Paul Lee, for Mouser Electronics
Global electrical energy demand is forecast to peak at 30 petawatts in 2020, and beyond that it’s set to increase further. The source might be fossil fuels or renewables–but regardless, the efficiency of power-conversion equipment is critical to minimize costs and energy lost to the environment.
Industry consumes over 50 percent of global energy in the form of electric motors, but data centers are also power-hungry, and electric-vehicle (EV) charging is an increasing load. All of these applications, and many others, have seen innovations that make them “smarter” with less power draw, but the associated power-conversion equipment has to keep pace too, with ever-lower losses. Here, we will review the wide-bandgap semiconductor technologies that make this possible.
Power conversion topologies and challenges
Power-converter designers have the goal to convert voltage from a distribution system, either utility AC or a DC bus, to a different DC or AC level with maximum efficiency. Galvanic isolation might be necessary for safety or functional reasons, and the output voltage might be higher or lower, and regulated or not, but the “switched mode” power conversion technique is now universal.
Original bi-polar switch technology gave way to silicon MOSFETs, and insulated-gate bipolar transistors (IGBTs) still dominate high voltage/power, but now silicon carbide (SiC) and gallium nitride (GaN) are upcoming contenders in modern converter topologies, often “resonant” types for best efficiency, with three-phase bridges used for motor control.
Whatever the topology, losses accumulate from on-resistance and from switching transitions that can have high instantaneous peak power values.
Switching losses in semiconductors are proportional to transitions per second, so low frequency is better. However, high frequency allows for smaller, lighter and lower-cost passive components such as inductors and capacitors–so in practice, switching frequency selection is a compromise, varying from a few kHz in motor drives to several MHz in size-critical DC-DC conversion, in data centers for example.
Significant power dissipation during switching transitions is caused by the energy, EOSS, required to charge and discharge device capacitance COSS. EOSS and COSS are therefore critical parameters, along with on-resistance, RDS(ON). The product of on-resistance and die area, RDS(ON) ∙ A is a figure-of-merit (FOM) for total losses, as capacitances and their associated switching losses reduce as die area shrinks.
Introducing wide-bandgap semiconductors
Wide-bandgap (WBG) semiconductors, SiC and GaN, need relatively high energy to move electrons from their valence band to their conduction band. High bandgap values yield higher critical breakdown voltages and lower leakage currents, especially at high temperatures. WBG devices also have better electron saturation velocity, leading to faster switching, and SiC has particularly good thermal conductivity.
With a critical voltage breakdown value around 10 times better than Si for a given thickness, SiC for example can have a 10 times thinner drift layer with 10 times doping concentration. This gives much lower on-resistance than Si, and correspondingly lower dissipation for the same die area compared with Si. With SiC’s high thermal conductivity, the die can be very small, with–excellent RDS(ON)–and FOM.
SiC and GaN have much lower gate-drive power requirements compared to Si MOSFETs. In particular, IGBTs need a significant gate charge for efficient switching, which translates to watts of drive power for some larger IGBTs, further contributing to system losses. For WBG devices, the figure is in milliwatts, even at high frequencies.
WBG device benefits continue: They can operate at a much higher temperature than silicon, peaking at over 500°C. Although packaging limits practical operating values, the high peak capability shows the margin available. Variation of gate leakage and on-resistance with temperature is also much lower compared with silicon devices.
WBG device costs have been higher than silicon but are reducing, and knock-on system benefits offset this to a large extent. For example, with efficiency gains, expect a corresponding reduction in size, weight and cost of other components such as heatsinking, and inductors and capacitors in filters. System functional performance also benefits from faster switching, with quicker response to load changes and smoother motor control, for example.
Overall, the value of using WBG devices means they can be considered for any new applications in power conversion, with device manufacturers refining their technologies so that parts are easy to use and robust, particularly under fault conditions such as short-circuits and over-voltages. Infineon, for example, has chosen a trench construction, which enables a low channel resistance at low gate electric field strengths, increasing the reliability of the gate oxide interface. Enhancement-mode GaN HEMT devices from Infineon use a planar construction and these, unlike SiC MOSFETs, have no intrinsic body diode, making them particularly suitable for “hard switching” applications. GaN devices have a 600V rating compared with 1200V and higher for SiC, but the theoretical limit for GaN RDS(ON) at a particular voltage rating is about 10 times better than SiC.
STMicroelectronics claims the highest temperature rating in the industry at 200°C for its 1200V SiC MOSFETs with class-leading, very low on-resistance over the temperature range. A very fast and robust body diode avoids the need for an external diode, saving space and cost in circuits where commutation occurs, such as in motor drives.
ROHM also offers products in the SiC MOSFET market, with cost-effective, breakthrough performance from its latest devices. ROHM claims the industry’s first SiC MOSFETs with a co-packaged anti-parallel SiC Schottky barrier diode for demanding, commutating switch applications where the lower forward voltage drop of the parallel diode (1.3V) results in lower losses than the body diode at 4.6V.
GaN Systems, another company in the WBG arena, has concentrated on patented packaging techniques, leveraging the speed and low on-resistance of GaN to the maximum. Its proprietary “Island Technology” connects a matrix of HEMT cells vertically with a lateral arrangement of metal bars to reduce inductance, resistance, size and cost. Additionally, its GaNPX packaging technology with no wirebonds gives optimum thermal performance, high current density and low profile.
Panasonic, another pioneer in the GaN market, has introduced its X-GaN devices with patented technologies to achieve “normally off” operation with no “current collapse”–an effect in GaN in which trapped electrons between drain and source can transiently increase on-resistance during the application of high voltage, potentially leading to device failure. Panasonic’s Gate Injection Transistor (GIT) technology yields a true “normally off” GaN device that can be driven with gate voltages compatible with Si MOSFET levels.
WBG wins over silicon in functional respects and the barriers to adoption are now just cost, ease of use and demonstrated reliability. WBG device manufacturers have addressed these concerns and mass production is now a reality, with active applications in all market areas.