can secure a predetermined heating state for various materials to be 
heated which may change a resonance frequency of the series resonance 
circuit composed of the heating coil 19 and capacitor 21. 
FIGS. 11a and 11b show various materials to be heated and corresponding 
resonance frequencies fO. In FIG. 11a, the heating coil 19 has 21.5 turns 
(T) and the capacitor 21 is of 1 .mu.F, while in FIG. 11b the heating coil 
19 has 30 turns and the capacitor 21 is of 0.55 .mu.F. 
Each material has specific input impedance. When a pan made of non-magnetic 
stainless steel is heated under a resonance state, i.e., with the inverter 
voltage VIN and voltage VCl having a phase difference greater than 
90.degree., excessive input power may be applied to the inverter circuit 
9, as indicated by curve "a" in FIG. 12. This may cause trouble in 
inverter circuit 9. A curve "b" of FIG. 12 is for heating a pan made of 
iron and indicates relation of an oscillation frequency to input power of 
the inverter circuit 9. 
To avoid such trouble, the embodiment of FIG. 9 controls input power 
according to the material of an object to be heated. 
A current transformer CT1 is disposed in a passage of a current that flows 
to the capacitor 21 of the inverter circuit 9. The current transformer CT1 
provides a signal correlating to the inverter current IIN. According to 
the signal, the material detecting circuit 33 provides a signal voltage, 
which may change in response to the material, i.e., impedance of the 
object. 
A comparing circuit 35 compares a reference value defined by resistors R11 
and R12 with the signal voltage from the material detecting circuit 33, 
and when judged that the material of the object is, for example, iron or 
magnetic stainless steel, provides an output signal to the phase 
difference setting circuit 27. 
A comparing circuit 37 compares a reference value defined by resistors R13 
and R14 with the signal voltage from the material detecting circuit 33, 
and when judged that the material of the object is, for example, 
non-magnetic stainless steel, provides an output signal to the phase 
difference setting circuit 27. 
A comparing circuit 39 compares a reference value defined by resistors R15 
and R16 with the signal voltage from the material detecting circuit 33, 
and when judged a no-load state that no object is placed on a top plate of 
the cooker, provides an output signal to the phase difference setting 
circuit 27. 
In this way, a phase difference in the phase difference setting circuit 27 
is changed according to the material, so that constant input power may be 
secured irrespective of the material to be heated. When a pot made of 
non-magnetic stainless steel having low impedance is placed on the top 
plate of the cooker, the phase difference is increased to oscillate the 
inverter circuit 9 at a frequency greater than the resonance frequency fO 
of the series resonance circuit, thereby controlling the input power. 
The phase difference setting circuit 27 may follow an externally given 
instruction signal SIN to set the phase difference of the first and second 
signals VIN and VCl greater than 90.degree. in controlling the input 
power. 
FIG. 6 shows an induction heating cooker according to another embodiment of 
the present invention. 
The cooker comprises an input current setting circuit 41; an input current 
detecting circuit 43; a comparing circuit 45 for comparing output signals 
of the circuits 41 and 43 with each other; a phase difference restricting 
circuit 47 for restricting a phase difference to put a series resonance 
circuit, formed by a heating coil 19 and a resonance capacitor 21 in an 
inductive state; and oscillation frequency restricting circuit 49 for 
restricting an oscillation frequency not to be lowered below a 
predetermined value; a current restricting circuit 51 for restricting a 
current flowing to the capacitor 21 not to be lowered below a 
predetermined value; and an initial setting circuit 53 for gradually 
lowering the oscillation frequency of an inverter circuit 9 from high to 
low at the start of operation of the cooker. 
The input current detecting circuit 43 detects an input current from an AC 
power source 1 according to a signal from a current transformer CT2. The 
comparing circuit 45 compares a value set by the input current setting 
circuit 41 with the value detected by the input current detecting circuit 
43, and provides a resultant signal to a phase difference setting circuit 
27. 
The phase difference setting circuit 27 changes a phase difference 
according to the signal from the comparing circuit 45, thereby securing 
constant input power irrespective of the material and shape of an object 
to be heated. 
If the oscillation frequency of the inverter circuit 9 is decreased to put 
the series resonance circuit in a capacitive state, a transistor 11 or 13 
may be turned on to cause an excessive -circuit current to flow during an 
inverse recovering period for diodes 15 or 17. The inverse recovering 
period is a shifting period from a period T22 to a period T23 or from a 
period T24 to a period T21 (T25), during which carriers remaining in the 
diode 15 or 17 disappear. 
To avoid an excessive short-circuit current, the phase difference 
restricting circuit 47 of the present invention restricts a phase 
difference to exceed 90.degree. so that the series resonance circuit may 
be kept inductive. As a result, the oscillation frequency of the inverter 
circuit 9 is greater than the resonance frequency fO of the series 
resonance circuit. As shown in FIG. 13, when the base of the transistor 11 
receives a signal Q1, an inverter current IIN flows through a passage LP11 
during a period T11. In the next period T12, the inverter current IIN 
flows through a passage LP12. In periods T13 and T14, the inverter current 
IIN flows through passages LP13 and LP14. 
