[SI-LIST] Re: Experience with Power supply collapse on boards due to excessive decoupling.

Hi Vipul,

There is much information missing regarding your set-up/tests, like:

- is your board hosted in a PC or any other std (VXI, PXI, VME etc) 
platform?

- if standalone, are you using a commercial power supply or a custom one? In
a standalone set-up, check the connections/wires from the power supply 
to your
board, their impedance could be your only problem.

- in both cases, is the PSU rated for your needs?

- do you have an additional power supply module (step-up, step-down, 
etc) on
your board;does it have an inrush current limiter?

- does the power supply collapse immediately when you connect your board
or after you enable some of its functional blocks? When you say 
"collapse" does
it drop/swing its output going "slowly" down to ~0 mV or does it go 
immediately?

- have you tried more than one boards to see if you get the same results?

- have you tried an alternative PSU to see if you get the same results?

-.......
-.......

- is there any special reason why you think that the problem comes from the
decoupling capacitors?

In general, I have used more capacitance than you are using without any 
problem.
Furthermore, I usually use "bulk" capacitors say 16uF or so, nearby 
"large switch
current loads" (i.e. CPUs/MCUs, FPGAs, Memory Banks etc). Maybe you 
could try
this as a 1st workaround.

Cheers,

John



Vipul Badoni wrote:
> Hello SI-Folks,
>  
>
> I was wondering if some of you could share with me your experiences with
> power supply collapse due to excessive decoupling of only a few values
> of capacitor on boards drawing a lot of switching current. The situation
> I am dealing with has ~400 in number of 0.1uF caps and ~200 in number of
> 1uf caps. The power supply shows a lot of noise ~400mV. 
>
>  
>
> Regards,
>
> Vipul Badoni.
>
>  
>
>
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