Small-signal measurement techniques in switching power supplies

As the power supply dynamic specifications become tighter, the control bandwidth has to be increased which interferes with the stability. Therefore, the feedback design becomes critical and should be optimized. In many practical cases, the accuracy of the power stage small-signal model is not enough for the adequate design of the loop gain. The optimal feedback design requires measurement of the plant transfer function and of the loop gain. The limitations of the conventional closed-loop measurement setup are shown and a measurement procedure, which is free of those limitations, is proposed. The plant identification in several common types of power supplies is non-trivial and is discussed in detail. An example of the small-signal measurement in the multiple-loop computer power supply is presented. Finally, the loop gain measurement of unstable power supplies is considered.