IV. System requirements
The designer of a distributed power system must take into account the input requirements to the system as well as the intermediate bus between the isolated converter and the POL converters. The input to the isolated converter must meet several requirements including conducted EMI emissions and susceptibility which are governed by MIL-STD-461. The EMI performance requirement of the system can be met with the addition of an appropriately sized EMI filter module.
A power bus feeding the onboard power distribution system will typically have transient limits on the voltage level as called out in MIL-STD-704. The VPT isolated DC-DC converter modules are designed to be compatible with the steady state and transient limits that are called out in this specification. Power bus specifications may also limit the surge current that can be drawn during start up. VPT also offers input protection modules that can limit the inrush during startup.
One important consideration for the intermediate bus between the isolated converter and the POL converters is impedance interaction that can lead to instability [2
]. This can be corrected by placing bulk capacitance on the intermediate bus [3
]. There is no limit to the amount of capacitance that can be placed on the input of the POL converters, but the design maximum capacitance for the isolated converter that feeds the intermediate bus should not be exceeded.
The key to creating an efficient distributed power system is implementing a proper architecture using qualified off-the-shelf front end modules, isolated DC-DC converters, and POL converters. Using this method will save your avionics, military, or space power system significant time, cost, and weight while increasing reliability and flexibility. VPT Incorporated offers an extensive range of isolated and non-isolated DC-DC power converters, EMI filters, and input protection modules that are built for rugged duty as defined in MIL-PRF-38534 and MIL-STD-883. Along with the standard converter modules, VPT also offers the “SV” series of isolated and non-isolated power converters that are designed for the added harsh environments of space flight. The same designs outlined in this paper can be developed for space applications.
] F. Nome, G. Hariman, L. Sheftlevich, “The Challenge of Pre-Biased Loads and the Definition of a New Operating Mode for DC-DC Converters,” Power Electronics Specialists Conference, 2007, IEEE, pp 319-325.
] S. Abe, M. Hirokawa, and T Zaitzu, “Stability Design Consideration for On-Board Distributed Power System Consisting of Full-Regulated Bus Converter and POLs,” Power Electronics Specialists Conference, 2006, 37th IEEE, pp. 1-5.
] S. Abe, M. Hirokawa, M. Shoyama, T. Ninomiya, “Optimal Intermediate Bus Capacitance for System Stability on Distributed Power Architecture” Power Electronics Specialists Conference, 2008, IEEE, pp. 611-616.
About the author:
Leonard G. Leslie,
Jr. received a B.S. degree from Virginia Tech in 2001 and M.S. degree from the Center for Power Electronics Systems (CPES), Virginia Tech 2002. He is currently the Manager of Space Product Engineering at VPT Incorporated in Blacksburg, VA.
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