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UCC2897A: UCC2897 Duty setting issue

JJ
Prodigy 50 points
Part Number: UCC2897A

Tool/software:

Hi Ti,

I have some questions for you about UCC2897 when turn off transient .

1. I measure the AUX pin duty value about 100ns ,the duty almost > 95% it will excess the set valure (86%)?( Ron/Roff setting : Ron=130kohm , Roff=18kohm , Tsw =5.133us , Ton/(Ton+Toff) = 0.8681 )

2. And I have no idea about the clamp MOSFET have abnormal VDS waveform. 

3.The active clamp forward max duty have limit under the UCC2897A?

*clamp mos vgs have inverse circuit form aux pin

thanks,

JJ

  • 0
    TI__Guru 74115 points

    Hi,

    1.

    Dmax should be measured on OUT pin instead of AUX pin.

    Dmax calculation needs to count in tDEL - it looks you did not. You need to follow the below equations.

    2.

    You need to adjust clamp capacitor value to change the shape of the Vds.

    3.

    Yes, it is limited by the MOSFET capability of turn on or off time and also MOSFET Vds rating.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Hi,

    thanks for response~

    1. Rdel is setted 20kohm, Tdel 0.237us

    Ron/Roff setting : Ron=130kohm , Roff=18kohm , Tsw =5.133us , Ton/(Ton+Toff) = 0.8681(Dmax)

    Does the Aux pin have its own duty cycle limit? it is independent of the OUT pin?

    2. Are there any related documents regarding this abnormal VDS and the impact of the clamp capacitor?

    3.So  the 2897A can increase the duty beyound 90% as long as it is acceptable for the external MOSFET?

    thanks,

    JJ 

  • +1 in reply to JJ
    TI__Guru 74115 points

    Hi,

    1.

    AUX duty is affected by the dead time. Duty is defined by the OUT pin.

    2.

    I do not know a document. But our support experience helps how to change the shape by adjusting the clamping capacitance.

    3.

    Yes, if MOSFETs turn on /off still ok and if MOSFET voltage rating is ok, then 90% duty cycle is ok. It is related to MOSFETs, tDEL, minimum off time and maximum on time and frequency in use.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Thanks,

    your response was very helpful ! 

    If there are any issue with the VDS waveform improvement , I will update the information!