GaN DC/DC Converters for High-Efficiency Power Supplies

Sep 16, 2022
A GaN chip on a circuit board with blue and violet light rays
GaN is the chemical symbol for gallium nitride, a III-V semiconductor material commonly referred to as a wide bandgap (WBG) material because it has a relatively high energy requirement (compared to silicon or Si) to knock an atom’s electron from the valence band (e.g. – insulator) to the conduction band (e.g. – conductor).

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When it comes to an electronically controlled switch, you want a material that has a high breakdown electric field (e.g. – blocking voltage) when off and very low-resistance conduction channels when on, which is why WBG materials make excellent semiconductor devices. Some other WBG semiconductors you may have heard of are silicon carbide (SiC), gallium arsenide (GaAs), and aluminum nitride (AlN).

Comparison of properties of Si (silicon), GaN (gallium nitride) and 4H-SiC (silicon carbide)

Fig. 1: Radar Chart Benchmarking Figures of Merit for WBG Materials vs. Si, courtesy of PowerRox [1]

GaN also has other interesting properties that make it attractive for several applications. Its electron mobility and melting point enable high-current channels and higher operating temperatures (or increased reliability at the same or lower temperatures), respectively. When fabricated into transistors, the resulting devices can have lower gate charge and equivalent on-state channel resistance (RDS_ON) than Si-based metal–oxide–semiconductor field-effect transistors (MOSFETs). Though there are many types of GaN-based switches, let’s focus on the GaN high-electron-mobility transistor (HEMT) as one example, with its structure shown in Figure 2. With the gate activated, current flows very quickly through the shallow GaN layer in what is sometimes called a two-dimensional electron “gas” (2DEG) [2], as represented by the dotted line in the figure.
GaN Power Transistor Structure
Fig. 2: Cross Section of GaN-on-Si Lateral Transistor, courtesy of EPC [3]
While GaN has been used for many decades in light-emitting diodes (LED) and RF applications, it is only in the last decade or so that its use in switching applications, such as switch mode power supplies and inverters, has become more common. The attractive properties described above enable power supplies designed with GaN switches to address many of the key size, weight, and power factors (a.k.a. – SWaP factors) that tend to be critical drivers of practically any power solution design.

Lower RDS_ON and gate transition times help reduce conduction and switching losses, respectively, which can lead to higher efficiency across the power train. These characteristics offer the added benefit of enabling control of the switches with lower duty cycle (D), which supports higher direct conversion ratios that were impractical with MOSFETs (e.g. – direct 48V to 1V power conversion).

When GaN Switching Speeds Become a Design Challenge

WBG switches can be fast, really fast. In fact, they come about as close as possible to the kind of ideal (e.g. – zero transition time) switches we first learn about in textbooks. These rapid transitions are due to the very low gate charge and very high electron mobility of materials like GaN. Turn-on and turn-off transitions can occur in <1ns (1ns = 10-9s) even in some fairly high-power applications.

These transitions are so fast that most engineers trying to measure them on their boards are likely not even using an oscilloscope with the appropriate bandwidth (BW) to capture them sufficiently (e.g. – see Nyquist–Shannon Sampling Theorem [4]). If you need to properly measure and characterize a signal with nanosecond transitions, then scope BW needs to be in the GHz range. These kinds of scopes are generally very expensive and usually designed for high-speed data applications rather than power stage analysis.
Oscilloscope traces: 350 MHz ringing, fast rise and fall times
Fig. 3: Switch Node Waveforms for the EPC2100, VIN = 12V to VOUT = 1.2V, IOUT = 25A, 1MHz Showing Rise/Fall Times, courtesy of EPC [5]
The negative ramifications of these extremely fast switch node transition rates include increased electromagnetic interference (EMI) and overshoot/oscillation events, both of which are attributed to undesired energy dumps, or more specifically, improper flow of high-energy transition currents to ground into parasitic inductance or equivalent series inductance (ESL). For the context of this blog, we shall only touch on these topics briefly to support the main points, but one should look to additional resources to truly dig into them with the rigor they deserve.

