“Gallium nitride (GaN) high electron mobility transistors (HEMTs) offer power system designers an exciting new option. GaN HEMTs enable them to significantly reduce switching losses and improve power efficiency compared to silicon MOSFETs, and support higher switching frequencies, reducing system size and weight.

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By Vito Prezioso Field Applications Engineer, Power Specialists, Future Electronics (Nordic)

Gallium nitride (GaN) high electron mobility transistors (HEMTs) offer power system designers an exciting new option. GaN HEMTs enable them to significantly reduce switching losses and improve power efficiency compared to silicon MOSFETs, and support higher switching frequencies, reducing system size and weight.

But superior performance doesn’t come without a price: GaN HEMTs are harder to drive than silicon MOSFETs. Silicon MOSFETs require a simple +10 V drive voltage and can handle transients up to 20 V without risk of damage, while GaN HEMTs typically only accept a maximum gate drive voltage of +6 V and specify a +5 V optimum gate drive voltage. Shutdown conditions must also be carefully managed, some HEMTs require a negative drive voltage to ensure that the device does not turn on unexpectedly. Therefore, GaN HEMTs require more tightly controlled gate driver operation than silicon MOSFETs.

This makes an integrated system-in-package (SiP) combining a HEMT and a gate driver very attractive: HEMT manufacturers can choose the best driver for the HEMT. In an integrated SiP, the manufacturer will also implement an optimized gate drive circuit. The main benefit of this optimized circuit is that performance and reliability are unaffected by parasitic inductance, which is apt to occur in circuits built with discrete components.

Figure 1 shows where the parasitic inductance occurs:

・ LS1 is the parasitic inductance caused by the gate trace, which connects the drive pin of the gate driver to the gate of the transistor through a resistor
・ LS3 is the inductance generated by the feedback loop that connects the source pin of the transistor to the COM pin of the gate driver
・ LS2 is the stray inductance of the source branch, which also affects the power loop

Figure 1: Simplified gate driver circuit showing sources of parasitic inductance

Parasitic inductance combined with Miller capacitance at fast switching transitions can cause ringing and overshoot or undershoot voltage spikes at the gate. At best, this just creates EMI and reduces efficiency; at worst, it actually damages the transistor.

In general, wider traces and shorter gate drive loops are better. By combining the driver and transistor in one package, the gate-source loop is kept very short, resulting in much lower parasitic inductance than circuits with discrete HEMTs and drivers.

In fact, the best integrated devices have extremely low stray inductance in the gate drive loop. This virtually eliminates gate-source voltage ringing and has the following effects:

・Reduces the stress on the gate structure, thereby improving the reliability of HEMTs
・ Reduce the damping resistance at the output of the driver. Faster switching can be achieved, resulting in lower switching losses.

The integrated device also provides low stray inductance in the power loop, which greatly reduces drain-source voltage spikes. in order to fulfill:

・ Lower switching loss
・ Low EMI
・ Lower drain-source voltage stress, higher reliability

Most importantly, integrated devices reduce component count and board space. Some of the 650 V, 150 mΩ HEMTs that designers can find in the market today come in packages measuring 8 mm x 8 mm. In application, they require discrete gate drivers and gate drive resistors. In contrast, Infineon’s IGI60F1414A1L, a CoolGaN™ integrated power stage (IPS) device that combines a half-bridge power stage consisting of two 600 V/140 mΩ enhancement-mode GaN switches with dedicated gate drivers, Packaged in a thermally enhanced 8 mm x 8 mm QFN-28 package.

Different products for different design requirements

These advantages, including easier design implementation, lower parasitic inductance, and smaller board footprint, have prompted all major GaN HEMT manufacturers to start building integrated device portfolios while offering discrete HEMTs and GaN drivers.

But some trade-offs are related to the use of integrated devices. The first is that the customer’s production is more closely tied to the manufacturer and device: Unlike discrete HEMTs and discrete drivers, which in many cases have industry-standard packages, integrated drivers may have fewer pin- or package-compatible alternatives.

Beyond that, integration also requires manufacturers to decide how to compromise and meet the needs of different types of applications. This means that there are important differences between integrated GaN products on the market today.

The most obvious product differences in the market are:

・ Is it optimized for a specific topology
・ Does the device provide a means of adjusting operation to minimize electromagnetic emissions at the expense of efficiency
・In addition to the driver and HEMT, additional functions integrated in the device

Topology-specific integrated GaN products

With the introduction of the MasterGaN family, STMicroelectronics occupies a unique position in the market for integrated GaN products. This is because these GaN SiPs are the first to feature integrated half-bridges in either symmetrical or asymmetrical configurations, paired with optimized 600 V half-bridge drivers.

