MAXREFDES1194: Five-Output, No-Opto Flyback DC-DC Converter Using the MAX17690

The MAXREFDES1194 is a no-opto flyback DC-DC power supply that delivers five outputs from a 6V to 60V supply voltage. It is designed for equipment that needs multichannel, isolated power supplies with a wide input voltage range.

The MAXREFDES1194 employs the no-opto flyback control technique of the MAX17690. This document explains how the MAX17690 peak current-mode PWM converter can be used to generate five isolated outputs from a 6V to 60V 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. The MAX17690 also compensates for the negative temperature coefficient of the rectifying diode.