Miniature Flyback Converter Reference Design

The power supply design gives 12V output, works with many input voltages, and keeps parts separated for reliable use.

The MAXREFDES1247, a reference design from Maxim Integrated, is a compact, isolated power supply that delivers a 12V output at up to 1A. It uses the MAX17596 peak-current-mode controller in a discontinuous-conduction mode (DCM) flyback topology, operating at 125kHz. While the controller’s input range is 4.5V to 36V, a bias winding from the transformer extends the input voltage range to 60V for this design. The transformer provides 1500VRMS galvanic isolation between the primary and secondary sides. An optocoupler ensures voltage regulation by providing isolated feedback from the secondary side to the controller.

The reference design is suitable for engineers and designers developing compact, isolated power supplies for industrial, automotive, and embedded applications. This design is especially useful for applications that need galvanic isolation, such as communication interfaces, motor control systems, or sensor modules operating in noisy or high-voltage environments. Designers looking for a low-cost, efficient solution based on DCM flyback topology can use this reference design as a starting point for customization, taking advantage of its programmable features and robust protection mechanisms.

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A generic isolated power supply block diagram includes a power stage, isolation transformer, rectifier, secondary-side error amplifier, and an optocoupler for feedback to the primary-side controller. While the basic structure is similar, isolated power supplies can vary based on how the transformer is used in the design.

In a flyback configuration, a transformer operates differently from its typical energy transfer mode. Normally, both the primary and secondary windings conduct at the same time to transfer energy. But in a flyback setup, they conduct alternately — not simultaneously. This makes the transformer behave more like a coupled inductor rather than a traditional transformer. Flyback converters can run in either discontinuous (DCM) or continuous conduction mode (CCM). Each mode affects the choice of components, stress on power devices, and controller design. 

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A basic flyback topology uses a transformer with its primary winding connected to the drain of a switching MOSFET, while the source of the MOSFET is grounded. The secondary winding connects to the output capacitor through a rectifier diode. During the on-time of the switching cycle, current flows into the primary winding. When the switch turns off, current flows into the secondary winding, delivering energy to the output.

The reference design is designed for compact applications, offering a miniature form factor optimized for small size. It supports a wide input voltage range from 17V to 60V and delivers a 12V DC isolated output at up to 12W. The design includes a cycle-by-cycle current limit for protection and uses a resistor-programmable threshold for undervoltage lockout (UVLO) and overvoltage input (OVI). Built on a low-cost discontinuous conduction mode (DCM) flyback topology, the power supply also provides galvanic isolation up to 1500VRMS, ensuring safe and reliable operation across isolated domains.

Maxim Integrated has tested this reference design. It comes with a bill of materials (BOM), schematics, assembly drawing, printed circuit board (PCB) layout, and more. The company’s website has additional data about the reference design. To read more about this reference design, click here.