Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-12-12
2003-11-25
Berhane, Adolf D. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021120
Reexamination Certificate
active
06654258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to resonant power supply circuits, and more particularly, the present invention relates to a resonant power supply circuit for generating a high voltage to be supplied to a cathode ray tube (CRT) or other electronic apparatus.
2. Description of the Related Art
FIG. 8
shows an example of a resonant power supply circuit related to the present invention. A resonant power supply circuit
10
includes a flyback transformer
12
. The drain of a field-effect transistor (FET)
14
functioning as a switching device is connected to a first end of a primary winding of the flyback transformer
12
, which has a source that is grounded. Between the drain and the source of the FET
14
, a resonant capacitor
16
and a damper diode
18
are connected in parallel. The anode of the damper diode
18
is connected to the source side of the FET
14
, and the cathode is connected to the drain side of the FET
14
. A second end of the primary winding of the flyback transformer
12
is connected to a power supply
20
.
A secondary winding of the flyback transformer
12
is connected to a voltage divider circuit
24
through a diode
22
. A signal having a voltage that is divided by the voltage divider circuit
24
is input to a control circuit
26
, thus generating a control signal to be input to the gate of the FET
14
.
FIG. 9
shows waveforms of signals at sections of the resonant power supply circuit
10
. Specifically, trace (a) shows the waveform of a voltage at point A in
FIG. 8
; trace (b) shows the waveform of a current in the primary winding of the flyback transformer
12
; and trace (c) shows the waveform of a signal for controlling the FET
14
. When the FET
14
is turned ON at t
0
, current flows from the power supply
20
to the primary winding of the flyback transformer
12
and the FET
14
. As a result of the current, electromagnetic energy is stored in the primary winding of the flyback transformer
12
.
When the FET
14
is turned OFF at t
1
, current flows from the primary winding of the flyback transformer
12
to the resonant capacitor
16
, thus causing the primary winding of the flyback transformer
12
and the resonant capacitor
16
to resonate with each other. As a result, as shown in trace (a) in
FIG. 9
, a flyback pulse is generated. The flyback pulse reaches a maximum value when the entire electromagnetic energy stored in the flyback transformer
12
is converted into electrostatic energy in the resonant capacitor
16
.
When the flyback pulse reaches the maximum value, the electrostatic energy in the resonant capacitor
16
is inversely converted into electromagnetic energy in the primary winding of the flyback transformer
12
, and the voltage of the flyback pulse decreases. When the flyback pulse becomes zero at t
2
, the damper diode
18
is turned ON, thus causing current to flow from the ground side to the primary winding of the flyback transformer
12
. When the voltage at point A recovers to the voltage of the power supply
20
, the damper diode
18
is turned OFF, and the current becomes zero. When the FET
14
is turned ON at t
4
, current flows from the power supply
20
to the primary winding of the flyback transformer
12
, thus returning to the initial state at t
0
. The operation as described above is repeated to maintain the circuit operation. The flyback pulse is boosted by the flyback transformer
12
, which results in outputting a high voltage from the secondary winding.
At t
3
in which current becomes zero, resonance with the primary winding of the flyback transformer
12
occurs due to the capacity of the resonant capacitor
16
and parasitic capacity in the FET
14
. Between t
3
to t
4
, a ringing pulse is generated. Such a ringing pulse causes noise. In order to prevent the generation of a ringing pulse, as shown in
FIG. 1
that illustrates another related art device, a clamping circuit
28
is provided. When a ringing pulse starts to be generated, the clamping circuit
28
causes both ends of the primary winding of the flyback transformer
12
to have the same voltage, thus preventing resonance.
The clamping circuit
28
is defined by a series circuit including a diode
30
and an FET
32
that functions as a second switching device. The clamping circuit
28
is connected in parallel to the primary winding of the flyback transformer
12
. The operation of the FET
32
is controlled by the control circuit
26
.
Referring to
FIG. 10
, a control method includes a method of simultaneously turning ON the first FET
14
and the second FET
32
, the first FET
14
functioning as the first switching device for generating a flyback pulse and the second FET
32
functioning as the second switching device used in the clamping circuit
28
, when the voltage at point A is zero. After the first FET
14
is turned OFF, the second FET
32
is turned OFF.
According to this method, the second FET
32
is turned ON to cause both ends of the primary winding of the flyback transformer
12
to have the same voltage. Even when the first FET
14
is turned ON and current flows through the primary winding, the voltage at point A remains at zero. When the first FET
14
is turned OFF and electromagnetic energy in the primary winding of the flyback transformer
12
is converted into electrostatic energy in the resonant capacitor
16
, the voltage at point A becomes equal to the voltage of the power supply
20
. When the second FET
32
is turned OFF, a flyback pulse is generated.
According to the control method as described above, the first FET
14
can be operated when the voltage at point A is zero, thus preventing the generation of a ringing pulse. When the first FET
14
is turned OFF, current flowing through the primary winding of the flyback transformer
12
reaches its maximum level. This current flows backward by a route passing through the diode
30
and the second FET
32
. A circuit loss caused by the backflow is greater than that in a case in which no clamping circuit is provided.
FIG. 11
shows another control method. According to this method, the second FET
32
is turned ON when the voltage at point A is zero. In this state, when current in the primary winding of the flyback transformer
12
becomes zero, the voltage at point A becomes equal to that of the power supply
20
because the voltages at both ends of the primary winding are clamped. After the first FET
14
is turned ON, the second FET
32
is turned OFF.
According to the control method as described above, while backflow current as shown in
FIG. 10
is eliminated, a large switching loss is caused since the first FET
14
is turned ON at the same time the voltage at point A becomes equal to that of the power supply
20
. Although a voltage ripple is suppressed, current noise is generated. Such current noise in turn generates screen noise in a CRT and causes an increase in temperature of a flyback transformer. It is thus necessary to provide a damping circuit to remove the current noise. In such a case, a considerably great loss is caused in the damping circuit.
SUMMARY OF THE INVENTION
In order to solve the problems described above, preferred embodiments of the present invention provide a resonant power supply circuit that effectively minimizes a circuit loss and noise.
According to a preferred embodiment of the present invention, a resonant power supply circuit includes a flyback transformer, a power supply for supplying power to a primary winding of the flyback transformer, a first switching device for controlling current which flows from the power supply to the primary winding of the flyback transformer, a resonant capacitor for generating a flyback pulse by resonating with the primary winding of the flyback transformer when the first switching device is OFF, and a clamping circuit including a diode and a second switching device which is connected in parallel with the primary winding of the flyback transformer, whereby the voltage between both ends of the primary winding of the flyback transformer is clamp
Berhane Adolf D.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
LandOfFree
Resonant power supply circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Resonant power supply circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Resonant power supply circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3143218