This article describes the possibility of increasing the power output of a flyback converter by connecting multiple transformers in parallel to drive multiple phases. This configuration also helps reduce conducted emissions at the input of a flyback switch-mode power supply topology.

■ Multi-phase flyback converter, easy to design to maximize power limit

Multiphase flyback converters maximize available power limits, are easy to design, and reduce conductive interference.

A flyback converter is a good way to generate a controlled galvanic isolation voltage.

This voltage conversion technology is being used in various applications because its circuit is very simple and the technology is mature.

Figure 1 briefly illustrates the circuit structure of a flyback converter.

However, flyback technology has some limitations.

First, the maximum power that can be transmitted is limited. Let’s explain this using the circuit diagram in Figure 1.

While switch Q1 is on, current flows in the primary side of the transformer.

During this time, energy is stored in the transformer core T1.

During the off time of Q1, no current flows in the primary side, but current is formed in the secondary side of the transformer.

The energy previously stored in T1 is released through the secondary winding.

There is a limit to the maximum energy that can be stored in a transformer.

Therefore, the maximum power of the flyback converter is limited.

Although output powers exceeding 100 W can be achieved using special transformers, the flyback configuration is typically used for output powers up to about 60 W.


There is a rather unusual but clever way to drive the flyback topology effectively at higher power levels.

A method of controlling multiple channels to drive a flyback converter with two or more transformers to distribute the output power to each channel.

These transformers are readily available in a variety of options and can be used in parallel.

Figure 2 shows a two-channel flyback converter circuit. This circuit is controlled by a special controller IC, the MAX15159.

This IC is a two-channel flyback controller that operates in phase-shift mode, ensuring that current is evenly distributed across two parallel power paths.

It is also possible to drive a four-phase flyback with four transformers using two MAX15159 flyback controllers.

As a result, this circuit can generate very high power, exceeding 100 W, while using a small transformer.


Like single-channel flybacks, multiphase flybacks can also operate without an optocoupler in the feedback path.

The MAX15159 features no-opto technology.

This method controls the output voltage by evaluating the voltage across the primary winding during the off time.

One of the unique advantages of multiphase flybacks is their ability to reduce conductive interference.

On the input side, the flyback operates like a switch-mode step-down converter, i.e. a regulator using a buck topology.

Both topologies produce pulsed input currents.

To minimize input-side interference, each channel of the multiphase flyback converter is phase-shifted and operates at different times.

This not only improves EMI characteristics but also reduces the size and number of input-side capacitors required.

Figure 3 shows the input side current of a two-channel flyback converter.


An interesting advantage of multiphase flyback converters is that they can use several simple, inexpensive, and compact transformers instead of one large transformer.

■ Multi-phase flyback converter, no optocoupler required

When developing a galvanically isolated power supply, there are other options besides the typical solutions of flyback converters for power levels below 60 W and forward converters for power levels above 60 W.

It is possible to supply power of more than 60W using a multi-phase flyback converter.

Solutions such as the MAX15159 controller IC provide no-opto technology.

This not only eliminates the need for an optocoupler, but also minimizes conductive interference caused by the phase shift control method.

※ About the author
Frederik Dostal is a power management expert with over 20 years of experience in the industry. He studied microelectronics at the University of Erlangen in Germany and joined National Semiconductor in 2001, where he gained extensive experience implementing power management solutions on customer projects as a Field Application Engineer (FAE). During his time at National Semiconductor, he worked in Phoenix, Arizona, for four years as an applications engineer, focusing on switch-mode power supplies (SMPS). He joined Analog Devices in 2009 and has since held various positions related to product lines and European technical support. Currently, he is working as a power management specialist at ADI's Munich office, leveraging his extensive design and application knowledge.