[Amps] Series PC Power Supplies

[Will Matney]

Jim,

I think what he's getting at is this. The power supplies to be seriesed are calculated to provide a certain set operating voltage at a certain set current. Together these seriesed supplies total the amount needed. The load requires this set amount of voltage and current to operate properly. Ok, if one supply fails in the string, there is a voltage drop. This voltage drop creates a current rise from the remaining supplies feeding the same load. If this current rating is over the maximum rating they are designed for, which most likely it will be because we sized it to use the number being used, it can make the other two fail, IE current trips, etc. Worse yet, if there's not good over-current protection, then you start having pass transistors to fail, etc, and have a catastrophic failure of the complete system. Most regular power supplies that are used generally have just a regulated voltage system, and don't even have current limiting. The ones that do are generally the upper end supplies with adjustable current, or having current sensing which in turn controls the pass transistors. If they would use FET's as the pass device a person would be better off, but most still use transistors which can go into thermal run-away, etc.. Most now only use an IC to compare the output voltage and run a driver transistor which in turn drives the pass regulator(s). A few may have a current sensing circuit, but again that costs more and a lot of manufacturers don't use it. This being the regulated supplies like Astron, Pyramid, and several others without adjustable current. About the only safety precaution they have is over-voltage pretection by a crowbar circuit. Switching supplies on the other hand are different, but can burn out too. I am still leary of switching supplies due to all the parts involved just giving one an extra excuse to fail. Switching supplies, I think, should be limited to weight limited applications. The less parts one uses to do a job correctly, the less chance it has to fail. The simplest type of supplies are the transformer-rectifier-pass regulator type. One can build a good regulated series pass circuit using two small transistors, a large pass transistor (up to 10 amps), and a zener diode. The regulator IC's really only take the place of the sensing transistor/comparator and the zener diode unless it has a current function which a lot don't. However, when seriesing or paralelling several supplies, you still have to worry about the lead and lag or each regulator circuit to the other in time. This being eaches reaction time to the current change from the load. Then you get into trying to build an outside circuit to try and get each supply to react equally with the changing current. That is a hard thing to do which is discussed in detail in the one book I mentioned, and a few others. One would think that small amount of lead and lag wouldn't amount to much, but let's remember what happens with surge current in capacitors for only part of a 1/2 cycle. It would be almost totally impossible to build two or three exact matching power supplies with exact matching characteristics and reaction times. The most one can do to try and get by is use the diodes, but there are some other ways that are way more complicated and expensive to do. Then, your better off to buy one supply to do the whole thing anyhow.

Best,

Will



*********** REPLY SEPARATOR  ***********

On 6/1/06 at 6:41 PM Jim Forsyth wrote:

>Please explain that more clearly, especially the "opposite sense" part.
>Are 
>you suggesting that the current somehow flows in the opposite direction 
>through the supply in current limit?
>
>Perhaps you meant to say the current may charge the output capacitor to a 
>reverse voltage. That is where the diode could be helpful as I mentioned
>in 
>my previous email.
>
>Jim, AF6O
>
>> All is fine until that load current rises to the current limit of the 
>> 'weakest' supply. When this limit is reached, that power supply will
>shut 
>> down or limit. The remaiing load current is presented to that supply in 
>> the opposite sense.
>
>
>
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