Optimum GaN Gate Control for Greater Performance Gains

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

As the world looks to electrification to help utilize energy efficiently and switch to renewable sources, the time is right for wide-bandgap semiconductor technologies like gallium nitride (GaN). The performance of traditional silicon MOSFETs and IGBTs is now close to the theoretical limits of the material, and further development is realizing only small improvements, slowly and at a high cost. GaN transistors permit a significant and instant increase in power-conversion efficiency and can deliver additional advantages, including smaller size and greater reliability.

 

Image used courtesy of Adobe Stock

 

Accordingly, these devices pervade new designs for important applications like power adapters and wall chargers, electric vehicle charging systems, industrial and medical power supplies, and motor drivers. End users will experience this revolution as new generations of equipment enter the market in slimmer form factors that are easier to carry and run cooler than their predecessors. GaN technology also delivers advantages in class-D audio amplifiers, including longer battery runtime, smaller size in portable and mobile applications, and the potential for superior audio quality.

Several important benefits of GaN transistors derive from their generally lower parasitic effects than silicon equivalents. In particular, lower values of gate-source and gate-drain capacitance (C_GS, C_GD) translate into lower energy losses during switching. Figure 1 compares the efficiency of 48 V to 3.3 V buck converters implemented using silicon and GaN technologies, showing a significant efficiency advantage for GaN that becomes even larger at a higher output current.

 

Figure 1. Efficiency comparison between GaN and silicon technologies in a buck converter. Image used courtesy of Bodo’s Power Systems [PDF]

 

In addition, faster charging and discharging of the capacitances result in shorter delay and transition times, allowing engineers to design applications for switching frequencies into the MHz range. This permits the use of smaller storage passives, with a direct effect on increasing power density. In class-D amplifiers, high switching frequency enables increased audio fidelity. Moreover, a low value of CGS enhances switching control in applications that call for a low-duty cycle, such as buck regulators with a high step-down ratio.

 

Unlocking GaN Advantages

Power is nothing without control, and the principle applies well concerning driving GaN transistors in switching circuits. The role of the gate driver is critical in maximizing the efficiency advantages of GaN transistors while protecting the device structure to ensure reliability.

MinDCet has created the MDC901 driver IC with features that are specially designed to ensure secure, fast, and efficient GaN switching to maximize performance and energy savings, leveraging experience producing high-performance, high-reliability ASICs and systems for demanding applications, including rad-hard space-ready components. Figure 2 highlights that the PCB area required by the MDC901 gate controller is five times smaller than the external components needed for a comparable gate driver solution.

 

Figure 2. The PCB area used by the MDC901 gate controller is five times smaller than the required external components for an equivalent competitor gate driver solution. Image used courtesy of Bodo’s Power Systems [PDF]

 

Overcharging Protection

The gate oxide in GaN transistors is relatively fragile and can be damaged by excessive voltage. The behavior of parasitic inductance in the gate loop, charging/discharging of transistor capacitances during switching, and induced voltages appearing on signal lines are all factors that can expose the low-side transistor to potentially damaging excessive gate-source voltage (V_GS).

There are various ways to protect the gate against overvoltage. One is to add external clamping circuitry. However, this tends to increase the power consumption and circuit footprint. PCB parasitic effects also limit its effectiveness. Alternatively, protection can be built into the GaN transistor at the cost of increased device complexity and cost. MinDCet’s MDC901 half-bridge gate driver protects the GaN gate by integrating true floating voltage linear (LDO) regulators for both high-side and low-side driver circuits. These LDOs tightly regulate the voltage at a level that can be programmed to 5 or 6 V. Hence, the driver effectively prevents overvoltage while allowing designers a broader choice of GaN transistors without internal protection.

 

Dead-Time Control

To realize the full efficiency gains that GaN technology can deliver in power conversion, designers need to understand the behavior of parasitic capacitances and the physics that permit transistor reverse conduction when VGS = 0 V. In contrast, an ordinary silicon MOSFET has an intrinsic body diode that conducts freewheeling current; the GaN transistor has no body diode. The device self-commutates when reverse-biased with VGS = 0 V so the freewheeling current passes through the transistor drain-source channel. This has several advantages, including eliminating the losses associated with body-diode reverse recovery and internal noise generated during diode turn-on.

On the other hand, the voltage drop across the transistor is greater than the corresponding voltage across the body diode of a silicon MOSFET. In a half-bridge, the loss due to this voltage drop is incurred during the transistor dead time. Hence, a short dead time helps to minimize these losses and enhances efficiency. On the other hand, insufficient dead time incurs losses as drain-source capacitance is discharged through the complementary transistor.

