Topology Key to Power Density in Isolated DC-DC Converters

Feb 1, 2011 12:00 PM
Bob Bell and Ajay Hari National Semiconductor, Phoenix Arizona

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Efficiency and density (Watts/Volume) have long been the metrics used to compare the performance of isolated DC-DC power converters. When designing an isolated dc-dc power converter, the first and most critical choice is selection of the topology. Historically, topology selection was based upon the desired output power level. For the basic topologies, the order from lower power to higher power was usually Flyback, Forward, Push-Pull, Half-Bridge and Full-Bridge. While this basic power order still remains true, for designers to push to new heights in power density, topologies that were once used in much higher power applications are now finding their way into relatively lower power, small form factor high density power converters. Power management IC manufacturers are enabling this trend by adding not only more features but also by integrating high voltage gate drivers within the controllers.

While clearly subjective, historically, the output power range has often been used as the primary guide when selecting a topology. However, there are many other factors that play into the topology selection for an isolated dc-dc power converter such as cost, size, electrical stress, output noise and input voltage range. The size of an isolated power converter primarily depends on the transformer size and the number of active switches employed. The utilization of the power transformer affects the size of the power converter. Isolated power converter topologies can be classified as either single-ended or double-ended depending on the usage of the B-H curve. During the operation, if the flux swings in only one quadrant of the B-H curve, then the topology is classified as single-ended. If the flux swings in two quadrants of the B-H curve, then the topology is classified as double-ended. For a given set of requirements, a double-ended topology requires a smaller core than a single-ended topology and does not need an additional reset winding. Table 1 lists several of the most popular isolated topologies and the power range these topologies had been historically employed.

CONVENTIONAL USE OF VARIOUS ISOLATED TOPOLOGIES

The Flyback may be the most commonly used isolated topology. It is generally found in low cost, low power applications. Flyback topology requires only a single active switch and does not require a separate output inductor in addition to the transformer. This makes the topology easy to use and low cost. The disadvantages of the flyback topology are poor transformer utilization, as it is a single-ended topology, and extra capacitors are required at both the input and the output due to the high input and output ripple currents.

The Forward and Active Clamp Forward topologies are often employed in medium power applications. The Forward topology also suffers from poor transformer utilization due to the limited duty cycle and as it is also single-ended topology. The active clamp forward transformer does operate in two quadrants during steady state operation however peak flux can reach high levels during startup and transient conditions. In order to reset the transformer the maximum duty cycle is limited in both the forward topology and the active clamp forward topology.

The remaining three topologies; Push-Pull, Half-Bridge and Full-Bridge are true double-ended topologies whereby power transfer occurs in two quadrants of the BH curve and does not require special provisions to reset the transformer. These double-ended topologies are the best choice for applications where the highest power density is desired, since the transformer core can be fully utilized. Another advantage of double-ended topologies is the transformer can be further optimized because of the larger available duty cycle range. Double-ended topologies can operate at a maximum duty cycle of almost 50% per side which equates to an effective maximum duty cycle of nearly 100% at the output filter inductor. Designing the transformer turns ratio to maximize the effective duty cycle greatly reduces the RMS current in the transformer and reduces the size of the output filter.

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