Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-01-27
2002-10-01
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S127000
Reexamination Certificate
active
06459600
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to voltage converters and, more particularly to a system level solution for overcoming paralleling problems with synchronous rectified modules.
BACKGROUND OF THE INVENTION
The topology selection for the next generation on-board DC/DC converters is driven mainly by the necessity to have high power density. To achieve high power density the efficiency of the non-board DC/DC module has to be maximized. In applications where a large step-down ratio, such as 48V to 5V, 3.3V, 2.5V, etc., is required, the secondary rectifier losses dominate. In order to reduce these losses, synchronous rectification can be used. Synchronous rectification has gained great popularity in the last ten years as low voltage semiconductor devices have advanced to make this a viable technology, as described in the following references: Tabisz, W., Lee, F. C., Chen, D. “A MOSFET Resonant Synchronous Rectifier for High Frequency DC/DC Converters”, IEEE PESC 1990 Proceedings, pp. 769-779; Jitaru, I.D., “Constant Frequency, Forward Converter with Resonant Transitions”, HFPC 91 Preceedings, pp. 282-292; Cobos, J.A., et al. “Several alternatives for low output voltage on board converters”, IEEE APEC 98 Proceedings, pp. 163—169; Bowman, W., Niemela, V.A., “Self-synchronized drive circuit for a synchronous rectifier in a clamped-mode power converter”, U.S. Pat. No. 5,590,032, Dec. 31 1996; Loftus, Jr. T.P., “Zero-voltage switching power converter with lossless synchronous rectifier gate drive”, U.S. Pat. No. 5,274,543, Dec. 28, 1993; Rozman, A.F., Low loss synchronous rectifier for application to clamped-mode power converters, U.S. Pat. No. 5,625,541, Apr. 29, 1997; Murakarni, N., et al., “A highly efficient, low-profile 300 W power pack for telecommunications systems”, IEEE APEC 1994 Proceedings, pp. 786-792; Yamashita, N., Marakami, N.,and Yachi, T., “A compact, highly efficient 50 W on board power supply module for telecommunications systems”, IEEE APEC 1995 Proceedings, pp. 297-302; Djekic, O., Brkovic, M., “Synchronous rectifier vs. shottky diodes is a buck topology for low voltage applications”, IEEE PESC 1997 Proceedings, pp. 1374-1380; Nakayashiki, et al., “High Efficiency Switching Power Supply Unit with Synchronous Rectifier”, IEEE INTELEC 1998 Proceedings, pp. 398-403; Kohama, T., et al., “Analysis of Abnormal Phenomena Caused by Synchronous Rectifiers in a Paralleled Converter System”, IEEE INTELEC 1998 Proceedings, pp. 404-411; Svardsjo, C., “Double ended isolated DC-DC converter”, U.S. Pat. No. 5,907,481, May 25, 1999; Cobos, J., et al., “New Driving Scheme for Self Driven Synchronous Rectifiers”, IEEE APEC 1999 Proceedings, pp. 840-846.
Synchronous rectification adds a new level of complexity to the implementation of DC/DC converters. First, the synchronous rectifiers have to be turned on and off with precise timing. Second, the operation of the rectifier stage using synchronous rectification is not limited to a single quadrant, which complicates the system solution when more than one module is required to operate in a parallel or redundant configuration. Having the ability of placing two or more modules in a parallel configuration is becoming more important as logic voltages continue to decrease and current requirements continue to increase. Therefore, the ability to configure modules in parallel is becoming a necessity.
When typical synchronous rectifier modules are placed in a parallel configuration, one or more of the modules will sink current instead of sourcing it, which causes a fault in the system output bus voltage. The problems encountered when trying to parallel modules using synchronous rectification are well-understood as described in the following reference: Kohama, T., et. al., “Analysis of Abnormal Phenomena Caused by Synchronous Rectifiers in a Paralleled Converter System”, IEEE INTELEC 1998 Proceedings, pp. 404-411. Even though the problem is well understood, a simple solution is apparently not available. It is not sufficient to prevent the fault condition during normal or steady state operation. The fault condition also needs to be prevented during start-up and shutdown conditions.
SUMMARY OF THE INVENTION
The present invention provides a simple solution for paralleling modules with synchronous rectification. The present solution takes advantage of the self-correcting properties of the selected system solution. This solution does not try to sense the current through the synchronous rectifiers with the intent of disabling the synchronous rectifiers when this current tries to reverse itself. Disabling the synchronous rectifiers under this condition changes the system behavior considerably since both continuous and discontinuous conduction modes of operation need to be dealt with. These two modes have stability. Furthermore, during light load operation such a system can easily oscillate between theses two modes of operation. The proposed solution avoids all this problems simpler and better overall system solution. The present invention is applicable to push-pull type topologies and other topologies, such as two-switch forward, conventional forward, (no active clamp), etc., as long as the synchronous rectifiers are self-driven.
The present invention provides a method for preventing a fault condition in a DC-DC converter having a first secondary winding coupled to a first synchronous rectifier and a second secondary winding coupled to a second synchronous rectifier. The first synchronous rectifier is turned on based on a voltage across the first secondary winding and is turned off based on a first driver signal. The second synchronous rectifier is turned on based on a voltage across the second secondary winding and is turned off based on a second driver signal.
The present invention also provides a DC-DC converter that includes a primary transformer, a first and second synchronous rectifier, and a first and second control circuit. The primary transformer has a primary winding, a first secondary winding and a second secondary winding wherein the first and second secondary windings coupled together. The first synchronous rectifier is coupled to the first secondary winding and the second synchronous rectifier is coupled to the second secondary winding. The first control circuit is coupled to the first synchronous rectifier, and turns the first synchronous rectifier on based on a voltage across the first secondary winding and turns the first synchronous rectifier off based on a first driver signal. The second control circuit is coupled to the second synchronous rectifier, and turns the second synchronous rectifier on based on a voltage across the second secondary winding and turns the second synchronous rectifier off based on a second driver signal.
In addition, the present invention provides a DC-DC converter that includes a power transformer, a signal transformer, a first and second output terminal, an output inductor, an output capacitor, a biasing voltage terminal, first and second synchronous rectifiers, and first, second, third and fourth switches. The power transformer has a primary winding, a first secondary winding and a second secondary winding wherein the first and second secondary windings coupled together. The signal transformer has a primary winding, a first secondary winding and a second secondary winding wherein the first and second secondary windings coupled together. The second output terminal is coupled to the connection between the first and second secondary windings of the signal transformer. The output inductor is coupled between the connection between the first and second secondary windings of the power transformer and the first output terminal. The output capacitor is coupled between the first and second output terminals. The first synchronous rectifier is coupled between the first secondary winding of the primary transformer and the second output terminal. The first switch is coupled between the biasing voltage terminal and a control of the first synchronous rectifier, a control of the first switch i
Farrington Richard W.
Hart William
Svardsjo Claes
Chalker Daniel J.
Ericsson Inc.
Gardere Wynne & Sewell LLP
Sterrett Jeffrey
Warren, Jr. Sanford E.
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