Wide Bandgap Now


Dan Kinzer, COO/CTO at Navitas Semiconductor

Dan Kinzer

Five years ago, in this column, I wrote an editorial entitled, “When Wide Bandgap?”  In that piece, I predicted, “The key to success in silicon carbide (SiC) is to accelerate the cost and material defectivity learnings, expand substrate and epitaxial capacity, and to transition to 150mm diameter.” In addition, “The key to success in gallium nitride (GaN) is improved high volume and lower cost MOCVD processes on silicon in the 150mm to 200mm range, with device and material designs that can withstand the high operating voltage and surface electric field stresses,” predicting successfully that this would happen in the next 2-3 years. Cost is still an issue, especially for SiC materials but there are still tremendous opportunities for cost learning through material and system improvements, and economies of scale. 
Not everything happened as predicted. While there are still proponents of the BJT, the focus has been to advance the state-of-the-art SiC power MOSFET. Vertical GaN devices have not flourished as wafers are too small, too expensive and the devices are extremely difficult to fabricate. For applications above 1KV, SiC is the way to go.

I concluded, “As system designers learn to use the high frequency capability… system performance, size, and cost advantages will emerge and drive a gradual shift in the industry through the rest of this decade and into the next.” The shift has begun, with WBG representing 1-2% of the power market. Now, the time has come for WBG to break strongly into the market and displace Si in multiple applications.   In fact with GaN, revolutionary - not evolutionary - changes are now happening that were not predicted.

System designer learning was not the barrier; tools were lacking. Mainstream controllers and magnetic materials continued to be designed for frequencies around 100 kHz. . The market asked for efficiency and low cost, and the system designers delivered. When Si switches and drivers were pushed to higher frequency in existing topologies attempting to achieve higher density, the power losses increased unacceptably. Even the early wide bandgap devices were hard to drive, difficult to use, and lacking evidence of reliability.

Power density - the reduction of size - has become a major selling point. Today, 1MHz+ controllers are becoming widely available to support proven, accepted, soft switching topologies. 1MHz+ magnetic materials are available in traditional wire wound and PCB-embedded planar winding configurations. Critically, lateral GaN power devices are now available and operate at those frequencies, at higher efficiency than the low frequency conventional designs. They are reliable, volume production ready, and cost-effective. Moreover, GaN power integrated circuits have emerged that are extremely straightforward to drive, because input is no longer high current analog, but low power digital. 

AllGaN™ is the industry’s first eMode GaN Power IC process, allowing monolithic integration of 650V GaN IC circuits (drive, logic) with GaN FETs . This is an impossible dream for superjunction Silicon MOSFETs, dMode GaN, or SiC with their complex, expensive gate drive and protection systems which restrict switching frequency. With AllGaN, the GaN FET gate is driven safely, precisely and efficiently by the upstream integrated GaN driver. Simple, robust, low-current digital signals, from standard, low cost, low voltage ‘no driver’ control ICs are fed directly into the GaN Power IC for a simple, low component count design. Waveforms exhibit a true “textbook” feeling with very clean rising and falling edges, no ringing, and extremely fast turn-on and turn-off propagation delays. Integration eliminates gate overshoot and undershoot, while zero on-chip inductance ensures no turn-off loss and tight control of deadtime in half-bridge circuits. The internal gate drive voltage can be precisely controlled, and even the turn on speed can be programmable. 

How does AllGaN™ performance translate into real-life application benefits? High critical electric field capability means reduced device dimensions, smaller die size, lower capacitances and lower losses. In turn, less cooling is needed and converters use smaller magnetic components, leading to large savings in application size, weight and cost. The eMode HEMT device relies on electron current exclusively, so there is no recovery loss in switching and integrated gate drive enables 1MHz operation easily. In a 150W adapter example, a small step from switching at 65/100kHz to 300kHz with 650V GaN Power ICs in a ‘CrCM PFC plus LLC’ topology yields a 2x increase in power density compared to best-in-class Si-based systems . For 25W smartphone / tablet chargers, GaN Power ICs in a soft-switching active clamp flyback achieves 70% more power density than benchmark Si designs.
Additional GaN Power IC features, expanded voltage and current capabilities are forecasted. Combined with controller and magnetic advances, lateral GaN will deliver unprecedented and cost-effective performance, driving the rapid and widespread adoption of this fantastic new technology. 

References

  1. “Wide Bandgap When?” D. Kinzer, Bodo’s Power Systems, Sep 2011.
  2. “Adoption of Wide Bandgap Power Devices Increases”, A. Bhindra, IEEE Electronics 360, July 2016. 
  3. “Breaking Speed Limits with GaN Power ICs”, D. Kinzer, plenary APEC 2016
  4. “Make it Easy, with GaN Power ICs”, T. Ribarich, Bodo’s Power Systems, May 2016,

Source: Bodo's Power Systems, October 2016