Silicon Carbide (SiC) Enhances Auxiliary Power in Transportation

With an all-in-one SiC solution incorporating power semiconductor die, digital programmable gate driver, and low-inductance power module, designers can finally achieve disruptive system-level benefits of SiC technology to shrink the size noise, and field failures of Auxiliary Power Units (APUs). 

APUs are in a variety of transportation vehicles including: 

• Subway and metro trains (light rail) 

• Electric buses 

• Construction, farming, and mining equipment 

• Heavy-duty cargo and delivery vehicles 

• Aircraft 

All electrified vehicles have two high-power conversion systems: the Traction Power Unit (TPU), which provides vehicle propulsion, and the APU that supplies power for all other on-board loads including lighting, air conditioning, doors and more. Unfortunately, both power units are big, heavy, inefficient and noisy because of the limitations imposed by silicon insulated gate bipolar transistors (IGBTs).  

Microchip SiC MOSFETs 

Today’s transportation APUs are based on silicon IGBT technology. Compared to IGBTs, SiC Metal Oxide Semiconductor Field-Effect Transistors (MOSFETs) enable the following system-level benefits to APUs:  

• Smaller – opens more cabin space for passengers/cargo  

• Lighter – extends vehicle range by reducing fixed on-board weight  

• More efficient – especially critical for continuously operating APUs  

• Quieter – eliminates tiresome electronics whine by switching outside the audible range. 

Because the IGBT’s high switching losses generate so much heat, large, expensive heat sinks are required, and a switching frequency limit is imposed (typically 10 kHz). Due to this limitation, the isolation transformer – by far the largest, heaviest part of the APU – cannot be reduced in size or weight. It is also worth mentioning that 10 kHz is very unpleasant to the human ear.  

With Microchip’s SiC MOSFETs, switching losses are slashed by up to 80%, and lower conduction losses at light load are optimal for continuously operating APUs. Higher efficiency means smaller, lighter heat sinks. Further, SiC MOSFETs switch with ease beyond the audible range; using 40 kHz as an example, one could reduce the size of the isolation transformer by 75% and simultaneously eliminate the tiresome whine. 

Microchip SiC Gate Drivers 

While the high switching speeds of SiC MOSFETs are the key to lower switching losses, they also make the system susceptible to EMI failure and voltage spikes. New gate driver technology is required for SiC MOSFETs to optimize the switching transients associated with switching such high levels of power at unprecedented speeds. In addition, the gate driver must quickly detect and respond to short circuit conditions, as SiC MOSFETs have shorter withstand times than most silicon IGBTs. These challenging requirements have motivated the design of Microchip’s AgileSwitchÒ family of software-configurable digital gate drivers.  

Unlike conventional turn-off (left), Microchip’s patented technique of Augmented Switching™  begins with an on-stage voltage (20 V, in this case), moves to a user-programmed intermediate level for a specified dwell time, and finally to the off-state voltage (-5 V, in this case). This allows designers to dial in overshoot voltage and switching losses simply by changing the gate driver settings. In addition, short circuit events are quickly arrested, reducing peak voltage and current by 60% and 10%, respectively.  

As a further benefit, the digital programmability of Microchip’s SiC MOSFET gate drivers will accelerate your time to market. With accompanying software, designers can fine-tune their driver settings in seconds with the click of a mouse – as opposed to countless hours in the lab with a soldering iron and bin of gate resistors. 

Table 1: Si IGBT vs. SiC MOSFET and benefits to the transportation end user.  

Table 2: Example of how augmented switching resolved customer challenges.   

For transportation auxiliary power SiC power applications, Microchip’s AgileSwitch SiC Gate Drivers are a proven solution – with thousands of drivers in service. 

Microchip SiC solutions allow APUs to dramatically shrink in size and weight and eliminate noise in ways that are not possible with silicon IGBTs. While these disruptive improvements are possible, care must be taken to ensure complete capitalization of these system-level benefits.  

Only Microchip unifies in-house die production with low-inductance power packaging and  digital programmable gate drivers so that our clients can make the most efficient, compact, and reliable end products.  

Microchip provides unrivaled SiC MOSFET avalanche and short circuit ruggedness alongside total system solutions designed to streamline your SiC development – from benchtop to production.