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H. Ertl, T. Wiesinger, J.W. Kolar, F. Zach:
"A Simple Active Method to Avoid the Balancing Losses of DC Link Capacitors ";
Poster: PCIM, Nürnberg, Germany; 2003-05-20 - 2003-05-22; in: "Proceedings of the forty-seventh International Power Electronics", ZM Communication, (2003), ISBN: 3-928643-35-5; 459 - 464.

DC voltage links of three-phase power converters
very often are equipped with a series connection of two electro-lytic
capacitors due to the high voltage level. In general,
resistors arranged in parallel to each capacitor are necessary for
balancing the capacitor partial voltages. The balancing resistors
have to be dimensioned regarding the worst-case condition of
the capacitor leakage currents, resulting in high permanent
dissipative losses.
To avoid these losses to a large extent, a novel simple and
low-cost active circuit to replace the passive balancing resistors
by a cascode array of low-cost small-signal low-voltage bipolar
transistors is presented. The paper describes the operating
principle of the circuit, analyzes the fundamental relationships
being relevant for the balancing characteristic and gives
guidelines concerning component selection. Furthermore, simu-lation
results as well as measurements taken from a laboratory
prototype are presented.

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04403660