Effectively, the ideal dead time is application dependent. Hence, dead-time control is a desirable feature of a suitable GaN driver to help designers optimize performance and efficiency. Moreover, control also ensures that the dead time is known and constant for the application’s lifetime.

The MDC901 provides digital inputs that allow setting the dead time for both the turn-on and turn-off phases of half-bridge operation. The driver can also set the dead time automatically if required. Closed-loop sensing of the GaN gate voltages provides a failsafe by ensuring the high-side or low-side transistor can only turn on when the complementary device is off.

 

Output Drive Strength

A key strength of GaN technology comes from its ability to transition quickly between the off and on states and thereby minimize dissipation. Achieving a short switching transition time is dependent on providing adequate gate current. The MDC901 has maximum gate-drive strength of 10 A, which maintains the ability to ensure fast switching transitions even where multiple GaN transistors are connected in parallel.

While fast switching is typically a priority, care must be given to moderate the speed to avoid ringing. This is typically achieved using resistors chosen according to the inductance of the gate circuit and the transistor gate capacitance. It is common for the driver to integrate these resistors to facilitate control of the turn-on/turn-off current.

The MDC901 takes a different approach that emphasizes using external resistors, which move power dissipation outside the driver IC, easing thermal management and enhancing reliability. The driver provides separate pull-up and pull-down outputs for gate-drive tuning. In addition, the driver is designed to operate with output voltage down to -4 V to ensure correct operation when the voltage swings below the supply ground, which a combination of source inductance and load conditions can cause.

 

High Duty Cycle

Another important advantage of GaN transistors’ fast switching capabilities is their ability to operate efficiently at low-duty cycles. This can be true in applications like power conversion with a large step-down ratio. GaN makes it possible to convert a 48 V bus directly to 1 V, at the point of load (POL), with high efficiency and no intermediate stage required. This enables bill of materials savings and a smaller circuit footprint and eliminates intermediate-conversion losses. The GaN transistors’ ability to minimize switching losses by performing fast transitions can raise the overall conversion efficiency by 10-15% compared to equivalent silicon MOSFET technology at the same switching frequency.

Conversely, GaN’s fast switching capability makes the technology suitable for applications requiring extremely high duty cycles. These include class-D amplifiers and motor drivers, particularly when operating at high rpm. When operating at a sustained high duty cycle, the bootstrap voltage and, thus, the voltage applied to the GaN transistor gate can be reduced due to leakage effects and biasing other loads in the system. To combat this, the MDC901 driver integrates a charge pump to sustain the necessary gate-drive bias. This enables operation at up to 100% duty cycle, thus permitting the high-side switch to be held on for extended periods. The MDC901 also integrates bootstrap diodes that help ensure adequate gate-drive strength.

Figure 3 shows the driver’s internal features, including the charge pump, dead-time generator, and floating regulators. Essential system safety features are also integrated, including die-temperature monitoring, gate-signal output monitoring, and gate undervoltage lockout (UVLO).

 

Figure 3. Block diagram of MDC901 GaN gate driver IC. Image used courtesy of Bodo’s Power Systems [PDF]

 

To accelerate development, MinDCet has created three half-bridge evaluation boards. The MDC901-EVKHB, MDC901-15I-EVKHB, and MDC901-2E-EVKHB combine the MDC901 driver with GaN Systems’ 100V GS61008P GaN HEMTs, Innoscience’s 150V INN150LA070A FETs, and EPC2215 200V eGaN FETs, respectively, in a buck-converter topology. A fourth half-bridge evaluation board MDC901-15NEVKHB using Nexperia’s 150V GAN7R0-150LBE GaN FET is under development and will be available soon. Each board measures 80mm x 90mm and is ready for use out of the box, delivering a compact solution ready for testing.

 

GaN Gate Control Conclusions

GaN transistors can drop directly into established power-conversion topologies and deliver advantages, including greater energy efficiency, higher power density, more compact product dimensions, cooler operating temperatures with easier thermal management, and greater reliability.

Maximizing these benefits requires some re-engineering, particularly in controlling the transistors. Ideal gate-driver characteristics include a large current-sink capability to control multiple GaN devices in parallel, configurable dead time, and protection against gate overcharging. With additional functions, including an integrated charge pump to serve applications requiring a high duty cycle and built-in system protection features, the MDC901 addresses demanding, energy-conscious applications in medical, industrial, consumer, and automotive markets.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine and is co-authored by Mike Wens, Jef Thoné, and David Czajkowski of MinDCet NV Belgium.