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ISO5851: SiC MOS Gate Drive Help Needed

John Currie
Prodigy 185 points

Part Number: ISO5851
Other Parts Discussed in Thread: ISO5852S, , TIDA-00446, TIDA-00917, TIDA-00195, TIDA-01606

I need to develop an isolated high side gate drive for SiC MOSFET.  I am looking for a smart chip that provides isolation, high CM rejection, Desat, Miller clamp, 2 level soft shutdown with the ability to also accept an input from a current sense circuit that if the level from the sense circuit is above a threshold the chip will begin a shutdown or fault.  It needs to be able to output if it is in a fault or undervoltage or over temp condition and have the ability for me to reset it.  I plan on taking this drive up to 500 kHz so the desat and fault report times need to be fast.  I plan on driving MOSFET single modules and half bridge modules in high frequency AC-DC-AC inverters with gate charges around 1100 nC so I will make it a dual channel gate drive with two identical drive circuits on it but isolated from each other.  I will use parallel BJT current booster between the smart chip output and the SiC MOS gates.  I don't know what TI offers as a suitable chip so I just picked one for the title part number.  Any suggestions and reference designs would be greatly appreciated.

Thank you - John

  • 0
    TI__Intellectual 1665 points

    Hi John,

    ISO5851 has most of the functions like high voltage isolation up to 1.5kVrms working voltage and 5.7Vrms VISO, high CMTI of 100V/ns min, active Miller clamp circtui, and DESAT protection. The DESAT pin can be connected to the output of the current sense circuit to implement over current or short circuit protection. The DESAT threshold value is around 9V that the current sense output needs to be properly set. In case of DESAT detection, ISO5851 would pull-down the driver output and report the fault to /FLT pin. The fault can be reported within a couple of micro-seconds after fault detection. Supply voltages are monitored with UVLO functions. Power supply UVLO faults are reported to RDY pin. ISO5851 does not have soft turn-off function. So I would recommend ISO5852S which has soft turn-off function and all the functions ISO5851 does. That said, ISO5852S seems to be able to support most of the needs of your application. ISO5852S is a single channel gate driver. So you will need two ISO5852S to drive a half bridge circuit. I am not quite sure about the over temp protection function. Could you please elaborate more?

    With Regards,

    Xiong

  • 0 in reply to Xiong Li
    Prodigy 185 points

    Xiong,

    Thank you for your reply.  Regarding temperature protection; I have seen on some of these smart chips that the manufacturer has self protected the chip by shutting it down in the event that the internal temperature of the chip gets too high.  So you think the ISO5852S will best meet my needs.  Are there any reference designs that I can see that would help me in design and layout for that chip?

    John

  • 0 in reply to John Currie
    TI__Intellectual 1665 points

    Hi John,

    ISO5852S does not have integrated thermal shutdown circuits. We have lots of reference designs with ISO585X gate drivers. You can search for TIDA-00195, TIDA-00446, TIDA-00917, and TIDA-01606 for more details. Please let us know of any questions.

    With Regards,

    Xiong

  • 0 in reply to Xiong Li
    Prodigy 185 points

    Xiong,

    I was needing a current boost stage between the 5852S chip and the SiC MOS. I need about +/-30 Apk. I saw in TIDA-00917 that they used a current boost stage but very little information.  I was wanting to parallel 2 or 3 such booster stages together and so I am inquiring about how to do that.  I was looking at Diodes #ZXGD3006E6 and I would parallel 3 of them.  I assume that when I parallel the stages together I would need separate input resistances and output resistances to help current sharing. The output resistance would be my gate drive resistor but if I have to parallel the total amount of resistance for each stage that is going to take a lot of room and resistors for the paralleled value to equal want I need for total gate resistance. Maybe I could parallel just a portion. For example, if I want 3 stages, and I am wanting 10 ohms as total Rgate then could I parallel just a portion of that in each stage; like leave 7.5 ohms bulk but then put 7.5 ohms in each stages output? Their parallel resistance would be 2.5 then added to the 7.5 bulk would give me 10. I assume I would do something similar on the input of each stage but I don't know how to figure it. Can you assist me?

    Also, I am needing +19/-5 V gate drive and it seems I saw one of your app notes that showed an on board power floating power supply but it seemed pretty low watts.  I need a similar supply but more like 20 - 30 watts.  Are there any documents that could help me in that regard?

    Thank you - John

  • 0 in reply to John Currie
    TI__Intellectual 1665 points

    Hi John,

    As you pointed out, it is better to use separate resistors on the base of the BJT buffer and gate resistors for better current sharing. I would not recommend to use a single 7.5 Ohm resistor as this may over heat the gate resistor. 

    Regarding the power supply, I would recommend you to use TI online power supply design tool Webench for assistance. Flyback and half bridge converters can be good topology options for your application. 

    Please let us know of any further concerns.

    With Regards,

    Xiong