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TIDM-1007: Control method for Totem pole PFC Sync Mos

BensonChen
Prodigy 160 points
Part Number: TIDM-1007

Hi experts 

 we're evaluate a CCM totem pole solution . Referenced from  TIDM-2008/TIDM-1007 . The AN shows the control timing of active mos and sync mos ,the sync mos is turned on after active mos finished duty-softstart.

   I also read some other solutions ,they verify the inductor current ,when current is continues  ,then turn on the sync mos ,otherwise turn off the sync mos .

  Is it right to distinguish CCM and DCM mode to control the sync mos or    Do I need more hardware circuit to avoid current flow back during DCM ? 

  

  • 0
    TI__Expert 4200 points

    Hi,

    The soft start algorithm was designed to avoid current spikes near zero crossing. It is not related to the transitioning between CCM and DCM.

    The article below (slyt650) has detailed analysis on switching sequence during soft start and hope this help you understand.

    https://www.ti.com/lit/an/slyt650/slyt650.pdf?ts=1620695014711&ref_url=https%253A%252F%252Fwww.google.com%252F

    Best,

    John

  • 0 in reply to John_K
    Prodigy 160 points

    Hi John 

        Thanks for your quick response ! I Read this article ,and it's very effective to solve current spike when AC crossing. 

         My questions is , When PFC work at light load , the PFC will work at DCM for a while then enter to CCM . How to control the sync Mos during PFC works at DCM ?

  • 0 in reply to BensonChen
    TI__Expert 4200 points

    In bridged PFC, Q1 and Q2 are diodes instead of MOSFETs and therefore it will become DCM under the light load condition. However, in totem-pole PFC, Q1 and Q2 are MOSFETs and it conducts the negative current.

    Please check the waveform below

    As totempole always operates in CCM,  there is no specific control algorithm for Sync FET in this solution.

    Best,

    John

  • 0 in reply to John_K
    Prodigy 160 points

    Hi John 

         I understand this AN is bi-directional TTP PFC .How it will be the negative current flow back to grid or AC Source ?

       To prevent negative current flow back to grid ,is it right to turn on sync mos at CCM or turn off sync mos when discharging current reach 0?

  • +1 in reply to BensonChen
    TI__Expert 4200 points

    Considering the inductance and voltage level, the negative current is relatively small and it won't impact much on ac source.

    Furthermore, there are few more issues related to DCM.

    First of all, the hall effect sensor has bandwidth around 120 kHz and the switching frequency is at 100 kHz. it is impossible to sense the accurate ripple measurement from the sensor which has to be used for sync FET forced turn off. If you want this feature, one way is to add a shunt resistor based measurement to get the enough bandwidth. We don't have way to measure 100 kHz current ripple in the existing HW.

    Second, DCM requires forced turn off of the sync FET and it may lose ZVS turn on of the control FET in the next switching cycle and increase the switching losses. Although it can reduce the conduction loss from the negative current but again, it is relatively small and I'm not sure if it can offset the additional lose from losing ZVS.

    We had an initial evaluation of DCM activation on totem-pole PFC but due to HW sensing limitation and low expectation of the improvement, we don't have plan to include that in SW.

    Best,

    John

  • 0 in reply to John_K
    Prodigy 160 points

    Hi John 

          Thanks for your kindly help!

    Benson