Easy-to-build high-performance, high-reliability isolated power supply circuit design

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Overview:
Pre-fabricated isolated DC/DC converters and modules provide a simple but relatively expensive solution for efficiently converting power distribution bus voltages to isolated low voltages. However, off-the-shelf designs may not provide all of the required functionality, so in many applications, custom designs are required. Using discrete PWM controllers to develop their own solutions is much cheaper and flexible enough to meet demanding requirements such as special form factors or non-standard input and output voltages.

Recent advances in controller technology have provided more features that have also simplified the task of designing isolated power supplies. Therefore, designers are now considering whether discrete design makes sense in their end applications. In particular, forward converters with active clamp reset have been widely used in custom power supplies due to their superior efficiency and lower component pressure. Although the forward converter is the simplest in an isolated buck topology, the active clamp reset method introduces performance limitations and concerns about potential reliability issues that limit the use of forward converters. range.

Linear Technology's second-generation LTC3765 / LTC3766 chipset uses a patent-pending approach that eliminates the performance limitations of forward converters with active clamp resets and concerns about their reliability. In addition, this extraordinary set of chips greatly simplifies the design of high-performance forward converters, making them easy to set up and becoming an attractive and alternative prefabricated solution.

LTC3765 / LTC3766 Chipset
The LTC3765 and LTC3766 combine to form an isolated synchronous forward converter chipset that includes an active clamp transformer reset and Direct Flux LimitTM, both of which prevent power transformer saturation under all conditions. The LTC3765 is a master-side intelligent controller that works with the LTC3766 for a rugged and simple self-starting isolated power supply. After startup, the LTC3765 receives timing signals and bias supplies from the secondary LTC3766 controller through a tiny pulse transformer. This secondary control method brings intelligence to the load, ensuring reliable monitoring and control of the output voltage and current while providing the fastest transient response. In addition, this architecture eliminates the need for opto-isolators or bias supplies, simplifying design and reducing component count. Figure 1 shows a typical application schematic and Figure 2 shows the associated transient response and efficiency/power curves.

Figure 2 Typical LTC3765/LTC3766 related efficiency/power curve and transient response (please read the PDF for details)