Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-04-27
2002-08-06
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S127000
Reexamination Certificate
active
06430063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synchronous rectification circuit of a DC-DC converter to commutate the voltage induces in a secondary winding of a transformer by implementing the on-off control of a commutation side field-effect transistor and a flywheel side field-effect transistor synchronously with the on-off operation of a switching transistor connected to a primary winding of the transformer.
2. Description of the Related Art
A diode has been generally employed in a commutating circuit of conventional DC-DC converters. However, the power loss due to the forward voltage drop of the diode is not negligible but large. Thus, in recent years, a Schottky diode small in power loss due to the forward voltage drop has been extensively employed.
An FET (Field-Effect Transistor) of at most several m&OHgr; in ON-resistance was developed. Thus, the power loss can be more reduced by using such an FET in a commutating circuit than by using the Schottky diode.
Thus, a configuration was proposed, in which the FET is connected to a primary winding of a transformer as a switching transistor, and is used in the commutating circuit of the DC-DC converter. In this case, the FET as the switching transistor implements the on-off control of the FET of the commutating circuit in a synchronous manner with the on-off timing. The commutating circuit can thus commutate the induced voltage in the secondary winding of the transformer with less power loss.
FIG. 1
is a circuit diagram of a conventional DC-DC converter, indicating a synchronous rectification circuit applied to a forward converter. In
FIG. 1
, T denotes a transformer, n
1
denotes a primary winding, n
2
denotes a secondary winding, Q
1
denotes a switching transistor (FET, Q
2
denotes a commutating side FET, Q
3
denotes a flywheel side FET, and L and C
1
denote a reactor and a capacitor to constitute a smoothing circuit, respectively. A control circuit
1
detects the output voltage Vout, and controls the ON-period of the switching transistor Q
1
by controlling the pulse width or the like. This means that the control circuit
1
controls the ON-period and the OFF-period of the switching transistor Q
1
so that the smoothed output voltage Vout is the preset voltage. More specifically, the switching transistor Q
1
turns on/off the current flowing in the primary winding n
1
of the transformer T based on the input voltage Vin. The control circuit
1
shortens the ON-period when the output voltage Vout is higher than the preset voltage, and extends the ON-period when the output voltage Vout is lower than the preset voltage. The control circuit
1
controls the output voltage Vout to be constant by the above operation. The detailed operation is described below.
When the switching transistor Q
1
is turned on, the commutating side FET Q
2
is turned on by the induced voltage in the secondary winding n
2
of the transformer T. As a result, the current flows in the capacitor C
1
via the reactor L. In this condition, the flywheel side FET Q
3
is turned off.
When the switching transistor Q
1
is turned off, the polarity of the induced voltage in the secondary winding n
2
of the transformer T is inverted, and the commutating side FET Q
2
is turned off. As a result, the flywheel side FET Q
3
is turned on, and the current attributable to the accumulated energy in the reactor L flows in the capacitor C
1
.
However, the transformer T is reset (to make the accumulated energy in the transformer T zero) through the resonance effect of the parasitic capacitance or the like or the switching transistor Q
1
with the inductance of the transformer T. After the transformer T is reset, the induced voltage in the secondary winging n
2
becomes zero. This means that the voltage applied to the gate of the flywheel side FET Q
3
becomes zero during the OFF-period of the switching transistor Q
1
, and the flywheel side FET Q
3
is turned off. Thus, the flywheel current flows via a parasitic diode (a body diode) of the flywheel side FET Q
3
, raising a problem of not taking advantage of the low ON-resistance of the FET.
Thus, the DC-DC converter shown in
FIG. 2
was proposed. In the figure, the same symbols as those in
FIG. 1
show the same parts. In
FIG. 2
, n
3
denotes a tertiary winding of a transformer T, CT denotes a current transformer, Q
4
denotes a transistor, D
7
and D
8
denote diodes (body diodes), Cgs denotes the gate-source parasitic capacitance (input capacitance), and Cgd denotes the gate-drain parasitic capacitance (input capacitance), respectively. The control circuit
1
used to implement the on-off control of the switching transistor Q
1
by detecting the output voltage Vout is omitted in the figure.
The tertiary winding n
3
is provided on the transformer T, and the induced voltage in this tertiary winding n
3
is applied to the gate of the commutating side FET Q
2
. When the switching transistor Q
1
turned on/off thereby, the commutating side FET Q
2
is turned on/off in a synchronous manner with the on-off operation of the switching transistor Q
1
.
The primary winding of the current transformer CT is connected in series to the flywheel side FET Q
3
, and the induced voltage in the secondary winding is applied to the gate of the flywheel side FET Q
3
via the diodes D
7
and D
8
. The resistor R
3
corresponds to the terminating resistor of the current transformer CT. The induced voltage in the secondary winding of the current transformer CT is applied to the Zener diode ZD
7
. The Zener diode ZD
7
suppresses the voltage across the resistor R
3
to the Zener voltage, and applies it to the base of the transistor Q
4
. Further, the Zener diode ZD
7
suppresses the voltage across the resistor R
3
to the Zener voltage, and applies it to the gate of the flywheel side FET Q
3
via the diode D
8
.
When the switching transistor Q
1
on the primary side of the transformer T is turned on, the commutating side FET Q
2
is turned on by the induced voltage in the tertiary winding n
3
of the transformer T. As a result, the current by the induced voltage in the secondary winding n
2
flows via the turned-on commutating side FET Q
2
. In this condition, the flywheel side FET Q
3
is turned off.
When the switching transistor Q
1
is turned off, the polarity of the induced voltage in the tertiary winding n
3
of the transformer T is inverted, and the commutating side FET Q
2
is turned off. The flywheel current flows via the body diode Dq
3
of the flywheel side FET Q
3
. This current flows in the primary winding of the current transformer CT. As a result, the induced voltage in the secondary winding of the current transformer CT is applied to the gate of the flywheel side FET Q
3
via the diodes D
7
and D
8
to charge the input capacitances Cgs and Cgd. When this charging voltage exceeds the threshold, the flywheel side FET Q
3
is turned on. Even when the induced voltage in the secondary winding n
2
becomes completely zero by resetting the transformer T, the flywheel side FET Q
3
continues the ON condition thereof by the induced voltage in the secondary winding of the current transformer CT.
FIG. 3
shows the flywheel side FET Q
3
illustrated in
FIG. 2
as a three-terminal element.
FIG. 4
, similar to
FIG. 3
, shows the flywheel side FET Q
3
illustrated in
FIG. 2
as a two-terminal element. More specifically, the induced voltage in the secondary winding of the current transformer CT is applied to the gate of the flywheel side FET Q
3
. This means that the flywheel side FET Q
3
is turned off by the induced voltage in the secondary winding of the current transformer CT if the forward, current flows in the body diode thereof. Thus, in this case, the flywheel side FET Q
3
can be used for the two-terminal element diode of the low voltage drop characteristic as illustrated in FIG.
4
.
FIG. 5
is a schematic representation of the current waveform and the voltage waveform on the flywheel side FET Q
3
shown in FIG.
2
.
In
FIG. 5
, Id denotes the current flowing in the primary wi
Kobayashi Kazuo
Kubota Yoshiki
Nishimura Katsuhiko
Berhane Adolf Deneke
Fujitsu Denso Ltd.
Jenkens & Gilchrist A Professional Corporation
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