PCIM was big – again
This year PCIM filled three of the Messe exhibition hall in the Nuremberg. There were a large number of exhibitors. This large turnout of exhibitors and the crowds attending shows that power electronics is going well in Europe and the world. The recent reasonable GDP growth in Europe suggests that the financial crisis of 2008 may finally no longer be a drag on the European economies.
Always at PCIM there are new and exciting devices launched. There is always lots of talk of how these devices will solve all the power loss and control problems. Better devices are always worth having and they add to the toolbox for power electronics engineers. Typically these new and improved switching devices allow higher power density by reducing losses and increasing the operating temperatures. As an end in themselves new devices are often a bit of a distraction. The fundamentals of the power converters job stay the same. Thermal design to keep the heat out, EM design to keep the noise in and control the converter to be stable and useful.
Gallium Nitride GaN
The GaN story is a good one. It is easy to be cynical about why GaN has appeared in the commercial market after being used extensively in military application for some time. The key issues that were clear in talking to GaN people was that the expected improvement from silicon is not as large as expected or as was initially indicated and that driving the devices is a challenge. As GaN devices are FET type devices with ON resistance they will be limited to lower voltages possibly up to 700VDC to 800VDC.
Another issue with GaN devices is a perceived low reliability. This may resolve and my guess is it will as I remember when I first started working with IGBTs in 1990 they too were considered “unreliable.”
Silicon carbide devices are well established and there are lots of switching devices and diodes available. These devices are being used to move the switching frequency up which is often assumed to be a good thing. The question that industry veteran Marty Brown so eloquently asks about this is “why go faster?” Time will tell whether faster switching gives the advantages that it should.
Silicon and stacks
Silicon devices are going from strength to strength. The effort being put into system design with gate drives and cooling is high. It is now possible to buy megawatt converters in cabinets ready for deployment into wind and solar applications. Vendors like Semikron, Danfoss and Infineon are leading the way with smaller vendors like Oztech and Agilestack either following quickly and at times leading.
It is still possible to use discrete devices. When talking with a vendor of an impressive graphite thermal interface material it was clear that they were surprised by the large numbers of TO247 packages being used in high volumes.
The trade off between required capacitor size and the switching frequency of three phase inverters is one of those design iteration choices that defines the physical size of the converter. If the switching is faster then there is less DC side capacitance needed but the switching losses are higher. And as we all switch faster is this optimization is starting to be limited by the inductance of the commutation loop. The available DC bus capacitors are very low inductance and the laminated DC bus bars are also extremely low inductance. There are opportunities to connect the capacitors directly onto the DC bus bars reducing the inductance to a very low value. SBE Capacitors have an excellent solution
Safety Critical Controllers – Functional Safety
The use of safety critical approaches in digital power control is recent. The automotive power electronic people have ISO 26262 requirements for how the gate drives and how the hardware need to behave in a fault. Medical device compliance requirements have long required risk management and safety critical partitioning of the system. Even household appliances have safety critical requirements in their product standards like IEC/EN 60335 and IEC/EN 60730. We have had 60730 code libraries for a number of processors for a while now and have IEC 61508 as the basis for our high reliability controllers. And the work that we do with controllers in equipment covered by the Machinery Safety Directive gives us a good insight into product risk management over the complete product lifecycle.
For a long while, and until recently, there has been little support for safety critical systems in power electronics. Partitioning of the controller is the way to meet the fault detection requirements of safety critical systems was a challenge. With the advent of ISO26262 in the auto industry there is now a demand for safety critical assessment and traceability inside the power converter controller. The use of safety critical techniques has long been useful in power converter control due to the inherent ability of bridge power converters to self-destruct.
There were microprocessors at PCIM which have (or will have) safety critical function as key to their function. These microprocessors have multiple cores that can implement either dual redundant systems or primary and secondary control to implement the safety critical control. This development is a great acknowledgement that there is need to treat power electronic systems as part of the safety critical development. Often times in the past the digital control in the power converter was not subjected to the same level of review and revision control. And often the digital control in the power converter can be updated in the field, leading to security issues for the digital power converter controller.
While micros with the partition and the safety critical features are a good step toward this there is probably still risk of hackers compromising the security of network connected safety critical systems. Engineering to avoid a Stuxnet type vulnerability in power converter controllers will be the challenge.
ELMG Control Platform for Safety Critical Systems
The ELMG Digital Power Control Platform allows safety critical partitioning of the FPGA function. The Xilinx isolated design flow ensures that each part of the the system can be separately verified and maintained. ELMG Digital Power experience in digital control for safety critical applications such as medical, household appliances and automotive traction allows safety critical analysis and design to appropriate process and performance standards.
Contact us to discuss your safety critical system.