Scope lead noise with ripple testing

[eem2am]

Hello,

I have just done output voltage ripple testing on a 40W offline flyback (switching at 67KHz).
Vout = 16V

I find that when i use the scope probe with "dangling" ground lead, i get 400mV peak to peak ripple at the switching frequency.

.....When i remove the "dangling" ground lead, and "bring" the output voltage rail off the board on twisted "flying" wires, and then physically hold these wire ends against the scope probe ground "barrel" and the scope lead tip, then i see only 50mV peak-to-peak ripple.

.however, holding these wires against the scope probe is very awkward and fiddlesome.

I wonder if there is some equipment that i can use to allow me to use the scope probe in this effort more effectively, without having to physically hold wires against the scope probe ground "barrel" etc?

 

[enjunear]

The dangling ground lead is giving you extra inductance on the line, which is causing the extra ripple seen by the scope. The only real solution is to shorten the lead length, as you discovered. You might try using a FET probe with a very short lead to ground, located nearby the output node you are measuring. However, FET probes have limited voltage ranges, so check that out first.

 

[Orson Cart]

You can get a bit of 50 ohm coax and solder the (short) ends of it to the 16V to be measured - the other end plugs into the scope - 1x, set to AC coupled, this will give a pretty good & realistic reading, in commercial tests they add a 10uF electro and 100n MLC cap right at the point of measurement - this gives a lower reading for diff mode ripple - but is more indicative of what a real circuit or load would see - it if had the same de-coupling nearby. You also need to check how much common mode noise is getting into the 'scope reading - do this by shorting the 'scope probe tip to the ground clip and touch to the pos o/p and observe the screen (and the neg o/p for comparison), if you see a fair bit of noise on the scope - then there is a fair bit of common mode noise being generated by the psu which will affect your reading, Regards, Orson Cart.