Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2003-02-27
2004-08-03
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S663000, C219S665000, C363S097000
Reexamination Certificate
active
06770857
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an induction heating apparatus such as an induction heating cooking unit in which load of high conductivity and low permeability, e.g., an aluminum pot, can be heated efficiently; and a induction heating type water heater, humidifier, an iron or the like.
BACKGROUND OF THE INVENTION
As for a conventional induction heating apparatus, e.g., an induction heating cooking appliances, a technology capable of preventing both a pot vibration noise and reduction of power factor while heating an aluminum pot is disclosed, e.g., in Japanese Patent Laid-Open Publication No. 1989-246783, and a technology for reducing a switching loss and for heating an aluminum pot with high-frequency wave is disclosed, e.g., in Japanese Patent Laid-Open Publication No. 2001-160484.
FIG. 9
is a circuit included in Japanese Patent Laid-Open Publication No. 1989-246783 supra. In
FIG. 9
, bridge circuit
2
, which rectifies AC(alternate current) power supply voltage of 100V to output DC(direct current) voltage, includes two thyristors
3
,
4
and two diodes
5
,
6
. Thyristors
3
,
4
control a conduction angle and, upon initiating the operation, reduce the DC voltage down to about 20V to set a low output power. And if load detector
24
detects an existence of a suitable load, output controller
26
controls the output power by varying the DC voltage.
Furthermore, input waveform shaper
23
drives transistor
10
to make an input current of a predetermined waveform based on signals outputted by input setting unit
25
and input current detector
22
, thereby increasing the power factor. The enhancement of the power factor is achieved by accumulating energy in choke coil
8
when transistor
10
is turned on and then by transferring the energy to capacitor
11
via diode
9
when transistor
10
is turned off.
Also, in order to heat an aluminum pot, a frequency of a current passing through heating coil
18
is increased from 20 kHz to 50 kHz by varying the number of turns of heating coil
18
and the capacitance of resonant capacitor
19
.
However, the prior art described above has many problems: that is, there is required a costly and complicated circuit structure capable of changing the number of turns of heating coil
18
in order to selectively heat both an aluminum pot and an iron pot; and there incurs a large switching loss in switching devices
15
,
17
because the driving frequency thereof is required to be set at same 50 kHz in order to accommodate the resonant frequency of 50 kHz; and if a resonance point tracking method is adopted to decrease the switching loss, additive circuits, such as a control circuit therefor and a power supply voltage varying circuit for output power modification, are required.
Japanese Patent Laid-Open Publication No. 2001-160484 addresses the above-mentioned problems as in
FIGS. 10
to
12
.
In Japanese Patent Laid-Open Publication No. 2001-160484, a frequency of a resonant current passing through heating coil
18
and resonant capacitor
19
is set to be at least twice as high as that of driving signals fed to transistors
15
,
17
, in response to the signal from resonant current detector
30
for detecting a current passing through heating coil
18
, thereby allowing for the heating of the aluminum pot by raising a frequency of the current supplied to heating coil
18
, while suppressing the switching loss of the transistors
15
,
17
.
In an output control method for a low output power mode as shown in
FIG. 11A
, transistor
15
is turned off at a first instant when sign of collector current Ic
1
thereof varies from positive value to zero and transistor
17
is turned off at a third instant when the sign of collector current Ic
2
thereof varies from positive value to zero. Also, in a high output power mode as shown in
FIG. 11B
, transistor
15
is turned off at a second instant when the sign of collector current Ic
1
thereof varies from positive value to zero and transistor
17
is also turned off at a second instant when the sign of collector current Ic
2
thereof varies from positive value to zero.
Alternatively, in the low output power mode as shown in
FIG. 12A
, transistor
15
is turned off when time t
1
, which is shorter than a half period of the resonant current, elapses after transistor
15
is turned on and transistor
17
is turned off at a third instant when collector current Ic
2
thereof decreases to zero from positive value. However, in the high output power mode as shown in
FIG. 12B
, transistor
15
is turned off at an instant when collector current Ic
1
thereof drops to zero from positive value for the first time (turn-on time of transistor
15
corresponding to one half period of the resonant current) and transistor
17
is turned off at a third instant when the sign of collector current Ic
2
thereof varies from positive value to zero.
The prior art induction heating apparatus of Japanese Patent Laid-Open Publication No. 2001-160484, however, suffers from certain drawbacks as follows. That is, a continuous output control cannot be achieved by the control method in
FIGS. 11A
,
11
B, and a fine output control cannot be achieved by the control method in
FIGS. 12A
,
12
B, because the variation of turn-on time produces too much variation of output power. Furthermore, because the envelope of current passing through heating coil
18
is not smoothed by the control methods of
FIGS. 11A
,
11
B and
FIGS. 12A
,
12
B, there occurs a pot vibration noise having a frequency of twice that of the commercial input power.
Japanese Patent Laid-Open Publication No. 1989-246783 addresses the problem of pot vibration noise generation, in which the output power is controlled by decreasing an input power fed to the inverter. However, even if this scheme is combined with the method disclosed in Japanese Patent Laid-Open Publication No. 2001-160484, suitable output control cannot be achieved because the resonant current is attenuated and thus cannot be maintained.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an induction heating apparatus capable of heating an aluminum pot with a sufficiently large output power, in which the output power can be continuously adjusted with a fine controllability, while suppressing the generation of the pot vibration noise and switching loss in switching devices.
In accordance with the present invention, in case a load with a high conductivity and a low permeability is heated by a magnetic field generated by the heating coil, the resonant current passing through a switching device or a inverse-parallel diode (function as a reverse conducting device) resonates with a shorter period than a driving time of the switching device and further the DC voltage is boosted and smoothed by a boosting and smoothing circuit, and then provided for the inverter in order to maintain an amplitude of the resonant current to be higher than a certain value during the driving time, so that a switching loss of the switching device can be suppressed by lowering a driving frequency thereof, and at the same time the resonant current with higher frequency than the driving frequency thereof can be provided for the heating coil. Therefore, a load with a high conductivity and a low permeability, e.g., aluminum etc. can be heated with high output power.
Moreover, since the boosting and smoothing circuit for boosting and smoothing the input DC voltage fed to the inverter is provided to restrain the peak-to-peak value of the resonant current from attenuating to zero during the driving times of the switching device, in case of heating the load of high conductivity and low permeability, the output power can be stably controlled by varying the driving time of the switching device to be greater than one period of the resonant current and/or the burden (turn-on loss) of the switching device can be reduced.
In accordance with a first aspect of the present invention, there is provided an induction heating apparatus including:
an inverter having a switching device,
Fujii Yuji
Fujita Atsushi
Hirota Izuo
Kitaizumi Takeshi
Miyauchi Takahiro
Bacon & Thomas
Leung Philip H.
Matsushita Electric - Industrial Co., Ltd.
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