Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-11-05
2001-02-13
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S021030, C363S127000
Reexamination Certificate
active
06188592
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to logic integrated circuits, and more particularly to a simplified externally-driven synchronous rectification scheme for a DC to DC power converter, easily adapted to all types of circuit topologies. More particularly, the present invention provides a scheme for synchronous rectification that simplifies the complexity of the timing circuitry.
BACKGROUND OF THE INVENTION
As logic integrated circuits (IC) have migrated to lower working voltages in the search for higher operating frequencies, and as overall system sizes have continued to decrease, power supply designs with smaller and higher efficiency power modules 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 has gained great popularity in the last ten years as low voltage semiconductor devices have advanced to make this a viable technology.
Synchronous rectification refers to using active devices such as the MOSFET as a replacement for diodes as rectifier elements in circuits. 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.
Most of these self-driven 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 power transformer signal in the secondary winding has the correct shape and timing to directly drive the synchronous rectifiers with minimum modifications.
In topologies such as the hard-switched half-bridge (HB) and 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. Using the transformer voltage to drive the synchronous rectifiers results in conduction of the parasitic anti-parallel diode of the MOSFETs used for the synchronous rectifiers for a significant portion of the freewheeling interval, negatively affecting the efficiency of the module, which is undesired. Some self-driven implementations for the resonant reset forward have been reported. See Murakami, N. et al., “A highly efficient, low-profile 300 W power pack for telecommunications systems”, IEEE APEC 1994 Proceedings, at pp. 786-792 and Yamashita, N. et al., “A compact, highly efficient 50 W on board power supply module for telecommunications systems”, IEEE APEC 1995 Proceedings, at pp. 297-302. In these implementations, the resonant reset interval has been adjusted to provide the correct gate-drive signal during the freewheeling interval. Therefore, the externally-driven implementation offers a better solution for synchronous rectification in many instances. However, the prior art externally-driven synchronous rectification is both complex and costly.
The implementation of an externally driven scheme for non “D, 1-D” type topologies, for example, requires a timing network that will allow the proper adjustment for the synchronous rectifier driving pulses relative to the primary drive, a signal transformer or opto-coupler to transfer the timing information between primary and secondary, an inverting stage, and a driving stage. The inverting stage is required to generate the proper driving pulses for the synchronous rectifier that handles the freewheeling current. The complexity and cost of such an external driving scheme has deterred the electronics industry from embracing the externally driven synchronous rectifier. Thus, what is needed is a simplified implementation of the externally driven synchronous rectifier.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as an externally driven synchronous rectification scheme that can be easily adapted to all types of topologies, but particularly adaptable to push-pull converters, two-switch forward, conventional forward converters ( hard-switched half-bridge (HB) and the full-bridge (FB) rectifiers), and non-“D, 1-D” type topologies (e.g. clamp forward with passive reset) for which no efficient externally-driven synchronous rectification scheme was previously available.
In one embodiment, disclosed is an externally-driven synchronous rectifier circuit for a DC to DC power converter. The circuit includes a primary transformer having a primary and secondary winding, the secondary winding having a first terminal and a second terminal. The circuit includes a first synchronous rectifier, having a gate, coupled to the second terminal of said primary transformer and having a control terminal, and a second synchronous rectifier coupled to the first terminal of said primary transformer and having a control terminal. An external drive circuit includes a timing transformer having a primary and secondary winding, the secondary winding having a first and second terminal. A first switch is controllably coupled to the second synchronous rectifier control terminal, and a second switch is also controllably coupled to the second synchronous rectifier control terminal. The circuit further comprises an inductor in series with the first terminal of the primary transformer and the output voltage terminal, as well as a capacitor coupled in parallel with the inductor. Because the first synchronous rectifier is not coupled to the timing transformer, only the second synchronous rectifier can receive timing information from the external drive circuit.
In another embodiment, disclosed is an externally-driven synchronous rectifier circuit for a DC to DC power converter. The circuit, similar to the embodiment described above, further comprises a third and fourth switch wherein the third switch is coupled to the second synchronous rectifier and the fourth switch coupled to the first synchronous rectifier. Each switch contains a gate, drain, and source. The secondary winding of the timing transformer comprises a center tap connected to the return voltage terminal. The gate of the first switch is connected to the first end of the secondary winding of the timing transformer while the gate of the second switch is connected to the first end of the primary transformer, therefore both switches can receive timing information from the external drive circuit and thus both synchronous rectifiers can receive timing information from the external drive circuit.
Other embodiments of the present invention include implementation as a full wave rectifier. Further embodiments include utilizing current limiting resistors for limiting the drive current of the circuit, additional switches for limiting the gate voltage, and additional capacitors for minimizing voltage overshoot across the synchronous rectifiers.
Also disclosed is a method of rectifying a varying DC signal of a DC—DC power converter using an externally driven synchronous rectifier circuit with a 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 the varying DC signal to the primary winding of the transformer, a first synchronous rectifier controllably conducting current via a second terminal of the second winding and a first switch controlling the first synchronous rectifier. A second synchronous rectifier controllably conducts current via t
Farrington Richard W.
Hart William
Svardsjo Claes
Ericsson Inc.
Navarro Arthur I.
Patel Rajnikant B.
Wong Peter S.
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