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Power Integrations Engineer Power Integrations ModeratorTNY 280 / Output FET damaged after switch on
Hello,
We are using the TNY 280 for a low-power application with an input voltage range of about 100...240 V AC, the output power is about 1 W and everything works fine. So far, so good.
From time to time we get customer complaints about our product and the reason is usually a defect in the TNY 280, the output FET for switching the transformer is damaged (low resistance = short circuit). We figured out, that this problem occurs in a case of over-voltage (270V) in combination with a low source resistance of the voltage source. I had made some tests to find a solution (e.g. limit the switch-on current by a NTC or resistor), but it doesn't help. Could you give me some further instructions, please? What are the most frequently reasons for such a defect?
Thank you for your help in advance.
With kind regards,
IsaacNewton2012 (unfortutately I am not a genius like he was)
P.S. If a circuit diagram will help you to find out the reason, please let me know.
Some new information: In the meantime I have figured out, that an improvement of the clamp circuit (which is in parallel to the transformer input)will solve my problem. However, do you have any other further ideas? Thank you in advance.
IsaacNewton2012
Isaac -
Given the proper design, running at 270 Vac shouldn't be a problem. We have quite a few reference designs for TinySwitch that are wide-range compatable (85-265 Vac).
I don't know if it's a source impedance issue as typical wall outlets are incredibly low impedance. Also, any bulk caps (unless damaged) are going to present a much lower impedance than your voltage source.
With 270 Vac input, your bulk cap peak voltage is going to be around 380 volts. This is far less than the 700 BVDSS of the internal MOSFET. I'd say it's unlikely the input voltage is the direct culprit.
Areas I would be investigating:
transformer design - What does the leakage inductance look like? How hard are you pushing the core? How hot is the core getting? Is it borderline saturating and something is pushing it over the edge?
Clamp design - This is an obvious thing to check.
If you can send me your PI expert files, schematic, layout, transformer documentation, etc, I can take a look at it and see if anything stands out.
-The Traveler
PI - Traveler | 204 posts 