GaN based PFC power supply with bi-directional power flow

In 2017, ABB conducted a research project that examined solutions and challenges using gallium nitride (GaN) semiconductors in power electronics applications. Compared to silicon (Si), GaN has a higher band-gap (3 times), a higher electric breakdown field (10-15 times), and a larger saturated electron drift velocity (2-2.5 times). These characteristics make GaN an attractive alternative to silicon-based devices.  

Totem-pole bridgeless Power Factor Correction (PFC) single phase rectifier topology uses fewer components than conventional Boost PFC topology, and it can be used in both, hard or soft switching modes. With GaN HEMT devices, the absence of reverse recovery and low switching loses means higher efficiency compared to conventional Boost rectifier topology.

In 2018, ABB took the next step in its investigation of GaN’s potential by initiating a research project to look at using GaN devices in interleaved totem pole power factor correction (PFC) power supplies. The device tested included:  

  • 600 V, 70 mΩ GaN with integrated gate driver in 8-mm × 8-mm QFN package
  • Integrated over current and over temperature protection
  • One phase leg on a daughter board
  • 3.3 kW totem-pole PFC reference design
  • Three GaN phase legs interleaved
  • 100 kHz switching frequency
  • >400 LFM fan required  

First, the R&D team conducted power tests in rectifier mode at input AC voltages of 208 VAC and 120 VAC. The results showed power at 2.7 kW and 1.3 kW respectively with peak efficiency more than 99%. This promising outcome was tempered, though, by the need to optimize control to deal with high line impedance. Thermal design also needed to be optimized to manage higher peak power.

The team next implemented bi-directional power flow functionality to simulate how equipment such as motor drives draw power from the local grid in some instances and feed it back in others (e.g., regenerative braking). Control was modified to reach fast response times and minimize DC voltage overshoot during transient. The tested configuration changed modes from 2kW rectifier to 1.8kW regeneration within 2ms. The test demonstrated system response in less than 1.5 line cycles. 

Besides two GaN devices used in a high switching frequency phase leg, two Silicon MOSFETs are used in the line frequency switching leg. These MOSFETs replace diodes in the conventional totem-pole bridgeless PFC. With lower conduction losses, they improve the efficiency of the converter.

By interleaving multiple high switching frequency phase legs (consisting of GaN HEMTs), higher power rating of the PFC can be achieved. The control algorithm uses a fast current control loop that is responsible for regulating AC current while maintaining low THD and high power factor. A slow DC voltage control loop maintains output voltage at the set point level.  

These preliminary results show that GaN-based devices can deliver superior performance in applications where a piece of equipment shifts rapidly from consumer to supplier of electricity on a power distribution system. As noted above, this has been realized on large scale equipment like gantry cranes used in ports. ABB’s research has demonstrated that the same functionality is available for smaller-scale applications found in other industrial facilities.

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