Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
1999-03-11
2001-07-03
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S021030
Reexamination Certificate
active
06256214
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to power converter circuits, and more particularly to self-driven synchronous rectifiers easily adapted to all types of circuit topologies.
BACKGROUND OF THE INVENTION
As logic integrated circuits (ICs) have migrated to lower working voltages in the search for lower power consumption and higher operating frequencies, and as overall system sizes have continued to decrease, power supply designs with smaller size and higher efficiency are in demand. In an effort to improve efficiencies and increase power densities, synchronous rectification has become necessary for these type of applications. Synchronous rectification refers to using active devices such as the MOSFET as a replacement for Schottky diodes as rectifier elements in circuits to reduce conduction power losses in the secondary rectifiers. Recently, self-driven synchronous schemes have been widely adopted in the industry as the desired method for driving the synchronous rectifiers in DC/DC modules for output voltages of 5 volts and below. Self-driven synchronous schemes provide a simple, cost effective and reliable method of implementing synchronous rectification.
Most of these schemes are designed to be used with a very particular set of topologies commonly known as “D, 1-D” (complementary driven) type topologies. See Cobos, J. A., et al., “Several alternatives for low output voltage on board converters”, IEEE APEC 98 Proceedings, at pp. 163-169. See also U.S. Pat. No. 5,590,032 issued on Dec. 31, 1996 to Bowman et al. for a Self-synchronized Drive Circuit for a Synchronous Rectifier in a Clamped-Mode Power Converter, and U.S. Pat. No. 5,274,543 issued on Dec. 28, 1993 to Loftus entitled Zero-voltage Switching Power Converter with Lossless Synchronous Rectifier Gate Drive. In these types of converters, the gate of the devices is referenced to ground, and the power transformer signal in the secondary winding has the correct shape and timing to directly drive the synchronous rectifiers with minimum effort. Furthermore, the rectifier is configured to insure the synchronous rectifier gate signals do not float relative to secondary ground and are easy to drive.
FIG. 1
shows an example of this family of converters, with an active clamp forward circuit
10
and self-driven synchronous rectification provided by synchronous rectification circuitry
12
comprising two synchronous rectifiers SQ
1
and SQ
2
coupled between the secondary winding of the transformer
18
and the output, V
out
. As shown in
FIG. 2
, the transformer signal
20
for these types of converters has a square shape with two very recognizable intervals, each corresponding to the “on” time of one of the synchronous rectifiers SQ
1
and SQ
2
.
In topologies such as the hard-switched half-bridge (HB), the full-bridge (FB) rectifiers, and the push-pull topologies and non-“D, 1-D” type topologies (e.g. clamp forward with passive reset), the transformer voltage has a recognizable zero voltage interval, making it undesirable to implement self-driven synchronous rectification. As a result, it is necessary to use an external drive circuit with these circuit topologies. Changing the placement of the synchronous rectifiers relative to the transformer to simplify the driving scheme may result in a floating transformer winding with respect to ground, which generally increases common mode current between the primary and secondary circuits, causing increased EMI noise. Rectifier circuits employing synchronous rectification generally are reconfigured away from the EMI-preferred configuration.
What is needed in the art is a circuit and method for providing synchronous rectification for the secondary side of a transformer that is suitable for use with a wide range of circuit topologies and has low EMI noise.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as a self-driven synchronous rectification scheme with synchronous rectifiers having a floating gate. The scheme may be easily adapted to all type of topologies, including hard-switched HB, FB and push-pull converters, for which no efficient self-driven synchronous rectification scheme was previously available.
The present invention is a self-driven synchronous rectifier circuit for a power converter, the circuit including a first transformer having a primary winding and a secondary winding, the secondary winding having a first terminal and a second terminal. A first synchronous rectifier is coupled to the first transformer secondary winding first terminal and has a control terminal floating relative to ground. A first drive circuit is coupled to the first synchronous rectifier floating control terminal and controls the first synchronous rectifier. A first control signal is coupled to the first drive circuit, wherein the first control signal controls the first drive circuit as a function of a voltage polarity reversal across the first transformer. The first control signal may be a signal from the first transformer secondary winding second terminal, or may be a signal from a second transformer secondary winding terminal of a signal transformer.
The circuit may also further include a second synchronous rectifier coupled to the first transformer secondary winding second terminal having a control terminal floating relative to ground, and a second drive circuit coupled to the second synchronous rectifier floating control terminal and controlling the second synchronous rectifier. A second control signal may be coupled to the second drive circuit, wherein the second control signal controls the second drive circuit as a function of a voltage polarity reversal across the first transformer. The first drive circuit may include a first switch and a second switch in a totem pole arrangement, and the second drive circuit may include a third switch and a fourth switch in a totem pole arrangement, where the switches are MOSFETs.
Also disclosed is a method of rectifying a varying voltage from a power converter using a self-driven synchronous rectifier circuit with a first transformer having a primary winding and a secondary winding, where the secondary winding has a first and second terminal. The method includes the steps of providing a varying signal to the primary winding of the first transformer, and a first synchronous rectifier having a control terminal conducting current via the first transformer secondary winding, where the control terminal floats relative to ground. A first drive circuit controls the first synchronous rectifier, and a first control signal controls the first drive circuit as a function of a voltage polarity reversal across the first transformer. A second synchronous rectifier having a control terminal conducts current via the first transformer secondary winding, and the control terminal floats relative to ground. A second drive circuit controls the second synchronous rectifier, and a second control signal controls the second drive circuit as a function of a voltage polarity reversal across the first transformer.
REFERENCES:
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Tabisz, W., Lee, F.C., Chen, D., “A MOSFET Resonant Synchronous Rectifier for High Frequency DC/DC Converters”, IEEE PESC Aug. 1990 Proceedings, pp. 769-779.
Jitaru, I.D., “Constant Frequency, Forward Converter with Resonant Transitions”, HFPC Jun. 1991 Proceedings, pp. 282-292.
Cobos, J.A., et al., “Several Alternatives for Low Output Voltage on Board Converters”, IEEE APEC Sep. 1998 Proceedings, pp. 163-169.
Murakami, N., et al., “A Highly Efficient, Low-profile 300 W Power Pack for Telecommunications Systems”, IEEE APEC May 1994 Proceedings, pp. 786-792.
Djekic, O., Brkovic, M., “Synchronous Rectif
Farrington Richard W.
Hart William
Ericsson Inc.
Navarro Arthur I.
Nguyen Matthew
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