Determining Which PoE Transformer Is Best: Flyback or Forward?

May 25, 2023

The transformer is an electrical piece of equipment that has redefined power electronics, utility power, LED lighting designs, and now, the wireless LAN sector known as Power-over-Ethernet (PoE). There are two types of transformer topologies to consider for your next PoE-based design: forward and flyback.

PoE is a fairly new method of providing power without looking for a 120 V outlet or nearby circuit breaker to supply power to low-voltage devices. When this method was introduced, developers were only able to generate less than 5 W of power via PoE. Now, there is a way to provide power designs with up to 100 W of PoE.

There is a lot to consider when designing equipment in IT rooms. One of the challenges engineers face is where to source power from. And, if there is only a medium voltage (for instance, 480 V) available, is there enough space to add a step-down transformer to obtain a low-voltage supply?

Couple these questions along with electrical safety codes, building codes, and equipment costs, and a designer may be faced with a number of constraints to create a fully-functional system in common data rooms.

This is what makes PoE an attractive idea for such projects. It supplies low-voltage DC power directly to the device and eliminates the complexity of locating AC power sources.

The PoE method supplies the necessary power for designs that have video IP phones; multi-channel, wireless LAN access points; and multi-direction IP cameras.

There are two types of PoE transformers that are heavily debated when used in DC-DC converters: flyback and forward topologies. Forward and flyback transformers can isolate the powered device interface from the rest of its circuitry while stepping down the PoE input voltage to power the PD circuits.

For designs that have output voltages greater than 2.5 V, a flyback converter-based transformer may be the most useful option. A flyback transformer is a coupled inductor with a gapped core; this means that during each duty cycle, the input voltage is applied to the primary winding.

Once the switch, bipolar transistor, or MOSFET opens and closes, the energy that is stored in the gap of the core flows over to the secondary windings of the transformer—delivering power to the load side. A disadvantage for flybacks is the distortion that will appear due to higher output current ripples. A required filter adds to the cost of the design.

For power designs that typically require output voltages less than 12 V, a forward PoE transformer may be a productive choice. The difference between the flyback and the forward transformer is the additional discrete components. Since the flyback transformer can store energy, an output filter, energy storage inductor, and additional rectifying device (such as a diode) are needed to achieve the same results.

Both flyback and forward transformers are equipped for multi-output voltages; however, there is a lot of unwanted noise in the flyback transformers due to the large ripples of current. Larger discrete components such as inductors and capacitors would need to be added to the design to achieve the same output as a forward transformer.

Neither transformer is inherently superior to the other. As with most principles of design, an engineer's choice between a flyback or forward topology will largely depend on the application and footprint of the PCB.

As an example of the burgeoning transformer industry, recently, Bel Fuse, a leading manufacturer in networking, telecommunications, and high-speed data transmission, announced a new series of surface-mounted transformers, dubbed SPoE.

Bel Fuse's SPoE utilizes a flyback methodology. The company claims that this device is useful for high-density boards. With high-density PCBs, designers are able to use both sides of the board, which allows for additional components to be added.

This is beneficial since complex low-voltage designs may generate unwanted distortion, and the flyback topology requires additional components. Overall, the SPoE may be a candidate for designs that have an input voltage of 60 V.

Today the Internet of Things (IoT) is increasing the market of computing devices that are requiring more than 5 W to operate. Developers must stay ahead of the demand by continuing to present new power solutions for low-voltage, high-powered network devices. PoE transformers provide engineers with another tool to hook on their design belt.