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LM5113: Question regarding driving floating GaN device

Part Number: LM5113

Hello,

I am using the LM5113 gate driver to drive an EPC2034 in a DC-DC converter. The IC is connected as required and an isolated DC-DC converter is used to provide 5 V voltage for the IC. The Vss port of the IC is connected to the power ground. The IC only needs to drive one switch and thus LI, LOH and LOL ports are not connected. The source of EPC2034 is connected to a point with 5-10 V potential over power ground. It is found that the gate-source voltage of the device is negative and the device is not turned on. However, if the source of the switch is connected to power ground, the gate-source voltage is about 5 V and switch is turned on. In short, the IC seems not capable of providing an isolated gate-source voltage that ensures the turn-on of switch.    

I would appreciate any idea/advice on why this happens and how I can drive such a single floating GaN device. Thanks!

  • Hi, Liyao,

    Thanks for your interest in our products.

    The LM5113 depends on a bootstrapping technique to generate the gate drive voltage for the high-side FET. It requires the switch node to be pulled to VSS in order to charge the bootstrap capacitor, which is connected between HB and HS.

    Since your switch node, HS, is never driven to ground, the capacitor is never charged.

    Would it be possible for you to connect VSS of the IC to the same potential as your load? This would then allow the bootstrap to charge normally.

    The downside to this is, the logic inputs will be have to be level shifted to that potential as well.

    Best regards,

    Don Dapkus

    Gate Driver Applications Engineering Manager

    Dallas, TX USA

  • In reply to Don Dapkus:

    Thank you Mr. Dapkus for your reply! It makes sense to me and I will check if I can manage to do that. If I understand it right, for the IC to work with VSS connected to the load potential, I will need a VDD about 5 V higher than THAT potential and a logic input referenced to THAT potential as well. Is this right?

    In addition, if I do this, shall I use HOH/HOL or LOH/LOL to drive the device? Or will they be both OK?

    Thanks again!
  • In reply to Liyao Wu:

    Hi, Liyao,

    Please call me Don!

    I am not sure I follow exactly what you're trying to do. Can you make a drawing and upload it so I can help you with the connection?

    Best regards,

    Don Dapkus

    Gate Driver Applications Engineering Manager

    Dallas, TX USA

  • In reply to Don Dapkus:

    Hello Don,

    Please check below the two drawings that can better explain my question. I assume we have a load voltage of 20 V over the power ground. According to your suggestion, I am now using the load voltage as the VSS input. I hope to check three things. First, in this case, do we need to use a VDD around 25 V to make the IC work properly? Secondly, should the HI or LI input also carry a logic signal with load voltage (20 V) as the reference level (e.g., 25 V when high and 20 V when low)? Thirdly, will there be any differences between the following two ways of connection, i.e., using HOH/HOL and using LOH/LOL to drive the device?

    Thank you for your time! I hope this makes my question clearer. 

    Sincerely,

    Liyao Wu

     

  • In reply to Liyao Wu:

    Hello Liyao,
    thanks for providing the diagrams as reference.
    To answer your questions:
    1.VDD would have to be 5V above your VSS, so in your case with VSS at 20 you are correct in setting VDD at 25V.
    2.Both HI and LI are referenced to VSS, so again you are correct: 20V would be "low", 25V would be "high" .
    One suggestion is that instead of grounding the other one, you should connect it to VSS.
    3.If you are using HOL HOH you would need to ensure that the bootstrap cap is charged up to, which in this case it would be.
    In this particular application there should not be a difference in operation.
    I would recommend to use the low side (LOL LOH) as the supply voltage does not have to go through extra elements (bootstrap diode)before powering the FET.
    If this is your application though, I would suggest you to used a low-side gate drive like the LM5114.
    If the reason you picked the LM5113 is because you require level-shifting the logic input then the best way to use this is as shown in figure below (differences circled in maroon).
    Best regards,
    Alberto