• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 38 subscribers
  • Views 234 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TIDA-010042: Need of turning on synchronous FET in DEM mode for charging bootstrap capacitor

Cyril Sebastian
Prodigy 220 points
Part Number: TIDA-010042
Other Parts Discussed in Thread: CSD19531Q5A

Hi,

Please tell me the reason for periodically turning on synchronous fet(low side fet) in diode emulation mode in tida-010042 old reference design (based on CSD19531Q5A). In the firmware, it is mentioned that periodically turn on is for charging the bootstrap capacitor. I am not able to understand the reason for this. 

Regards

Cyril

  • 0
    TI__Intellectual 1130 points

    Hi Cyril,

    The bootstrap capacitor needs to be re-charged to a normal voltage every switching period.

    If the load is too light, which means the lower FET turns on for a very short time (Vsw=0), bootstrap capacitor starts to re-charge. But this period of time is too small, it may be not enough to re-charge the bootstrap capacitor to a normal voltage. So the lower FET needs to periodically turn on for some time to re-charge the bootstrap capacitor, this is not very much mandatory, but can be seen as an additional function.

    Regards,

    Bowen

  • 0 in reply to Bowen Ling
    Prodigy 220 points

    Hi Bowen,

    Thanks for the reply.

    In my firmware, initially while turning ON the charger, it will be starting in DEM mode. It was not able to turn on the high side fet after the removing periodical turn on function of low side fet from the code.

    But I have a doubt, whether this phenomenon is only applicable to converters with the battery as load.

    What about asynchronous buck converters? How it is achieved in light load conditions.

  • 0 in reply to Cyril Sebastian
    TI__Intellectual 1130 points

    Hi Cyril,

    No, this is a common phenomenon of BUCK. For sync buck, it will turn on lower FET regardless of the reverse current of the inductor, so the bootstrap capacitor can be charged.

    Bowen

  • 0 in reply to Bowen Ling
    Prodigy 220 points

    Hi Bowen,

    Is there any application note related to this topic.

    If I am using a non synchronous buck, how bootstrap capacitor will be charged in this case.

    Regards,

    Cyril

  • 0 in reply to Cyril Sebastian
    TI__Intellectual 1130 points

    Hi Cyril,

    For non-sync buck, the bootstrap capacitor will be charged when the current flows through the diode of lower FET. 

    Bowen