Isolated discrete forward DC/DC design: Page 3 of 5

June 24, 2016 // By Bruce Haug
Isolated discrete forward DC/DC design
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.

This device can be used in either synchronous or non-synchronous applications. For synchronous operation, the LT8310 sends a control signal via a pulse transformer to a secondary-side MOSFET driver for synchronous rectification timing. Synchronous designs are most advantageous for higher power or lower output voltage applications. An output voltage regulation of ±8% can be attained without the use of an opto-coupler as shown in figure 2.

Fig. 2: LT8310 output voltage regulation of figure 1 schematic.

When an opto-coupler is used, ±1.5% regulation can be realized. A programmable volt-second clamp provides a safeguard for transformer reset that prevents saturation and protects the MOSFET. This function optimizes the transformer and MOSFET, reducing overall solution size. The LT8310 is available in a TSSOP-20 package with several pins removed for high voltage spacing.


Flyback converter design

For an even simpler isolated DC/DC converter solution at lower power levels, the flyback topology could be used. Flyback converters have been widely used in isolated DC/DC applications for many years; however, they are not necessarily a designer’s first choice. Power supply designers unwillingly select a flyback converter out of necessity for lower power isolated requirements, not because they are easier to design. The flyback converter has stability issues due to the right-half-plane zero in the control loop which is further complicated by the aging and gain variation of an optocoupler.

A flyback converter requires a significant amount of time devoted to the design of the transformer, a task further complicated by the normally limited selection of off-the-shelf magnetics and the possible necessity for a custom transformer. Recent advances in power conversion technology have made isolated converters much easier to design. Linear Technology’s LT8302 isolated flyback converter solves many of these flyback design obstacles.

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