Electric heating – Microwave heating – With control system
Reexamination Certificate
2002-02-15
2003-04-22
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With control system
C219S702000, C219S723000
Reexamination Certificate
active
06552313
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high-frequency heating apparatus using as the power unit a semiconductor power converter for generating high-frequency power.
BACKGROUND ART
Conventional circuit configurations of high-frequency heating apparatus are shown in FIG.
7
and
FIG. 9
while their respective current control schemes are described in FIG.
8
and FIG.
10
.
That is, there are roughly two classes of input current control schemes: the first scheme is achieved by the configuration shown in
FIG. 7
, where current control is made based on the primary-side current, following the control characteristics shown in FIGS.
8
(
a
) and (
b
) (see Japanese Patent Application Laid-Open Hei 11 No. 283737); and the second scheme is achieved by the configuration shown in
FIG. 9
, where current control is made based on the secondary-side current (magnetron current), following the control characteristic shown in FIG.
10
. These will be explained in this order.
First,
FIG. 7
shows a circuit configuration of a high-frequency heating apparatus using a conventional semiconductor power converter.
In the circuit configuration, a power unit
1
is configured so that the input from a commercial power supply
4
(with an overcurrent circuit breaker
4
a
disposed in the power line) is rectified through a rectifier
5
and the output is smoothed by the combination of a coil
6
and a capacitor
7
. A power converter
2
is comprised of a frequency changing circuit made up of a semiconductor device
9
, diode
8
, step-up transformer
11
and capacitor
12
for the electric power supply from power unit
1
and a high-voltage rectifying circuit made up of step-up transformer
11
, a capacitor
14
and diode
13
. The voltage which is obtained by high-voltage rectification through this rectifying circuit is converted into a high frequency by a magnetron
15
so as to output and emit microwaves over the food to be cooked. The circuit further includes an inverter controller
10
for ON-OFF control of semiconductor
9
.
In the above configuration, in order to implement input current control, the voltage output from an input current detector
16
and input to inverter controller
10
is compared to the current control signal output from a control circuit
20
that governs the high-frequency heating apparatus as a whole, so as to determine the input current to the high-frequency heating apparatus. Inverter controller
10
also provides a protecting function for semiconductor device
9
and will stop the operation or take an appropriate action when an anomaly has occurred to stabilize the operation of semiconductor device
9
.
Control circuit
20
as the circuit system for input current control is usually connected to a potential (on the secondary side), insulated from the primary side, and hence outputs a signal via a photocoupler
21
.
Now, the input current control system for the conventional high-frequency heating apparatus will be described.
In the high-frequency heating apparatus based on the conventional primary-side input current control, the output signal from control circuit
20
and the output from input current detector
16
are compared, so that the input current will be kept constant with respect to the elapsed time of heating as shown in FIG.
8
(
a
) or so that the ‘short-time high power’ control signal for setting the output at the maximum during only the initial period Tmax (about 1 min. 30 sec. to 3 min.) from the start of heating and reducing it to a lower level after that as shown in FIG.
8
(
b
) will be output.
As a high-frequency heating apparatus based on secondary-side current control, a circuit configuration as shown in
FIG. 9
is present, which includes a magnetron drive circuit configuration equivalent to the high-frequency heating apparatus shown in FIG.
7
. Hence like components are allotted with like reference numerals without description.
The configuration in
FIG. 9
differs from the configuration shown in
FIG. 7
in that the detecting position of an input current detector
16
A is moved from the primary side to the secondary side (the magnetron current side) so as to perform control based on the secondary-side current. This secondary-side current control will regulate the magnetron current so as to be constant, whereby the input current is controlled presenting the operating characteristic indicated at
8
A in FIG.
10
.
However, if such a conventional input current control as shown in FIG.
8
(
a
) is implemented, there occur cases where the input current will not lower even when the temperature has been elevated since the input current is controlled to be constant, so that the high-frequency heating apparatus is forced to operate at high temperatures. In the case of the short-time high power configuration shown in FIG.
8
(
b
), the high power only lasts about 1 min. 30 sec. to 3 min. Therefore, this configuration is in its way effective in heating for a short period with light loads (such as heating cooked rice, etc.) because of the shortness of cooking time. However, heating up frozen foods or the like needs a heating time of about 4 min. to 8 min., hence, on the contrary, the cooking will take up a longer time because the heating power is lowered when the short-time high power operation is switched into the normal operation. This is the drawback of this configuration. Accordingly, this configuration is not able to make the best use of the input power of the high-frequency heating apparatus, so results in the problem that high-frequency output cannot be used effectively to the maximum.
Most of the magnetron drive circuits for high-frequency heating apparatus currently put on the market use a commercial a.c. power supply transformer, which has the characteristic shown in FIG.
6
(
a
), in that the input current declines with the passage of time from the start of heating. This characteristic is adapted to have the appearance similar to the current cutoff characteristic of a typical current breaker for home use, with a constant margin secured relative to the cutoff current.
The conventional, primary-side current control systems (indicating the so-called switching systems using a semiconductor device, herein), however, are adapted to have the characteristics shown in FIGS.
8
(
a
) and
8
(
b
), having inconstant margins relative to the cutoff current of the current breaker. Hence there has been a possibility that the current breaker might operate at times when some other appliance is activated.
Further, since the switching system differs from the commercial a.c. power supply transformer system in input current control characteristic or high-frequency output characteristic over the elapsed time of heating, there is no correlation as to cooking time in the operations of auto-cooking menu between the two systems. Therefore, if system change from the high-frequency heating apparatus of the commercial power supply transformer system to that of the switching system is attempted, cooking methods should be once again studied. This makes system change difficult.
Next, the problem with the use of the current control scheme based on the secondary side current (magnetron current) will be mentioned. In this case, the current through the magnetron is controlled so as to be constant, which means that the power consumption of the magnetron should be controlled to be constant because the following relation holds:
(Magnetron Current)×(Magnetron Voltage)=(Magnetron Power Consumption).
Here, if it is assumed, for example, that the power supply voltage to the high-frequency heating apparatus drops by 10%, the input current increases by 10% because the apparatus is controlled so that the power consumption will be kept constant, presenting the current control operation shown at
8
B in FIG.
10
.
This will induce temperature rise in the parts of the high-frequency heating apparatus because the power consumption is kept constant, despite the fact that the cooling capability of the cooling fan in the high-frequency heating apparatus is lowered due to the
Masuda Shin'ichi
Takashige Yutaka
Ueda Koji
Leung Philip H.
Sharp Kabushiki Kaisha
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