In half-bridges, high-voltage level shifters or digital isolators must be used 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, the control ground and the power ground must be connected 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, > 50 V/ns, in hard-switching applications, TI highly recommends to use isolators with high common-mode transient immunity (CMTI) and low barrier capacitance. Isolators with low CMTI can easily generate false signals, which could cause shoot-through. The barrier capacitance is part of the isolation capacitance between the signal ground and power ground, which is in direct proportion to the common mode current and EMI emission generated during the switching. Additionally, TI strongly encourages to select isolators which are not edge-triggered. In an edge-triggered isolator, a high dv/dt event can cause the isolator to flip states and cause circuit malfunction.

Generally, ON/OFF keyed isolators with default output low are preferred, such as the TI ISO77xxF or ISO67xxF series. Default low state ensures the system will not shoot-through when starting up or recovering from fault events. As a high CMTI event would only cause a very short (a few nanoseconds) false pulse, TI recommends a low pass filter, like 300-Ω and 22-pF R-C filter, to be placed at the driver input to filter out these false pulses.