Multi-Mode PWM Modulator for a 4-Switch DC-DC Converter Operating with Tri-State

The 4-switch bi-directional (Buck+Boost) dc-dc converter is a good choice for interfacing DC buses where the voltage of one bus can be higher or lower than the other. It can operate with four different states determined by a pair of ON switches. The simplest and most common modulation schemes employ only two of them, in a dual-state logic. Tri-state operation can eliminate the right-half plane zero from the transfer function output-to-control variable in Boost-derived modes of operation. It can also reduce the inductor current ripple and rms value. A similar effect can be obtained by changing the order of the states used in a switching period as the power flow reverses. The issue of switching losses can be addressed with zero voltage switching (ZVS) but requires the inductor current to reverse in every switching cycle. Operation with ZVS frequently employs all four states. This paper describes a new carrier-less PWM modulator that allows the change of the mode of operation and sequence of states in a simple way. Its performance is verified by simulation of the 4-switch converter in a typical application: As the interface of a supercapacitor connected to a DC bus employing a tri-state scheme with possible reversal of the states of operation.