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UCC27614: IN-, IN+ not referenced for GND

Jimmy Palmer
Prodigy 80 points

Part Number: UCC27614
Other Parts Discussed in Thread: UCC57108, UCC57102

Tool/software:

Hi, 

I'm interested in using the UCC27614 with a negative GD for SiC MOSFET. Do I need to reference the IN- and IN+ to the UCC27614 GND or could I offset these by +5V? 

  • +1
    TI__Expert 8791 points

    Hi Jimmy,

    The input signals will need to use the same reference as the gate driver GND. Otherwise, the output will be stuck high.

    See this FAQ to learn how you can level shift the input signal to utilize bipolar drive for the UCC27614: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1083720/faq-ucc27614-how-to-drive-a-sic-fet-or-igbt-with-a-negative-bias-using-a-low-side-gate-driver-bipolar-drive

    Alternatively, we have the UCC57108, which can do bipolar drive without the need for external circuitry.

    Thanks,
    Rubas

  • 0 in reply to Rubas Khalid
    Prodigy 80 points

    Hi Rubas, 

    I checked the UCC57108, but it seems like it has the same verbiage as the UCC27614. Do you mind giving a little more information why the UCC27614's IN+ and IN- cannot accept a different ground reference?

    Here's the bit I'm referring to:

    "The inputs are independent of supply voltage and can be connected to most controller outputs for maximum control flexibility"

  • 0 in reply to Jimmy Palmer
    TI__Expert 8791 points

    Hi Jimmy,

    The bit you are referring to is stating that the input signal voltage does not need to follow VDD voltage. Some gate drivers require the input voltage to be within the voltage of VDD, but this is not the case for the UCC27614.

    To explain the concept of different GND references, let's first state that the gate driver has a GND pin, which acts as the driver's reference point as a whole. With this point being made, let's start with the typical application of setting GND = 0V and using VDD as a simple example. In the datasheet, the UCC27614 has an absolute maximum VDD rating of 30V. If we set VDD = 40V, the absolute maximum rating of VDD is broken. However, if we set GND = 10V, the potential difference between VDD and GND is now 30V, which does not exceed the absolute maximum rating of VDD. This concept also applies with using a negative bias on GND, with an example being that setting GND = -10V will only allow a max VDD = 20V to remain within the 30V absolute maximum VDD rating.

    Applying this concept to the input signal, if we keep gate driver GND = -10V and use a typical input signal of 0-5V (0V being the "GND" reference for the input signal), the gate driver will reference that 0-5V signal to its own GND of -10V. This means that the 0-5V signal is equivalent to 10-15V in a gate driver that has GND = 0V, and it will result in the output being permanently high. The FAQ I sent in my previous response shows how you can circumvent this with external circuitry, and essentially make that 0-5V signal into -10 to 5V signal for the gate driver to operate properly.

    The UCC57108/UCC57102 has it's own separate VEE pin (also referenced to gate driver GND), which allows for negative bias without external circuitry like the FAQ shows. For example, you can set VEE = -10V and VDD = 30V, but keep GND = 0V and IN = 0-5V. Because the GND = 0V, the VDD potential difference is still 30V and not 40V like in the UCC27614 example. This results in a -10 to 30V output signal without external circuitry to modify the 0-5V input signal to become -10 to 5V.

    Please let me know if this helps or if you need more clarification.

    Thanks,
    Rubas