TOP250 explosive failure - high line
I have a power supply that is failing on power up at high line with explosive results. I've reviewed the layout and see the lack of separate retun path for FBK winding return. Could the attached scope plot of of control voltage (ch1)& Drain voltage (ch2)at turn on cause this type of failure (and this plot is only at 120Vin...)? I would suspect massive currents are present due to a temporary "loss of control" by the noise on the control pin. The result is a vaporized die... could someone add their 2 cents ? thanks
Comments
Thanks Traveler.
This is for a customer of mine... I don't have the design files for the xfmr and I can't release the schematic.
-Yes I do have concerns with their layout, serious concerns.
-I've asked questions about recent MFG changes in the transformer.
-The clamps seem to be working. I haven't seen any spikes over ~450V (although I need to check some extreme conditions). The failures the customer is seeing are immediate on startup, or a restart due to a brown out.
-The Ilim resistor on pin x was essentially not doing anything because of it's low value. I've adjusted to slightly under 10K, so I still get the proper output at min Vin.
-still looking into the control loop.
-I did see something else where there's a burst of pulses for the ~10mS I'm assuming the internal soft start, then nothing for ~1ms, and continues after that. Is that normal as the device starts up / changes modes? pls see the scope plot.
If this is a design that is currently in production and only recently started experiencing these failures...something has changed and you'll need to track that down. I'd look at purchase orders and build information to track down lot numbers, date codes etc. It might be something as random as the magnetics vendor omitting one layer of tape in a batch of transformers.
From your scope plot, I'd be looking at the transformer and clamp components. You have spikes over 350V at only 120V input. Some basic troubleshooting techniques of swapping components might be helpful...swapping transformers between a known good unit and a known bad unit and see if the problem follows the transformer or the PCB.
If you have the equipment available, I would definitely look at both a good and bad transformer on an impedance analyzer and see what differences pop up.
Without some additional information I'm very limited in what I can assist with.
-The Traveler
A couple things come to mind:
- Have you verified proper transformer construction (could cause excessive leakage inductance)?
- Have you verified that the clamp circuitry is working correctly?
- Have you tested/analyzed your control loop at all?
If the design is running fine at low-line, my concern would be the transformer design and/or clamp circuitry.
I really want to make sure you get this problem solved. To help me out, please provide the following:
-PI Expert/PI XLS design files
-Your transformer documentation
-Your schematic
-Your PCB layout
-The Traveler