MAXREFDES1100: 90% Efficient Size, 12V/500mA, Optocouplerless Flyback DC-DC Converter

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.

Key Features

• 4.5V to 60V Input Voltage Range
• Programmable Switching Frequency from 50kHz to 250kHz
• Programmable Input Enable/UVLO Feature
• Programmable Input Overvoltage Protection
• Adjustable Soft-Start
• 2A/4A Peak Source/Sink Gate Drive Capability
• Hiccup Mode Short-Circuit Protection
• Fast Cycle-by-Cycle Peak Current Limit
• Thermal Shutdown Protection
• Space-Saving, 16-Pin, 3mm x 3mm TQFN Package
• -40°C to +125°C Operating Temperature Range

This board is not available for purchase.

An isolated no-opto flyback DC-DC converter using the MAX17690 is demonstrated for a 12V DC output application. The power supply delivers up to 500mA at 12V.

Designed, Built, Tested

This document describes the hardware shown in Figure 1. It provides a detailed systematic technical guide to designing an isolated no-opto flyback DC-DC converter using Maxim’s MAX17690 controller. The power supply has been built and tested.

Design Resources