Q&A on the next generation of power electronics
Will everything be GaNtastic or are you just SiC of the hype?
Q: The new wide bandgap (WBG) semiconductor materials gallium nitride (GaN) and silicon carbide (SiC) promise to revolutionise power electronics. Are the days of the traditional silicon-based technologies of super-junction MOSFETs and insulated-gate bipolar transistors (IGBT) numbered?
Both SiC and GaN transistors switch faster and cleaner, and have overall better thermal performance than IGBT (SiC through its chemistry as silicon carbide has about 3.5 times better thermal conductivity than silicon and GaN through its very low losses and efficient SMD packaging), but for many applications, super-junction MOSFETs and IGBTs still offer an acceptable performance at a much lower price. So, for many price-sensitive industrial applications, silicon-based technologies still have the edge over the new chemistries — for now, at least.
Furthermore, as WBG transistors are still in early development, the more mature IGBT technology offers higher switching voltages and currents than its younger rivals:
However, there are several key applications where the performance advantages of WBG are crucial: electric vehicles (EVs) require higher efficiency (higher switching frequency) and better thermal performance (lower switching losses) than is currently available from silicon-based technologies.
The overall efficiency of a plug-in EV is currently around 60% (conversion of mains power to kinetic energy of the vehicle). WBG offers the prospect of improving the power control and battery charging efficiency so that the overall efficiency increases to 72%. This represents an effective range increase of more than 20% without changing the existing battery technology. This prospect of high-volume EV applications is very attractive for SiC and GaN technologies, if only they can lower the cost and improve the wafer yield.
|Current overall electric vehicle efficiency = 60%|
|Improvements to charger, battery and power control efficiency promise overall efficiency = 72%|
For military or space applications, the inherent Rad Hard capability of the lateral GaN over the vertical Si/SiC construction is also a major pull-factor.
Q: Hold on! What’s all this about vertical and lateral constructions?
Conventional power transistors are constructed of layers of variously doped compounds stacked vertically on to a substrate. Current flows from top to bottom controlled by a gate electrode.
As SiC has a 10x higher breakdown voltage resistance and 3x higher thermal conductivity than silicon, the layers can be made much thinner, which means lower parasitic capacitances and faster switching. A typical IGBT requires about 10 millijoules (mJ) of energy to switch on and 12 mJ to switch off. These losses limit the switching frequency to a few 10s of kHz. An equivalent SiC transistor requires only 3 mJ to switch on and the same to switch off, which means that 50 kHz switching can be achieved without the transistor overheating internally.
GaN uses a different construction and switching mechanism. A normally off GaN is an example of a high electron mobility transistor (HEMT) in which the source and drain are positioned laterally. The crystal structure of GaN allows electrons to move very easily through it — so easily that it is called an electron gas. Thus the two terminals would be effectively connected together, were it not for a special depletion region formed under the gate electrode:
Thus to turn on (enhance) a GaN HEMT, the depletion region needs to be ‘cancelled’ by applying a small voltage to the gate, unblocking the connection between source and drain. This happens very fast, making switching speeds from 100 kHz up to the MHz regions easily realisable. Faster switching allows smaller inductors and capacitors to be used, so making the power converter more compact and more efficient.
Q: OK, so what are the design considerations using SiC and GaN?
It is not possible to simply take an existing IGBT design and drop in a SiC or GaN transistor and hope it will work. For example, the gate driver voltages differ greatly between the different technologies. While an IGBT might require a +15/-9 V isolated supply, a SiC design needs +20/-5 V for first-gen devices or +15/-3 V for second-gen devices. GaN HEMTs might need typically +6/-0 V or +6 /-1 V depending on the manufacturer. Also the gate driver power consumption differs greatly. While GaN switches so cleanly that 1 W is sufficient, an IGBT might need between 2 and 3 W. SiCs switch faster than IGBTs, but are typically used at higher switching frequencies, so 2 up to 6 W might be needed. If the driver voltages are too high, the sensitive gate input can be quickly damaged, but if the voltages are too low, then the transistors are not fully enhanced, and losses will be higher. Get the gate drive voltage wrong, and the expensive SiC or GaN transistors can be easily damaged.
Q: Are there any new skill sets required by engineers using these new technologies?
The very high speed switching with fast slew rates means that a good understanding of the effect of AC impedances, PCB layout and coupling artefacts is needed to avoid ground bounce, shoot-through or false-triggering events. Not only is the PCB layout absolutely critical, but for low-side isolation as well as high-side isolation, it makes sense to use kelvin contacts to reduce the effect of parasitic inductances. To solve both of these issues, RECOM has released a universal half-bridge reference design called the R-REF01-HB to make prototyping easier and has published free white papers explaining new design rules.
Q: What are the EMC implications of SiC and GaN circuits?
A track-to-track capacitance of only 10 pF can have sufficient coupling that it becomes impossible to meet the EMC regulations. The major problem is finding where these coupling capacitances are on a complex layout. There are often several overlapping sources of interference. At the high switching frequencies of SiC and GaN circuits, the harmonics can spread over a very wide range of frequencies, making it difficult to detect noise sources. Anything that can eliminate even a single source of RF interference helps a lot. All isolated DC/DC converters used in gate driver circuits also contain oscillators that will add to the overall conducted EMI, so this is why RECOM publishes class A and class B EMC filter suggestions for its DC/DC modules, information that is very often missing from IC datasheets. RECOM also uses shielded inductors and transformers to reduce radiated RF emissions.
Q: What next?
Progress on WBG transistors is so rapid that the support technologies are struggling to catch up; controllers need to be developed with shorter dead times and propagation delays, magnetic materials need to be improved to offer better power performance at very high switching frequencies and the high dv/dt stresses on the isolation barriers may limit lifetimes if new low isolation capacitance products are not developed. RECOM has conducted research on the long-term DC/DC converter isolation barrier reliability under high-frequency switching stress as an isolated gate driver power supply is a critical part for all of these new technologies for both low-side and high-side switches. The report is available for download here.
On a longer term (the next year or two), GaN technology will also offer massive reductions in AC power supply dimensions. Power densities of 40 W/in² are already being developed, double or three times the power densities of conventional Si-based technologies. This means that industrial and medical customers can expect a new generation of AC/DC power supplies that fit into much smaller enclosures with minimal heat generation. As GaN is especially suitable for low-EMI, highly efficient resonant or active-clamp flyback topologies, many bulky power supplies will be replaced with sleek new designs that will mean that future products will be designed more around the user interface than around the bulky power supply.
The future is bright — it really will be GaNtastic!
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