It should be clearly noted that in nearly all comparable applications (we should at least confine this discussion to non-RF switching power supply applications), WBG components are NOT drop-in replacements for their Si counterparts. The greatly decreased switching energy and high electron mobility of a GaN HEMT compared to a Si FET can allow for transitions in the nanosecond range, but these extreme current transitions can cause previously benign parasitic loop inductances to produce catastrophic voltage overshoot, as shown in the simple calculation examples below.
Voltage overshoot equation with parasitic inductance
Equation 1: Showing the relationship between overshoot voltage, parasitic inductance and rate-of-change of current.
DC supply with load inductor
Fig. 4: Representation of Current Flows (red/yellow/green) with Parasitic Inductance in Boost dc-dc Topology, courtesy of PowerRox [1]
Parasitic inductances of only a few nanohenries may be considered negligible with current slew rates (di/dt) in Si-based designs, but they can be catastrophic in a GaN-based design.

The equation above clearly shows how even a small amount of ESL, from the component package alone, can have catastrophic effects on your design. This is before one has even invested the extreme time and effort required for a very clean, tight layout that contains these current flows as effectively as possible. Make no mistake, though: proper layout techniques and best practices for GaN circuits are your best weapon in the fight against EMI and hard converter failure (i.e. – self-destruction due to uncontrolled oscillation eventually building up to an electrical overstress or EOS scenario).


Comparison chart of IGBT, TO247, and SMD electronic components

Fig. 5: Calculation of Parasitic-inductance-induced Voltage Overshoot by Common Device Packages and Characteristics, courtesy of PowerRox [1]

Gate Drive Challenges in GaN DC/DC Converters

WBG gate thresholds (Vth) tend to be lower than those of their Si counterparts and have lower absolute maximum voltage levels, so the gate drive requirements needed to take advantage of GaN’s potential also come with a fairly steep learning curve when designing and implementing these solutions robustly. There are a variety of solutions on the market to address these challenges, from integrated gate drives (or even full power stages) to fully qualified power modules.

This means more care must be taken in the gate drive circuit because there is a risk of shoot-through (or false turn-on) due to high transition rates (dV/dt) acting on the switch’s gate-to-source capacitance (a.k.a. – Miller capacitance or CGS), which can apply a potential across the gate-to-drain capacitance (CGD) and trigger unwanted turn-on. If this occurs when a synchronous device is also on, then a shoot-through (a.k.a. – cross-conduction) event may occur. At best, this will decrease overall efficiency, and at worst, it will lead to failure of the DC/DC converter.

Different types of GaN can have different gate drive requirements, and this can be one of the most challenging aspects of designing with gallium nitride components. Some can be driven directly and are normally off devices, while some use what is known as a cascode arrangement in which an enhancement-mode (i.e. – normally off) MOSFET is used to drive the depletion-mode (i.e. – normally on) gate of the GaN device. Some can require negative or offset gate drive voltages. For this reason, it can be highly advantageous to acquire a qualified GaN driver, even if you are designing your own DC/DC converter solution.

GaN Design Resources and Best Practices

There are plenty of great resources available for learning about, acquiring, and implementing GaN solutions. Some of these have already been identified above. Take advantage of them and really do your homework if you are new to WBG and GaN solutions. You will need to go through many build and test iterations to achieve a truly robust, high efficiency, GaN-based design, particularly if you are newer to the space.

As a reminder, GaN is not a drop-in replacement for Si and therefore should not be approached as such. The early days of investigating the use of GaN in power supplies involved many people learning this lesson the hard way and even being dissuaded about WBG’s viability because of early failures that did not embrace the importance of carefully managed layout practices and robust gate drive design.

References

[1] E. Shelton, P. Palmer, A. Mantooth, B. Zahnstecher, G. Haynes, “WBG Devices, Circuits and Applications,” APEC 2018 Short Course, San Antonio, TX, March 4, 2018.
[2] Wikipedia contributors, "Two-dimensional electron gas," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Two-dimensional_electron_gas&oldid=955419012 (accessed May 27, 2022).
[3] “eGaN® Technology," EPC FAQs. [Online]. Available: https://epc-co.com/epc/FAQ/eGaNTechnology.aspx.
[4] Wikipedia contributors, "Nyquist–Shannon sampling theorem," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Nyquist%E2%80%93Shannon_sampling_theorem&oldid=1086141927 (accessed May 27, 2022).
[5] “GaN Integration for Higher DC/DC Efficiency and Power Density," EPC Application Note AN018. [Online]. Available: https://epc-co.com/epc/DesignSupport/ApplicationNotes.aspx.