Phase-modular buck-boost dc/ac inverters extend the voltage conversion range of conventional buck-type inverter topologies, and accordingly offer significant advantages for variablespeed motor drives powered from dc sources with wide-voltageranges like fuel cells or batteries. In this letter, two new modulation schemes are applied to the six-switch buck-boost Y-inverter (6YI) topology, with an analysis of the efficiency improvement for the proposed third harmonic injection pulsewidth modulation (TPWM) and discontinuous PWM (DPWM) schemes relative to conventional sinusoidal PWM (SPWM). Third harmonic injection PWM (TPWM) substantially reduces conduction losses and discontinuous PWM (DPWM) extends this gain by eliminating high-frequency switching for one-third of the fundamental period. These gains are validated on a 1 kW hardware demonstrator across a dc input voltage from 80 to 240 V, with DPWM reducing total converter losses by 31% over SPWM for a peak efficiency at nominal load of 95.6% with a power density of 62.3 W/in(3).