Simplifying higher current split-rail designs with Fly-Buck converters

Since 2011 TI has recommended using the Fly-Buck™ topology (or isolated buck topology) to simplify isolated bias designs for industrial and communication applications. The Fly-Buck design adds coupled windings to the inductor to provide single or isolated bias supplies without an opto-coupler. The LM5017 family is simple to use, reduces bill of materials (BOM) costs and increases performance, which is why it has gained popularity in the last few years.

Even though it’s popular, there is one constraint to existing Fly-Buck products – they are limited on total output power. The LM5017 is a 100V, 650 mA part, but in a typical 24V system, it can only support up to 5W output power before triggering the current-protection circuit. This limitation prevents the technology from expanding to more industrial applications. For example, a programmable logic controller (PLC) input/output (I/O) module requires 24VIN, isolated 24VOUT @ 500mA; in this case, the LM5017 could only support half of the required output power.

To meet these design challenges, TI has introduced the LM5160A, which expands the Fly-Buck portfolio and triples the power capability. The LM5160A not only easily solves the power-limitation problem; it also expands the technology to designers of broader applications like three-phase AC motor drives, power over Ethernet (POE) IP cameras and automotive.

Take the three-phase AC motor drive as an example to show how LM5160A simplifies the gate driver bias design. Figure 2 shows a typical AC motor drive system and bias supplies for the high- and low-side IGBT gate drivers. Positive and negative bias voltages are usually required due to IGBT characteristics.  

The Wide Input Isolated IGBT Gate-Drive Fly-Buck Power Supply for Three-Phase Inverters reference design demonstrates the LM5160’s ability to generate eight isolated outputs from a 24V input bus. Three pairs of +15V and -8V are used to bias the high-side gate drivers, while the remaining pair (+15V and -8V) is capable of providing 300mA to bias all of the three-phase low-side gate drivers. The total output power is about 14W. Figure 3 shows the reference design board.

In addition to the AC motor drive, the LM5160A also addresses the higher bias power needed in PLC systems due to the increasing number of I/O channels. The Isolated Tri-output Fly-Buck Power Supply for Industrial PLC Applications reference design takes 24Vin and generates three isolated outputs: 5V@1A, +15V@200mA, -15V@200mA. All outputs remain +/-5% regulation over line and load regulation.

POE applications are booming these days, especially in the IP surveillance market. The LM5160A solution greatly increases power density and improves efficiency by more than 5% compared to traditional approaches. The Isolated Fly-Buck Power Module for POE Application reference design (PMP10572) demonstrates high density and high efficiency using a combined POE solution with the LM5160A and TPS2378.

These are just a few examples of the numerous options presented by TI’s expanded Fly-Buck capabilities. This approach will be successful in building automation; heating, ventilation and air conditioning (HVAC); elevator control; and automotive bias power in hybrid electric vehicles/electric vehicles (HEV/EV).

Additional Fly-Buck resources