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UCC24612-1EVM: UCC24612 timing error

Chris Kuppens
Prodigy 20 points
Part Number: UCC24612-1EVM
Other Parts Discussed in Thread: UCC24612,

Hello,

I am using the UCC24612-1DBVR in a POE supply. This power supply, a TI-design (TIDU190-January 2014), has a 12V 15W fly-back topology.

In the DCM mode (Discontinuous Conduction Mode) everything works fine.

When the power supply has to provide more than approx. 10W, the converter enters CCM (Continuous Conduction Mode).

In the CCM the PowerFET is switched off too late, which introduces a momentary short that leads to a very strong spike.

All this leads to the PowerFET and snubber resistor becoming very hot.

Leaving out the snubber doesn't change behaviour of UCC24612, PowerFET still becomes very hot when converter enters CCM.

The PowerFET type is FDMC102L, VDD and REG voltages are stable and the gate has a 2 ohm resistor as recommended in the data sheet.

Is there something I forgot?

Best regards,

Chris

  • 0
    TI__Mastermind 26840 points

    Hello Chris,

    Thank you for your interest in the UCC24612-1 SR controller.  To get the best performance from this controller, the synchronous rectifier (SR) MOSFET should be well matched to the current that it must conduct.  Following the UCC24612 datasheet equation 1 example, the Rds(on) for your application should be approximately 40-50mR (based on an estimated 2-2.5 Apk secondary current).  If the SR MOSFET is too large (Rds(on) too low) it can be difficult to turn it off quickly in CCM.

    You mention that your SR FET is FDMC102L, but I could not locate a datasheet or any reference to this part number in a web search. Can you verify that number and provide its parameters? 

    Also, can you clarify your application?  The title of this E2E post refers to the UCC24612-1EVM.  Are you using this EVM in place of the SR FET used in the TIDU190 design?  If so, have you disconnected the existing SR FET and are using the EVM in its entirety, or are you using only the controller on the EVM and disconnected the EVM MOSFET?  Which snubber are you talking about: the snubber on the TIDU190 or the one on the EVM?
    Can you provide a clear schematic of the SR section of your design to show exactly what you have implemented?

    Please make sure that the SR FET that you are using is rated for 80-100V and about 50mR, and that only one FET is being driven (not the TIDU FET + EVM FET in parallel).

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Hi Ulrich,

    Thanks for the quick reply and suggestions.

    The reason I choose the SR is that I don't have to use the drive winding. Should the transformer become obsolete I have more choices to replace it.

    Therefor I only use a SR UCC24612-1DBVR in combination with a MOSFET.

    The SR MOSFET is FDMC86102L (23 mohm at Vgs 10V and 34 mohm at Vgs 4.5V). In my current design I have no option to modify this quickly.

    In a re-design I will add a small resistor to meet the required optimal MOSFET resistance. My snubber is a 470pF with a 47E resistor.

    Thanks again for the help.

    Chris