In power applications, gallium nitride (GaN) devices have significant performance and efficiency advantages over traditional silicon MOSFET devices. Gallium nitride devices can meet the needs of various industries, with higher density, faster switching speed, and higher energy efficiency. But for some applications, they will face significant design challenges.
From compact USB-C chargers and electronic car chargers to solar and data center applications, designers are eager to utilize GaN semiconductor technology to create smaller, lighter, and better cooling products.
Given the fast switching speed of GaN devices, designers will face multiple challenges, including parasitic inductance, more precise gate control requirements, gate leakage current, and reverse conduction voltage drop.
A dedicated GaN controller is an ideal choice for designing certain GaN based applications. For example, Analog Devices, Inc. offers a range of GaN power controllers. Designers can utilize simple dedicated GaN FET drivers, such as the LT8418 100V half bridge GaN driver with an integrated intelligent bootstrap switch (Figure 1).
Figure 1: ADI's dedicated LT8418 half bridge GaN driver. (Image source: Analog Devices, Inc.)
This device uses a separate gate driver to precisely control the slew rate of GaN FET during on and off periods, thereby suppressing ringing and enhancing EMI performance. The device also uses wafer level chip level packaging (WLCSP) to minimize parasitic inductance.
In addition, more complex controllers can be chosen, such as the LTC7890 and LTC7891 (Figure 2) high-performance dual buck DC/DC switching regulator controllers for GaN FET.
Figure 2: High performance ADI LTC7891 DC/DC switching regulator controller suitable for GaN FET. (Image source: Analog Devices, Inc.)
Unlike silicon MOSFET solutions, LTC7890/LTC7891 devices do not require protective diodes or other external components. The gate driving voltage of these devices can be precisely adjusted between 4 V and 5.5 V to optimize performance and support the use of other GaN FETs or logic level MOSFETs.
When the silicon controller is the only option
There is currently no dedicated GaN controller for key components such as 4-switch buck boost controllers. With careful operation, engineers may be able to use controllers originally designed for MOSFETs to drive GaN FETs, thereby improving power and efficiency. If controllers for silicon devices are directly used in GaN applications, extra caution must be taken when selecting components and designing circuit boards, and other circuits may also be required.
In high-power converters, the output voltage of traditional gate drivers is usually higher than 5 V, typically between 7 V and 10 V, and sometimes even higher. When driving GaN FET with this voltage, it can cause problems because the maximum rated gate voltage of GaN FET is usually only 6V. Even if this limit is briefly exceeded due to voltage spikes or ringing caused by stray inductance on the PCB, it may permanently damage the GaN device.
To avoid these issues, designers need to choose the controller correctly and closely monitor the PCB layout, especially around the gate and source return paths, in order to maintain low inductance as much as possible and reduce unnecessary voltage overshoot.
Many MOSFET drivers use non regulated silicon gate drivers, but their voltage may drift above the absolute maximum voltage of GaN FET. When designing, consideration should be given to managing gate drive voltage, regulating bootstrap power supply, and optimizing dead time.
The 4-switch buck boost device must use a 5V gate controller to prevent unexpected overvoltage in GaN FET. It is also important to introduce protective components such as clamp circuits or gate voltage limiters to protect the gate from accidental overvoltage.
By using a 5.1V Zener diode in parallel with a bootstrap capacitor, ADI's LT8390A can be used as a 5V gate controller (Figure 3). This will clamp the gate voltage at the recommended driving level, so the device is always within the safe operating range. To provide more protection, a 10 Ω resistor can be connected in series with a bootstrap circuit to reduce any ringing phenomenon that may be caused by very fast high-power switching nodes.