Part Number: TPS54360
Tool/software: WEBENCH® Design Tools
I'm experiencing repeated catastrophic failure of the TPS54360 when it is plugged into an active power source.
In particular, I have the TPS54360 setup to convert 48V DC to 6.5V DC for power over Ethernet. When the Ethernet cable is connected with the system off, turning on the power supply results in normal operation. However, when the Ethernet cable is connected after power is already supplied to the system, the TPS54360 fails dramatically. This happens with some regularity, though not every time the device is hot plugged. I have been able to reproduce this behavior.
I'm using a recommended WEBENCH configuration for the TPS54360 and a Mean Well supply for the 48V. Any recommendations about the potential problem (and how to solve it) are appreciated.
Check for overshoot on VIN. You would see this sort of issue if the VIN voltage goes above the abs max spec (65V in this case).
Hot plug will be worse than turning the supply on for a situation like this. The supply ramps up as it charges its output caps when you turn it on. But hot plugging immediately connects those charged caps to the input of your board which will cause worse transients.
If this is the case you can remedy it by reducing inductance from Vsupply to VIN which can include shortening the connecting wires, move the VIN caps closer to the IC, and more. Let me know what you find and we can carry on from there.
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In reply to Samuel Jaffe:
Thanks for the quick reply Sam.
I agree, it seems like an overshoot problem at VIN. I don't have an oscilloscope on hand to be able to verify that at the moment, but I think operating on that assumption is correct.
The wires are already quite short between VIN and Vsupply (just a few feet) and the VIN caps are immediately adjacent to the VIN pin (just a millimeter or two). Perhaps larger caps would mitigate the issue? Currently using the WEBENCH recommended 2 x 2.2 uF.
I'm happy to share photos of the board, but I don't see an option to upload attachments (possibly because I'm a new member) I also have a video of the most recent failure if that might be helpful.
I just reviewed the video of the failure, and the board survived several hot plugs until the last one. On the last hot plug, the board was unplugged for only for 2-3 seconds (much shorter than the other tests). Perhaps the VIN caps retained their charge and where thus unable to protect from the initial onrush of current? Would a resistor across the VIN caps allow them to dissipate quickly and protect from the surge?
In reply to Patton Doyle:
Ah, I see the option now. See a number of files attached.
I will add a large electrolytic capacitor the board and see if that resolves the issue. Is there a good rule of thumb for the proper size? I don't have any 100V on hand, so I'll have to place an order.
Let me know if you see any other issues in the schematic or video that I missed. I removed the audio from the video as it just involves the fan and me swearing at the end. The failure in the video occurs at 40 seconds.
Also, it's important to note that there is a resistor across the 6.5V line - the remainder of the board beyond the power supply has been disconnected to isolate this issue. On the schematic, this corresponds to just the power supply portion of the schematic up to C7.
Note: On this board, there is a linear regulator to bring the 6.5V down to a clean 5V. In this test, the linear regulator has been replaced with a large resistor. On the next draft of the board, we've decided to change TPS54360 to 12V for general use with a linear regulator for the 5V controller. Let me know if you believe this might affect operation significantly.
Board Schematic.pdfBoard Top.pdfBoard Bottom.pdf
Reposting the video at a human scale.
After doing some digging, it looks like an inrush current limiting circuit is the next best option if the larger capacitance doesn't do the trick.
I mocked up this circuit which should prevent inrush until the capacitor is mostly charged (delaying the full power to the board about 0.05 seconds). Would this circuit interfere with the operation of the TPS54360? Are there other unintended consequences we should consider?
Ah, excellent suggestion! And much simpler to fit onto our already crowded board.
I will perform a few more tests with the larger cap and the TVS diode to see if it solves our problem.
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