Transient load testing with AC coupled scope to view Vout?

[cupoftea]

Hi,
We need to do transient load testing on a 9Vin, 20Vout , 60W boost converter.

There is no need for any great transient response, since the 20V rail is just supplying some battery chargers.
We just want to check that the Boost doesn't become unstable after the transient.
Also, for the full load to no load transient, we want
to check that the vout does not go above 25V.

So do you agree that the scope should best be on DC coupling? It certainly answers all the questions that are needed to
be answered.

I have been places where they do transient load testing with scope on AC coupling
, and I never really saw the advantage. (unless of course the transient undershoot/overshoot
is like, 100mV and you just cant see it on DC coupling)

However, in our case, if the undershoot is <100mV, then its not of any importance anyway.

So , do you recommend AC or DC coupling for transient load testing?

 

[KlausST]

Hi,

* absolute voltage, like "25V", which means w.r.t. GND --> DC mode
* relative voltage, like "5V overshoot", which means w.r.t. nominal output voltage --> AC mode

Klaus

 

[Easy peasy]

you need AC coupling to zoom in and see overshoot - undershoot - settling time & any oscillations

 

[cupoftea]

Yes, I see the benefits of AC coupling, though as you know, it depends on the size of the coupling capacitor
etc etc. Do you think the attached is a good rendition of the circuit that you end up
with when doing an AC coupled transient test?
As you know, the capacitor value affects it.....too small and you loose signal.
Too big and there are other issues...the scopes don't tell you what size
capacitor they are using or "emulating".