When it comes to wide-bandgap semiconductors like Silicon Carbide (SiC), the markets that usually come to mind are traction, solar and wind inverters, electric vehicles, and military applications, basically any needing high-performance power modules. An often overlooked aspect of these is the part about driving them properly, with optimal switching and built-in protection. One solution is to use plug-and-play drivers such as those from AgileSwitch to give you optimum performance while protecting the power module in your inverter.
By Alix Paultre, Editor Power Electronics News (based on a technical paper presented by Cliff Robins, Nitesh Satheesh, and Rob Weber, AgileSwitch)
We’ve all heard today about the fantastic benefits of silicon carbide MOSFETs about switching losses, high power density, high thermal conductivity, and high temperature operation, and of course they can switch much, much faster than IGBTs. Because they’re very fast switching you can get a high turn off di/dt, but that causes high voltage Vds overshoot because of the strain within the module. It also causes oscillation in the Vds, which is a real problem, which can cause false triggering or faults back to your controller.
So how did we drive these silicon carbide modules more efficiently? Typically, with an IGBT driver we would get around the di/dt by putting a larger gate resistor in, and if we do that we would get a design of trade off of obviously smaller di/dt with a larger resistor, but less efficient. With IGBTs we normally see customers using these at about one to 15 kilohertz, normally one to five kilohertz, so you don’t see much effect on the efficiency of the inverter by putting a larger gate resistor in.
If we were to do the same thing for silicon carbide MOSFET module, we would have much higher switching losses because these large gate resistors are much slower response time because we have to react much faster now. And it wouldn’t work because fast enough to shut down that silicon carbide module. So neither of these things are really of any benefit to a silicon carbide module.
AgileSwitch has come up with a technique called Augmented Turn-Off, that provides precise control while using very low gate resistors. So for the Rg(on) and Rg(off), the maximum value we would use is .50, controlling the di/dt by using the Augmented Turn-off. In a conventional solution, we would put a high gate resistor in, and there’ll be a slow transition rate to switch the device off, and we’d have much higher switching losses. We’d also have the need for active Miller clamp and we’d effectively have uncontrolled di/dt, the high voltage overshoots.
Figure 1: Augmented Turn-Off provides precise control while using very low gate resistors
When you optimize the voltage drop and the dwell time for each module, and understand the impact of high efficiently total response time and high voltage overshoot, and the compromise of all of those, you can achieve very low gate resistor values. This enables us to have a very fast response time and lower switching losses across because of that very small gate resistor value and no need for active Miller clamp with this technique either, controlling the di/dt and reducing the EMI significantly as well at the same time.
Another issue that comes up quite frequently is controlling the short circuit condition, particularly in expensive silicon carbide modules where you’re switching at anything from 40 kilowatts up to 300 kilowatts. We want to try and safely shut down the silicon carbide module without losing efficiency, and we could do that with a large gate resistor, again with very slow transition, but also with very high stress on that power device, and a greater chance of going into avalanche. So now we have very low value gate resistors, softer gate transition, less power, less stress on the device, and a greatly reduced chance of it going into avalanche.
Figure 2: A conventional solution going into avalanche vs. Augmented Turn-off
In Figure 2 the left image shows a conventional solution going into avalanche, as it couldn’t react fast enough to shut down in a safe manner. On the right the short-circuit detection is 20% faster than conventional methods, and the voltage overshoot was reduced by 80% compared to traditional methods. Augmented Turn-off can reduce total system losses up to 50%.
Another advantage to using a finished solution is added functionality. For example, AgileSwitch’s boards can deliver up to seven fault codes, as well as provide DC link and temperature monitoring, which is a low voltage one- to five-volt oscillating output from the driver card back to our controller. And you can set the temperature that you want the fault trigger to go at, or the DC link voltage, and you can change that voltage and temperature setting via software updates. Another advantage to a software-defined driver is the ability to update and change the performance in real time and immediately get results back of how that performance change has happened, enabling a very fast optimization of the inverter for the application.
Figure 3: The EDEM3 for the Econo-Dual silicon carbide module
One example of one of our drivers the EDEM3 for the Econo-Dual silicon carbide module. Features include seven fault condition output and up to +/-15 current amp drive (also available in versions providing (+/-20 amp) for the power module, with software-programmable parameters. To address tough applications like traction, the driver has been upgraded to meet IEC standards for vibration and shock, and is also available with conformal coatings.
It’s always useful to have a reference design to aid in evaluation and design-in, and this technology is available in kits from several sources. Based on a three-phase half-bridge into one interface card providing all of the sensing, temperature monitoring, and software update capability, these reference designs, are available for you to build your own solution and quickly put it into an application.