Feature: Power
Figure 3. EVAL-LT8390A-AZ maximum output current vs. input voltage. The board can produce 120W through a wide input range at high frequency.
Table 1. DC-to-DC Controllers Compatible with GaNFETs
buck side FET, and D2 is across the synchronous boost side FET. For a simple buck converter, only D1 is required. For a simple boost, use D2.
Higher Power with Higher Frequency Te LT8390A has a switching frequency up to 2MHz. GaNFETs have significantly lower switching losses compared to Si MOSFETs, enabling similar power losses at higher switching frequencies and voltages. Te EVAL-LT8390A-AZ GaNFET board demonstrates the efficiency and compact size advantages of using GaNFETs by setting the switching frequency to 2MHz. With an output of 24V, the GaNFETs can produce 120W of power
Figure 4. EVAL-LT8390A-AZ GaN controller efficiency vs. DC2598A Si MOSFET controller efficiency. GaNFETs provide higher efficiency at higher voltage.
dead time, can lead to overheating and damage to the FETs. Te best solution is to minimise the dead time. However, controllers meant for silicon FETs design the dead time around the fact that silicon FETs have slow turn-on/off characteristics (in the tens of ns). Terefore, the dead time is set longer to prevent shoot through. Te LT8390A has a set 25ns dead time, which is a shorter dead time
compared to many synchronous controllers on the market. While this is suitable for high frequency, high power MOSFET control, it is still too long for GaNFETs. GaNFETs can turn on extremely quickly (in the ones of ns). Terefore, to mitigate additional conduction losses during the dead time, it is recommended to add a catch Schottky diode in antiparallel with the synchronous GaNFET to divert the conduction to a less lossy pathway. D1 and D2 in Figure 2 show which FET to place the Schottky diodes across. D1 is placed across the synchronous
at room temperature. Te board size is comparable to the previous LT8390A evaluation board: the DC2598A, which uses silicon MOSFETs and provides a 12VOUT with 48W power. Figure 3 shows the maximum power capability of a 2MHz GaN
buck-boost, while Figure 4 compares the efficiency of both boards. Even at higher voltages, and 2.5× output power, the GaNFET board produces better efficiency than the Si MOSFET board. Te utilisation of GaNFETs allows operation at higher voltages and power with a similar board area. If there are no DC-to-DC controllers that specifically have GaNFET
driving capabilities, it is still possible to drive them effectively. Even using a controller originally meant to drive Si MOSFETs, the EVAL-LT8390A- AZ can easily outpower and achieve higher efficiency in a similar board area. Table 1 shows a wide selection of recommended controllers for driving GaNFETs. For even higher power requirements, such as paralleled buck-boost GaNFET control, please contact the factory. By researching a controller that offers a 5V gate driver and incorporating additional external protection circuit components, it is possible to drive GaNFETs safely and explore more options in power conversion design. Analog Devices:
www.analog.com
www.electronicsworld.co.uk Dec 2024/Jan 2025 41
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