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UCC28C56H-Q1: UCC28C56H - As flyback PSR - Aux control

manoj kumatr
Prodigy 180 points
Part Number: UCC28C56H-Q1
Other Parts Discussed in Thread: UCC28C56H

Hi Team,

I can see in the webench ckt for the UCC28C56H PSR, the VDD and FB are connected at the same node and not separated by the diode. could you pls explain more how does this connection works for regulating the Flyback. As par we can see it only regulate the VIN supply not the AUX. Pls advise ...!!

  • +1
    TI__Guru 66015 points

    Hi,

    Rfbt resistor divider is to set VDD and regulate VDD so Vout is cross regulated.

    The diode after Rfbt is optional but can help improve regulation.

    I do not think the VIN can be regulated from this circuit.

  • +1 in reply to Hong Huang
    Prodigy 180 points

    Hi Hong, 

    Thanks for the response .

    I have tested the below ckt recently whcih is designed accordance with Ti webench and UCC28C56H Q1 EVB references. pls verify.

    I have observed huge VDS spike across the Flyback switch, also the Vout of flyback is held at 234V.  here I am sharing the VDS & Vout waveforms.

    pls advise possible causes.

    Tested Waveforms at Vin 21.21V dc 

    Apart from this couple of questions:

     1. How do we assue that the UCC28C56H takes VDD from the AUX? Why not from the VIN? 

    2. WE have observed the VFB fluctuating though there is no connection to any RFBs, pls advise why?

    3. IF the IDD is not sufficient, Does IC still tries to turn on and off like below ?

    pls avise above and thanks in advance !!

  • +1 in reply to manoj kumatr
    TI__Guru 66015 points

    Hi,

    1. How do we assure that the UCC28C56H takes VDD from the AUX? Why not from the VIN?

    The IC needs about a few mA to make control. If Vin is high, then the associated power losses are high so use AUX is to help reduce power losses. If Vin is low then you can use Vin to bias VDD. 

    2. WE have observed the VFB fluctuating though there is no connection to any RFBs, pls advise why?

    You need to make loop compensation to stablize the operation from proper phase margin above 45 degree.

    3. IF the IDD is not sufficient, Does IC still tries to turn on and off like below ?

    The IC will turn on then off if IDD is not sufficient.

    I am not sure what else support you need. It sounds like you want to make the spikes lower which are related to your layout and the transformer leakage inductance. You need to tune up these.