Measure Power-supply-loop Transfer

摘要

Power supplies use control-loop circuits to produce constant voltage or current. The transfer function-gain and phase as a function of frequency-provides valuable information about a control loop's speed and stability. Knowing a control loop's transfer function, as well as the poles and zeros of the transfer circuit, can help you select the right compensation and power-stage components.rnYou can measure gain and phase shifts and plot them with a network analyzer that sweeps the frequency of an injected signal and automatically computes the control loop's phase difference and gain. Such an instrument is nice to have and convenient-but is also expensive. If you don't have one available, you can make the measurements with an oscilloscope, a signal generator, and a standard transformer.rnTo perform the measurements, you inject a small ac signal into the power supply's control-loop circuit and measure the loop's gain and phase shift. By measuring the gain and phase, you can plot them with a Bode plot. The gain and phase differences between the injected signal and the control loop's output are the transfer function.