Feature: Power
Figure 1. EVAL-LT8390A-AZ 24VOUT 5A 4-switch buck-boost GaN controller schematic.
Figure 2. Simplified 4-switch buck-boost GaN controller schematic with GaN control protection components.
overvoltage. In particular, the bootstrap supply for the top FETs on silicon gate drivers is unregulated, which means that the top gate driver can easily driſt up above the absolute maximum voltage of the GaNFET. Figure 2 shows how to address this: a 5.1V Zener diode (D5 and D6) is placed in parallel with the bootstrap capacitor to clamp that voltage at the recommended drive level of the GaNFET. Tis ensures that the gate voltage remains within the safe operating range. Additionally, for even more protection, a 10Ω resistor is added in
series with the bootstrap diodes (D3 and D4) to reduce any ringing that might be caused by the very fast and high-power switch node.
Dead Time and Body Diode Challenges In traditional converters, a catch diode is present to conduct during the off-time. Synchronous converters replace the catch diode with another switch to reduce the forward conduction loss of a diode. However, a
40 Dec 2024/Jan 2025
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problem arises if the top and bottom switches turn on simultaneously, resulting in shoot-through. In the event of a shoot-through, both FETS can be essentially short to ground, which can lead to component failures and other disastrous consequences. To prevent this, controllers implement dead time, a period where neither the top nor bottom switch is turned on. Typical synchronous DC-to-DC controllers implement dead times of up to 60ns. Tis dead time is not a significant concern with silicon MOSFETs since the body diode conducts during this period. GaNFETs do not have body diodes and switch on/off significantly
faster than silicon MOSFETs. Instead of body diodes conducting during the dead time, GaNFETs can conduct with 2V to 4V compared to the typical 0.7V of a diode. Tis conduction voltage, multiplied by the conduction current, can result in nearly 6× more power loss during the dead time. Tis increased power loss, combined with a long
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