Powering Bipolar Rails with Wide-VIN Fly-Buck Converter

I’m often asked by power system design engineers, how do you provide bipolar (positive and negative) voltage rails while keeping cost and complexity to a minimum? And at the same time, how should one deal with a variety of challenges―from galvanic isolation and widely-ranging input voltage to small solution size and electromagnetic compatibility (EMC)? Consider, for example, building and factory automation, test and measurement equipment, and isolated RS-485 and CAN transceivers in industrial communications applications.

Fly-buck Power Converter

Count on the Fly-Buck™ topology for low-current auxiliary and bias rails, especially if both isolated and non-isolated outputs are required. Shown in Figure 1 and Figure 2 are Fly-Buck converter schematics based on the 100V LM5017 regulator.

Figure 1: Fly-Buck converter supplying non-isolated ±12V rails


Figure 2: Fly-Buck converter supplying isolated ±12V rails

 The circuit in Figure 1 provides non-isolated ±12V rails based on a buck-boost topology and unity turns ratio coupled inductor. The feedback loop regulates the net of VOUT+ and VOUT– for symmetric startup behavior. Conversely, the circuit in Figure 2 delivers isolated ±12V rails, apropos a need to reduce noise, break a ground loop, or provide user safety. A primary-side aux rail sends bias power to the LM5017 VCC input to minimize quiescent loss at high VIN. Bifilar winding of the split-rail secondaries balances winding parameters (such as DC resistance and leakage inductance) for better load regulation.

Powering Precision Analog Applications

Ideal for powering high performance analog applications where low noise and signal integrity are key (for example, PLLs, VCOs, bipolar op amps, A/D converters), Figure 3 details a Fly-buck converter with post-regulated ±12V rails. Here, the secondary-side slave outputs are derived from positive- and negative-output LDO post regulators. Both LDOs provide high DC accuracy over line and load as well as excellent AC performance in terms of PSRR and spectral noise density. The Fly-Buck converter in this example runs at 300kHz, and the LDOs’ wide-bandwidth PSRR attenuates the outputs’ switch-mode ripple and noise.

Figure 3: Fly-Buck converter supplying post-regulated ±12V rails for precision analog applications

 Indeed, many of the LDO control and protection features complement the Fly-Buck’s system-level performance. Examples of this are secondary-referenced ON/OFF, programmable soft-start, along with current limit and thermal shutdown protection functions. Also, as the Fly-Buck’s isolated output voltages hinge on the selected transformer turns ratio, post-regulation offers an easy way to fine-tune an output to a target voltage setpoint.

Fly-Buck Solution Versatility

A good understanding of Fly-Buck circuits should be established for all power engineers. With that in mind, I recently wrote an article, “Post Regulated Fly-Buck Powers Noise-sensitive Loads,” in Power Electronics Technology that delves into a low-noise solution for powering high-precision op amps and data converters. In summary, the Fly-Buck topology provides a cohesive feature set to meet a variety of power solution needs:

  • Reliable synchronous buck or buck-boost converter based design
  • Multiple regulator- or controller-based IC solutions depending on input voltage and output current specification
  • Well-proven constant on-time control technique with excellent transient dynamics
  • Straightforward BOM, no loop compensation or feedback opto-coupler components
  • Small-size magnetic component ideal for space-constrained designs
  • No primary-side voltage spike from transformer leakage inductance.

Visit ti.com/widevin to learn more about the Fly-Buck topology and its position within our purpose-dedicated portfolio of wide VIN controllers, converters and power modules.

Additional Resources:

  • Read “Post-regulated Fly-Buck powers noise-sensitive loads” in powerelectronics.com.
  • Check out the “Wide VIN power solutions for industrial automation” app note.
  • Refer to the “Wide VIN power management ICs simplify design, reduce BOM cost, and enhance reliability” whitepaper.
  • Review these designs from the TIDesigns reference design library:
    •  “LM5017 Fly-Buck quad output isolated power supply without opto-coupler,” TI Design
    • “Isolated synchronous serial communication module,” TI Design
    • “1W small form factor power supply with isolated dual output for PLC I/O module,” TI Design.
    • Choose a wide VIN DC/DC power solution here.
    • Order the isolated and non-isolated EVMs for the LM5017 600-mA synchronous buck regulator.
    • Start a design now with WEBENCH® Power Designer.

  • Thanks for sharing your experience and knowledge

    At Fig1 what's the difference if we connect the RFB1 to Ground instead of connecting to the Vout+?

    in case when I connected the RFB1 to the ground and under asymmetric loads (I mean Iout+=200mA and Iout-=20mA) the |Vout+| is not equal to |Vout-|. I have more drop-out voltage at the Vout+.

    if I connect the RFB1 to the Vout+ like at Fig1 the problem will solve? is it effective for asymmetric loads?

    Best Regards

  • Hello,

    Thanks for your question regarding the feedback resistor connection for Fly-Buck dual outputs. By connecting RFB1 to VOUT+ in Figure 1 above, the total of the two outputs is effectively regulated. This is in contrast to just regulating the negative output when RFB1 is tied to GND, implying more variation on the positive output in terms of load regulation. A more balanced regulation profile is thus achieved when configured for feedback of the sum of the two outputs. However,  a relative imbalance can still occur in terms of output loading / cross regulation given the DCR of the coupled inductor windings and the voltage drop of the Fly-Buck rectifying diode.

    If the input voltage is less than 5.5V and the outputs are non-isolated, you may also consider the TPS65130/1/2/3/5/6/7/8 split-rail, single inductor, dual-output converter family. See more detail here: