A Clever Voltage-Fed Half-Bridge Converter Architecture (.PDF Download)

Design engineers often turn to push-pull, half-bridge, and full-bridge power-converter designs for higher power output. These architectures show up from 200 W to 500 W and beyond. When space or weight is a constraint, it may also make sense to use a cascaded converter, where a buck regulator applies a voltage to those architectures to do the regulation. This means you set up the power transformer to use a 50% duty cycle—it’s a simple dc-dc transformer with no pulse width modulation (PWM).

Because the transformer is working at 50% duty cycle, you can minimize its weight and size. The wide duty cycle might allow you to use switching frequencies and magnetic materials that will also reduce weight, size, and cost. In terms of reducing transformer size, the push-pull architecture does not make as much sense. It lacks the winding efficiency, sometimes called copper efficiency, of the half- and full-bridge designs. With a push-pull design, you are only using half of the primary winding at any given time. Two coils are used to reverse the magnetic flux in the core, as opposed to reversing the current in a single coil by reversing the applied voltage.