Electronics > Projects, Designs, and Technical Stuff
LM25085: Vout dropping out at higher Vin
magic:
Disconnect all sensitive loads and halve the value of R14 or R17 to see if the problem is with FB or ISEN?
SiliconWizard:
Also, one obvious thing to check would be the soldering. This IC comes in packages with a thermal pad. Have you properly soldered it?
Phoenix:
--- Quote from: magic on August 23, 2020, 06:42:11 am ---Disconnect all sensitive loads and halve the value of R14 or R17 to see if the problem is with FB or ISEN?
--- End quote ---
This looks like a good path of investigation. The chip has OVP and OCP comparators - if noise gets in either of them it can trip early like you're seeing. You can also try simplifying the feedback network down to two resistors to see if that's a path for noise too.
Kristoff:
I built another board and only populated the 5V supply circuit. I also simplified the feedback circuit (did not load R15, C28, and C29). I removed the fuse and bridge rectifier and I'm providing VS from a benchtop supply but still seeing the same behavior.
I reduced R14 to 0.05 Ohms but didn't see any change in behavior.
One new thing I noticed is that as I increase VS a volt at a time I can hear a slight buzz/whine when VS gets close the the point where the output cuts out. I pressed on caps and think the noise is coming from C15. I replaced the 1nF cap with a 10nF but still no change in behavior.
I then replaced Cout (C30) with 3x 10uF 10V caps in parallel. The previous Cout was 22uF 6.3V. Still no change in behavior.
Finally I halved the value of R17 to 1.66K. This resulted in the expected output voltage around ~8.8V. I was also able to increase VS beyond the previous point of failure.
With Vout =5V and no load the output was stable to Vin ~19V.
With Vout = ~8.8V and no load the output as stable at least to 25V (this is where my benchtop supply maxed out). However when generating VS from a 24 VAC supply with the bridge rectifier to get a VS of ~36VDC the output voltage dropped to ~2V.
Any suggestions on how to interpret these findings and what to try next?
Thanks!
Ton:
Hi all
I once had a nice fight with the same SMPS controller, in our case it was the switching noise coupling to the current measurement.
and as your scope captures show then you have a quite a lot of ringing in your circuit (depending on how you have used your oscilloscope probes.)
For us it was solved by increasing the Rsen (R14/R20) from around 20mOhm to 82mOhm (and adjusting Radj (R13/R19)) for a better current value signal, and adding a RC snubber on the junction between the Mosfet drain(T1/T2), coil (L1/L2) and diode Cathode (D6/D9) to GND, the snubber values has to be calculated based on the ringing frequency.
you could also try to lover the dV/dt of the switching node by inserting a smal resistor (around 2-4 ohm) in the gate signal. in my case there was no room for a gate resistor, so we had to fix it with a snubber.
I think in my case we ended up using a 30 ohm resister to gnd, in series with a 470pF to the switching point, this was with a ringing frequency of 124Mhz, the ringing frequency wil depend on componnts and layout.
it ended up running pretty well in the end.
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