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CSD18540Q5B

Salah Zayouna
Intellectual 485 points
Other Parts Discussed in Thread: UCC27210, CSD18540Q5B, CSD18563Q5A

I have a PWM Buck DC-DC converter that runs fine with FETs like BSC27N04LSG or BSC039N06NS or Si7848bd. Recently TI introduced new superior FET, CSD18540Q5B, which offers lower Rds on, Lower Qcharge, Higher Current and voltage, etc. This transistor gets shorted immediately or in seconds when replace the other FETS mentioned above. The driver used is UCC27210 which can output max 4A (This one also get shorted when the FET get shorted too).

The PWM circuit runs fine and cool with other FETs, so I think I might missing something in its datasheet that makes it unable to work.

From datasheets, CSD18540Q5B, is an ideal replacement for the FETs mentioned above, but in practical it is not, why? ....What I am missing?

  • 0
    Intellectual 485 points
    Vin is 24V, max current is 5A output at 5V
  • 0 in reply to Salah Zayouna
    TI__Mastermind 18200 points
    Let me discuss this with our applications team and get back to you. In general, we recommend the CSD18563Q5A as our low side buck solution but let me try to nail down a hard answer for you as to why the CSD18540Q5B isn't working.
  • 0 in reply to Brett Barr1
    TI__Mastermind 18200 points
    Salah,
    See the response from our applications engineer:

    "For 24Vin,5A load only, CSD18540Q5B is grossly overkill. Way too large device for 5A load.
    There is a simple answer to the failure: use data-sheet Qg, Qgs, Qgd and Qg_th parameters together with Rg and Vth to calculate turn ON, turn OFF times, gate delays, Peak current etc.
    As a result , being such a large device turn on and turn OFF times are longer than for the BSC039N06NS Fet.
    Converter dead time should be designed at least 1.2 to 1.5 longer than FET commutation times to allow for Temp variation, Vin, load variation etc…

    If the converter dead times are Not adjusted properly, the LS device enters in shoot-through with HS device. Hence, the device failure. Shorting the Vin leads to excessive large currents and device hot spots."
  • 0 in reply to Brett Barr1
    Intellectual 485 points
    Hi Brett,

    Sorry for delay..... CSD18540Q5B is always better than BSC039N06NS according to the datasheets related, but in "Turn Off delay", they are equal. Lower Qgt, lower RDS on, faster turn on , etc. Now I am suspicious that its very low Rds on might cause not enough voltage feedback to inform the PWM IC for the switching point voltage status after high side FET turned off. In all cases, I cant give more time to it, will consider another FET. Thanks for help.