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Re: [Amps] Series PC Power Supplies

To: "Jim Forsyth" <>, <>
Subject: Re: [Amps] Series PC Power Supplies
From: "Roger Glover" <>
Date: Fri, 2 Jun 2006 02:21:32 +0100
List-post: <>
----- Original Message ----- 
From: "Jim Forsyth" <>
To: <>
Sent: Thursday, June 01, 2006 10:21 PM
Subject: Re: [Amps] Series PC Power Supplies

> In what way is the load unbalanced and how would the diodes help? In a
> series connection all supplies have the same current and each contributes
> whatever voltage it is set to. How does one get to be overburdened? How do
> the diodes ever get to conduct? If they don't conduct then they are having
> no effect on anything.
> I can see that the diodes might help at turnoff to prevent a supplies 
> output
> capacitor from charging up in reverse but other than that I must be 
> missing
> the point.
> Jim, AF6O

Suck egg time, sorry.
In series the total load current passes through all the power supplies.

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.

On PC type supplies this is not so much of a problem as the regulation is 
performed by the switcher. The secondary side being rectifier, C, L, C. 
Although undesirable, the rectifier diodes clamp the reverse voltage to a 
low level.

However, when a similar event occurs with a PSU with series element control, 
the pass device will tend to switch off at limit, resulting in (unless 
protection measures are taken) the remaining supply's combined voltage being 
presented, in the reverse direction across that series element, resulting in 
failure of that device.

The problem is exacerbated as when that element fails, the tendancy is for 
it to short, re-presenting a full unregulated output on that supply. The 
total load voltage now increases usually creating an increased load current 
which will trip another supply's current limiter - blowing that series 
..and so on, until all series elements of all the supplys are shorted and 
the sum of the unregulated outputs are presented to the load.

My money says that diode across the output of each supply is worthwhile.
Been there, had to pick up the pieces.


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