Three-loop feedback control of fault-tolerant power supplies in IBM Enterprise System/9000 processors
by K. R. Covi
In an Enterprise System/9000 (ES/9000) processor, a fault-tolerant power system composed of multiple power supplies connected in parallel provides thousands of amperes of current to low-voltage (1-2 V) logic circuit boards, monitors the voltage at each board, and immediately responds to compensate for failure of a supply. If a supply fails, the very fast closed-loop response redistributes the current uniformity among the remaining supplies and allows the normal functioning of the processor logic to continue uninterrupted. This rapid response is not obtained from a conventional two-loop (current-mode) feedback power supply because the loop bandwidth is restricted by a resonance that develops in the power distribution. A third feedback loop that is added to each supply controls this power distribution resonance and makes possible the wide loop bandwidth necessary to achieve the required power system control. Analysis is presented of a three-loop control system, and a simulation of its application to a typical ES/9000 power system is described.