Protect Polarised Cap From reversed voltage? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2017, 08:06:07 pm
content: I have an EMC and noise shaping filter, for a DC device, and there is the very occasional possibility that this device may be accidentally connected backwards by the installer. Normally, the dc voltage supply is of course not reversed and is fundamentally "DC" with a small AC component (say <0.5v AC on ~30V dc).As it stands, downstream of the filter the device is protected from reverse connection by a Mosfet acting as a low loss diode, but currently the filter itself, being the "first in line" is not protected, and it contains electrolytic polarised caps (for cost and sourcing reasons).My options for make the device withstand a continuous (but very infrequent) reverse connection seem to be:1) swap to non polarised caps (expensive, and makes sourcing harder, currently there is around 100uf at 50V rating)2) Fit two polarised electro caps Back-to-Back - opinions differ as to the robustness of this approach3) Clamp across the supply with a big fat diode so it crowbars the supply to close to 0 v and blows a fuse upstream (id rather not do this as customers tend to still complain "your device blew up my supply" despite the fact it was them that fitted it backwards!)4) Move the Mosfet "diode" to upstream of the filter (rather not do this as all the current emc testing has been with the filter first in line)What i'm interested in is approach 2), ie two back-to-back caps, especially in a predominately DC environment ie for 99.99999% of the operating life the device will sit with a fairly constant, correctly oriented DC potential across those caps. Does anyone have real world experience of this approach?

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2017, 08:08:15 pm
content: Quote from: max_torque on December 19, 2017, 08:06:07 pm

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2017, 09:11:46 pm
content: I personally prefer the first option. I don't know what margin you have on costs. You can find Kemet film capacitors (100µF, 100V) for around $10. Not cheap for a capacitor, I agree.What kind of polarized capacitor are you using? I suppose electrochemical, which frankly I don't like in this kind of setting. They tend to end up leaking and fail after a few years, especially if exposed to (even moderately) high temperatures. That may not be your case.The two polarized back-to-back topology may or may not work well for your needs. You will of course need to double the capacity of each capacitor. That would typically be two 220 µF polarized capacitors back-to-back. You have to consider that in terms of cost and overall space. But aside from the reliability and space issues, which may or may not be relevant in your application, I'd be even more concerned with the frequency response. Big electrochemical capacitors have impedances that become essentially inductive at a relatively low frequency (due to their ESL), and in a back-to-back topology, that will get worse, as both ESR and ESL will add up, and since you will use caps with double the capacity, the ESR and ESL for each will probably be bigger as well, so you'll end up with more than double the ESR and ESL of a single capacitor. So for EMC filters that's a concern, and that topology may invalidate your previous EMC testing...

author: mrkev
author_rank: Regular Contributor
timestamp: December 19, 2017, 10:12:55 pm
content: Two electrolythic caps connected back-to-back is something, that was used for AC in amplifiers. But in that application, both of them are positively polarized (even tho each one for different half of a period). I definitely wouldn't use this in a scenario with one cap reversely polarized all the time.Best solution seems to be to either use different kind of cap (deppends on the value you need), to move the MOSFET "upstream" as you said or to add another one. I doubt that Mosfet can change the EMC performance, since it is just a switch that is either dead ON, or OFF (I expect you are using P-FET and it's body diode is conductively polarized anyway).

author: AndyC_772
author_rank: Super Contributor
timestamp: December 19, 2017, 10:25:26 pm
content: How about a diode across the supply, and a PTC in series, which will limit the fault current so nothing goes bang?I've a feeling I've solved this problem before by careful choice of DC connector. Make it as hard as you can to screw up the installation, and accept that it's impossible to make anything completely idiot proof.

author: Damianos
author_rank: Frequent Contributor
timestamp: December 19, 2017, 10:52:38 pm
content: Some possible solutions:- A polarized connector may helps here.- The reverse polarized diode with a fuse. Anyway if the installer can connect it reversely, what prevents him to make a short circuit? So, the power supply line must be protected. Also the fuse can be "local", dedicated to this device.- A diode in series to the input...- A diodes' bridge to correct the polarity...- Use non-polarized electrolytic capacitors or two back to back (requires studying the data sheets and application notes from the manufacturers, most are for this type of operation and not for AC applications).

author: JPortici
author_rank: Super Contributor
timestamp: December 19, 2017, 11:31:29 pm
content: Quote from: AndyC_772 on December 19, 2017, 10:25:26 pmmechanics always find the most creative ways to insert connectors backwards. Idiot proof is imposible.

