Simplifying GaN Power Conversion with ADI Controllers

Power converter applications using gallium nitride (GaN) field-effect transistors (FETs) deliver significant improvements in efficiency and power density over silicon FETs but also introduce new design considerations. The rapid switching capabilities of GaN devices amplify even small timing errors or voltage mismatches, which can impact performance and reliability.

GaN switching events can occur at speeds that exceed the capabilities of traditional control methods and measurement tools. Designers must maintain tight control of the gate voltage—typically within a narrow range of about +6 V to -4 V, while managing voltage transitions with slew rates exceeding 30 V/ns.

GaN FETs inherently experience less power loss than their silicon counterparts. For instance, a 12 V step-down converter using 100 V GaN FETs operating at 500 kHz can achieve 97% efficiency. This equates to approximately 40% less power loss (a 2% overall efficiency gain) compared to using 100 V silicon FETs.

A key application focus for GaN-based power stages is high-efficiency DC/DC conversion in systems such as 48 V to 12 V point-of-load designs. GaN-based solutions can achieve significantly higher switching frequencies while maintaining high efficiency, often in the 500 kHz range and beyond, allowing designers to increase power density and reduce solution footprints.

Realizing these benefits requires more than a simple device swap from silicon to GaN. The low gate charge and extremely fast switching edges that boost performance also make the application more sensitive to gate drive accuracy, timing, and layout parasitics. Without careful control, issues such as overshoot, ringing, and electromagnetic interference (EMI) can quickly erode efficiency gains.

Many traditional controllers and standard test equipment struggle to keep up with the challenges of GaN power conversion, making it harder to ensure reliable operation and accurately measure performance in real-world designs. In some cases, designers can find themselves “chasing ghosts” as they try to distinguish true gate behavior from high delta voltage/delta time (dv/dt) switching noise.

In many GaN designs, controllers require additional components to ensure reliable operation, including circuits to limit gate voltage, control switching timing, and reduce noise and ringing. Analog Devices, Inc. (ADI) offers a family of GaN power controllers with integrated features to meet those requirements and simplify overall designs.

GaN optimized controllers

Transitioning to GaN-based designs is less about reinventing power conversion and more about refining implementation details, especially around layout, gate control, and measurement.

Gate drivers designed specifically for GaN often provide tighter control over rise and fall times, better noise immunity, and more precise timing alignment. In addition, layout techniques such as minimizing loop area and carefully managing return paths become more critical for achieving expected efficiency gains.

ADI's high-performance DC/DC switching regulator controllers share a common architecture that’s focused on precise gate drive control, regulated bootstrap management, and protection against gate overvoltage conditions. Integrated dead-time control helps minimize the risk of shoot-through, while reducing the need for additional external gate drive components.

The LTC789x family is differentiated primarily by topology (buck vs boost) and channel count (single vs dual), enabling flexible system-level power architecture selection:

• The LTC7890 is a 100 V dual-channel buck (step-down) controller designed for multi-rail or multi-phase power conversion applications. It enables designers to control two independent buck stages or operate them in parallel for higher output current.
• The LTC7891 (Figure 1) is a 100 V synchronous buck (step-down) controller designed for single-output power conversion applications such as core or high-current point-of-load rails. It is optimized for straightforward single-stage step-down designs where regulation accuracy and efficiency are primary requirements.

Figure 1: ADI's LTC7891 controller with an internal smart bootstrap switch. (Image source: Analog Devices, Inc.)

• The LTC7892 (Figure 2) is a 100 V dual-channel boost (step-up) controller intended for multi-rail or flexible step-up power architectures. It supports two independent boost stages, enabling compact multi-output or system power conversion designs where voltage must be elevated across multiple rails.

Figure 2: The gate drive voltage of ADI's LTC7892 can be precisely adjusted from 4 V to 5.5 V to optimize performance and allow the use of different GaN FETs. (Image source: Analog Devices, Inc.)

