Abstract
This article discusses GaN technology in detail and explains how such wide band gap switches can be used in switch-mode power supplies. It introduces circuit examples and discusses the advantages of using dedicated GaN drivers and controllers. LTspice® is shown as a tool to understand the usage of GaN switches in power supplies. Finally, an outlook is given to the future of GaN technology.
Introduction
Wide band gap technology is becoming more and more popular for switch-mode power supplies. For circuit designers who are interested in using this relatively new technology in future designs, it is necessary to understand the benefits and the challenges, as well as gain experience with this technology.
Wide Band Gap Semiconductors
Silicon is the most amazing material for electronics. The ability to grow pure, bulk silicon and dope for both p-type and n-type properties has resulted in the incredible infrastructure and industry of microelectronics. This leads to low cost, highly available devices that endlessly permeate our lives. However, as engineers inevitably push their tools to the boundary of performance, we constantly search for the better transistor. While silicon performs great for many use cases in various applications, certain material traits limit advances in speed, power density, and temperature range. While many other semiconductor technologies, such as gallium arsenide (GaAs), silicon carbide (SiC), and gallium nitride (GaN) are available, the experience designers have with building circuits out of silicon is enormous. This applies to research, the development toolchain, and production. According to SEMI.org, in the year 2023, 12,602 million square inches of wafers were shipped, which equals the size of enough to cover 1,000 football fields. Quite the feat for an industry always pushing itself for smaller solutions!1
Familiarity with silicon enables the industry to push the boundaries of silicon further and further over time. However, the pressure to seriously consider an alternative semiconductor technology has increased so much that research on wide band gap semiconductors has produced tangible results.
GaAs is a III-V band gap semiconductor and is useful in high frequency applications such as microwaves, laser diodes, and solar cells. The high saturated electron velocity and mobility allow GaAs to function at frequencies above 100 GHz.
SiC has been used in electronics for a very long time. Early applications were light emitting diodes. Silicon carbide is used as power stage components in power supplies due to its high temperature and high voltage capability. Switches and diodes with voltage ranges way above 1000 V are available.
One specific technology to replace or enhance silicon circuitry, in power applications, is GaN. In the early 1990s, GaN was mostly a research grade material, yet by 2003, GaN ranked in the top three semiconductors by volume after silicon and GaAs. The early use cases include solid-state lighting and radio frequency electronics.2
In 2012, GaN prototypes were used for the first time as power switches (pGaN HEMT devices) as a replacement for silicon field effect transistors (FETs) in switch-mode power supplies. This enabled a higher power conversion efficiency compared to heritage silicon FET devices in such power supplies. The main difficulty with producing GaN devices has been, and still is, the ability to grow large single crystals for generating high quality large wafers.
GaN has many advantages over silicon. The main advantages are lower capacitance drain and gate for a given current and voltage rating. In addition, GaN switches are physically smaller than silicon, resulting in a small solution. GaN material has a high breakdown voltage, which is useful in applications running at voltages of 100 V and above. However, below 100 V, the power density and the capability of fast switching can give this technology advantages, such as higher power conversion efficiency, when designing different power supplies.
GaN is a wide band gap semiconductor, which means that the band gap voltage is 3.4eV vs. the 1.1eV of silicon. However, figures of merit matter differently in power supply design. A valuable use case would be in 400 V intermediate bus applications, such as in 240 V AC power converters where we use 650 V breakdown voltage FETs, with a drain source current of roughly 30 A. This system requires a gate charge of 93 nC when using a silicon FET vs. only 9 nC using a GaN FET.3 An application using such switches would run in power levels roughly between 1 kW and 8 kW. The benefit of using the GaN device with the small gate capacitance results in much faster switch transition times and reduced switching losses, which ultimately result in higher power conversion efficiency, especially at higher switching frequencies with smaller magnetics.
Using Wide Band Gap Semiconductors in Switch-Mode Power Supplies
There certainly are some challenges when replacing silicon-based MOSFETs with GaN devices. These challenges relate to GaN gate drive, fast voltage change during switching, and high conduction loss during dead times.
