Feature: Power
Can you drive GaNFETs with a DC-to-DC Controller originally designed for Silicon MOSFETs?
By Kevin Thai, Applications Manager, Analog Devices
H
ow can I design a 4-switch buck-boost DC- to-DC converter using GaNFETs when there are no controllers specifi cally made to drive GaNFETs available? GaNFETs are notoriously more diffi cult to drive and may require extra protection
components if using a driver meant for silicon (Si) MOSFETs. Proper care in choosing the correct drive voltage and some small protection circuitry can provide a safe, all-in-one, high frequency GaN drive for a 4-switch buck-boost controller.
In the never-ending quest to reduce board size and increase
effi ciency, gallium nitride fi eld eff ect transistor (GaNFET) power devices have become an ideal candidate to push these boundaries. GaN is an emerging technology that promises higher power with ultrafast switching and reduced switching losses. T ese advantages allow for more power dense solutions. T e current market is saturated with a myriad of diff erent Si MOSFET drivers, and new GaN drivers and controllers with built-in GaN drivers are some years away from becoming accessible. Along with simple, dedicated GaNFET drivers (such as the LT8418), complex buck and boost controllers targeted for GaN exist on the market (LTC7890, LTC7891). T ere is still no straightforward 4-switch buck-boost solution. However, driving GaNFETs is not as diffi cult as it may seem. With
some simple background knowledge, Si MOSFET targeted controllers can be adapted to drive GaNFETs. T e LT8390A is a great candidate as it is a unique 2MHz buck-boost controller with very low dead time (25ns) (see Figure 1). T e buck-boost scheme has the sense resistor in line with the inductor and outside of both hot loops - a novel feature for buck-boosts. T is allows the controller to operate in peak current mode control in both boost and buck regions of operation (as well as 4-switch buck-boost). While the article delves into 4-switch buck-boost GaNFET control, the information can be extended to simple buck or boost controllers.
5V Gate Driver Is a Must For high power conversion, silicon drivers typically operate above 5V, with typical silicon MOSFET gate drivers ranging from 7V to 10V or even higher. T is poses a challenge to GaNFETs, as they commonly have an absolute maximum gate voltage rating of 6V. Even the ringing caused by stray PCB inductances on the gate
and source return lines that exceed the maximum gate voltage can lead to catastrophic failures. Careful layout considerations are necessary to safely and eff ectively drive a GaNFET by minimising inductances in the gate and source return signals. In addition to layout, implementing component-level protection is crucial in preventing catastrophic overvoltage of the gates. T e LT8390A provides a 5V gate driver specifi cally designed for
lower gate drive FETs, making it an ideal choice for GaNFETs. T e issue is silicon FET drivers oſt en lack protection against accidental
www.electronicsworld.co.uk Dec 2024/Jan 2025 39
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