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The VS pin of the gate driver ic shifts above a vds of 120V.

Amal
Expert 1140 points
Other Parts Discussed in Thread: UCC27712, UCC27714

Hai,

i am designing an pv based inverter.I have implemented h-bridge in my design.I have attached My High side driver schematics with this.As a part of testing i just drove a half cycle with two mosfets,one from high side and other from low side so that the path completes from vdd to ground.I happen to probe the VB and VS pins of the gate driver IC.I found it normally working until a particular Vds of 120v.When the VDS increased from 120V , the voltage on VS pin with zero reference ,level shifts above zero proportional to the increase in voltage.I mean to say that if the VDS is 120v the VS pin rises from a zero level to a particular voltage.But when VDS is increased from 120v....if the VDS is 150v, then the pulse level on vs shifts above zero level with a dc value of 30v.We found proportional increase in the level shift from zero level when we increase from 120v.I have attached rough sketch of the H_bridge with this.what could be the possible reason for the DC value or the level shift from the reference level in the VS pin in this design?Why isn't it not level shifting for VDS values below 120V?Please note that i am using max 200v variable power supply across the H-bridge.Is it due to my driver or is it due to the Mosfet i am using in my design?

  • 0
    TI__Genius 12990 points
    Hi Amal,

    Thanks for reaching out to us on this gate driver question!
    I see from the sch you are using FAN73711MX. Have you considered using UCC27714 or UCC27712? I recommend it because its the most similar and might help with the issue you are seeing thanks to a detailed datasheet.

    Check out section 8.2.2.2 from the UCC27714 datasheet about sizing the boot cap.
    Please let me know if you have any more questions.
  • 0 in reply to Jeffrey Mueller
    Expert 1140 points

    Hai,

    thanks for  your suggestion.I will surely try this ic.But my doubt still prevail.Why is the dc shift seen in the VS pin of the driver ic above120V.You can see that in the yellow channel of the dso output.You can clearly see that the voltage at VS pin is level shifted.What could be the possible reason.Can you please share me your opinion regarding this.

  • 0 in reply to Jeffrey Mueller
    Expert 1140 points
    Please can you see what could be the possible reason for the shift in the voltage reference level on vs pin.You can see the shift clearly on the DSO image...it is the Yellow channel.Why is the reference level rising after 120v not below 120v.The Blue line represents VB pin probed.
  • 0 in reply to Amal
    Expert 1140 points
    i will surely try using UCC27714 in my design.But i feel that without having clear idea of the reason behind level shift from reference only at 120v and above ,i cant complete this design.This issue may occur again while using UCC27714.
  • 0 in reply to Amal
    TI__Genius 12990 points
    Hi Amal,

    Thanks for your update,

    I believe this issue is due to the size of your bootstrap cap. You bootstrap cap should be 10 times your Ciss of the fet. Not only this but the Cvdd is to be 10 times your bootstrap. This is because all the driver is is a mechanism to slap charge from the Cvdd to charge the boot cap during LS on time then the boot cap slaps the fet with some charge during HS on time so the value of the cap doing the slapping should be 10 times the value of the cap being slapped....:)

    check out 5.2.2.5 Grounding Considerations
    www.ti.com/.../slua618.pdf

    can you tell me why you are using FAN73711MX? what advantage does it serve in your system?
    Thanks,
  • 0 in reply to Jeffrey Mueller
    Expert 1140 points

    Hai,

    Thankyou very much for your support.We had chosen FAN73711MX driver mainly looking at the floating channel bootstrap operation up-to +600v feature.And the cost was satisfactory.We already have IX4427NTR for low side driving.The maximum voltage which would appear across the H_bridge would be around 400V.Since we are designing it for a large scale production we opted for low cost drivers capable of driving High side alone.Do you have any suggestions regarding the gate driver ic.

  • 0 in reply to Amal
    TI__Genius 12990 points
    Ok Sounds good Amal,

    Just remember if you use our part you have me and the rest of TI will be there to support you the whole way. And as you can see we just want your system to work.

    For low side driver we have plenty of driver that should give you better efficiency with tighter edges from greater driver strength than IX4427NTR . Check out UCC272531 for example.

    Let me know how the 714 performs after you try it out.

    Thanks,