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DRV8323

hello TI partner

Recently  I read the article of "slva84" , some  question need to be confirmed ;

pls check the following content on the  red line, 

--why  need to wait until the LS_MOS body diode fully conduct,and turn on the HS_MOS ??  

pls  help me to explain it 

thanks

slvaf84-MOS dead time.pdf

  • Hi!

    Thanks for posting your question in MD forum. As mentioned in the document, when LS FET is pulled low, current will momentarily be pulled through the body diode and this current flows out of the phase. On top of this current, when we turn ON the HS FET, there will be more current flowing out of the phase which will generate a large change in the current. To avoid large change of current, we turn ON the HS FET after the LS FET body diode fully conducts.

    Regards,

    Vishnu.  

  • thanks for your reply 

    -sorry , I still can not understand the content  "when we turn ON the HS FET, there will be more current flowing out of the phase which will generate a large change in the current."

    why will generate the large change in the current ? as I know , in the inductance circuit, the current  will not change quickly 

    check the following pic, the I_Phase don't change quickly 

    also another question: if the LS_MOS body diode doesn't conduct fully , then how the current change ,can you show me some photogragh;

    plz help me 

  • Hello,

    The large change in current is not the current flowing out of OUTx but rather the internal current direction of the HS/LS MOSFETs. Because they share the same switch node internally, inductive spiking can occur if the FETs are turned on too quickly, so by design, the MOSFETs are turned on more slowly in this scenario (longer propagation delay time) for robustly switching OUTx and avoiding ringing or shoot through conditions. 

    I_phase does not instantaneously change because the motor phases are inductors, which resist changes in current. 

    Thanks,
    Aaron

  • thanks 

    maybe I know your meaning ^^,plz check

    -when the LS_body diode is not fully conduct, is that meaning the LS_Mos does not fully turn off ,is that right  ?

    if this time , turn on the HS_MOS , will cause Vol_Spike  at the switch node 

    plz  check 

    thankyou 

  • Hello,

    Yes, that is correct, by design the internal gate drive source and sink current is controlled, and low internal gate sink current means it will take time for the LS FET to turn fully off (VGS below the threshold voltage). Once the FET is off, the LS body diode will conduct for a short amount of time. If the HS FET is turned on while the LS body diode is conducting, this will cause a voltage spike at the switch node. 

    Thanks,
    Aaron