DC/DC Power ICs

The full-bridge topology is popular because, although the IC driver is more complex (four power transistors instead of two), the transformer does not need a split primary winding, which reduces cost and complexity. The transformer can also be made very compact, as this topology eliminates the risk of DC flux bias, allowing more power to be transferred with the same core size compared to push-pull drivers. Furthermore, the input-output voltage ratio is set by the number of turns on the windings, so fixed 1:1, step-up or step-down power solutions can be easily built.

The push-pull topology is popular because it is the simplest and most cost-effective transformer driver option, but the transformer requires a split primary winding (center tap), which slightly increases cost and complexity. However, the transformer can be made very compact, as this topology maximizes the power transfer ratio by utilizing both quadrants of the BH curve. Furthermore, the input-to-output voltage ratio is set by the number of turns on the windings, so fixed 1:1, step-up or step-down power solutions can be easily built.

The single-ended topology is popular because the power is stored in the transformer while the transistor is on and only transferred while the transistor is off, so the output voltage ratio depends not only on the turns ratio but also on the duty cycle, which is adjustable over a very wide range. Thus, 1:1, step-up or step-down power solutions can be easily built by selecting the turns ratio, and the output voltage can be regulated by adjusting the duty cycle to accommodate wide variations in supply voltage or load.

The feedback pin for the output voltage regulation supports three separate functions to enable the optimal feedback technique for the given application:

• Primary-side sensing: the reflected output voltage is derived from the primary winding signal to determine the output voltage without the bridging the isolation barrier.
• Secondary-side sensing: the output voltage is monitored via a photocoupler placed across the isolation barrier. This allows a more precise feedback control at the additional cost of an external photocoupler.
• Direct secondary-side sensing: where isolation is not required, the output voltage can be directly monitored by the feedback pin to provide the most accurate output voltage regulation.

In addition to primary-side transformer driver solutions, RECOM also offers secondary-side smart rectifier ICs. The RVS series are self-powered ICs that integrate control, protection and timing circuits for synchronous rectifier solutions, either with integrated switches or for driving external MOSFETs.

Compared to discrete diodes, they offer:

• Lower losses and higher efficiency
• Compact SMD package requiring less board space than four separate diodes
• Intelligent voltage limiting to clamp the output under no-load conditions
• Parallelable for higher current