Electronic – Why aren’t there any 400V ultracapacitors

filteringhigh voltagesupercapacitor

I thought that ulracaps could be better than electrolytic ones for filtering after a bridge rectifier in a compact switching power supply due to their higher energy density. But, for some reason, I can't find any high-voltage supercapacitors on ebay or amazon. I could, of course, put a bunch of low-voltage ultracapacitors in series but that would defeat the whole purpose of a power supply being compact. Is there any reason why there are no high-voltage supercapacitors? There already are 5.5V ultracapacitors, which are basically two 2.75V caps in series, so why they can't go higher? Did I miss something? Are they hard to manufacture?

Best Answer

A high-voltage super-capacitor/ultra-capacitor would be a contradiction in existing technologies. They achieve their high capacitive values by having a super thin dielectric of special materials, hence the low voltage limit.

To build a 400 volt capacitor means having a thicker 'solid' dielectric with more common materials, which 'fattens' up the size a lot.

There are large 450 volt electrolytic capacitors to 20,000 uF or more. To have a super-capacitor with the same voltage rating would be duplicating the large can-type electrolytics.

Super-capacitor/ultra-capacitors have been around a couple of decades now, mostly changing in materials to cut down on self-leakage. Graphene is the latest trend. The laws of physics and chemistry limit the size of such capacitors, or we would already have them to buy.

Engineers would love to have giant super-capacitors for cars, etc, so there is an on-going effort to reach that goal.

This is a snippet from Wikipedia:

A supercapacitor (SC) (also electric double-layer capacitor (EDLC), also called supercap, ultracapacitor or Goldcap) is a high-capacity capacitor with capacitance values much higher than other capacitors (but lower voltage limits) that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

Supercapacitors are used in applications requiring many rapid charge/discharge cycles rather than long term compact energy storage: within cars, buses, trains, cranes and elevators, where they are used for regenerative braking, short-term energy storage or burst-mode power delivery. Smaller units are used as memory backup for static random-access memory (SRAM).

Unlike ordinary capacitors, supercapacitors do not use the conventional solid dielectric, but rather, they use electrostatic double-layer capacitance and electrochemical pseudocapacitance, both of which contribute to the total capacitance of the capacitor, with a few differences:

Electrostatic double-layer capacitors use carbon electrodes or derivatives with much higher electrostatic double-layer capacitance than electrochemical pseudocapacitance, achieving separation of charge in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. The separation of charge is of the order of a few ångströms (0.3–0.8 nm), much smaller than in a conventional capacitor.

Electrochemical pseudocapacitors use metal oxide or conducting polymer electrodes with a high amount of electrochemical pseudocapacitance additional to the double-layer capacitance. Pseudocapacitance is achieved by Faradaic electron charge-transfer with redox reactions, intercalation or electrosorption. Hybrid capacitors, such as the lithium-ion capacitor, use electrodes with differing characteristics: one exhibiting mostly electrostatic capacitance and the other mostly electrochemical capacitance.