GATE DRIVE TECHNOLOGIES How have gate drivers evolved with SiC/GaN semiconductors?

More than 10 years ago, we started seeing lots of SiC and GaN activities and an influx of new product launches, aiming to dethrone the incumbent silicon IGBT and Power MOSFET solutions that have been on the market for more than 50 years. This will take longer than anticipated as these existing solutions are in every power electronics engineer’s design.

SiC and GaN have lots of benefits, however it has taken more than 10 years for the market to consolidate and align on SiC and GaN specifications and standards. One of these specifications is the definition of gate driving and fault protection of these new wideband gap power switches.

Today all these wideband gap disparities have been aligned to very much how we will drive IGBT and Power MOSFET. Most importantly, gate drive technologies have also improved tremendously to catch up and enable the adoption. Broadcom´s newly released 10A gate drive optocouplers are able to fulfill the demanding requirements of driving and protecting SiC and GaN.

How Broadcom's gate driver has evolved with SiC and GaN

The variations of the gate voltage required by SiC and GaN have been standardized over the years. The definition of Broadcom's gate driver power supply range and under voltage lockout or UVLO threshold has been established as a result of this.

Broadcom's gate drivers have a wide supply range from 0 to 30V, which make them very versatile for either uni-polar gate driving or bipolar gate driving. This ensures that the SiC and GaN switches are firmly switched on or off. Broadcom offers two UVLO levels for their new basic gate drivers. The single channel ACFL-3161’s UVLO is at 13V which is suitable for driving 15V SiC’s gate and the dual channel ACFJ-3262’s UVLO is at 8V which is suitable for 10V GaN’s gate.

Besides having low RDSON to conduct higher current, one other important benefit of SiC and GaN is their ability to switch on and off very quickly. This is critical to reduce the switching losses and increase the operating frequency of the switches. The 10A peak current of Broadcom's new gate drivers is extremely useful here. The 10A gate current helps to overcome the input capacitance and charges up the SiC and GaN gate very quickly. This optimizes the potential of the SiC and GaN and improves the overall system efficiency.

Next, is the gate driver’s noise immunity, sometime also known as slew rate or dv/dt suppression. The fastest SiC/GaN switch, the higher the dv/dt, the worse the noise. In the past, Broadcom's gate drivers have noise immunity of less than 50V per ns which is more than sufficient for the slower IGBT. The new 10A gate drivers can guarantee a noise immunity of more than 100kV/us.

SiC and IGBT need to be protected

In terms of protection, the 10A smart gate drive optocoupler, ACPL-355JC has an over current protection feature, similar to DESAT feature of the IGBT. The ACPL-355JC monitors the drain and source of the SiC and triggers a soft shutdown when a high fault current raises the drain source voltage. One critical difference is SiC and GaN switches faster than the IGBT and hence fault current needs to be extinguished earlier before the switch get destroyed. Typically, the rule of thumb is 1 to 2us for SiC and GaN, vs 5 to 10us for IGBT.

Now, to address the difference in time SiC and IGBT need to be protected. The detection voltage, detection time and shut down time of the ACPL-355JC can be adjusted and fine-tuned to accomplish the task whether it is 1us or 10us.

Application examples of SiC/GaN components

The applications that can reap the potential of SiC/GaN must leverage on their ability to deliver higher efficiency and operating frequency. One of them is the renewable energy power converter. To harness energy with the least loss, it is important for solar inverters to squeeze out every single percentage of power conversion efficiency. It is very common for solar inverter manufacturers to claim 99 % conversion efficiency when they are enabled by SiC or GaN.

Another application is fast battery charging that will require high frequency switching on and off of the the SiC and GaN. The high frequency operation also reduces inductor size and this make it perfect for electric vehicle on-board charger where small size matters.

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