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TPS23756EVM Broken

Other Parts Discussed in Thread: TPS23756

Having just purchased this eval board and tested it on POE. The output was 5V and POE input after the bridge rectifer was around 52V.  Next a 2R2 dummy load was applied across J1 to simulate about 12.5W, the voltage remained at 5V so it was left on for a while to check which parts got warm. On returning (20-30minutes later) the output voltage had slumped to about 3.2V.

The load was removed and board left to cool down as L3 and Q5 were very hot (too hot to touch). Now even without a load attached it only gets to about 4V and L3 is still getting very hot. L3 has about 6.2V on the xformer side and 5.6V on the IC side just measured with a DCM so there may be some higher peaks if viewed with a scope.

The PCB HPA479 Rev B and has a sticker with QR code and 4417900004.

Any ideas why this has happened? The design should handle 5A so if it broke at 2.3A while in a air conditioned room in free space....

  • Hi Lyndon,

    I'm not sure what might've damaged while you were away during the test. 5.6V at the Vc means the converter is not on or in some fault condition which explains the unregulated output and heating of L3 which should be a very low current circuit.

    You may want to see if there is any shorts on your board or damage to the FETs/gate drive control on the EVM.
  • OK so I checked both the GAT and GAT2 and they are switching and both the FETs are switching too. I decided there must be a large current draw into the VC pin so swapped the TPS23756 and the board is working again, perfect 5v output.

    I'm looking at the junction of L3/D5/D6 and I'm seeing a switching peak voltage of 123V that settles to 100V then falls low for the rest of the cycle. This seems like a very high voltage?

    On the other side of L3 is a steady 11V and VC is around the same voltage. I'll reconnect the 2R2 load and see what happens.