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UCD7138: UCD7138

Part Number: UCD7138
Other Parts Discussed in Thread: UCD3138

While going around the forum, I bump into this conversation about "3rd quadrant conduction"

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Hi Jongwan,

yes you are correct, the third quadrant operation is similar to a body diode conduction as far as current direction and gate state are concerned.

GaN HEMTs don't have a body diode, therefore this mode is caused by the gate voltage being biased by Cdg/Cgs  which turns on the FET in the linear region and keeps the voltage at the Drain constant.

In the LMG5200 this voltage is at 2V.

Therefore if you have current flowing from the source to the drain while the gate is off, you can equate this with a body diode that has 2V of forward bias. This mode of conduction is suboptimal, and want to minimize this by turning on the gate as soon as possible, thus minimizing the dead time.

Best regards,

Alberto

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I can extend my imagination to internalize this event of turning ON the gate drive as soon as the 3rd quadrant spontaneous conduction has started- as in a diode-- but still, current flows from source to drain, no matter.

In MOSFETs perhaps there is symmetry in the enhanced channel to create a much lower impedance path for the current but not sure this is true for GaN. Because, no matter, Vds is negative.

So how does the SYNC rectifier operate in low conduction loss mode with Vds negative? If this is so, I conjecture that there are transfer curves with very low -Vds with  +Vgs that behave similarly to the 1st quadrant conduction.

If there is Physics of this mode , then we will be greatly relieved: we have several designs that blindly follow this hook up without a full understanding of the mechanism of how it works.

  • if your question is how UCD7138 works, it is simple, it measures mosfet body diode conduction time, and report to controller UCD3138, then UCD3138 adjusts the deadtime to optimize the sync operation. For more detail, please refer UCD7138 datasheet.

    if your question is related TI GaN, I will ask an expert from GaN team to help you. 

  • thnx much Bosheng.

    I have discussed this issue with EPC2024 app engineer at EPC & his detailed explanation was amazing and elegant. & of that, I was not aware.

    So we can close this issue.

    That said, it is always educational to hear TI experts' positions.

    So we will also read what he/she has to say.

    -robin