How to use GaNFETs with ordinary buck-boost controllers? - Page 1
Forum section: electronics / general

author: katzohki
author_rank: Frequent Contributor
timestamp: March 06, 2025, 08:21:43 pm
content: I use LT's (now AD's) buck-boost regulators pretty regularly (hah) in our designs. Lately in the range of 1500 Watts. Of course the struggle is to shrink everything all the time and lately I'm getting feedback to start moving to GaNFET technology. Most of LT's stuff seems to be geared towards using MOSFETs and looking at Renesas theirs that use GaNFETs are all either buck or boost. The applications I work on all need to be buck-boost due to their wide input range. The only thing I've been able to find recently is the LT8390A which makes mention of using it with GaNFETs in one of its demo boards (see EVAL-LT8390A-AZ). This paragraph:Quotegives me a clue as to what is going on to use the GaNFETs, but I'd really like something more definite that tells me what the pitfalls and requirements of using GaNFETs with an "ordinary" buck-boost controller are. Like an app-note or something. Your help is appreciated!Bonus points for SiCFET tech.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 06, 2025, 11:16:49 pm
content: PWM controller manufacturers like LT/ADI/TI have been very slow in offering PWM controllers which are really suitable for low-voltage GaN devices. Here's the major things to be aware of IME:1.The primary pitfall is avoiding gate overvoltage. Let's assume your gate driver has a supply voltage of 5V. This is generally fine for GaN FETs, but overvoltage can still occur for half bridges with bootstrapping gate drivers. When the bottom FET turns off the freewheeling current will force the bottom FET to conduct in its third quadrant. GaN FETs have a high voltage drop in this quadrant though, maybe 2V as opposed to 0.7V for a MOSFET. So if your switch node is -2V, that means that the bootstrap cap for the high side driver may charge up to 5V-(-2V)-0.4V = 6.6V (the -0.4V is for the bootstrap diode). That's too much for most GaN FETs, so that top FET is likely going to see overvoltage on its gate. Parasitic inductance can cause the switching node to spike even further negative, making things even worse. Dedicated GaN gate drivers usually avoid these problems by adding a switch in series with the bootstrap diode which prevents it from charging the bootstrap cap during the FET dead time. If you don't have that, then you can try and add clamps to the bootstrap supply and schottky diodes in parallel with the bottom FET (like the LT8390A and LTC7800 eval boards do).2. Also the extremely fast switching of GaN FETs can be problematic for controllers and gate drivers not designed with GaN in mind. Look up common mode transient immunity (CMTI). A good GaN driver will have a CMTI of at least 100V/ns.3. Practically all pwm controllers are going to have dead times on the order of 50-250ns. For GaN FETs 5-10ns is usually plenty, any more is just a waste of power (especially given the higher third-quadrant conduction losses of GaN). Some faster controllers like the LT8390A and LTC7800 are better, like 20-25ns.4. Another nice feature dedicated GaN drivers (and also some higher end MOSFET and IGBT drivers) have is split gate drivers. This allows one to tailor the drive strength for rising and falling edges independently, but without needing a messy diode.5. General PWM controllers will have limitations on minimum pulse widths and PWM frequencies, meaning you can't take advantage of GaN's speed.The LTC7890 is more optimized for GaN, with built-in protection against boostrap cap overcharging and split gate drivers. It's a dual buck controller, not a buck-boost though. I'm not aware of any buck-boost controllers really suited for GaN at the moment.When I use GaN FETs I just use a dedicated half bridge driver (LMG1210 or LMG1205) and interface that to the PWM controller. Usually the controller chip doesn't notice the fact that it's not directly driving the FETs.EPC maintains a handly list of controllers and gate drivers that they see as relevant for their GaN devices:https://epc-co.com/epc/products/gan-drivers-and-controllers

author: katzohki
author_rank: Frequent Contributor
timestamp: March 08, 2025, 12:18:28 am
content: Thanks mtwieg, that's pretty much exactly what I needed!