Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-05-16
2001-08-28
Berhana, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S131000
Reexamination Certificate
active
06282102
ABSTRACT:
TECHNICAL FIELD
This invention relates to a drive circuit for a switching power supply, and more particularly to a drive circuit for a switching power supply, for delivering a drive signal to a main switching element via a drive transformer.
BACKGROUND ART
Conventionally, a power supply unit
41
shown in
FIG. 6
is well known as a switching power supply unit including a drive circuit of the above-mentioned kind. The power supply unit
41
is comprised of a control circuit
2
for generating a switching signal SS, a drive circuit
42
for carrying out power amplification of the switching signal SS, and a main circuit
4
for generating a DC voltage Vo by performing switching operation in synchronism with a drive signal SD delivered from the drive circuit
42
.
The drive circuit
42
is comprised of a pair of an npn transistor
11
and a pnp transistor
12
forming a complementary circuit, a pull-up resistor
13
, a capacitor
14
for blocking DC current, and a drive transformer
15
having a primary winding
15
a
and a secondary winding
15
b
at a turns ratio of 1:1. Further, the main circuit
4
is comprised of an n-channel MOS FET
21
serving as a main switching element, bias resistors
22
and
23
, a switching transformer
24
, and a rectifying and smoothing circuit
25
.
In the power supply unit
41
, the control circuit
2
receives a feedback signal SF delivered from the main circuit
4
, and outputs a switching signal SS having a pulse width (or frequency) dependent on the feedback signal SF. Then, the transistors
11
and
12
operate alternately in synchronism with the switching signal SS delivered from the control circuit
2
, to carry out the power amplification of the switching signal SS for generation of the drive signal SD, and delivers the generated drive signal SD to the primary winding
15
a
of the transformer
15
via the capacitor
14
. As a result, the drive signal SD is induced in the secondary winding
15
b
of the transformer
15
and supplied to the FET
21
. In this case, if the duty ratio of the switching signal SS is 50%, the FET
21
has a voltage VGS of VCC/2 between its gate and source during the ON time period TON of the switching signal SS, as shown in
FIG. 5
, and a voltage VGS of −VCC/2 during an OFF time period TOFF of the same. As a result, during the ON time period TON over which the voltage VGS is above a threshold voltage Vth, the FET
21
is controlled to an ON state to output a drain current ID shown in the figure to the primary winding
24
a
of the transformer
24
. Then, the rectifying and smoothing circuit
25
rectifies and smoothes a voltage induced in the secondary winding
24
b
of the transformer
24
, to thereby generate a DC voltage Vo.
DISCLOSURE OF INVENTION
From a study of the above prior art, the inventor found out the following problems. In the conventional power supply unit
41
, if the duty ratio of the switching signal SS is e.g. 50%, the FET
21
has the voltage of ±VCC/2 applied between the gate and source thereof during operation of the control circuit
2
. Accordingly, since the transformer has the turns ratio of 1:1, the voltage of VCC/2 is constantly applied between opposite ends of the capacitor
14
. This means that the capacitor
14
constantly stores energy. For this reason, when the control circuit
2
stops operating, the stored energy is released from the capacitor
14
, whereby a series resonance phenomenon dependent on a capacitance of the capacitor
14
and an excitation inductance of the primary winding
15
a
of the transformer
15
occurs in a closed circuit formed by the capacitor
14
, the primary winding
15
a
, and the emitter and collector of the transistor
12
. In this case, if the capacitance of the capacitor
14
is represented by a value C and the excitation inductance of the primary winding
15
a
by a value L, a series resonance frequency f in the series resonance phenomenon is expressed by the following formula (1):
f=
1/(2·&pgr;·(
L·C
)
0.5
) (1)
Therefore, during production of the series resonance, a series resonance voltage having a voltage waveform W
11
is induced in the secondary winding
15
b
of the transformer
15
, and the voltage waveform W
11
is applied between the gate and source of the FET
21
. In this case, since the excitation inductance of the primary winding
15
a
is large, the series resonance frequency f provides a period which is extremely longer than a period (TS) of the switching signal SS. For this reason, during time periods between times t
11
and t
12
and between times t
13
and t
14
, over each of which the voltage waveform W
11
is above the threshold voltage Vth, an excessively large amount of drain current ID flows to the FET
21
as shown by respective current waveforms W
12
and W
13
. As a result, magnetic saturation occurs in the transformer
24
, which, for instance, causes a further excessively large amount of drain current ID to flow, setting up a vicious circle. Thus, the conventional power supply unit
41
suffers from a problem that the FET
21
and the transformer
24
can be broken due to the series resonance phenomenon which occurs when the control circuit
2
stops operating. To avoid this vicious circle, the conventional power supply unit
41
controls the duty ratio of the switching signal SS within a range below 50% to reduce the energy stored in the capacitor
14
to thereby reduce resonance energy during production of the series resonance. For this reason, the conventional power supply unit
41
is not allowed to control the duty ratio of the switching signal SS to 50% or more, and hence the range of switching control is narrow.
The invention has been made to solve the above problems, and it is a main object of the invention to provide a drive circuit for a switching power supply, which is capable of preventing breakage of a main switching element and at the same time permits an increase in the range of switching control.
The drive circuit for a switching power supply includes a driving element for generating a drive signal for driving a main switching element, and a transformer for delivering the drive signal input via a capacitive element to a primary winding thereof, to the main switching element from a secondary winding thereof, the drive circuit being characterized by comprising an inductive element having an inductance value smaller than at least a value of an excitation inductance of the primary winding and allowing series resonance to be produced between the capacitive element and the inductive element.
This drive circuit increases the resonance frequency in the resonance phenomenon caused by the capacitive element and the inductive element upon stoppage of operation for generating power. Therefore, since the period of the series resonance frequency is shortened, a time period over which the resonance voltage waveform is above a threshold voltage of the switching element becomes shorter. For this reason, the switching element becomes difficult to turn on, and even when the switching element is turned on, the maximum value of a current flowing is reduced, whereby magnetic saturation of the switching transformer connected to the main switching element is prevented and hence breakage of the main switching element and the switching transformer is prevented. Further, by setting a capacitance value of the capacitive element to a suitable value, it is possible to minimize the amount of energy stored in the capacitive element. In this case, series resonance energy itself is reduced, which ensures prevention of the magnetic saturation of the switching transformer. As a result, the duty ratio of the drive signal can be increased, whereby it is also possible to increase the range of switching control.
Preferably, the capacitive element and the inductive element have respective element constants that determine a resonance frequency exceeding one fifth of a frequency of the drive signal. By this construction, the ON time period of the switching element is further shortened. In this case, since the ma
Matsuzaka Atsushi
Minamisawa Toshitaka
Berhana Adolf Deneke
Greenblum & Bernstein P.L.C.
Nagano Japan Radio Co. Ltd.
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