In half-bridges, use high-voltage level shifters or digital isolators to provide isolation for signal paths between the high-side device and control circuit. Using an isolator is optional for the low-side device. However, using and isolator equalizes the propagation delays between the high-side and low-side signal paths, and provides the ability to use different grounds for the GaN device and the controller. If an isolator is not used on the low-side device, connect the control ground and power ground at the device and nowhere else on the board. For more information, see Layout Guidelines. With fast-switching devices, common ground inductance can easily cause noise issues without the use of an isolator.

Choosing a digital isolator for level-shifting is important for improvement of noise immunity. As GaN device can easily create high dv/dt, > 50V/ns, TI highly recommends using isolators with high common-mode transient immunity (CMTI) and low barrier capacitance. Isolators with low CMTI can easily generate false signals, which can cause shoot-through. The barrier capacitance is part of the isolation capacitance between the signal ground and power ground, which is directionally proportional to the common mode current and EMI emission generated during the switching. Additionally, TI strongly encourages selecting non-edge-triggered isolators. In an edge-triggered isolator, a high dv/dt event can cause the isolator to flip states and cause circuit malfunction.

Generally, the preference is ON/OFF keyed isolators with a low default output. Default low state establishes that the system does not shoot-through when starting up or recovering from fault events. A high CMTI event causes a very short (a few nanoseconds) false pulse, TI recommends placing a low pass filter (such as 50Ω and 150pF R-C) at the driver input to filter out the false pulses.