author: calexanian
author_rank: Super Contributor
timestamp: December 19, 2017, 11:43:46 pm
content: We use them in some products. I see 50 volt bipolar at 100uf are fairly reasonable and from good companies on Mouser.https://www.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Leaded/_/N-75hqw?P=1yx4avvZ1z0wrkm&Keyword=Bipolar&FS=True

author: mrkev
author_rank: Regular Contributor
timestamp: December 20, 2017, 12:26:49 am
content: Quote from: calexanian on December 19, 2017, 11:43:46 pmStill, I don't think it's a good component for aplication where it is connected in just one polarity for the entire time. Some datasheets even specify life time:"The specifications listed at right shall be met when the capacitors are restored to 20°C after the ratedvoltage is applied for 2000 hours at 85°C with the polarity inverted every 250 hours."http://www.mouser.com/ds/2/293/e-uvp-1219438.pdf

author: calexanian
author_rank: Super Contributor
timestamp: December 20, 2017, 12:34:50 am
content: Then the real solution is a series protection diode. I might recommend a shotkey type for low voltage drop. That is your only real solution.

author: jbb
author_rank: Super Contributor
timestamp: December 20, 2017, 01:16:42 am
content: I suggest option 4) reverse polarity protection before filter. A MOSFET polarity protector should generate very little noise so it probably won’t be an issue on the far side.Also consider option 5) redesign filter to use lower value capacitor.

author: max_torque
author_rank: Super Contributor
timestamp: December 20, 2017, 02:44:02 pm
content: Hmm pretty much as i suspected really, but good to know people were thinking along similar lines to me!The connector set is heavily polarised and scaled to prevent the wrong connector going into the wrong hole, but as the loom supplying the unit is often not supplied by us (it is the responsibility of the systems integrator in most cases) we can't prevent basic f-ups like people getting the + and - backwards (it happens more than it should!)On balance, i think i might just move the transient voltage protection diode (a massive 8kW peak device) upstream, so backwards connection blows the upstream external fuse (again the systems integrator is responsible for that bit, we just say "supply must be fitted with Fuse of rating x,y or z") That way at least when they mess up it's obvious.......

author: SeanB
author_rank: Super Contributor
timestamp: December 20, 2017, 05:17:43 pm
content: Move the TVS upstream and put in a really overrated polyfuse, so that it will only operate when the supply is reversed, or the TVS diode has had enough of being connected to the mains direct.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2017, 12:09:33 am
content: I think a non-polarized aluminum electrolytic capacitor would work fine or a pair of them with protection diodes. The datasheet note about operating them for a limited time with one polarity have to do with meeting the datasheet specifications for things like leakage which do not matter here.A pair of tantalum capacitors with protection diodes would work but would also be a lot more expensive than a suitable film capacitor.

author: max_torque
author_rank: Super Contributor
timestamp: December 21, 2017, 01:21:03 pm
content: One other solution springs to mind, namely put a diode in parallel with the polarised capacitor (so it is reversed biased most for the correctly connected supply, ie doing nothing) and then put a low esr (if such a thing exists) poly fuse in series with both.That way, when the supply is the right way around, the cap charges and discharges as normal (but always remains correctly polarised) but should the supply be reversed, the diode conducts, clamps the max reverse voltage across the cap (to <1v if i use a schottky) , and the excess current flow, causes the poly fuse to trip preventing excessive thermal damage to the diode? As the DC current (and in fact the rms AC current too) is going to be pretty low, that polyfuse could have quite a small rating, meaning it would trip before the main (external) supply fuse would blow?The downside, is that the ESR of the polyfuse is now added to that of the cap, which will of course change the AC performance, but if i can find a low esr poly fuse and also if i added a bit more low value non polarised ceramic capacitance in parallel i might be able to get back to status quo??

author: duak
author_rank: Super Contributor
timestamp: December 21, 2017, 09:00:46 pm
content: I vaguely remember reading something from a manufacturer of non-polarized electrolytics that a significant DC bias wasn't good for them. If you think about it, during typical operation one plate's oxide layer will have a voltage impressed upon it that's trying to remove it. This could also apply to back to back electrolytics even with protection diodes.Cheers,

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2017, 03:26:23 pm
content: Quote from: max_torque on December 21, 2017, 01:21:03 pmThat sounds reasonable.Depending on the size of the capacitor and polyfuse, you might even be able to skip the diode, since electrolytic capacitors can withstand reverse voltages and high currents for a short period of time, before blowing up. The only other issue I can see with this is the capacitor might need to be reformed, after the power has been reversed, even with a reverse diode in parallel, which might draw a high enough current to trip the polyfuse. It's something which will need to be tested.