STMicroelectronics has created a MasterGaN product family consisting of five series to meet the various topologies used by most customers and the power rating range required by their applications. Therefore, as shown in Figure 2, the MasterGaN2 and MasterGaN3 products are only used in active-clamp flyback converters because this topology requires the low-side on-resistance to be lower than the high-side on-resistance. STMicroelectronics’ new reference design EVLONE65W demonstrates how much space can be saved when using MasterGaN2 with ST-ONE’s all-in-one digital power controller. EVLONE65W is a 65W USB Power Delivery 3.1 charger board based on Active Clamp Flyback topology. The EVLONE65W measures 5.8 cm x 3.2 cm x 2.0 cm and can achieve a high power density of 30 W/in3.

Figure 2: STMicroelectronics launched the industry’s first integrated GaN half-bridge product MasterGaN series

For LLC resonant topologies, the MasterGaN1, MasterGaN4 and MasterGaN5 families offer symmetrical configurations supporting power ratings up to 400 W.

In fact, Future Electronics has developed a feature-rich development platform called GaNSTar that can drive loads up to 500 W using MasterGaN1. GaNSTar implements a 96% efficient LLC resonant DC-DC converter. It benefits from a precise digital control scheme running on the board’s STM32G4 microcontroller and an exemplary thermal design.

The MasterGaN product is then optimized for one of two soft-switching topologies. In contrast, integrated GaN devices from other important suppliers of GaN switches, such as ON semiconductor‘s 650 V integrated driver GaN product to be released by the end of 2022, and Infineon’s IGI60F1414A1L CoolGaN IPS device, can handle hard switching and apply in any application topology. For example, an evaluation board that ON semiconductor is developing implements a converter design that includes a 500 W totem-pole power factor correction converter, a 65 W flyback converter, and a 300 W LLC converter.

Managing EMI and Efficiency Tradeoffs

On the other hand, however, Infineon’s CoolGaN IPS family is different from all other integrated GaN devices on the market. These Infineon devices enable the power system designer to access the gate of the transistor and configure the gate drive resistor/capacitor to adjust the dV/dt ratio as shown in Figure 3. This unique feature enables designers to manage the balance between switching losses, electromagnetic emissions, and overshoot, which is valuable in applications that are highly sensitive to EMI. However, there is a tradeoff: adding an optional external resistor lengthens the gate-source loop, which, as mentioned above, increases parasitic inductance.

Figure 3: External resistors control the dV/dt ratio in CoolGaN IPS devices

Like ON Semiconductor, Infineon has a range of reference board designs based on the CoolGaN IPS family, including a 65 W quasi-resonant flyback converter and a 65 W active clamp flyback converter for high-density power adapters.

Deeper integration to reduce component count and board space

The integration of an optimized driver with its GaN HEMT provides partial value as it reduces development time and effort. However, as Power Integrations demonstrates, this advantage extends beyond the integration of drives. As its name suggests, Power Integrations specializes in multifunctional products. For example, the InnoSwitch™3 and InnoSwitch4 low-power AC-DC converters are flyback controllers that integrate PowiGaN™ GaN transistors, a synchronous rectifier controller, and a FluxLink isolated feedback link. These devices minimize the length of the gate-source loop, enabling designers to achieve very compact and efficient designs at power levels up to 110 W.

Power Integrations also offers the HiperPFS-5, a power factor correction controller with an integrated 750 V PowiGaN GaN switch.

An effective way to evaluate Power Integrations products is to use Future Electronics’ TobogGaN power board, as shown in Figure 4. This is a 60 W AC-DC converter based on the InnoSwitch3-Pro integrated flyback controller module, which includes a PowiGaN GaN switch on the primary side. The TobogGaN system operates from a universal mains input with up to 92% efficiency at full load and provides a programmable output between 5 V and 20 V.

Figure 4: Future Electronics’ TobogGaN board is a flyback converter rated up to 60 W

Power Integrations doesn’t have this part of the market to itself: Also notable is STMicroelectronics’ VIPerGaN offering. The VIPerGaN50 is a quasi-resonant flyback controller paired with a GaN power switch. It operates from mains input and supports loads up to 50 W. ST also offers the VIPerGaN65 rated at 65 W and the VIPerGaN100 rated at 100 W.

Market Response to Demand Stimulus

The diversity of integrated GaN drivers reflects manufacturers trying to keep their finger on the pulse of the market: as demand for GaN products grows rapidly from a low base, it is currently uncertain whether customers will prioritize efficiency and minimization of parasitic inductance over free control of dV/ dt rate.

To be sure, the market will continue to grow rapidly, and GaN device manufacturers are investing heavily in development and production to meet this demand. Happily, customers can look forward to a growing selection of products to optimize their high-efficiency, high-density GaN-based power system designs.

development kit

Supported components: MasterGaN1
Board Type: GaNSTar from Future Electronics

illustrate:
This 96% efficient LLC resonant DC-DC converter benefits from a precise digital control scheme running on STMicroelectronics’ STM32G4 microcontroller.

Supported components: InnoSwitch3-Pro
Board type: TobogGaN from Future Electronics

illustrate:
This 60 W AC-DC converter is based on the Power Integrations InnoSwitch3-Pro integrated flyback controller module. The TobogGaN system operates from a universal mains input with up to 92% efficiency at full load and provides a programmable output between 5 V and 20 V.