Kettering University researchers working on more efficient Level 2 charger for HELLA
16 July 2015
Researchers in Kettering University’s Advanced Power Electronics Lab (APEL) are working with HELLA on a more efficient Level-2 EV charger. Presently, level-2 EV chargers on the market have a three-stage design—converting AC grid voltage to 400 VDC; inverting this DC to high frequency AC to feed the transformer; and then rectifying AC to DC again to charge the battery.
Assuming that each stage of that process leads to about a 2% loss of overall power, the overall wall-to-battery efficiency is 94%.
A research team led by Dr. Kevin Bai, associate professor of Electrical Engineering at Kettering University, is working with HELLA to develop a next-generation charger that has a 2-stage design rather than 3-stage design, which would offer 97% efficiency, an improvement of 3%.
By using the novel gallium-nitride devices, the charger switching frequency is also significantly higher, nearly double of the present charger. The design will make the charger ultra compact and light, which eventually will be a game changer for the EV charging industry.—Kevin Bai
The research team includes Bai, a research graduate and a research engineer working on the system design and test. A prototype of the charger is expected to be finished by October and several patents have already been applied for related to this project.
Bai and researchers in his lab have worked with numerous industry partners to develop charging technology, including development of a 24-kilowatt charger for Turkish automaker Derindere Motorlu Araclar (DMA) and development of a 10-kilowatt charger for Magna E-car.
This seems silly. Unless isolation is required, only rectification and DC-DC voltage conversion are required. There are already boost-buck converters which can produce a given output voltage from sources which are either higher or lower voltage.
Posted by: Engineer-Poet | 16 July 2015 at 11:34 AM
Isolation is usually an absolute requirement, silly poet. ;)
Integrated charging (using the regenerative braking circuitry to charge the battery from the grid) is rarely used despite its enormous cost advantages, precisely because it lacks galvanic isolation. The double insulation, isolation monitoring, and other techniques such as performing an emergency power down if someone opens the hood while DC high voltage is active do not satisfy most specifications. I think this is likely to change, someday. But there are other complications to integrated charging such as poor efficiency at low power (such as 120 VAC source), multiple optimizations required (surviving power grid transients during electrical storms is quite different than regenerative braking), etc. I am guessing here but you get the idea.
See the Renault Zoe as an example of integrated charging (43 kW with a cheap 3 phase AC EVSE!) and the BMW Mini E of years past (AC Propulsion drive train, still almost 50 cars running in Delaware in V2G power trials).
Posted by: James McLaughlin | 16 July 2015 at 02:42 PM
Usually the HV battery and its wiring are fully isolated anyway so that any contact between wiring and a part of the vehicle doesn't make it "live" or cause a short. The AC150 drivetrain that Tesla used for its first efforts connects directly to the mains for charging, using the motor as the inductor.
Surge suppression can be provided upstream of the charging electronics. If your electronics are optimized for high blocking voltage, you're going to have issues with high voltage drops and lower efficiency at 120 VAC anyway. In a world where higher charging power is being pushed hard, 120 V charging will be increasingly rare (you can't even get 120 V in G. Britain and most of Europe, no?) and much less of a factor.
Posted by: Engineer-Poet | 17 July 2015 at 05:33 AM
EP, surely you heard that Tesla dropped the AC Propulsion drivetrain due to reliability problems? Surge suppression is not enough apparently. My personal experience with AC Propulsion in the Mini E was roughly a charging system failure about every 6000 miles, a total of 4 such failures over 11 months. Maybe you can fix it for them with surge suppressors, good luck. ACP is still selling many hundreds of drivetrains per year from what I have heard, maybe you should send them your resume? I have to admit that car was an absolute hoot, as much trouble as it was. Perhaps the main issue was the older analog controls, I dunno.
Still I agree that things will probably go towards integrated charging in the long run, especially for heavy duty applications. But it is not hard to see why most everyone (including Tesla) is shying away from it for now. I would not be surprised to see it come back in the Model 3. That would be cool.
And the isolation issue is about the need to remain safe even after a single failure point, you do understand that requirement? I have the impression that permeates all of the automotive world as well as the National Electrical Code world.
But certainly this article is about the conventional isolated charger design with a small high power high frequency transformer. As I read it, they are talking about converting the incoming 60 Hz (or 50 Hz) AC directly into high frequency AC to feed the isolation transformer, without rectifying it first. Sounds neat, what is wrong with that if your project management insists on galvanic isolation?
I think Ideal Power has an interesting approach to isolation without a transformer with their time division technique, they call it Power Packet Switching. But the questions of isolation after a single point failure remain. Transformers are pretty good about isolation, it will take time to build confidence in alternatives.
Posted by: James McLaughlin | 17 July 2015 at 05:37 PM
No, I haven't followed Tesla closely.
Were the problems due to surges or something else? There are lots and lots of ways to build flaky electronics (ask any auto company).
That's done with ground-fault detection and disconnection. You still need it even with isolated HV systems, so there's not a whole lot extra involved with direct connection to the mains. Transformers are the gold standard, but you can't afford to make everything out of gold.
Posted by: Engineer-Poet | 18 July 2015 at 08:02 AM
EP, ground fault interruption is a mitigation technique, not a prevention method. At the voltages commonly used in EVs, a GFI (called an RCD in Europe, residual current detector) might not prevent a death in the event of a fault, it may only prevent the corpse from being burned beyond recognition. Bad joke, gallows humor and all. But the point is that prevention is a requirement, mitigation is not enough.
I don't know what the reliability problems were with the ACP integrated charging method. I hear that the Renault Zoe sales are taking off, and the Zoe uses essentially the same techniques, as far as I can tell. (The ACP patents are long expired.) So I presume Renault has fixed the issues.
Posted by: James McLaughlin | 20 July 2015 at 05:30 PM
I'm not sure what your point is. Isolated HV systems are a prevention method, like double-insulation of electric tools; detection of faults to ground with automatic shutdown is a backup. No matter how much prevention you build in, I can find a way to damage things to get around it. What ultimate prevents harm is detection of that damage and shutting things down.
Okay, that makes two of us. If you get any firm information I am now greatly interested.
Posted by: Engineer-Poet | 20 July 2015 at 09:25 PM