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UCC29910A: -730_EVM

Kambiz Mahboubi
Prodigy 20 points
Part Number: UCC29910A

Tool/software:

Hi,

I am using the UCC29910A-730 design. The output of this board is fed into a uC based flyback SMPS with variable output (see attached).

Measuring the voltage between the output ground (SGND in the circuit) of the flyback-SMPS and the chassis ground (a crate mount module)

I see about 40 volts. in an attempt to connect the SGND to the chassis ground with a capacitor the fuse at the input of the UCC29910A-730

blew and now it is not working and has to be fixed. What I do not understand is why the secondary winding of the flyback transformer/supply 

can not be tied to a reference level to have a well defined ground or reference voltage? The output of the flyback SMPS is between 15V and

65V.

flyback-SMPS.pdf

  • 0
    TI__Mastermind 40550 points

    Hello Kambiz, 

    If the secondary winding of the flyback transformer is truly isolated from the other windings, I see no reason that you cannot connect the SGND node to any reference potential that you want, including earth-GND (which is what chassis-ground typically is).  

    If the secondary winding is NOT isolated (for some reason) from the primary side, then there is the possibility of excessive voltage between SGND and Earth-GND (chassis-ground). Even if you measure only 40V this may be an average or rms measurement (with DMM) and not a peak voltage measurement.   

    Please verify whether the flyback transformer windings are isolated or not.  If not, then high primary voltage may have damaged your capacitor from SGND to chassis-ground and allowed fault current to flow through the UCC289910 EVM in a way that caused damage to the EVM, then blows the fuse.  (The fuse is not fast enough to save over-stressed semiconductors, it is mainly there to limit fault energy to avoid starting or sustaining a fire.)

    If the windings themselves prove to be fully isolated (can withstand high voltage between them without breakdown), then examine your pcb to see if SGND or Vout is somehow connected to the primary side of the flyback circuit. 
    Also check to see if your chassis-ground is connected to earth-GND or to some node on the primary side. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Hi Ulrich,

    thanks for the prompt response. I am using the UCC29910A-730 EVM. The secondary winding of the flynback transformer is isolated from the primary. The only connection (if you can call it that) is the safety capacitor (c9 in the schematics) sitting between the two grounds. I will remove this and try it again. At the moment I am trying to fix the UCC29910A-730 EVM. It is not clear what is damaged. Replacing ZR2, Q5 didn't help unfortunately. It seems like more is damaged. I am replacing the UCC29910A chip itself . Let's see what happens. Do you by any chance have any idea what else might have suffered from the voltage/current surge? I'll post here the progress.

    Regards,

    Kambiz

  • 0 in reply to Kambiz Mahboubi
    TI__Mastermind 40550 points

    Hello Kambiz, 

    If Q5 was damaged, it is possible that the current sense resistor R19 may be damaged as well.

    Also check gate-drive R21, driver IC U2, Q4 and R14. 
    High voltage has a way of propagating through low volage circuitry if one of the main isolations fail.  In this case it could be the drain-gate insulation breakdown in Q5.  Also check CS resistor R23. 
    This not to say that these have gone bad, but there is possibility that one or more of them may be damaged.

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Hi Ulrich,

    sorry for the late reponse. We have replaced several components, such as the driver IC U2 and Q5 and almost all active devices. but no success so far. We still have not replaced R19, R21, R23 and Q4. We will do that too and I'll get back to you with the result. Otherwise I have to order another board to assemble and try again. 

    Regards,

    Kambiz