FROM EDN EUROPE: À la carte intermediate bus power
By Graham Prophet -- EDN, April 1, 2004
With its BCM (Bus Converter Module) concept, Vicor is proposing a modification to the power-distribution architecture usually referred to as IBA (Intermediate Bus Architecture, Picture). In IBA, you use the standard (nominally 48V) "telecom" power bus for distribution around the card cages of circuit boards. At board level, you carry out dc/dc conversion, with isolation, down to an intermediate level, usually 12V; then, at point of load, you dc/dc convert again to the final circuit voltage with multiple nonisolated converters. The problem Vicor identifies is that 12V is increasingly a less-than-ideal compromise. If you have a high power system, you can still be routing uncomfortably high currents to circuit boards; if you have heavy loads at today's processor core voltages (1 to 2V), then 12V is too far away from the final voltage for efficient conversion.
The solution? Choose the intermediate voltage to suit the system. Vicor bases its proposal on a conversion topology called an SAC (Sine Amplitude Converter), a fixed-frequency resonant converter switching at 3.5 MHz. It uses zero-voltage switching on the primary switches and both zero-voltage and -current switching on the output synchronous rectifiers. Using a low-Q, low-profile transformer, Vicor says it achieves greater than 1-kW/in.3 power density and 97% efficiency, with additional gains in noise and speed of response. Vicor's V{I modules, which the company in turn builds into its BCMs, use this technique; you get 300W from a 32×21×6-mm package. From a 38 to 55V-dc input, you can generate an intermediate bus of 3, 4, 6, 8, 9.6, 16, 24 or 48V, according to your power-load needs. Final regulation will be with an ni-POL (nonisolated point-of-load) converter, as before. You can get 600W from a module using two such converters that still occupies the outline of a standard quarter-brick format.
Vicor, +44 1276 678222, www.vicr.com.