Firstly, GaN switches typically have lower gate voltage ratings than silicon FETs. Most GaN manufacturers recommend a typical gate drive voltage of 5 V. Some have an absolute maximum rating of 6 V, which does not give much headroom between the recommended gate drive voltage and the critical threshold, above which would damage the device. Recommended gate drives vary from manufacturer to manufacturer. This limitation, along with the fact that the gate charge in GaN devices is so small, means that driver stages must strictly limit the maximum gate drive voltage to avoid damage to the GaN device.
Also, one must deal with the fast voltage change (dv/dt) of the switch node of the power supply. This may cause false turn-on of the bottom switch. The gate of GaN devices is quite small. Any fast voltage changes in the vicinity, such as the switch node, may capacitively couple onto the small gate of the GaN switch and turn it on. To have more control of the turn-on and turn-off profiles, a separate pull-up and pull-down pin and a carefully designed printed circuit board layout are needed.
Lastly, GaN FETs have a higher conduction loss during dead times. These are the times when both the high-side and low-side switch of a bridge configuration are turned off. Dead times are necessary to prevent a short circuit from the high-side voltage rail to ground. During the dead time, typically, the low-side switch develops current flow through a body diode of the low-side switch. One way of solving this problem of high conduction losses during such dead times is to strictly minimize the length of these dead times. This needs to be done without generating overlapping times of the high-side and low-side switches to avoid a short circuit to ground.
One other item to mention is the fact that GaN offers a wider conversion range. The fast rise and fall times provide smaller duty cycle than with silicon MOSFETs.
Using a Different Switch Than the Silicon
For many years silicon switches have been used as power stage switches in the power conversion industry. Now that GaN switches are available for power supply designers, how can they be used to replace silicon switches? Are they simply a drop-in replacement or does the power stage design with them look different?
Figure 1 shows a power stage of a typical buck regulator switch-mode power supply. The red arrows indicate additional components that may be necessary when using GaN switches in a switch-mode power supply. GaN switches do not offer the convenience of a body diode. The body diode in a silicon MOSFET is a p-n junction, which occurs by the structure of the silicon process. The process of GaN technology is somewhat different so a simple p-n junction body diode is not available.4 However, GaN switches have a different mechanism to yield similar results. Only majority carriers are involved in GaN device conduction so there is zero reverse recovery.5 However, the GaN FET does not have the forward voltage of the body diode, as with silicon MOSFETs, so the voltage across the GaN FET may get quite large. Thus, the power losses during the dead time are quite high. This is the reason for the importance of reducing the dead time when using GaN switches compared to silicon switches.
Silicon MOSFETs have a body diode, which power designs use excessively during the dead time in a switch-mode power supply. In a buck regulator’s low-side switch, the current flow through its body diode provides the continuous current flow the inductor demands. Without a body diode in the low-side switch, every bit of dead time would cause the switch node in a buck regulator to go to minus infinity voltage. Most certainly the circuit would lose energy and eventually blow up due to voltages outside of the rated voltage of the switch, before reaching minus infinity.4
If the source and gate are at the same potential when using a GaN switch, but with a continuous current source such as the inductor, the GaN FET will turn on in reverse.
Since GaN switches do not include a p-n junction body diode, like a silicon MOSFET, the low-side switch needs to be constructed with an alternate current path around the low-side switch allowing for current flow during the deadtime. Figure 1 shows a simple Schottky diode (D2) placed between drain and source of the lowside GaN switch. This diode will quickly take over the inductor current flow during the dead time of the circuit.
In the GaN FET, during the reverse conduction, the drain and the source get flipped due to the symmetry of the GaN FETs. The gate remains at ground potential, but the switch node is selfbiased to be the minimum turn on threshold of the GaN FET. This low voltage is the minimum threshold needed to turn the GaN FET on (typically GND-2V to GND-3V). Since the VGS is not optimized, the RON suffers in the reverse conduction. The external Schottky is an alternative path without turning the GaN FET on in the reverse conduction.
The second modification to the circuit when using GaN switches in Figure 2 is the resistor in series with diode D1, supplying the basic voltage to the high-side driver of the circuit coming from the INTVCC supply voltage. This resistor may be needed to limit peak currents for the high-side driver.
