Because of their high transistor density and thin gate oxides, the latest high-performance microprocessors, FPGAs, and ASICs combine low operating voltages with currents that swing rapidly from milliamps to tens of amperes and back. To accommodate this, most high-end equipment, especially servers and telco (telephone company) switches, use some form of distributed power. A number of schemes exist for distributed power. The most common one in new designs is the intermediate bus architecture (IBA), which can have as many as three stages of power step-down.
Elements Of Distributed Power Schemes
A front-end supply (or rectifier in telco terminology) in the equipment rack steps down and rectifies the ac mains voltage from the power company to semi-regulated 48 V dc. It isolates the boards and circuits downstream from the lethal characteristics of the ac mains.
Each board in the system has its own step-down supply, or bus converter, that changes the front-end's 48 V to 12-, 8-, or 5-V bus voltages. The actual voltage is a tradeoff between the efficiency of the buck regulator at the point of load and the bus IR drop and I2R losses.
The bus voltage is stepped down and regulated by buck converters located at each load. Depending on the application, this converter may be termed a voltage-regulation module (VRM) or a point of load (POL) converter. For consistency, they'll be called POLs in this article.