Thursday, September 1, 2016

SSPA Power Supply - Protection Diodes


I've seen a few (probably real smart) folks out on the web running power supplies in series like I am.  Some of them are using diodes bridged between the positive and negative on EACH power supply in the series.  

The idea is to protect the other supplies in the series-chain if one of them should short or fail.  At least that's how I understand it.  Honestly, in my humble (unknowing) opinion this is probably going a bit far for protection.  BUT then again if it prevents something catastrophic like a fire...I'm all for it :-)

If you've been following this project and many of my other ones I try to error on the side of safety and over-sizing.  :-)  I just don't like that 'pending doom' feeling every time I strenuously exercise my equipment. 

So...I'm going try to add diodes on each supply in the series of each 'bank'.  As you may remember there are four 12.5 vdc supplies in series in each 50 vdc 'bank' within this system.  Then there are two 50 vdc 'banks' run in parallel for a total of 200 amps @ 50 vdc.

Each of my 12.5 vdc supplies is capable of 100 amps.  So I have 100 amp diodes.  (it's NOT LIKELY that I would ever run this system at maximum as long as the zombie invasion never happens.  SO this is probably safe.  SIZE is an issue with diodes this big.  So 100 amp are the ones I chose.  

This is one I'm going to try to span the positive and negative of each supply.  It's a Vishay p/n: VS-100BGQ100. 100 (If) amp 100 volt with a Vf of 0.82 @ (If).

Then to separate the two 'banks' of 50 vdc I got two of these.  IXYS p/n: DSS2X101-015A.  100 (If) amp 150 volt with a Vf of 0.77 @ (If).

So I think that because the diodes in the series supplies would 'sum' their respective Vf's at Max If that I'll need to adjust the Po of each supply (with a mod, discussed below).  0.82 (Vf) vdc * 4 (supplies) = 3.28 vdc dropped at max current out.  + 0.77 vdc at the final for a total = 4.05 vdc.

That would mean that I'd have to run this system at 54.05 vdc at idle so that at Max load output it would only drop to 50.0 vdc.  (and that's likely being generous assuming that the power supplies can really maintain regulation at their set output at max load.

OK lets say I'm OK with that.  So in order to even get close to this...I'll have to go back into each power supply and modify a potentiometer that's in there with a 2k ohm pot.  I have those on hand and was thinking I would probably do this anyway.  

Once the pots are all changed out then instead of running 12.5 or (max unmodified) of 12.7 vdc per supply, I would be able to run 13.7 vdc.  13.7 * 4 = 54.8 vdc!

That's more than enough to overcome the voltage drop caused by all these protection diodes.  

So...that's probably the route I'm gonna go.  I have all the parts I need to do this.  I just need to do it.  So I'll be working on accomplishing that over this coming weekend, and I'll report on the results and provide pictures along the way.

I swear I'm pretty close to be all done with this power supply.  LOL just a few more things to do after this.  (honest)