Control, Drive, Sense: High-Power Density SiC and GaN Power Conversion Applications
New power switch technologies are key to success with the next generation of motor control, solar inverters, energy storage and electric vehicles. Just as important—the ability to drive these technologies safely and sense them more accurately.
Sensing current within these systems while operating at these higher switching rates is becoming more challenging.
Electricity consumption and its generation, which adds to our carbon footprint and affects climate change, is one of the key problems the world faces. The largest global consumption of electricity is from electric motors and the systems they drive. These systems consume more than twice as much electricity as the next largest consumer, lighting. A 2011 International Energy Agency report estimates that electric motor systems account for between 43 and 46 percent of the world’s electricity consumption.
Farther on Less
The need to further shrink our carbon footprint by reducing the CO2 emissions from transportation is a key driver for the electrification of vehicles. With the electrification of vehicles comes the need for them to be able to travel greater distances with less energy consumed. At the same time, we must ensure that the electricity generated for charging these vehicles comes from clean sources. As important as reducing electricity consumption is improving electricity generation methods. Generating energy through renewable resources like the sun requires efficient solar farms that are becoming mainstream in implementations worldwide.
We’ve seen the emergence of Wide Bandgap semiconductor technologies like Silicon Carbide (SiC) and Gallium Nitride (GaN) and the use of power MOSFETS in applications such as solar inverters, motor drives, and electric vehicles. Along with these technologies comes the need for gate drivers that have the capability of driving them efficiently and safely at higher data rates with less dead time in the system. Sensing current within these systems while operating at these higher switching rates is becoming more challenging.
Moving to these new technologies makes electric motors and driving electronics smaller and lighter. Increasing the range of the electric vehicle and decreasing its charging time becomes possible. Higher switching frequencies in solar inverters, as specified in IEC62109-1, will improve the overall efficiency of the systems as well as reducing the size of the line filters. Industrial automation applications where motors are commonly used, as specified in the variable frequency motor drive standard IEC61800-5, will become less bulky and more efficient, reducing the overall energy footprint.
Greater Robustness, Reliability
Isolation is mandated for safety and operation. Implementing the isolation barriers within these applications without compromising on performance is critical. These systems often have long lifetimes and could be implemented in harsh environments, so high levels of component robustness and reliability are a must.
“Sensing current within these systems while operating at these higher switching rates is becoming more challenging.”
One example of a solution for driving new Power switch technologies is Analog Devices iCoupler® digital isolation integrated with gate drivers like the ADuM4121 (Figure 1). It has the capability of driving these new Power switch technologies because of its low industry leading propagation delay of 38ns typical, allowing faster switching and the ability to withstand high Common Mode Transients up to 150kV/µs during fast turn on and turn off events.
Integrating Analog Devices iCoupler digital isolation with industry leading sigma delta analog to digital converters, such as the AD7403, makes it possible to accurately sense the current in high-voltage applications across a smaller shunt resistor, improving system efficiency. This enables the use of higher accuracy shunt-based current measurement architecture rather than Hall Effect systems. Selecting smaller resistors reduces the overall size of the solution.
To demonstrate system performance benefits, Analog Devices has developed a new Half Bridge GaN evaluation platform in collaboration with GaN Systems, as shown in Figure 2. On this platform we have the ADuM4121 isolated gate driver driving the GS66508B GaN MOSFET from GaN Systems that is rated to 650V at 30A. The gate charge requirement of the GS66508B is very low, making it much easier to drive at higher frequencies with a much lower supply voltage on VDD2 of 6V. The ADuM4121 also includes an internal Miller clamp that activates at 2V on the falling edge of the gate drive output, supplying the driven gate with a lower impedance path to reduce the chance of Miller capacitance induced turn on.
Making use of three of these half bridge evaluation boards combined with the Analog Devices Motor Control evaluation platform a demonstration system showcasing a three-phase inverter driving a three-phase motor was built (Figure 3). Within the three-phase inverter, large currents are being switched at high frequencies that can cause radiated and conducted emissions. To reduce the conducted and radiated emissions in the system while operating efficiently, it is critical to slew the edges of the switching waveforms sufficiently by selecting an appropriate gate resistance. This series resistance can further help with dampening the output ringing by matching the source to the load.
In this demonstration platform, the ADSP-CM409 generates the PWM signals required to drive the power switches, while the integrated SINC filters allow for direct connection of the Isolated Sigma delta ADC used for accurately sensing the current. The reinforced isolation provided by the isolated gate drivers can withstand up to 5kVrms as well as working voltages as high as 849Vpeak according to VDE0884-10. The isolation AD7403 offers can achieve 5kVrms withstand with a working voltage 1250Vpeak, also according to VDE0884-10.
Implementing a three-phase inverter using GaN suits systems operating up to 650V. SiC, having much higher breakdown voltages, more closely matches systems going up as high as 1200V and 1700V because it will have more margin within three-phase systems with 690Vrms line voltages.
Hein Marais is a System Application Engineer at Analog Devices, Inc.