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Technical Article

The benefits of low-power GaN in common AC/DC power
topologies

Noah Rodriguez

Consumers want portable, fast and efficient chargers for the various electronic devices they carry with themdaily. With a majority of electronics moving to USB Type-C® chargers, demand for compact power adapters thatcan charge all of their devices is increasing rapidly.
The challenge in designing modern consumer USB Type-C mobile chargers, PC power supplies and TV power
supplies is shrinking solution size while maintaining or increasing the power level. TI’s low-power gallium nitride(GaN) devices help address this problem in a wide variety of the most popular topologies while simultaneously
offering thermal, size and integration benefits. Along with the development of wide-bandgap technologies such
as GaN over the past couple of decades, there have also been new improvements in AC/DC topologies toimprove efficiency and functionality. In this article, I’ll dive into the benefits and compatibility of these devices
within popular topologies for these applications, as well as some enticing new topologies.
Maximizing efficiency and power density with ACF and AHB topologies
A couple of newly developed half-bridge topologies optimize for efficiency while offering variable output voltage
capabilities. The active clamp flyback (ACF) and asymmetric half-bridge (AHB) topologies as shown in Figure 1
can help maximize efficiency and power density in DC/DC stages. Instead of using a lossy snubber clamp, likein a quasi-resonant (QR) flyback or a zero voltage switching (ZVS) flyback, the ACF and AHB topologies areable to recycle the leakage energy to the output instead, further improving efficiency. These two topologies arealso capable of completely eliminating voltage spikes on the low-side field-effect transistor (FET), which enables
lower-voltage synchronous rectifier FETs on the secondary side. Additionally, the AHB topology does not requirea second output filter, making for a lower-cost and smaller solution overall.
The LMG3622, LMG3624 and LMG3626 integrated GaN FETs have an integrated “lossless” current-sensing
feature that can help improve efficiency even further by reducing power losses, as shown in Figure 2. For
example, in a 65-W ACF, the integrated current sensing would contribute less than 10 mW of losses, whereas

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a traditional current-sensing scheme would contribute around 170 mW of losses. Any topology that requires
current-mode control, including ACF, AHB and others, would greatly benefit from this large reduction in losses
and would enable a more efficient overall solution.

Figure 1. The AHB and ACF topologies

Figure 2. Integrated current sensing vs traditional current sensing power loss
Totem-pole PFC topology for higher power designs
In most of the world, there is a requirement for a power factor correction (PFC) stage once you reach power
levels above 70 W. In this stage, if you really wanted to take advantage of the capabilities of GaN, youwould likely consider the totem-pole PFC topology, as shown in Figure 3. The removal of the bridge rectifier
strengthens the value of GaN FETs in this topology, given its zero reverse-recovery losses.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) have a body diode that makes them virtually
unusable in this topology because of their high reverse-recovery charge, with silicon carbide (SiC) offering only aminor improvement in reverse-recovery charge. On the other hand, the LMG3624 offers an adjustable slew rate,
helping you to find the appropriate balance of electromagnetic interference and efficiency in your system.

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Figure 3. The totem-pole PFC topology
Low-power GaN in QR, ZVS, LLC and boost PFC topologies
Although newer topologies have started gaining traction, there are still clear benefits of using integrated GaN
with traditional topologies. Introducing GaN into QR flybacks, ZVS flybacks and traditional boost PFC has
become common, because you only need to replace a single switching FET with a GaN FET to see theimprovements in efficiency and switching frequency capabilities (largely through GaN’s lower input capacitance,
which results in lower turnoff losses). In addition, the LMG3624 GaN FET has low quiescent current, whichstandby mode can reduce even further. The QR, ZVS and boost PFC topologies also benefit from the integrated
lossless current sensing in the LMG3624.
The LLC resonant converter topology has existed for decades, and is popular in fixed output-voltage applications
such as laptop adapters and TV power supplies where a USB Type-C controller does not dominate the outputvoltage. The LLC topology will also enable the highest transformer efficiency compared to most half-bridge
DC/DC topologies.
Conclusion
As demand for smaller and more efficient AC/DC solutions continues to grow, consumers prefer smaller
adapters for better portability. In industrial settings, highly efficient power-supply units (PSUs) are becoming
necessary for PCs as graphics processing units become more power-demanding. Thinner PSUs are also paving
the way for slimmer high-end TVs. The versatility of LMG3624 helps fulfill the requirements of these applications
by offering features and benefits that you can integrate into all of the topologies referenced in this article.
Additional resources• Read the LMG362x Family of Low-Power GaN FETs product overview.
• Read how GaN enables high efficiency in totem-pole PFC-based power designs.
• View the LMG3622EVM-082 65-W USB-C PD High-Density Quasi-Resonant Flyback Converter reference
design.

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