The current restricting circuit 51 comprises an inverter current detecting 
circuit 61 for detecting the inverter current IIN according to a signal 
from the current transformer CT1; an inverter current limit setting 
circuit 63 for setting a limit of the inverter current IIN; and a 
comparing circuit 65 for comparing output signals of the circuits 61 and 
63 with each other. 
In the phase difference setting circuit 27, a phase difference is changed 
according to an output signal from the current restricting circuit 65 to 
control the inverter current IIN smaller than a rated current of the 
transistors 11 and 13. Accordingly, an object having low impedance such as 
a pot made of stainless steel may be heated without causing excessive 
short-circuit current. Namely, without burning the transistors 11 and 13, 
an operation of the inverter circuit 9 is secured to heat the object. 
Under a normal operation, an inverter voltage VIN is in synchronization 
with the inverter current IIN as shown in FIG. 15. At the start of 
operation of a voltage-controlled oscillator (VCO) 29 or the cooker, 
oscillation of the VCO 29 is unstable. At this time, if an oscillation 
frequency becomes one third of the resonance frequency fO of the series 
resonance circuit as shown in FIG. 16, the PLL control mentioned before 
may be locked to disorder the operation of the inverter circuit 9. 
To cope with starting instability, the oscillation frequency restricting 
circuit 49 of the present invention controls the oscillation frequency of 
the VCO 29, so as to be lowered below a predetermined value. The 
predetermined value is set to be lower than the lowest oscillation 
frequency of the inverter circuit 9 according to the material of an object 
to be heated. Accordingly, the inverter circuit 9 is securely driven even 
when the oscillation of the VCO 29 is unstable. 
At the time when a power source is turned on, operations of the respective 
circuits are unstable, so that the oscillation frequency of the inverter 
circuit 9 must be set as high as possible to prevent an excessive current 
from flowing to the inverter circuit 9. 
To achieve this, the initial setting circuit 53 of the present invention 
gradually reduces a signal voltage VL given to a low-pass filter 25 at the 
start of the cooker or the VCO 29 as shown in FIG. 17. As a result, the 
oscillation frequency of the inverter circuit 9 gradually decreases from a 
value higher than the resonance frequency fO, and therefore, the inverter 
circuit 9 is securely driven even during the initial period where circuit 
operations are unstable. 
FIGS. 18a and b are circuit diagram showing the details of the 
above-mentioned embodiment of the present invention. 
The voltage-controlled oscillator (VCO) 29 changes its oscillation 
frequency in response to its input voltage, and if the input voltage is 1 
V, provides a rectangular pulse of 40 KHz. If the input voltage is 5 V, it 
provides a rectangular pulse of 170 KHz. 
A dead time generating circuit 30 divides the frequency of the rectangular 
pulse of the VCO 29. The dead time generating circuit 30 produces a dead 
time not to simultaneously turn on the two transistors 11 and 13. The dead 
time is so set that a driving current is not supplied to one transistor 
until the other transistor is completely turned off after a driving 
current for the other transistor is stopped. 
An upper arm driving circuit 31A for driving the transistor 11, and a lower 
arm driving circuit 31B for driving the transistor 13, constitute a 
driving circuit 31. Drive signals provided for the upper and lower arm 
driving circuits 31A and 31B have different operational potential levels 
from those of the transistors 11 and 13. The drive signals, therefore, are 
provided from the circuits 31A and 31B to the transistors 11 and 13 
through pulse transformers TRA and TRB, respectively. 
In the inverter circuit 9, the capacitor 21 is connected to a capacitor 71 
in series. A divided voltage of between the capacitors 21 and 71 is the 
second signal, whose phase correlates to the phase of a current flowing to 
the capacitor 21 and which is provided to a capacitor voltage phase 
detecting circuit 22. 
The capacitor voltage phase detecting circuit 22 comprises an operational 
amplifier 73, a photocoupler 75, etc. The circuit 22 receives the second 
signal and generates a rectangular pulse, and the photocoupler 75 adjusts 
the potential level. 
A phase comparing circuit 23 employs an exclusive OR circuit. The circuit 
23 receives a first signal Ca whose phase correlates to that of an output 
voltage of the inverter circuit 9 from the dead time generating circuit 
30, as well as a second signal Cb from the capacitor voltage phase 
detecting circuit 22. If the oscillation frequency of the inverter circuit 
9 is equal to the resonance frequency of the series resonance circuit, the 
phase comparing circuit 23 provides an output signal VPl having a duty 
ratio of 50%. as shown in FIG. 19. If the oscillation frequency of the 
inverter circuit 9 is higher than the resonance frequency, the output 
signal VPl of the phase comparing circuit 23 has a duty ratio greater than 
50% as shown in FIG. 20. 
The low-pass filter 25 has an operational amplifier 77 to smooth the output 
signal VPl and provide a smoothed signal to the VCO 29. 
