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UCC21750: Isolation bypassing

Part Number: UCC21750

Hi,

I would like to use this gate driver with a high voltage high-side switch with a bus DC voltage of 6kV. In the datasheet i found different isolation specs besides the 5.7kVrms but i am not sure about the compliance with my requirements.

Another solution i had is to use this isolated gate driver ignoring the isolation barrier, therefore i would connect the COM and GND terminals together to have a common ground. I would generate the isolation barrier before the driver through a isolated transformer to generate both the 5V for the logic side and the bipolar voltages for the power side, all referred to COM/GND. Is it a possible application?

If no, could you suggest another non-isolated driver with high current capability?

Thank you

Stefano

  • Stefano,

    What is the part number for the power switch you will be using? 6kV bus voltage is very very high and no gate driver has enough clearance to meet that isolation requirement for safety. Please double confirm your bus voltage is really 6kV

    The IEC 60664-1 dictates the creepage/clearance requirements for CE compliance, which for a working voltage of 6000V is approximately 30mm which between 3-4 times larger than what even our reinforced isolation drivers provide. 

    I would like to use this gate driver with a high voltage high-side switch with a bus DC voltage of 6kV. In the datasheet i found different isolation specs besides the 5.7kVrms but i am not sure about the compliance with my requirements.

    UCC21750 isolation is rated up to 2121V working voltage and would be unsuitable for this application without additional isolators.

    Another solution i had is to use this isolated gate driver ignoring the isolation barrier, therefore i would connect the COM and GND terminals together to have a common ground. I would generate the isolation barrier before the driver through a isolated transformer to generate both the 5V for the logic side and the bipolar voltages for the power side, all referred to COM/GND. Is it a possible application?

    This also works, but note that the requirements for the digital isolator are also extremely high. To my knowledge no single isolation IC exists which provides 30mm of creepage. 

    If your DC working voltage is truly 6kV, you might want to look at fiber optics for the digital communication which can easily provide the isolation you need here. 

    DC-DC modules suitable for 6kV operation are available off the shelf to my knowledge. 

    If no, could you suggest another non-isolated driver with high current capability?

    Our non isolated drivers do not have short circuit protection and other features like UCC21750. 

    For this application i do not think they would be suitable. 

    Please let me know if you have anymore questions. 

    Best

    Dimitri

  • Hi Dimitri,

    Thanks for your reply.

    The power switch we have and want to use is the FD250R65KE3-K from Infineon rated for 6.5kV and we want to use it with a bus voltage up to 6kV for generating  high voltage pulse train for testing of inductive loads. Currently we have a prototype in our lab which can drive correctly (as far as we tried) this module with a gate driver circuit using the UCC21750 with a bus voltage of 5kV.

    If using the driver ignoring the internal isolation connecting COM and GND and providing a proper external isolation (e.g. fiber optic as you suggested), I could try this solution. I had in mind this solution just because we already used that driver IC in previous project and we have enough confidence.

    Anyway i would not use the integrated protection circuits, so a suggestion for another driver with similar current capability even without protection as I asked previously is well accepted.

    Concerning the isolated DC-DC, i found the  REC3.5-2415DRW/R10/A from RECOM, declaring a 10kVDC isolation voltage tested for 1s. On RECOM website i found  a conversion tool and a 10kVDC isolation for 1s corresponds to a 4.95kVDC continuative isolation. Is it a reliable device for my scope? Are the DC-DC module you mentioned  similar to this or you had in mind other devices?

    Have a good day.

    Stefano

  • Stefano,

    If using the driver ignoring the internal isolation connecting COM and GND and providing a proper external isolation (e.g. fiber optic as you suggested), I could try this solution. I had in mind this solution just because we already used that driver IC in previous project and we have enough confidence.

    Connecting COM/GND is OK and driver will function properly.

    Concerning the isolated DC-DC, i found the  REC3.5-2415DRW/R10/A from RECOM, declaring a 10kVDC isolation voltage tested for 1s. On RECOM website i found  a conversion tool and a 10kVDC isolation for 1s corresponds to a 4.95kVDC continuative isolation. Is it a reliable device for my scope? Are the DC-DC module you mentioned  similar to this or you had in mind other devices?

    I would say this should not be used here based on the RECOM isolation rating and 14.2mm min creep/clear from datasheet make this unsuitable for providing isolation voltage. 

    Since this is a RECOM part, I suggest to reach out to them. My suggestion is that this would not be recommended by them since it could not provide safety isolation at 6kV DC

    They would be able to better comment and may have other solution. If it is possible for you to generate the bias supply voltage on the secondary side alone without going from primary to secondary, this would eliminate the creepage and isolation rating limitation of the recom module. 

    Other things to consider would be potting/sealing the leads or conformal coating plus adding a slot across the isolation to increase clearance. 

    Since our parts support DC voltages of ~2kV, I don't have expertise or experience with far higher voltage applications like yours, but If you have any other questions please let me know and I will try to help. 

    If this answers all your questions for now, please let me know by pressing the green button. 

    Best

    Dimit

  • Thank you very much for the quick response.