FreeLists / si-list / [SI-LIST] Re: ESR too low?

[John Matthews <john.matthews@xxxxxxxxxxxxxxxxx>]

JF,

In some topologies the ESR of the output capacitors place a zero in the 
loop, in other topologies
they don't. Or should I say that the zero affects the loop more in some 
cases.  I had this a while ago
and was looking at the difference between continuous and discontinuous 
modes in a constant current
flyback. In my case it was a 5 to -48V supply, and was complicated by 
the fact that the DSU load
had 3 poles not far above the switching frequency.

Again in my flyback, the ESR of the bulk output capacitors was one of 
the most important factor in
determining what the output ripple would be. I ended up using quite high 
value capacitors as it was the only
way to keep the ESR down. The ripple was more or less a function of the 
ESR. To reduce to acceptable
levels I had an LC post filter.

I came across a very good reference. It was from the Unitrode Design 
Seminar Notes Application
Handbook 1986-87, going right back to the eighties. Basically the paper 
listed all the most common topologies
and gave the key details related to the transfer functions. It gives you 
enough so that you can work out the rest 
yourself, and is definitely worth a read.

If you  can't get your hands on a copy, tell me the relavent  topology 
and I'll fax you the couple of sheets.

Linear Technology have some very good application notes with detailed 
relationships worked out for
ripple vs ESR.

John Matthews

 




Fasig, Jonathan L. wrote:

>Can anyone speak to the issue of bulk capacitor ESR being so low that it
>affects the stability of the voltage regulator?  I have a system with a
>well-know commercial 1.5V SMPS powering a board that was populated with
>numerous very-low ESR bulk caps.  The transient response rings beautifully
>for step load currents and the phase margin (supply+board) is typically less
>than 10 degrees (GM<4dB).  At certain low load currents the regulator
>oscillates spontaneously at 1.3KHz.  When these low-ESR caps are replaced or
>removed, the transient response settles nicely and phase margin approaches
>30 degrees (GM>12dB) but ripple magnitude is affected.
>
>Discussion or references appreciated.
>
>Regards,
>jf
>
>------------------------------------------------------------------------
>
>Jonathan Fasig                      Email:  fasig.jonathan@xxxxxxxx    
>Mayo Foundation 
>4001 41st Street NW 
>MSC Sn 2-132                        Phone:  (507) 538-5464
>Rochester, MN 55901                 Fax:    (507) 284-9171
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