Lastly, the Zener diode, D3, may be needed to prevent voltage spikes from becoming excessive on the high-side driver voltage supply.
While the additional components in Figure 1 look fairly simple and straightforward, ensuring that such a circuit will run reliably in all operating conditions requires fine tuning and thorough evaluations on the bench. Also, variations of component values over production and over aging will need to be considered. The ultimate risk is permanent damage to the GaN switches.
Using a Special GaN Controller
A simple way to avoid the critical evaluation process of protection functions in the power stage of the switch-mode power supply using GaN switches is to select a power supply controller IC. The LTC7891, single-phase, step-down (buck) controller is specifically designed for GaN power stage switches. Selecting a dedicated GaN controller makes a GaN power supply design simple and robust. All the challenges mentioned earlier are addressed and solved with such controllers. Figure 1 shows the simplicity of a step-down power design using GaN FETs controlled by a dedicated GaN controller.
While allowing for a simple design, such dedicated switching controllers still offer the flexibility needed to work with different GaN switches available on the market today. Also, GaN switch technology is far from having finished its path of development and innovation. Future GaN switches will be different and better than today’s offerings. However, they might require slightly different handling compared to the switches that are readily available today. Devices such as the LTC7891 in Figure 2 offer a dedicated up and down gate drive pin for both switches. With this, the rising and falling slope of the gate voltage of the GaN switches can be controlled separately. This allows for driving the power stage with GaN switches perfectly with minimal ringing and overshoots.
In Figure 2, the most prominently visible difference to a heritage silicon MOSFET buck controller is the separate gate drive pins for rising and falling edges. However, many additional differences exist between the LTC7891 and heritage controllers designed for silicon switches. There is an internal bootstrap switch to prevent the overcharging of the high-side driver during dead times. This is implemented reliably without the need for external components.
Another important feature is the smart near zero dead time control. This allows for reliable operation and yields significant power conversion efficiency improvements, while also allowing for high switching frequencies. The LTC7891 is rated for switching frequencies of up to 3 MHz.
One other unique feature is the possibility of accurately adjusting the gate drive voltage from 4 V to 5.5 V. This optimizes the VGS needed for various GaN FETs available in the market.
Using Any Controller IC
Besides using external passive fixes to get a heritage silicon controller to work with GaN switches or using a dedicated GaN controller, engineers may also consider using a heritage controller IC and utilizing a driver stage optimized for GaN usage. This takes care of solving the challenges with GaN and allows for a simple and robust design. Figure 3 shows the power stage of a buck regulator implemented with the LT8418 driver IC. This driver comes in a very small WLCSP (wafer level chip scale package) that enables very low parasitic resistances and inductances for low voltage offsets resulting from fast current changes.
Simulation to Help with the Circuit Design
Once suitable hardware, controller ICs, and GaN switches have been selected, a great way to get first evaluation results is to use a detailed circuit simulation. LTspice from Analog Devices offers complete circuit models that may be used for simulation free of charge. This is a convenient way to learn about using GaN switches. Figure 4 shows a simulation schematic with the LTC7891. A dual-channel version, the LTC7890, is also available.
Integrating Wide Bandgap
While GaN technology is brilliant for building FETs and using them in an advanced power stage, GaN is not necessarily able, nor cost-effective enough, to be used as the control circuitry for a switch-mode power supply. Therefore, we will see a hybrid approach for the foreseeable future. The control will be silicon based with highly optimized control and drive circuitry to drive a high power GaN switch. This approach is technologically available today and it is cost competitive. However, it will require the use of multiple die in one circuit. Either with the GaN switches separate, as shown within the examples of this white paper, or by integrating multiple die in one of ADI’s all integrated hybrid approach in a power converter IC or in a μModule® power supply solution with the integration of many passive components including the inductor.
As previously mentioned, growing large, high quality GaN remains a challenge. GaN on diamond is one way of processing GaN switches. However, since roughly the year 2010, the mainstream choice in GaN manufacturing is high electron mobility transistors (HEMTs) on silicon due to the larger possible wafer diameters and the lower associated cost with existing silicon processing infrastructure.2 Early technical challenges with this approach have been resolved. However the technology requires years of further development. In this case, GaN devices are made using GaN epitaxy on silicon wafers, so they are not grown as bulk crystals like silicon or SiC.
