Isolated discrete forward DC/DC design

June 24, 2016 // By Bruce Haug
High density isolated DC/DC converters have significantly changed during the course of the past 25 years. The introduction of the full and half-brick form factors back then created a rush to use them in distributed power architectures in telecom, datacom, industrial and medical systems where a bus voltage is routed to every board within a system and each board had its own isolated DC/DC converter.

Many of these bricks incorporated several hundred parts making it much easier to utilize them instead of designing a discrete supply on the PCB. At the time, there was much haste by several power supply companies to get into this market and catch up with the leaders in the field. Many went through several years of painstaking development to bring their products to market, creating their own magnetics, topologies and control schemes, always trying to outperform the competitive offerings. Many came out with the same footprint and others with their own patented sizes and pin-outs. Consequently, these footprints were expanded to include 1/4, 1/8 and 1/16 bricks, along with countless variations of other sizes depending on the output power required.

However, with the introduction of application specific DC/DC controllers and monolithic devices, along with off-the-shelf planar power transformers and inductors have made it much simpler to produce discrete designs. In fact, an isolated design can be done with 15 parts for a flyback and 20 parts for a forward converter. This new era of application specific controllers and monolithic devices has given designers a different avenue for the development of isolated DC/DC converters. Improved MOSFET switching, VDS ratings and RDS(ON) have also helped to make it easier to do discrete designs. Some designs also no longer require an opto coupler or signal transformer in the feedback loop, and many can be used for military applications with operation from -55°C to 150°C.

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