https://www.analog.com/ AC/DC DC/DC 27 www.power-mag.com Issue 3 2025 Power Electronics Europe current mode control in both boost and buck regions of operation (as well as 4switch buck-boost). While the article delves into 4-switch buck-boost GaNFET control, the information can be extended to simple buck or boost controllers. 5V Gate Driver Is a Must For high power conversion, silicon drivers typically operate above 5V, with typical silicon MOSFET gate drivers ranging from 7V to 10V or even higher. This poses a challenge to GaNFETs, as they commonly have an absolute maximum gate voltage rating of 6V. Even the ringing caused by stray PCB inductances on the gate and source return lines that exceed the maximum gate voltage can lead to catastrophic failures. Careful layout considerations are necessary to safely and effectively drive a GaNFET by minimising inductances in the gate and source return signals. In addition to layout, implementing component-level protection is crucial in preventing catastrophic overvoltage of the gates. The LT8390A provides a 5V gate driver specifically designed for lower gate drive FETs, making it an ideal choice for GaNFETs. The issue is silicon FET drivers often lack protection against accidental 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 drift 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. This 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 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. This 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. This conduction voltage, multiplied by the conduction current, can result in nearly 6? more power loss during the dead time. This increased power loss, combined with a long dead time, can lead to overheating and damage to the FETs. The best solution is to minimise the dead time. However, controllers meant for silicon FETs design the dead time around the fact that silicon Figure 3. EVALLT8390A-AZ maximum output current vs. input voltage. The board can produce 120W through a wide input range at high frequency. Figure 4. EVALLT8390A-AZ GaN controller efficiency vs. DC2598A Si MOSFET controller efficiency. GaNFETs provide higher efficiency at higher voltage. Figure 2. Simplified 4-switch buck-boost GaN controller schematic with GaN control protection components.
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