Due to the ever increasing demands to drive up DC-DC power converter density, topologies that had previously been used only in high power applications are now finding their way into medium and lower power applications. The power density, expressed as Watts-per-cubic inch, of modern DC-DC power converters has increased dramatically over the past few years. The benchmark 100W / in3, once an elusive target is now commonplace. In order to successfully design a high density power converter, topology selection is the first and most important choice. The largest components in a DC-DC power converter are the energy storage elements; transformer, input and output filtering. Single-ended topologies such as Flyback and Forward, operate in only the first quadrant of the transformer B-H loop and therefore have a flux swing limited to half that of a double-ended topology. In order to maximize the utilization of the transformer, a double-ended topology is the optimal choice. The most popular double-ended topologies are push-pull, half-bridge and full-bridge.
Half-Bridge Circuit Operation
Historically, the Half-Bridge topology was only used in applications greater than 200W, it is now finding wide use in high density applications as low as 50W. Shown in Figure 1 is a power stage diagram of a Half-Bridge topology power converter. The input capacitors C1 and C2 are equal value and act as a voltage divider and input filter. The voltage at the mid point of the capacitors is ½ Vin. The two primary side switches Q1 and Q2 are arranged in a totem pole configuration. Each switch operates alternately each clock cycle. The upper switch requires a level shift circuit to control the upper gate (with respect to the upper source). The transformer secondary is arranged in a center tapped configuration with the center tap connected to the output LC filter. For high density applications, synchronous rectifiers are used for secondary rectification.
Figure 1. Half-Bridge Power Converter