Bringing up SMPS..Low impedance input filter needed?

I'd consider that any switched mode PSU design that cannot tolerate random voltage variations at least a few percent beyond its designed input range, is fundamentally flawed and not worth pursuing!

Input supply variation tolerance is one of the most fundamental requirements.

Flyback said:

Hi,
Do you agree, that when bringing up an SMPS, the bear minimum needed is an input source voltage which remains flat-in-voltage no matter if the SMPS goes unstable?
Do you agree, without that, you cannot even start your SMPS bring-up testing?

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Absolutely NOT. In practice, an input voltage source with a near zero rise time is a practical impossibility.

Thanks, but i apologise, as both of your good selves are not on the same page as me....and its my fault....i am saying if youre testing a smps which requires 5vin, and you set 5vin.....then vin goes from 2v5 to 5v5 at 1khz when you connect the smps , then that input source is not suitable for the testing in the first place. Do you agree?

If the power source significantly exceeds the tolerances of the PSU being tested, it's obviously unsuitable.

However that's a very different thing to your original
"an input source voltage which remains flat-in-voltage"

Switched mode supplies generally have a fairly wide input range; the power source should not exceed that significantly, but it must be tested outside the specified limits to guarantee correct operation to those limits.

Any SMPSU that needs a tightly-toleranced input voltage is a very strange and obscure device, that I'd not want anything to do with or would include in any design or product!

Flyback said:

Thanks, but i apologise, as both of your good selves are not on the same page as me....and its my fault....i am saying if youre testing a smps which requires 5vin, and you set 5vin.....then vin goes from 2v5 to 5v5 at 1khz when you connect the smps , then that input source is not suitable for the testing in the first place. Do you agree?

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If your input source is doing this, then it is not behaving as a voltage source or there is a fault in the SMPS that it is trying to power. To test the input voltage source connect it to a fixed resistance and see if you get the expected current.

You can also try using a battery. That should tell you something.

You can also try using a battery. That should tell you something.

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...Yes, good point..i now suspect, from what you kindly say, that the feedback loop of the bench power supply (CPX400D in this case) , is interacting (fighting) with the feedback loop of the Boost SMPS...and this is the cause of the problem.

Flyback said:

...Yes, good point..i now suspect, from what you kindly say, that the feedback loop of the bench power supply (CPX400D in this case) , is interacting (fighting) with the feedback loop of the Boost SMPS...and this is the cause of the problem.

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One thing for sure -- batteries do not attempt to regulate the output.

The other thing you might want to try is a filter made from a differential mode choke and a common mode choke with Y-capacitors in between.