Frequency compensation and power stage design for buck converters to meet load transient specifications

The output voltage transient of a buck regulator during a load current step is often a key performance specification. However, many practical design approaches are based on frequency domain specifications like bandwidth and phase margin, without explicitly considering the constraints of the output filter. This paper derives an analytical expression for the peak transient output voltage due to a load step in a buck converter considering bandwidth of its control loop. Utilizing this, a simple design method is developed for a current-mode controlled buck converter with type II compensator that meets the time domain voltage transient specification. The compensator design is illustrated using a critically damped pole/zero cancellation method. The approach is extended to include a design method for voltage-mode buck converters. The critical bandwidth and critical inductance concepts are presented for design in this context. The results are verified using time and frequency domain simulations as well as experimental data.