Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-05-14
2002-04-09
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S089000
Reexamination Certificate
active
06370044
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a synchronous rectifier circuit, and more particularly to a self-driven synchronous rectifier circuit.
BACKGROUND OF THE INVENTION
Synchronous rectification is widely applied in a low voltage and high current DC-to-DC converter. Because the on-state voltage drop of a low voltage power MOSFET transistor is much lower than that of a diode, power MOSFET is used as synchronous switch to improve the overall conversion efficiency. As it is known in the art, in the customarily used forward DC-to-DC converter, the MOSFET synchronous switch is driven by the secondary windings of a transformer because the self-driven technique has inherent simplicity. Meanwhile, the duty ratio is so small that the continuous conduction of the MOSFET switch would not be effectively conducted. In such condition, the load current will be diverted through the body diode of the MOSFET switch, which causes additional loss and thus reduces the overall conversion efficiency. In order to solve the above drawbacks, a phase-lock loop circuit was developed by International Rectifier (U.S. Pat. No. 6,026,005). The application of the phase-lock circuit is restricted because a specific gate-driving chip and the corresponding peripheral circuit are required, which results in high cost.
Recently, a secondary-winding self-driving synchronous rectifier circuit is developed. FIG.
1
(A) is a simplified equivalent circuit illustrating the self-driving circuit according to the prior art. Referring to FIG.
1
(A), the capacitor C is a gate parasitic capacitance of a MOSFET switch, the switch Sa is an auxiliary MOSFET switch, and V
1
is a driving signal. FIG.
1
(B) is a timing diagram of waveforms in the circuit of
FIG. 1
(A). Please refer to
FIG. 1
(B). Before t=t
0
, the switch Sa is turned off and the initial voltage of the capacitor C is zero. At t=t
0
, the input signal V
1
is positive, and the positive current passes through the diode D
1
for charging the capacitor C to an amplitude of V
1
. At t=t
1
, the input signal is zero and the diode D
1
is biased off. The electric charges stored in the capacitor C is maintained at a voltage V
2
. At t=t
2
, the switch Sa is turned on; therefore, the electric charges in the capacitor C discharges through the switch Sa such that the voltage V
2
decreases to zero. It will be found that although the driving signal V
1
is disappeared from t=t
1
to t=t
2
, the synchronous rectifier MOSFET switch still keep conducting.
FIGS.
2
(A) and
2
(B) are respectively circuit diagram and timing waveform diagram of the self-driven synchronous rectifier for a forward DC-to-DC converter according to the prior art. The switch S is a main switch of a forward converter, the switches S
1
and S
2
are synchronous rectifier MOSFET switches and the switch Sa is an auxiliary MOSFET switch. The self-driving function for the gate of the MOSFET switch S
2
is performed by employing the auxiliary MOSFET switch Sa and the diode D
1
. The operation process will be described as follows.
From t=t
0
to t=t
1
, the main switch S is turned on. The voltage of the secondary winding is positively applied on the synchronous rectifier MOSFET switch S
1
and the auxiliary MOSFET switch Sa such that the MOSFET switch S
1
and the auxiliary MOSFET switch Sa are conducted. The conduction of the switch Sa causes the switch S
2
to be shorted and turned off. Therefore, the output current passes through the MOSFET switch S
1
.
At t=t
1
, the main switch S is turned off and the magnetizing current flows towards the magnetic reset (MR) circuit. The synchronous rectifier MOSFET switch S
1
and the auxiliary MOSFET switch Sa are biased off. The voltage on the secondary winding of the transformer T passes through the diode D
1
and charges to the gate of the MOSFET switch S
2
. Therefore, the output current passes through the MOSFET switch S
2
.
At t=t
2
, the reset of the transform T is finished. The voltage on the secondary winding changes to zero and the switch Sa is still off. Since the diode D
1
is biased off, the electric charges in the MOSFET switch S
2
maintains constant and thus the MOSFET switch S
2
continuously conducts.
At t=t
0
′, the voltage on the secondary winding of the transformer T changes to a positive value. The MOSFET switch Sa is turned on to discharge the gate capacitance of the MOSFET switch S
2
and allow the switch S
2
to be turned off. Therefore, the MOSFET switch S
1
is turned on because of the positive voltage on the secondary winding.
Then, a new switching cycle is repeated.
A main problem occurs at the time when the MOSFET switch S
2
is being turned off. When the voltage on the secondary winding of the transformer T changes from a negative value to a positive value, the MOSFET switch S
1
and the MOSFET switch Sa are simultaneously conducted, while the switch S
2
is turned off until its gate is discharged to a voltage below a turn-on threshold voltage. That is to say, the turn-off of the switch S
2
lags behind the turn-on of the switch S
1
. Therefore, a cross conducting period exists between the switch S
1
and the switch S
2
, which increases the conductive loss.
Therefore, the present invention provides a self-driven synchronous rectifier circuit for overcoming the problems described above.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a self-driven synchronous rectifier circuit to reduce the simultaneous conduction described above.
It is another object of the present invention to provide a self-driven synchronous rectifier circuit with simplicity.
It is another object of the present invention to provide a self-driven synchronous rectifier circuit for increasing the stability and reliability of the driving circuit.
In accordance with an aspect of the present invention, there is provided a self-driven synchronous rectifier circuit applied to a forward converter. The circuit includes a transformer, a first synchronous rectifier switch, a second synchronous rectifier switch and an auxiliary switch. The transformer has a primary winding and a secondary winding for converting an input voltage into an output voltage, wherein the secondary winding further includes a driving winding having a center tap. The first synchronous rectifier switch and the second synchronous rectifier switch are connected to the secondary winding for rectifying the output voltage. The gate terminal of the auxiliary switch is connected to the gate terminal of the first synchronous rectifier switch and the positive end of the driving winding, the source terminal thereof is connected to the drain terminal of the first synchronous rectifier switch and the negative end of the driving winding, and the drain terminal thereof is connected to the gate terminal of the second synchronous rectifier switch.
Preferably, each of the first synchronous rectifier switch, the second synchronous rectifier switch and the auxiliary switch is MOSFET switch.
Preferably, the circuit further includes a saturated inductor connected to the secondary winding.
Preferably, the positive end of the driving winding and the positive end of the primary winding have the same polarities.
Preferably, the forward converter further includes a dual switch forward converter.
In accordance with another aspect of the present invention, there is provided a self-driven synchronous rectifier circuit applied to a forward converter. The self-driven synchronous rectifier circuit includes a transformer having a primary winding and a secondary winding for converting an input voltage into an output voltage, a first synchronous rectifier switch and a second synchronous rectifier switch connected to the secondary winding for rectifying the output voltage and an auxiliary switch, wherein the gate terminal thereof is connected to the gate terminal of the first synchronous rectifier switch and the positive end of the secondary winding, the source terminal thereof is connected to the drain terminal o
Gu Yilei
Huang Guisong
Zhang Alpha J.
Delta Electronics , Inc.
Riley Shawn
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