See Table 1 for current offerings from ADI for wide band gap power supplies with GaN switches.
| Device | Description |
| LT8418 | 100 V Half-Bridge GaN Driver with Smart Integrated Bootstrap Switch |
| LTC7890 | Low IQ, Dual, 2-Phase Synchronous Step-Down Controller for GaN FETs |
| LTC7891 | 100 V, Low IQ, Synchronous Step-Down Controller for GaN FETs |
| EVAL-LT8418-BZ | LT8418 Evaluation Board |
| DC2938A | LTC7890 High Frequency, Dual Output, Synchronous Buck Converter Using GaN FETs |
| EVAL-LTC7890-AZ | LTC7890 High Frequency, Dual Output, Step-Down Supply with EPC GaN FETs |
| EVAL-LTC7891-AZ | LTC7891 High Frequency Step-Down Supply with EPC GaN FETs |
| DC2995A | LTC7891 High Frequency Step-Down Supply with GaN FETs |
The Future of GaN Technology
GaN technology for switch-mode power supplies has reached a solid development state in which many power supply applications can be designed with them. However, there will be further development with each new generation of GaN switches. The existing switch-mode power supply controllers and drivers for GaN from ADI are a flexible and dependable way to work with GaN FETs from different vendors now and in the future.
We are on a path to GaN usage with many aspects progressing today. The first aspect is that GaN switches by themselves are quite robust today. However, time and development are necessary for the reliability of the switches to be fully accepted by the users of this relatively new technology. Secondly, the manufacturing processes of GaN switches will improve further, increasing the yield and reducing defect density, thus reducing the cost, and improving the reliability of GaN switches. Thirdly, more and more specific GaN drivers, such as the LT8418, and switching controllers such as the LTC7890 and LTC7891 buck controllers, are released to the market to simplify the implementation of a GaN-based switch-mode power supply.
Common GaN voltages are 100 V and 650 V. This is why the first power supplies using GaN technology are designs in the 100 V and 650 V maximum voltage range. However, the GaN specific features, especially the small necessary gate charge needed, scale down to lower voltages also. In the future, we will also see lower voltage power supplies in the range of 40 V maximum voltage to make use of the advantages of GaN. It is also quite possible to see GaN switches all the way up to 1000 V. Especially at such high voltages, the fast switching is very beneficial.
Conclusion
Semiconductor materials that expand the operating range and power density of power supplies will continue to develop. Silicon was the exciting material. GaN is a great material for the next 10 to 15 years. Something else exciting will come next. Electronics has grown massively, with automotive, AI, connectivity, etc., and will keep growing to solve the major issues facing mankind. Each of these applications will continue to grow, requiring more power, higher density, greater robustness, and higher efficiency. GaN provides the opportunity to keep pace with these innovations.
参考电路
1 "Worldwide Silicon Wafer Shipments and Revenue Fall in 2023, SEMI Reports." SEMI, February 2024.
2 Felix Ejeckam, Daniel Francis, Firooz Faili, Daniel Twitchen, and Bruce Bolliger. "GaN-on-Diamond: A Brief History." 2014 Lester Eastman Conference on High Performance Devices (LEC)
3 Larry Spaziani and Lucas Lu. "Silicon, GaN and SiC: There’s Room for All." 2018 IEEE 30th International Symposium on Power Semiconductor Devices and ICs (ISPSD).
4 "Does the GaN Have a Body Diode? If So How Does It Compare with the Silicon MOSFETs with Respect to Forward Voltage Drop and Reverse Recovery Characteristics?" EPC, February 2022.
5 "The p-n Junction." Britannica.
6 Yaozong Zhong, Jinwei Zhang, Shan Wu, Lifang Jia, Xuelin Yang, Yang Liu, Yun Zhang, and Qian Sun. "A Review on the GaN-on-Si Power Electronic Devices." Fundamental Research, Vol. 2, No. 3, May 2022.
