Controlled Auxiliary Circuit to Improve the Unloading Transient Response of Buck Converters

In this paper, a controlled auxiliary circuit is presented to improve the transient response of a Buck converter. It is well established that for converter applications with a large input/output voltage ratio, voltage overshoots (due to step-down load transients) are much larger than corresponding voltage undershoots (due to step-up load transients). Therefore, the goal of the proposed method is to reduce the overshoot. The control method only activates the auxiliary circuit during step-down load transients and operates by rapidly transferring excess load current from the output inductor of a Buck converter to the converter's input. The proposed method behaves as a controlled current source to remove a constant regulated current from the output of the Buck converter. The duration of activation of the auxiliary circuit is also regulated. The proposed circuit has the following advantages: 1) predictable behavior allowing for simplified design; 2) inherent over-current protection; and 3) low peak current to average current ratio allowing for use of smaller components. In addition, the proposed auxiliary controller estimates the magnitude of the unloading transient and sets the auxiliary current proportional to the transient magnitude. This allows for greater design flexibility and increases the auxiliary circuit efficiency for unloading transients of lesser magnitude. In this paper, it is shown through analysis, simulation, and experimental results that a large reduction of voltage overshoot and output capacitor requirements can be realized through the addition of a small MOSFET, diode, and inductor.