Voltage-mode digital pulse train control for light load DC-DC converters with spread spectrum

Pulse train (PT) control improves light load efficiency in a DC-DC converter and also enables to achieve spread spectrum using bi-frequency operation (BFO). However, existing PT control shows existence of sub-harmonic oscillations; thus periodic BFO cannot be ensured. This paper proposes digital voltage-mode PT control in DC-DC converters, in which on-time of an active pulse is generated from DPWM output (similar to voltage mode control) for a predefined sequence of pulses. Discrete-time models are derived, which confirm stable BFO with predictable spectral behavior. This enables to further optimize efficiency, to meet EMI compliance, and to achieve smooth PT/PWM transition because of sharing the outer voltage loop. A buck converter prototype is made, and the proposed scheme is implemented using an FPGA device.