Transient Response Improvement of DC-DC Buck Converter With Adjustable Sawtooth Signal

Transient response performance is very important in DC-DC converters. It measures how steady the output voltage is maintained when the load changes suddenly. For single inductor dual output (SIDO) converters (1)~(4) that work in continuous conduction mode (CCM), the transient response affects cross-regulation: if the load of one sub-converter changes, the output voltage of the other sub-converter is affected. The conventional control strategy in switching power supplies is to use output voltage or inductor current feedback. Although feedback control can stabilize the system, its effectiveness is limited by control loop delay and by the increased cost of using a large capacitor to stabilize the output voltage. Since feed-forward control does not introduce control delay, combining conventional feedback control with an additional feed-forward controller is one way to improve transient response. Load current feed-forward control methods (5)(6) have been proposed for output voltage regulation. Although these improve transient response, the performance improvement is limited in large load current transient conditions as the duty cycle saturates to 100%. Digital non-linear control methods have been also proposed (7)(8). The main drawback of such approaches is difficulty of non-linear calculation, especially for SIDO converters. This paper proposes to use an adjustable sawtooth signal to realize a simple feed-forward controller for DC-DC buck converters. This approach employs sawtooth signal modulation based on charge balance of output capacitor. The adjustable sawtooth signal is compared with the error signal to provide the PWM drive signal. The proposed controller—whether used in single inductor single output (SISO) or SIDO converters—has the advantage of transient response improvement, and also it does not require complex and precise calculations that are required for digital controllers. 2. Proposed Adjustable Sawtooth Signal and Feed-forward Control