• The LTC7893 is a 100 V boost (step-up) controller intended for single-channel, high-voltage front-end or intermediate bus generation applications. It is suited for designs requiring a single, higher-power boost stage rather than multiple outputs.

From a system perspective, the basic function of a DC-to-DC converter remains the same—it still converts one DC voltage to another. However, GaN devices shift the design focus toward implementation details such as timing, layout, and parasitic control.

Integrating key functions

GaN devices make it practical to operate at higher switching frequencies than with silicon FETs. Higher frequencies allow the use of smaller inductors and, in many cases reduced output capacitance, increasing power density and shrinking overall solution size. The tradeoff is that tolerable parasitic inductance at lower frequencies can now contribute to ringing, overshoot, and EMI.

These challenges extend into both control and validation. Precise gate voltage regulation is required to stay within the narrow operating window of GaN devices, while high dv/dt switching behavior can make accurate measurement of high-side signals difficult using conventional probing techniques. As a result, both circuit implementation and test methodology must be adapted to match the speed of the device.

Instead of maximizing switching speed, designers increasingly focus on meticulously controlling it. With traditional silicon FETs, designers often improve efficiency by using stronger gate drive to switch the device on and off as quickly as possible. Faster transitions reduce switching losses, and any resulting noise is usually manageable.

GaN devices are already capable of extremely fast switching and pushing them further with aggressive gate drive can introduce new challenges. The rapid voltage transitions can excite parasitic elements within the circuit, resulting in ringing and EMI.

A typical 100 V GaN FET is driven at around 5 V, with a safe range of roughly +6 V to -4 V. Staying within this window requires a well-regulated gate supply and careful control of switching overshoot and undershoot.

This is straightforward for the low-side FET, where a stable 5 V supply is usually sufficient. The high-side FET is more challenging. Traditional bootstrap circuits can unintentionally raise the gate voltage beyond safe limits due to GaN’s reverse conduction behavior. Unlike silicon devices, which have a ~0.7 V body diode, GaN can exhibit an effective drop of 2 V to 3 V, increasing the voltage on the bootstrap capacitor and potentially overdriving the gate.

In a GaN-based buck converter, the high-side gate voltage can easily exceed safe limits if switching is not controlled. For example, without a small series gate resistor, the gate-source voltage may rise above the typical +6 V maximum. Adding a modest resistor (such as ~2 Ω) helps reduce this voltage and dampen ringing at both the gate and switch node.

Measuring GaN gate signals

Measuring gate signals accurately is challenging for GaN-based designs. While standard oscilloscope probes can capture ground-referenced signals like the low-side gate and switch node, the high-side gate is much harder to observe. Its source node swings rapidly between V_IN and ground, with very fast edges (over 30 V/ns) and high-frequency ringing.

These conditions exceed the practical limits of many conventional differential probes. As a result, designers often rely on specialized tools such as optically isolated probes, which provide the high common-mode rejection and bandwidth needed to accurately capture high-side GaN gate waveforms. Findings must be validated against manufacturer reference data to ensure precise performance characterization.

Poor layout can reduce or even negate the advantages of GaN-based power conversion. Long current loops, poorly placed input capacitors, and excessive gate trace inductance can all introduce losses and noise that offset the benefits of faster switching. From a design perspective, performance is no longer limited primarily by the device itself, but by how well the system controls parasitics and timing.

ADI’s controllers simplify high-side operation by regulating the bootstrap voltage to prevent the overvoltage conditions often seen in discrete implementations. This reduces the need for external clamping components and helps maintain stable gate drive conditions across different operating states.

Conclusion

Rather than relying on multiple external support components, ADI's LTC789x controllers integrate the key functions needed for reliable operation of GaN-based power conversion. By tightly controlling gate drive timing and voltage, the devices help ensure that GaN FETs operate within their safe limits even during high dv/dt switching events. Built-in control of dead time and switching transitions improves reliability and prevents efficiency loss, even when operating at high switching frequencies. Rather than managing the low-level complexities of GaN gate drive behavior, designers can focus more on optimizing system-level performance, such as layout, thermal design, and power density.