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. 
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. xe2x80x9cA MOSFET Resonant Synchronous Rectifier for High Frequency DC/DC Convertersxe2x80x9d, IEEE PESC 1990 Proceedings, pp.  769-779; Jitaru, I.D., xe2x80x9cConstant Frequency, Forward Converter with Resonant Transitionsxe2x80x9d, HFPC 91 Preceedings, pp.  282-292; Cobos, J.A., et al.  xe2x80x9cSeveral alternatives for low output voltage on board convertersxe2x80x9d, IEEE APEC 98 Proceedings, pp.  163xe2x80x94169; Bowman, W., Niemela, V.A., xe2x80x9cSelf-synchronized drive circuit for a synchronous rectifier in a clamped-mode power converterxe2x80x9d, U.S. Pat.  No.  5,590,032, Dec. 31 1996; Loftus, Jr. T.P., xe2x80x9cZero-voltage switching power converter with lossless synchronous rectifier gate drivexe2x80x9d, 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.,  xe2x80x9cA highly efficient, low-profile 300 W power pack for telecommunications systemsxe2x80x9d, IEEE APEC 1994 Proceedings, pp.  786-792; Yamashita, N., Marakami, N.,and Yachi, T., xe2x80x9cA compact, highly efficient 50 W on board power supply module for telecommunications systemsxe2x80x9d, IEEE APEC 1995 Proceedings, pp.  297-302; Djekic, O., Brkovic, M., xe2x80x9cSynchronous rectifier vs. shottky diodes is a buck topology for low voltage applicationsxe2x80x9d, IEEE PESC 1997 Proceedings, pp.  1374-1380; Nakayashiki, et al.,  xe2x80x9cHigh Efficiency Switching Power Supply Unit with Synchronous Rectifierxe2x80x9d, IEEE INTELEC 1998 Proceedings, pp.  398-403; Kohama, T., et al.,  xe2x80x9cAnalysis of Abnormal Phenomena Caused by Synchronous Rectifiers in a Paralleled Converter Systemxe2x80x9d, IEEE INTELEC 1998 Proceedings, pp.  404-411; Svardsjo, C., xe2x80x9cDouble ended isolated DC-DC converterxe2x80x9d, U.S. Pat.  No.  5,907,481, May 25, 1999; Cobos, J., et al.,  xe2x80x9cNew Driving Scheme for Self Driven Synchronous Rectifiersxe2x80x9d, 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.,  xe2x80x9cAnalysis of Abnormal Phenomena Caused by Synchronous Rectifiers in a Paralleled Converter Systemxe2x80x9d, 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. 
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 is coupled to the first secondary winding of the primary transformer.  The second switch is coupled between the control of the first synchronous rectifier and the second output terminal, a control of the second switch is coupled to the first secondary winding of the signal transformer.  The second synchronous rectifier is coupled between the second secondary winding of the primary transformer and the second output terminal.  The third switch is coupled between the biasing voltage terminal and a control of the second synchronous rectifier, the control for the third switch is coupled to the second secondary winding of the primary transformer.  The fourth switch is coupled between the control of the second synchronous rectifier and the second output terminal, the control for the fourth switch is coupled to the second secondary winding of signal transformer. 
Other features and advantages of the present invention will be apparent to those of ordinary skill in the art upon reference to the following detailed description taken in conjunction with the accompanying drawings.