High frequency heating apparatus

Details High frequency heating apparatus High frequency heating apparatus

2001-04-26

2002-03-26

Leung, Philip H. (Department: 3742)

Electric heating

Microwave heating

With control system

C219S716000, C219S723000, C363S017000, C363S098000

Reexamination Certificate

active

06362463

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a high frequency heating apparatus that performs induction heating using a magnetron, for example a microwave-cooking oven; more specifically to the structure of a circuit for driving a magnetron.
BACKGROUND ART
The power supply circuit incorporated in a home-use high frequency heating apparatus and various other such apparatus is bulky and heavy in weight. Reduction in size and weight of the power supply circuit has been one of the main tasks in the industry. Efforts have been made in many sectors to make it compact, light and inexpensive through adoption of the switching power supply. Also in the field of food processing high frequency heating apparatus using a magnetron, the magnetron driving circuit is requested to reduce its size and weight. There is a patent that intends to meet the requirement through introduction of the switching power supply (PCT/JP98/00751).
According to the above patent, the switching loss of semiconductor switching devices operating at high frequency is reduced through adoption of a resonance type circuit, which circuit being an essential technology in the switching power supply. Because of a high voltage in the circuit generated by the functioning of resonance circuit, the semiconductor switching devices and other relevant electric components are required to have high voltage specifications. This makes the circuit large and heavy. In order to evade such problems the above patent discloses the following structure.
As shown in
FIG. 28
, a conventional circuit comprises a DC source
1
, a leakage transformer
2
connected to one end of the DC source
1
, a first semiconductor switching device
6
connected in series to a primary coil
3
of the leakage transformer
2
and to the other end of the DC source, a first capacitor
4
connected in parallel with the primary coil
3
of the leakage transformer
2
, a series circuit of a second capacitor
5
and a second semiconductor switching device
7
, driving means
8
having an oscillator for driving the first semiconductor switching device
6
and the second semiconductor switching device
7
, rectifying means
10
connected to a secondary coil
9
of the leakage transformer
2
, and a magnetron
11
connected to the rectifying means
10
. The series circuit of the second capacitor
5
and the second semiconductor switching device
7
is connected in parallel with the primary coil
3
of the leakage transformer
2
.
A feature of the above circuit structure is that a voltage to be applied on the main first semiconductor switching device
6
can be lowered by the use of an auxiliary second capacitor
5
that has a capacitance greater than that of the first capacitor
4
which forms a resonance circuit in combination with the leakage transformer
2
.
Thinking of cases where the DC source
1
is provided by rectifying a commercial power supply. The commercial power supply comes in different voltages, for example 100V in Japan, 120V in the U.S., 240V in the UK and Peoples Republic of China, 220V in Germany. Even within Japan, many of the power consuming professional apparatus receive the power supply at 200V.
In a case where the commercial supply voltage is 100V or 120V, the voltage to be applied on the main first switching device
6
may be reduced by the above described circuit. However, if voltage of the commercial power supply is higher than 200V, the voltage to the main first switching device
6
goes high in the circuit disclosed by the above patent. Also, it is necessary to change inductance of the primary coil and the secondary coil of leakage transformer
2
, as well as capacitance of the first capacitor
4
and the second capacitor
5
.
Table 1 compares the constants of the leakage transformer
2
, the first capacitor
4
and the second capacitor
5
, and the voltages on the first switching device
6
in the 100V power supply and the 200V power supply. Table 1 teaches us, for example, that the inductance of primary coil of leakage transformer
2
increases to approximately 4 times, the number of coil turns increases, in proportion to the square root of inductance, to approximately two times. Thus the structure undergoes a substantial change.
Also, the withstand voltage of the first switching device
6
has to be raised to meet the two-fold voltage to be applied thereon.
The conventional circuit has two problems when it encounters a commercial power supply higher than 200V. One problem is that the voltage applied on the switching device goes high, the other problem is that the leakage transformer, the first capacitor and the second capacitor are compelled to have different constants respectively.
The conventional circuit is described further referring to
FIG. 28. A
parallel resonance circuit formed of the leakage transformer
2
, the first capacitor
4
and the second capacitor
5
makes the voltage of primary coil
3
to be higher than the DC source voltage by the resonance effect.
Therefore, when the DC source is provided from a high voltage commercial power supply, the voltage of primary coil
3
goes still higher. So, it becomes necessary to lower the step-up ratio of leakage transformer
2
(ratio in the number of turns between the primary coil
3
and the secondary coil
9
), and to increase the number of turns of primary coil
3
in order to lower the voltage of primary coil
3
.
TABLE 1
100 V
200 V
Leakage transformer
Primary coil inductance
45 &mgr;H
150 &mgr;H
Secondary coil inductance
14 mH
8 mH
Coupling coefficient of
0.74
0.74
Primary-Secondary coils
First capacitor capacitance
0.18 &mgr;F
0.05 &mgr;F
Second capacitor capacitance
4.5 &mgr;F
4.5 &mgr;F
Voltage on first switching device
430 V
930 V
The DC source
1
is composed of rectifying means for rectifying commercial power supply and a filter formed of an inductor and a capacitor to smooth the output.
The filter smoothes the voltage, removes a noise generated as a result of switching operation by switching device and avoids intrusion of a noise from outside.
However, the filter formed of an inductor and a capacitor generates an overvoltage twice as high as that of the DC source at the moment when the power is on.
There is another problem that is related with a sudden change of impedance caused by a discharge started within magnetron tube. Relationship between primary coil
3
and secondary coil
9
of the leakage transformer
2
is shown in FIG.
32
(
a
), in the form of an equivalent circuit diagram. The primary coil
3
may be divided into a leakage inductor and an exciting inductor; further the exciting inductor and the secondary coil
9
magnetically coupled with an ideal transformer (magnetic coupling coefficient
1
). Both ends of the secondary coil are connected with the rectifier circuit, the rectifier circuit is connected to a magnetron. In the drawing, L
1
represents inductance of the primary coil, L
2
inductance of the secondary coil.
When impedance of a magnetron becomes extremely small to an equivalence of short-circuiting of the secondary coil, an equivalent circuit of leakage transformer
2
is as shown in FIG.
32
(
b
). There is only a leakage inductor. The inductance L of which is given by the formula below.
L
=(1
−k
2
)×
L
1
  (Formula 1)
where:
L
1
is inductance of primary coil
k is coefficient of magnetic coupling between primary coil and secondary coil
When the secondary coil
9
is short-circuited, inductance of primary coil becomes small. Therefore, a large current as shown in Formula 2 flows to the first switching device
6
.
I
c
=V
DC
×T
on
/L
(1)  (Formula 2)
V
DC
: output voltage of DC source
1
T
on
: Time of conduction in the first switching device
4
Because of the small L the current is an overcurrent; furthermore, an overvoltage is generated when the first switching device
6
turns OFF. Thus the first switching device
6
is given with a great stress by the overcurrent and the overvoltage emerging continuously.
Still, there is a following problem with respect to driving of a magnetron. An app

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