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Towards GaN

For decades, Si has dominated the semiconductor market due to its abundance and low cost. Almost every circuit has some Si based component. Power Electronic circuits are no exception. To this date, most converters use Si transistors as switches. More than 60% of the space on a converter circuit board is occupied by bulk capacitors and magnetic components. Because these components transfer energy every second, their size depends on the operating frequency. Si fails to provide considerable size reduction in high power circuits because it struggles to operate efficiently at high frequencies (>100kHz). Hence, Si-based high-power solutions tend to be bulky. This is where GaN steps in

GaN enables efficient operation at high frequencies (>100kHz) which means that the size of magnetic components can be decreased allowing for power efficient high-power density circuits. GaN based converters are more compact, dissipate lesser heat and pack more power per unit volume than their Si-counterparts. The BLDC driver under development at IIT-B uses GaN transistors to operate at higher frequency because at high frequencies, the current ripple and hence the amount of DC-Link capacitance required decreases. This means that smaller film capacitors can be used instead of bulky electrolytic capacitors. With GaN transistors, the team was able to increase the operating frequency to 4x that of initial prototype and get 50% theoretical reduction in C-Link capacitance and 40% lesser losses per switch.

Owing to higher operating frequency, lesser losses and easier thermal management, the team was able to decrease the power stage size to 1/3rd that of the Si-based prototype. This allowed them to design modular half bridge cards (daughter boards). 3 such daughter boards when plugged into a motherboard (which carries DC-Link capacitors) complete the power stage of the BLDC driver. This plug-n-play features makes the motor driver highly modular. In case of a failure in any of the phases, the corresponding daughter board can be simply pulled out and replaced like a Lego block. This modularity facilitates easy debugging, quicker maintenance and shorter downtime.

Si vs GaN Technology (100-650V range device)
Criteria Si GaN Reason
Operating Temp Lower Higher GaN has higher Bandgap than Si
Operating Temp Lower Higher GaN device has lower CISS, COSS meaning faster turn-on, turn-off and lower switching losses
Device Size Bigger Smaller GaN has higher critical elec. field, which means for same breakdown voltage, GaN device can be much smaller (VBR = 0.5*wdriti*Ecrit)
RDS-ON Lower Higher Higher electron density and much smaller drift region in GaN
Device Parasitic Lower Higher Instead of Au bond wires, modern GaN devices have a “laminated structure” interconnected with thick Cu-filled vias for terminal connections
Cost Lower Higher Fabrication Complexity

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Figure:Power stage of driver with 3 daughter cards and DC-Link capacitors installed

B. G. Fernandes