Power Tips: how to power low voltage, low-power industrial applications with a Fly-buck™

Some industrial applications contain sub-circuits which require a small supply to power noise-sensitive circuits that cross an isolation boundary.  In applications like PLCs, data acquisition and measurement equipment, this isolation boundary provides noise immunity.  Typical sub-circuits that would require this isolated supply include isolated RS-232 and RS-485 communication channels, line drivers, isolated amplifiers, sensors, and CAN transceivers.  Similar power requirements are also found in applications that require an isolated supply to provide the gate drive power for IGBTs, and in some medical applications that require isolation for safety. 

The figure below shows a simple block diagram of the power requirements for these types of systems.  A low voltage rail (usually 3.3V or 5V) is available for the main system power.  This rail is used to generate the low power isolated voltage rail, which typically requires less than 2W and is usually unregulated.  

Simple block diagram of the power requirements for these types of systems

In these systems, the asymmetrical half-bridge or fly-buck™ topology can provide an efficient and well regulated solution.  A simplified schematic of the fly-buck topology is shown in the figure below.  At first glance it may seem complicated, but upon further inspection it is really quite simple.  The primary side of the circuit is composed of a controller, a high-side (S1) and a low-side (S2) power switch, an inductor, and an output capacitor (Cr). 

Often, the controller and FETs are combined into a single package for a more integrated solution.  This primary circuit looks and operates exactly like a buck regulator, where the voltage on Cr is regulated by the controller.  The secondary circuitry looks and operates similar to a flyback converter, where a second winding is added to the inductor to provide the isolated output voltage.  When S2 is on, the voltage on Cr is impressed across the inductor winding.  This voltage is coupled across the inductor and charges the output capacitor (Co) through D1.  The output voltage is simply determined by the turn ratio of the inductor, and the regulated voltage on Cr.

Simplified schematic of the fly-buck topology

Because the voltage on Cr is regulated, this topology is able to maintain a fairly tight regulation band on the output voltage compared to most other unregulated approaches.  The main factors degrading the regulation with the fly-buck are load dependent and associated with winding resistances in the inductor, the output diode forward voltage, and leakage inductance in the inductor.  With a little bit of pre-loading, it is usually possible to keep a 5V output within +/-5% of nominal.

Because of the synchronous nature of the primary circuitry, the efficiency of a fly-buck supply is also impressive.  Take PMP6813 for example, which provides an isolated 5V at 1W and is over 80% efficient.  Combined with integrated FETs, this high efficiency enables fly-buck solutions to fit into very small form factors.  The PMP6813 design mentioned previously fits on a 10mm by 20mm board area and is designed with a transformer that is hi-pot tested to 3kV.

Although the examples that I provided are for a 5V output, the output voltage can easily be changed by selecting an inductor with a different turn ratio.  It is also possible to generate an isolated split-rail supply like +/-15V.  In general, higher output voltages will also result in better efficiency.  There are several fly-buck designs loaded into the PowerLab library, like the few examples given below.  Also, be sure to check back in the future as more reference designs are added to PowerLab each month.

  • PMP6813 - 5V Input to 5V/1W Output, Isolated DIP Module
  • PMP6838 - Flybuck Isolated SIP Module 4.5-5.5Vin, 5V/1W
  • PMP7315 - Flybuck 18-30Vin, 24V/100mA Output
  • PMP7942.1 - Flybuck 17-32Vin, Dual 5V/0.25A, 15V/0.1A
For more design tips from PowerLab, check out our PowerLab Notes series.