MAXREFDES1244: 88% Efficient Tiny, 5V/1.2A, Photoless Flyback DC-DC Converter Based on the MAX17690

The MAXREFDES1244 is a No-Opto Flyback converter that delivers up to 1.2A at 5V from a 18V to 54V supply voltage. It is designed for industrial equipment that needs isolated voltage from wide input supply voltage.

The MAXREFDES1244 illustrates techniques using the No-Opto Flyback converter to generate isolated output. This document explains how the MAX17690 No-Opto Flyback converter can be used to generate 5V from 18V to 54V input voltage. An overview of the design specification is shown above.

Due to its simplicity and low cost, the flyback converter is the preferred choice for low-to-medium isolated DC-DC power-conversion applications. However, the use of an optocoupler or an auxiliary winding on the flyback transformer for voltage feedback across the isolation barrier increases the number of components and design complexity. The MAX17690 eliminates the need for an optocoupler or auxiliary transformer winding and achieves ±5% output voltage regulation over line, load, and temperature variations.

The MAX17690 implements an innovative algorithm to accurately determine the output voltage by sensing the reflected voltage across the primary winding during the flyback time interval. By sampling and regulating this reflected voltage when the secondary current is close to zero, the effects of secondary-side DC losses in the transformer winding, the PCB tracks, and the rectifying diode on output voltage regulation can be minimized.