7 Overvoltage Clamp Method

Bootstrap overcharge is a common issue in GaN half-bridges, so half-bridge GaN gate driver ICs such as LMG1205 and LM5113-Q1 integrate overcharge protection. These devices work by adding an internal switch in series with the integrated bootstrap diode. The device senses the voltage across Cboot and opens the series switch when the voltage goes above approximately 5 V. The bootstrap path becomes high-impedance because the switch is in series with the bootstrap diode. Rboot is infinite—infinite Rboot results in 0 Iboot and no more Qin. The bootstrap voltage gradually decreases due to Qout and eventually falls below the threshold where the driver closes the switch again, restoring normal function to the bootstrap path.

The overvoltage clamp is a simple and effective option to prevent overcharging bootstrap. Like the Zener diode, the clamp allows a certain amount of productive overcharging to cancel out the bootstrap diode drop. In addition, the clamp is more efficient than the Zener diode because the clamp prevents excess charge rather than dissipating the charge as heat.

There are some downsides to this overvoltage clamp approach. First, this circuit has a response time delay. In LMG1205, the response time is approximately 250 ns. While this delay is sometimes acceptable, the response is too slow in some applications to prevent damage. This delay is especially noticeable when using a small Cboot and significant dead time. Second, the clamping threshold voltage is fixed, which limits device flexibility because the voltage cannot support both 5 V and 6 V gate GaN FETs.