QUESTION:
How can I design a 4-switch buck-boost DC-to-DC converter using GaNFETs when there are no controllers specifically made to drive GaNFETs available?
Answer:
GaNFETs are notoriously more difficult to drive and may require extra protection components if using a driver meant for silicon (Si) MOSFETs. Proper care in choosing the correct drive voltage and some small protection circuitry can provide a safe, all-in-one, high frequency GaN drive for a 4-switch buck-boost controller.
Introduction
In the never-ending quest to reduce board size and increase efficiency, gallium nitride field effect transistor (GaNFET) power devices have become an ideal candidate to push these boundaries. GaN is an emerging technology that promises higher power with ultrafast switching and reduced switching losses. These advantages allow for more power dense solutions. The current market is saturated with a myriad of different Si MOSFET drivers, and new GaN drivers and controllers with built-in GaN drivers are some years away from becoming accessible. Along with simple, dedicated GaNFET drivers (such as the LT8418), complex buck and boost controllers targeted for GaN exist on the market (LTC7890, LTC7891). There is still no straightforward 4-switch buck-boost solution. However, driving GaNFETs is not as difficult as it may seem. With some simple background knowledge, Si MOSFET targeted controllers can be adapted to drive GaNFETs. The LT8390A is a great candidate as it is a unique 2 MHz buck-boost controller with very low dead time (25 ns) (see Figure 1). The buck-boost scheme has the sense resistor in line with the inductor and outside of both hot loops—a novel feature for buck-boosts. This allows the controller to operate in peak current mode control in both boost and buck regions of operation (as well as 4-switch buck-boost). While the article delves into 4-switch buck-boost GaNFET control, the information can be extended to simple buck or boost controllers.
Figure 1. EVAL-LT8390A-AZ 24 VOUT 5 A 4-switch buck-boost GaN controller schematic.
5 V Gate Driver Is a Must
For high power conversion, silicon drivers typically operate above 5 V, with typical silicon MOSFET gate drivers ranging from 7 V to 10 V or even higher. This poses a challenge to GaNFETs, as they commonly have an absolute maximum gate voltage rating of 6 V. 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 minimizing 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 5 V 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 5.1 V 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.
Figure 2. Simplified 4-switch buck-boost GaN controller schematic with GaN control protection components.
Additionally, for even more protection, a 10 Ω 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 60 ns. 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 2 V to 4 V compared to the typical 0.7 V 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 minimize 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). Therefore, the dead time is set longer to prevent shoot through.
The LT8390A has a set 25 ns 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). Therefore, 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 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
The LT8390A has a switching frequency up to 2 MHz. GaNFETs have significantly lower switching losses compared to Si MOSFETs, enabling similar power losses at higher switching frequencies and voltages. The EVAL-LT8390A-AZ GaNFET board demonstrates the efficiency and compact size advantages of using GaNFETs by setting the switching frequency to 2 MHz.
With an output of 24 V, the GaNFETs can produce 120 W of power at room temperature. The board size is comparable to the previous LT8390A evaluation board: the DC2598A, which uses silicon MOSFETs and provides a 12 VOUT with 48 W power. Figure 3 shows the maximum power capability of a 2 MHz 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. The utilization of GaNFETs allows operation at higher voltages and power with a similar board area.
Figure 3. EVAL-LT8390A-AZ maximum output current vs. input voltage. The board can produce 120 W through a wide input range at high frequency.
Figure 4. EVAL-LT8390A-AZ GaN controller efficiency vs. DC2598A Si MOSFET controller efficiency. GaNFETs provide higher efficiency at higher voltage.
Conclusion
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 5 V gate driver and incorporating additional external protection circuit components, it is possible to drive GaNFETs safely and explore more options in power conversion design.
| Recommended GaN Controllers | Topology | Max Input/Output Voltage | Switching Frequency | GaN Safe Features |
| LTC7890 | Dual buck GaN controller | 100 V | 100 kHz to 3 MHz |
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| LTC7891 | Buck GaN controller | 100 V | 100 kHz to 3 MHz |
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| LT8418 | Half-bridge GaN gate driver | 100 V | Up to 10 MHz |
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| LT8390/ LT8390A/ LT8392 |
4-switch buck-boost controller | 60 V | LT8390/LT8392: 150 kHz to 650 kHz LT8390A: 600 kHz to 2 MHz |
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| LT8391/ LT8391A/ LT8391D |
4-switch buck-boost LED driver controller | 60 V | LT8391/LT8391D: 150 kHz to 650 kHz LT8391A: 600 kHz to 2 MHz |
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