Advantages of low-power GaN in common AC/DC power topologies
Consumers expect portable, fast and efficient chargers for the various electronic devices they carry every day. As most electronics move to USB Type-C® Chargers, more and more users want to use compact power adapters to charge all their devices.
When designing modern consumer-grade USB Type-C mobile chargers, PC power supplies, and TV power supplies, the challenge is to maintain or even increase power levels while reducing solution size. Texas Instruments' low-power gallium nitride (GaN) devices help solve this problem in a variety of the most popular topologies while providing thermal, size and integration advantages. Over the past few decades, with the development of wide bandgap technologies such as GaN, new improvements in AC/DC topologies have emerged aimed at improving efficiency and functionality. This article will provide an in-depth look at the advantages and compatibility of these devices in popular topologies for this type of application, as well as some exciting new topologies.
Maximize efficiency and power density with ACF and AHB topologies
Some newly developed half-bridge topologies can optimize efficiency while providing variable output voltage capability. The active-clamped flyback (ACF) topology and asymmetric half-bridge (AHB) topology shown in Figure 1 help maximize the efficiency and power density of the DC/DC stage. ACF and AHB topologies do not use lossy snubber clamps like quasi-resonant (QR) flyback topologies or zero-voltage switching (ZVS) flyback topologies. Instead, they are able to recycle leakage energy back to the output, thus allowing further improvements. efficiency. Both topologies are also able to completely eliminate voltage spikes on the low-side field-effect transistor (FET), thereby enabling a low-voltage synchronous rectifier FET on the secondary side. Additionally, the AHB topology does not require a second output filter, making the overall solution less expensive and smaller.
Figure 1: ACF and AHB topology
The LMG3624 integrated GaN FET features integrated “lossless” current sensing, which helps further improve efficiency by reducing power losses, as shown in Figure 2. For example, in a 65W ACF, the loss of integrated current sensing is less than 10mW, while the loss of traditional current sensing scheme is about 170mW. Any topology requiring current mode control (including ACF, AHB, etc.) will greatly benefit from this greatly reduced loss and achieve a more efficient overall solution.
Figure 2: Power loss comparison of integrated current sensing versus traditional current sensing
Totem pole PFC topology enables higher power designs
In most cases, once power levels reach above 70W, a power factor correction (PFC) stage is required. At the PFC level, if you want to take advantage of GaN's capabilities, you need to consider a totem pole PFC topology, as shown in Figure 3. By removing the bridge rectifier, the value of GaN FETs in this topology is enhanced due to zero reverse recovery losses.
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) have body diodes that produce high reverse recovery charge, whereas silicon carbide (SiC) offers little improvement in reverse recovery charge, making MOSFETs almost impossible to use in this topology. The LMG3624, on the other hand, offers an adjustable slew rate that helps find a reasonable balance between electromagnetic interference and efficiency in the system.
Figure 3: Totem pole PFC topology
Low-power GaN in QR, ZVS, LLC and boost PFC topologies
Although new topologies have begun to gain traction, there are still clear advantages to using integrated GaN in conjunction with traditional topologies. The use of GaN in QR flyback topologies, ZVS flyback topologies, and traditional boost PFC topologies is becoming more common as improvements in efficiency and switching frequency capabilities can be seen by simply replacing a single switching FET with a GaN FET (Mainly due to GaN's lower input capacitance, which can reduce turn-off losses). In addition, the LMG3624 GaN FET has low quiescent current, and its standby mode can further reduce quiescent current. QR, ZVS and boost PFC topologies also benefit from the lossless current sensing functionality integrated in the LMG3624.
The LLC resonant converter topology has been around for decades and is popular in fixed output voltage applications such as laptop adapters and TV power supplies where USB Type-C controllers have not yet become ubiquitous to provide output voltages. The LLC topology will also achieve higher transformer efficiency than most half-bridge DC/DC topologies.
As the demand for smaller, more efficient AC/DC solutions continues to grow, consumers prefer smaller adapters that are more portable. In industrial environments, as the power requirements of graphics processing units become higher and higher, PCs have an increasingly urgent need for efficient power supply units (PSU). Thinner PSUs also pave the way for slimmer high-end TVs. The LMG3624 provides features and benefits that can be integrated into all topologies described in this article, and its versatility helps meet the requirements of these applications.
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