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Original question:

UCC256403: voltage transients abnormal on low side MOSFET

UCC256403: FB pin

Henry Wang3
Expert 2805 points
Part Number: UCC256403

Hi Sir,

My customer have the below questions.


1. What conditions it will turn off the AVDD and internal MOSFET of the FB pin?

UCC25640x LLC Resonant Controller Features Brief Overview and Bring up Guidelines


2. The IFB typical value of UCC256403 is 164uA, if the IFB current is over the value, may I know to have any side effects?
3. The CTR of the Opto-coupler(BPC-1009SC) is 200~400%, based on the below circuit, the current(If) flowing into the Opto-coupler, what are the values of minimum and maximum?

  • 0
    TI__Genius 13245 points

    Hi Henry,

    Henry Wang3 said:
    1. What conditions it will turn off the AVDD and internal MOSFET of the FB pin?

    1. The AVDD will be off during the fault state.

    opto coupler current = IFB + I_RFB

    From above equation, Internal PMOS will be off, when the opto coupler current is same as IFB source current.

    2. 

    Henry Wang3 said:
    The IFB typical value of UCC256403 is 164uA, if the IFB current is over the value, may I know to have any side effects?

    The opto coupler will be saturated when the current drawn equal to IFB. You cannot draw current more than 164uA. During this case, VFbreplica will be at its minimum value.

    When continues to increase and reaches the value of , the FB pin voltage starts to drop because there is no current flow through the PMOS. FB pin pulled low will impact the system transient response, due to the extra delay introduced by charging the parasitic capacitor of the optocoupler to pull up the FB pin voltage.

    3. 

    IF=Iopto/CTR 

    Ideally, IF can be anywhere between 0 to (164uA/CTR_minimum)

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Expert 2805 points

    Hi Sir,

    I_R712 = (18V-5.5V)/12K = 1.04mA, I_R719 = 1.5V/2K = 750uA

    If = I_R712 - I_R719 = 290uA

    CTR = 200~400%

    Ic = CTR(300%) x If = 870uA

    My customer hopes for system stability, could you help to review both calculation values of Ic and If, could they be used or not?

  • 0 in reply to Henry Wang3
    TI__Genius 13245 points

    Hi,

    Please see the attached app note draft (final section of the app note) where I have solved an example for compensator. Please follow those guidelines and find the compensator values accordingly.

    Tri-34b0609e-2c6e-40c5-8f6a-40c808450af2.pdf

    Let me know if you still have questions.

    Regards

    Manikanta P