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Posted on 01 October 2019

A 105°C Cooled DC Link Capacitor for Advanced Inverter Applications

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A Breakthrough in DC Link Capacitor Design

SBE Inc. has introduced its new Power Ring Film CapacitorTM to industry. The Annular Form Factor already provides the opportunity for a 35% increase in capacitor volume density vs. the traditional array of Film Capacitor Units currently used and has significantly lower temp rise under power (10°C in many cases), but can it be used with only 105°C engine coolant and remain reliable? SBE is investigating it with the help of the US Department of Energy and shares a status report of the technology for Bodo’s Power Systems magazine.

By Ed Sawyer, President & CEO, SBE

 

A Disruptive Design

SBE introduced its Power Ring Film Capacitor Technology in 2005. Originally conceived as an Extreme Pulse Power dry film capacitor, it was soon discovered by HEV, PHEV, EV inverter engineers that it also had a very desirable feature: The capacitor runs very cool while under the heavy ripple current loads typically seen in Automotive Powertrain Inverter systems. These systems typically run 100 – 250 Arms ripple current, some even approach short term ripple current needs approaching 400 Arms.

The typical film capacitor solution in use today is to assemble a bank of film capacitors with each capacitor taking 25 – 50Arms of the current load. Even at these levels, the standard film capacitor design will generate 20 - 30°C temperature rise due to dissipated heat from ESR losses. So not only does it require a bank of 4 – 8 film capacitors, or as many as 24 capacitor sections in an array, but the resulting assembly will still need to be cooled by 50 - 80°C coolant to remain reliable for any period of time approaching automotive standards.

The SBE Power Ring Film Capacitor changes this accepted engineering design practice in a number of very desirable ways:

1) The large surface area of the single ring capacitor provides enough electrode area to handle the entire 100 – 400 Arms.
2) A very low temperature rise resulting from very narrow film, which reduces the ESR.
3) The very low temperature rise while under this current load allows for much more reliable operation when cooled with the existing 50 – 80°C coolant lines.
4) If the existing 50 - 80°C coolant lines are cooling the Inverter IGBTs, a much denser inverter topology can be utilized, even allowing stacking of the Power Ring Film Capacitor above the IGBTs if appropriately rated. Up to a 50% increase in overall inverter power density could be achieved.
5) It could possibly allow for an elimination of the entire extra cooling line and allow for 105°C cooling of the inverter yet remain reliable to automotive standards when appropriately rated in the system.

Power Rings

What makes it work?

Why does this capacitor run so cool while more traditional capacitors heat up so significantly more under the same ripple current load? It is because of the advantageous geometry: a short path for the generated heat to escape, a very large conducting surface area vs. dielectric volume ratio, extremely low ESR, and optimized distribution of current across the device. The dielectrics used are the same Metallized Polypropylene (MPP) used in the rest of the automotive industry’s current capacitor solutions therefore no new reliability issues are introduced based on the materials employed. This is the most reliable capacitor dielectric available today and widely accepted as the most desired solution from a quality perspective in the automotive industry.

Unlike the traditional capacitor shapes used today, the Power Ring Film Capacitor annular shape provides for single digit temperature rise for many inverter applications. A design point of 10°C maximum temperature rise is significant because, as will be seen later on, it is this 0 - 10°C rise range which has the greatest chance of success for long term automotive use with only 105°C coolant.

The geometry is helpful in another way as well. You can fit up to 35% more capacitance in a given volume when the capacitor is wound up into one continuous ring vs. an array of various flattened or cylindrical shapes as employed by most systems fielded today. This is critically important in PHEV, HEV, and EV systems as every cubic inch which can be dedicated to battery volume is extremely valuable. Of course, the round geometry must be utilized efficiently to get all of this advantage, however, compared to efforts to squeeze energy out of cubic inches of battery space, this task should be considered considerably easier.

Thermal profile for an early prototype annular form factor capacitor with a symmetric contact

How does the Power Ring Film Capacitor compare to other typical arrays while under a number of different current loads? The following graph compares the temperature rise of a typical film capacitor array in use today for an HEV powertrain with that of the Power Ring Film Capacitor under currents ranging from 0 – 300Arms.

As you can see, the more power expected from inverters in the next generation PHEV, HEV, and EVs, the worse the temperature problem becomes for the typical array of film capacitors. This is not a good thing for today’s inverter designer being asked to “make it smaller, lighter, and less expensive”. Because of the greatly increased temperature rise exhibited under increased current load, the cooling requirements become much greater for the larger motor expectations of these vehicles.

Temperature Rise of Annular DC Link vs. Conventional Film Capacitor Array for a 25°C ambient example

The 105°C Coolant Breakthrough – Possible?

But an additional breakthrough in the inverter industry for the DC Link capacitor solution will come if such improved performance was also available with no additional cooling required beyond the already available 105°C engine coolant in an HEV or PHEV. There is so much interest in this possibility that the US Department of Energy is sponsoring research to enable SBE to answer this very question: Is 105°C coolant possible under expected inverter loads of 150 – 250 Arms and still provide the 10 – 15 year lifetime expected for today’s automobiles?

In order to provide this answer, we are approaching the reliability assessment 2 ways:

What are the combined affects of current load, temperature cycling, vibration, and shock on the larger annular shape and connection methodology? We want to be sure that the complete automotive environment is demonstrated reliably under the extreme load conditions presented.

What is the affect of MPP while operating on a 105°C cooling plate which will establish capacitor operating hot spot regions of 110 - 125°C? Accepted reliability acceleration tests will be performed to understand the 90% confidence factor MTBFs and to establish a reasonable ripple current rating for the industry which will still comply with the desired lifetime. And is there any advantage to using the available High Crystallinity structures which are now available? This film has some desired higher temperature characteristics however. SBE’s early research has shown that some characteristics may make it unusable as a reliable automotive inverter dielectric. Our DoE sponsored research will allow for a conclusive decision to be made on this material choice as well.

One of the things that will become an issue with any design approaching the 105°C design goal will be DC leakage as a function of voltage at the extremely elevated temperature and what the usage profile is of the inverter in this region. If the demand of the application becomes too strenuous from a high voltage transient standpoint, or the bus voltage is increased upwards of 400V, some trade-off may need to be made to maintain the needed reliability, usable Bus Voltage, acceptable DC Leakage, and total capacitance density. But regardless of this final outcome, an option could become available to eliminate the additional cooling loop and any reduction in ultimate capacitance efficiency should be more than offset by the system efficiencies gained.

The Future for Inverter Designs

SBE has demonstrated that the Power Ring Film Capacitor can already greatly exceed traditional film capacitor solutions using the expected cooling systems. Therefore, inverter designers are already incorporating the solution into new leading edge designs. Like any traditional MPP film capacitor DC Link solution, when cooled within expected norms, the Power Ring Film Capacitor is already the most reliable solutions available. It brings along the lowest temperature rise for inverter integration flexibility and the potential for a much greater use of volume for desired capacitance density.

Those systems designers working with the Power Ring Film Capacitor who have elected to truly integrate the new DC Link solution into their inverter bus structure have really reaped the most benefit from what this technology has to offer.

Current designs of any level using this technology can migrate to lesser cooling requirements in the future as the reliability data and resulting MTBF predictions become available in 2009/2010 with the DoE sponsored research.

If SBE is successful in proving automotive reliability expectations of 10 – 15 year life using only 105°C engine coolant, it will open up a whole new world of possibilities for the automotive inverter designer in the future.

 

 

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