Does anyone have experience with supercapacitors? What's the best way to learn more about them and what they're useful for? Any good web links with design examples?
We have a 1 kW transmitter for ultrasound that uses 12 electrolytic capacitors (2200uF each) in a bank, for 26.4 mF total capacitance. The ESR is very low so our system responds very well when transmitting pings (lengths ranging 0.1-1.0 ms). The caps are buffering a 48V power rail. I've thought about trying to scale the power rail down to 12 V and beefing up the transformer by a factor of 4.
Has anyone used a supercapacitor in a transmitter? Thanks for the help.
Most supercaps I used (some time ago now) have rather high ESR (in the order of tens to even hundreds of ohms), to say nothing of the fact that most of them are very low voltage devices.
I haven't used them recently, though, so things may have changed.
I wouldn't really call that a super capacitor. A few hundred uF or more per channel isn't that unusual. In my ultrasound designs the most important aspect was where the peak current would be coming from. This data plus ESR values is in the data sheets and if not you have to obtain it.
Real super caps are in the Farad capacitance range, more like short-term backup supplies. They come in various flavors. As Pete said they can be high in ESR, for example if their market is RAM backup. You would need the ones that are marketed into the short-term power supply sector.
A kilowatt is nothing to sneeze at. Especially if it is Doppler and more than just one short pulse, be careful. I have seen electrolytics explode with gusto when exceeding the max ripple rate. A short pulse can be cushioned off with several high quality smaller caps but long pulse trains really can't.
Check out Maxwell Technologies they make a super cap that is a cross between a lead acid battery and high capacity capacitor - very low ESR and can deliver 100's of amps
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I haven't used them in a transmitter, but in a special embedded system with very good results. It was a mission critical system powered from
24V DC which drawed over 1A continuous. The system had to operate 24/7 under poor power conditions, with very frequent power sags and outages. In order to guarantee an orderly shutdown in case of a long term power failure, I need to guarantee 15 seconds of backup power, but given the hostile environment, relatively high temperature and maintainance free requeriments, batteries should be avoided.
The solution was to use a bank of "ultracapacitors" (that's the name Epcos give them) in series connection. Six cells of 10[F] 2,3[V] gave us an equivalent capacitance of 1.66[F] at 13,8 [V]. This bank was charged from the 24[V] input by a DC/DC switchmode power supply while a secondary DC/DC converter generated the main 5[V] supply from the
13.8[V] rail of the capacitor bank.
This way, the 5 [V] can sustain 1 [A] load current over 15 seconds after a power failure, enough for a safe shutdown. Several hundreds of these systems are still working after several years of operation.
At design time, the cells employed (10[F]/2,3[V]) were the the smallest available, both in size and capacity, but there are some real monsters. You can take a look at Epcos ultracapacitor web page:
The Allied catalogue has some PowerStor Aerogel Capacitors. Supercapacitor types with low ESR. ESR ranges from 0.025 to ~0.4 Ohms. Capacities up to 50F (-20% to +80% tollerance) at 2.5V (3V surge). Very small too, the 50F is 18mm diam x 40mm high.
Nothing fancy there :-) Epcos offers a specially designed voltage equalisation modules for large capacitor banks, but since ours was relatively small (it used only six caps of the smallest value), I decided to go with a simple pasive equalization network made of resistors. If you are interested, there is a detailed Epcos application note in which compares both methods with calculation examples.
I'm just curious really. some guys I know are considering building an HVDC bus using supercaps, and my guess is that voltage sharing is pretty important, as there are so many in series (hundreds).
The application note I mentioned was part of an earlier Epcos Ultracapacitor handbook, but I think it is included in the current "UltraCap - Data sheet collection 2005" you can grab at:
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Additional application notes can be found at Maxwell Technologies:
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