Magnetron drive power supply

Details Magnetron drive power supply Magnetron drive power supply

2002-05-15

2003-09-23

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Combined load device or load device temperature modifying...

Distributed parameter resonator-type magnetron

C219S715000

Reexamination Certificate

active

06624579

ABSTRACT:

TECHNICAL FIELD
This invention relates to a magnetron drive power supply with a magnetron of a microwave oven, etc., as a load.
BACKGROUND ART
Magnetron drive power supplies in related arts will be discussed with reference to the accompanying drawings.
FIG. 29
is a circuit diagram of a magnetron drive power supply in a related art. The magnetron drive power supply in the related art once converts a commercial power supply
1
of AC into DC voltage through a diode bridge
2
, an inverter circuit
5
generates a high-frequency voltage in a primary winding of a high-voltage transformer
6
by turning on and off semiconductor switch elements
3
and
4
, and the high-voltage transformer
6
excites a high-frequency high voltage in a secondary winding. This high-frequency high voltage is rectified to DC high voltage by a high-voltage rectification circuit
7
and the DC high voltage is applied to a magnetron
8
. The magnetron
8
is driven at the DC high voltage and generates a radio wave of 2.45 GHz.
FIG. 30
is a drawing to show the operation waveform of the magnetron drive power supply in the related art. AC voltage V
1
of the commercial power supply
1
is rectified to a DC voltage through the diode bridge
2
. An inductor
9
and a capacitor
10
make up a smoothing circuit; the capacitor
10
has a capacity to such an extent that it can hold DC voltage with respect to the inverter circuit
5
operating in the range of 20 kHz to 50 kHz to miniaturize the inverter circuit
5
, and does not have a capability of smoothing for the frequency of the commercial power supply
1
(50 Hz or 60 Hz). Thus, voltage V
10
of the capacitor
10
shows a waveform provided by simply full-wave rectifying the commercial power supply
1
and shows a pulsation waveform fluctuating from almost 0 voltage to the maximum voltage of the commercial power supply
1
. Since the inverter circuit
5
operates based on the pulsating voltage V
10
of the capacitor
10
, the envelope waveform of the high-frequency voltage generated in the primary winding of the high-voltage transformer
6
becomes a waveform as shown in V
6
(Lp) and in the time period over which the voltage V
10
of the capacitor
10
is low, likewise only a low voltage can be generated.
On the other hand, the operation characteristic of the magnetron
8
shows a nonlinear voltage current characteristic such that an anode current does not flow if a predetermined voltage or more is not applied between an anode and a cathode, as shown in FIG.
31
. Therefore, in the time period over which the voltage generated in the primary winding of the high-voltage transformer
6
is low, the voltage excited in the secondary winding also becomes low at the same time and thus in the waveform of a voltage V
8
applied to the magnetron
8
, a time period over which the voltage does not reach VAK (TH) occurs, as shown in the figure. In the time period, the magnetron
8
stops oscillation and thus power is not consumed in the magnetron
8
of the load and thus a current I
1
of the commercial power supply
1
does not flow. Consequently, the waveform of the current I
1
of the commercial power supply
1
becomes a waveform having much distortion having time periods over which the current becomes 0, as shown in
FIG. 30
, causing the power factor of the magnetron drive power supply to be lowered and a harmonic current to be generated in input current.
To solve such a problem, a circuit configuration shown in
FIG. 32
is proposed wherein an active filter circuit
13
is placed preceding an inverter circuit
5
for improving the power factor of the input current and suppressing the harmonic. The active filter circuit
13
forms a so-called step-up chopper circuit and can control step-up voltage based on the on time ratio of a semiconductor switch element
17
.
The operation will be discussed with reference to FIG.
33
. The voltage of a commercial power supply
1
shows an AC voltage waveform as shown in V
1
. The active filter circuit
13
controls voltage provided by full-wave rectifying the AC voltage V
1
through a diode bridge
2
by turning on/off the semiconductor switch element
17
, thereby generating step-up voltage in a capacitor
15
. The step-up voltage V
15
changes in ripple factor depending on the capacity of the capacitor
15
, but can be prevented from lowering completely to 0 like V
10
in the configuration in FIG.
29
. Thus, voltage V
6
(Lp) generated in the primary winding of a high-voltage transformer
6
can be generated a predetermined value or more if the voltage of the commercial power supply
1
is in the proximity of 0. Consequently, it is made possible to always hold the voltage applied to the magnetron
8
at oscillation-possible voltage or more. Consequently, an input current
11
can be made a waveform roughly like a sine wave having no time periods over which the current becomes 0, as shown in the figure, and it is made possible to improve the power factor of the input and suppress a harmonic current.
However, in such a configuration, the active filter circuit
13
is added to the inverter circuit
5
and the power conversion process becomes rectification to boosting to harmonic generation (inverter circuit) to high-voltage rectification. Thus, the power conversion process grows and degradation of the conversion efficiency and upsizing of the circuitry introduce a problem.
Then, JP-A-10-271846 discloses a configuration intended for sharing components and circuit functions.
FIG. 34
is a circuit diagram to show the circuit configuration in JP-A-10-271846. According to the circuit configuration, the boosting function operation and the inverter function operation are performed at a time for improving the power factor of input and simplifying the circuit configuration.
FIGS. 35 and 36
are drawings to describe the circuit operation. FIGS.
35
(
a
) to (
d
) are drawings to describe energization paths as semiconductor switch elements Q
1
and Q
2
are turned on and off, and
FIG. 36
is an operation waveform chart corresponding thereto. The circuit operation will be discussed with reference to
FIGS. 35 and 36
. For convenience of the description that follows, the voltage polarity of a commercial power supply
1
is in the direction shown in the figure and the semiconductor switch element Q
2
is on in the beginning. When the semiconductor switch element Q
2
is on, a current flows over a path of a capacitor C
2
to the commercial power supply
1
to an inductive load circuit
19
to the semiconductor switch element Q
2
as shown in
FIG. 35
(a), and a current IQ
2
of the semiconductor switch element Q
2
increases monotonously as shown in FIG.
36
(
a
). If the semiconductor switch element Q
2
is turned off in a predetermined time, the current path makes a transition to the state in FIG.
35
(
b
) and a capacitor C
1
is charged as a current flows over a path of a diode D
2
to the commercial power supply
1
to the inductive load circuit
19
to a diode D
3
to the capacitor C
1
. When all energy stored in the inductive load circuit
19
is emitted, a current flow over a path of the capacitor C
1
to the semiconductor switch element Q
1
to the inductive load circuit
19
to the commercial power supply
1
to the capacitor C
2
in FIG.
35
(
c
) with the capacitor C
1
as a power supply. If the semiconductor switch element Q
1
is turned off in a predetermined time, the inductive load circuit
19
attempts to allow a current to flow in the same direction and thus a current flow over a path shown in FIG.
35
(
d
) (commercial power supply
1
to capacitor C
2
to diode D
4
to inductive load circuit
19
) and the capacitor C
1
is charged by energy stored in the inductive load circuit
19
. When all energy stored in the inductive load circuit
19
is emitted, again a current flows over the path in FIG.
35
(
a
) and the circuit operation is continued. Although not disclosed in JP-A-10-271846, the capacity relationship as shown in Expression
1
is required between the capacitors C
1
and C
2
to realize the operation:
C
1
&g

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