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. But, it will equalize propagation delays between the high-side and low-side signal path, and provide 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. See Section 12.1 for more information. With the fast-switching GaN device, common ground inductance could 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, it is highly recommended to use isolators with high common-mode transient immunity (CMTI). Isolators with low CMTI can easily generate false signal, which could cause shoot-through. On the other hand, it is strongly encouraged 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 malfunctioning.
Generally, ON/OFF keyed isolators are preferred, such as the TI ISO77xxF series, as a high CMTI event would only cause a very short false pulse, a few nanoseconds, which can be filtered out. To filter these false pulses, a low pass filter, like 1 kΩ and 22 pF R-C filter, is recommended to be placed at the driver input.