An Isolated DC–DC Converter Using a Cross-Coupled Shoot-Through-Free Class-D Oscillator With Low EMI Emissions

Abstract

A low electromagnetic interference (EMI) iso- lated dc–dc converter with a cross-coupled shoot-through-free Class-D oscillator and a voltage series-combining transformer is presented. The proposed Class-D oscillator features a fully symmetrical topology to greatly reduce the dipole emissions of the converter, and the shoot-through current is avoided by the cross-coupled structure that biases the gates of the PMOS and NMOS with different dc voltages. Moreover, a voltage series- combining transformer is proposed by using two 1:1-coupled coils to form a parallel-connected primary and a series-connected secondary, achieving a voltage conversion ratio (VCR) of >1 without undermining the coupling coefficient (k). The transmit- ter (TX) and the receiver (RX) were fabricated in a 0.18-µm BCD process and assembled in an LGA package. A peak efficiency of 51% and a maximum output power of 1.2 W are achieved, and the EMI performance meets the CISPR-32 Class B standard on a two-layer PCB without using any stitching capacitor.