7.5W / 12V Offline Flyback Converter Reference Design

October 29, 2019 by Paul Shepard

The MAXREFDES1188 reference design from Maxim Integrated Products uses the MAX17595, a peak-current-mode controller optimized for designing wide input-voltage flyback regulators. The MAX17595 offers optimized input thresholds for universal input ac-dc power supplies and telecom dc-dc converters (36- to 72-Vdc input range). It contains a built-in gate driver for an external n-channel MOSFET.

The MAX17595 houses an internal error amplifier with 1% accurate reference, eliminating the need for an external reference. The switching frequency is programmable from 100kHz to 1MHz with an accuracy of 8%, allowing optimization of magnetic and filter components, resulting in compact and cost-effective power conversion.

For EMI-sensitive applications, the MAX17595 incorporates a programmable frequency dithering scheme, enabling low-EMI spread-spectrum operation. Users can start the power supply precisely at the desired input voltage, implement input overvoltage protection, and program soft-start time.

A programmable slope compensation scheme is provided to ensure stability of the peak current-mode control scheme. Hiccup-mode overcurrent protection and thermal shutdown are provided to minimize dissipation in overcurrent and overtemperature fault conditions.

Key Features

  • Programmable switching frequency allows optimization of the magnetic and filter components, resulting in compact, cost-effective, efficient isolated and non-isolated power converters
  • 100kHz to 1MHz programmable switching frequency with optional synchronization
  • Peak current-mode control provides excellent transient response — offline (universal input ac) and telecom (36- to 72-Vdc) flyback controller
  • Programmable frequency dithering enables low EMI spread-spectrum operation
  • Integrated protection features enhance system reliability
  • Adjustable current limit with external current-sense resistor
  • Fast cycle-by-cycle peak current limiting hiccup-mode short-circuit protection
  • Overtemperature protection
  • Programmable soft-start and slope compensation
  • Input overvoltage protection

Designed, Built, Tested

This reference design describes the hardware. It provides a detailed systematic technical guide to designing an offline discontinuous conduction mode (DCM) flyback using Maxim's MAX17595 current-mode controller. The power supply has been built and tested, details of which are included in this document.