12V/500mA No-Opto Flyback 92.8% Efficient DC-DC Reference Design

March 30, 2018 by Paul Shepard

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 from Maxim Integrated Products eliminates the need for an optocoupler or auxiliary transformer winding and achieves ±5% output voltage regulation over line, load, and temperature variations. And it's the basis for the System Board MAXREFDES1095 reference design.

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. Other features include the following:

  • 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

One of the drawbacks encountered in most isolated dc-dc converter topologies is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated back to the primary side to maintain output voltage regulation. In a regular isolated flyback converter, this is normally achieved using an optocoupler feedback circuit or an additional auxiliary winding on the flyback transformer.

Isolated no-opto flyback converter topology with typical waveforms

Optocoupler feedback circuits reduce overall power-supply efficiency, and the extra components increase the cost and physical size of the power supply. In addition, optocoupler feedback circuits are difficult to design reliably due to their limited bandwidth, nonlinearity, high CTR variation, and aging effects.

Feedback circuits employing auxiliary transformer windings also exhibit deficiencies. Using an extra winding adds to the flyback transformer's complexity, physical size, and cost, while load regulation and dynamic response are often poor.

The MAX17690 is a peak current-mode controller designed specifically to eliminate the need for optocoupler or auxiliary transformer winding feedback in the traditional isolated flyback topology, therefore reducing size, cost, and design complexity. It derives information about the isolated output voltage by examining the voltage on the primary-side winding of the flyback transformer.

Other than this innovative method for regulating the output voltage, the no-opto isolated flyback converter using the MAX17690 follows the same general design process as a traditional flyback converter.