A phase difference setting section 27A includes the input current setting 
circuit 41, comparing circuit 45 and initial setting circuit 53. The input 
current setting circuit 41 comprises a resistor 81 and a variable resistor 
83. By adjusting the variable resistor 83, an output of the inverter 
circuit 9 can be changed. A signal from the variable resistor 83 is 
provided to a non-inverted input terminal of the comparing circuit 45. An 
inverted input terminal of the comparing circuit 45 receives a signal from 
the input current detecting circuit 43. The comparing circuit 45 compares 
the received signals with each other, thereby setting an output of the 
inverter circuit 9 to a required value. 
The initial setting circuit 53 comprises resistors 85 and 87 connected in 
series and capacitor 89 in parallel with the resistor 85. A voltage 
divided by the resistors 85 and 87 is a phase controlling voltage. 
Immediately after the power source is turned on, the control voltage is 
gradually decreased from high to low due to the capacitor 89 to gradually 
lower the oscillation frequency of the inverter circuit 9 from high to 
low, thereby realizing a so-called soft start. 
The phase difference restricting circuit 47 comprises an operational 
amplifier 91, resistors 93 and 95, etc. A divided voltage of the resistors 
93 and 95 is a phase difference lower limit VLL with which a lower limit 
of the phase difference is controlled so as not to put the series 
resonance circuit into the capacitive state. 
The oscillation frequency restricting circuit 49 comprises an operational 
amplifier 97, etc. The circuit 49 monitors an input voltage of the VCO 29 
to limit the oscillation frequency of the VCO 29 not to be smaller than a 
predetermined value. 
The current restricting circuit 51 comprises an inverter current detecting 
circuit 61 for detecting an inverter current, an inverter current limit 
setting circuit 63 for setting a limit value VUL of the inverter current, 
and a comparing circuit 65 for comparing the values of the circuits 61 and 
63 with each other. The current restricting circuit 51 limits the inverter 
current not to exceed a predetermined value. 
FIG. 21 shows an induction heating cooker according to still another 
embodiment of the present invention. 
This embodiment comprises a capacitor current phase detecting circuit 22A 
and a current transformer CT3. Based on a signal from the current 
transformer CT3, a current flowing to a resonance capacitor 21 is detected 
as a second signal. 
The phase of the current flowing to the capacitor 21 advances ahead the 
phase of a terminal voltage of the capacitor 21 by 90.degree.. 
Accordingly, the phase of a signal Cd provided by the capacitor current 
phase detecting circuit 22A advances ahead the signal Cb shown in FIG. 18 
provided by the capacitor voltage phase detecting circuit 22 of FIG. 18 by 
90.degree.. 
When the oscillation frequency of an inverter circuit 9 is equal to the 
resonance frequency of a series resonance circuit composed of a heating 
coil 19 and the capacitor 21, a phase comparing circuit 23 provides an 
output signal VPl having a duty ratio smaller than 50%, as shown in FIG. 
22. When the oscillation frequency of the inverter circuit 9 is higher 
than the resonance frequency of the series resonance circuit, the duty 
ratio of the output signal is larger than that of FIG. 22, as shown in 
FIG. 23. 
An input power setting circuit 41A sets required input power, and an input 
power detecting circuit 43A detects the actual input power. 
Other parts of FIG. 21 are the same as those of FIG. 6, and are represented 
with like numerals. 
The input power setting circuit 41A and input power detecting circuit 43A 
easily and securely set the required input power. 
In summary, according to the first aspect of the present invention, the 
oscillation frequency of an inverter circuit is controlled to set a phase 
difference between the phase of a first signal correlating to the phase of 
an output voltage of the inverter circuit and the phase of a second signal 
correlating to the phase of a current flowing to a resonance capacitor. 
With this arrangement, input power can continuously be changed for a wide 
range, and noise from a power source is eliminated. 
According to the second aspect of the present invention, a phase difference 
set by phase difference setting means is changed in response to a value 
set by input setting means. With this arrangement, the same input power 
may be secured for the same setting even for objects of different 
materials and different shapes. 
According to the third aspect of the present invention, a material 
information detecting means detects information identifying the material 
of an object to be heated, and a phase difference is changed according to 
the detected information. With this arrangement, input power can be 
stabilized irrespective of the material of the object. 
According to the fourth aspect of the present invention, a phase difference 
of first and second signals is restricted, so that a heating coil and a 
resonance capacitor may form an inductive resonance circuit. With this 
arrangement, the oscillation frequency of an inverter is set larger than a 
resonance frequency of the resonance circuit, thereby preventing a 
switching element from sustaining an excessive short-circuit current. 
According to the fifth aspect of the present invention, a frequency 
controlled by frequency controlling means is restricted not to be smaller 
than a predetermined value. With this arrangement, an inverter circuit can 
be securely driven even when an oscillating operation of the frequency 
controlling means is unstable. 
According to the sixth aspect of the present invention, a current flowing 
to a resonance capacitor is restricted not to be smaller than a 
predetermined value. With this arrangement, even an object of low 
impedance can be heated by securely driving an inverter circuit without 
burning a switching element due to an excessive current. 
According to the seventh aspect of the present invention, the oscillation 
frequency of an inverter circuit is gradually reduced from high to low at 
the start of operation of the frequency controlling means. With this 
arrangement, the inverter circuit can securely be driven even at the start 
of a cooker where circuit operations are unstable. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.