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LTCDriver Bottom Gate is not driving a square wave. Only Top Gate is firing

Category: Hardware
Product Number: LTC7060
Software Version: na

I am running this circuit at 20KHz square wave and will try to run it as high as possible in the future. It's a full bridge design. The LTC7060 has 8v supplied to it. For some reason only the TG of the LTC7060 is delivering a square wave pulse. The BG on each LTC7060 is not sending any pulse and is instead just a flat line. 

What should I try? What is the problem?

  • Hello,

    The schematic you sent is correct connection wise. Here are a few things to check:

    1. Check the PWM voltage. It should swing from a voltage less than the minimum BG turn-on threshold of 0.5V to a voltage greater than the maximum TG turn-on threshold of 3.6V. In a typical application PWM swings from GND to 5V.
    2. Check the left and right 8V bias supplies. To allow both circuits to work the 8V bias should be between 6V and 15V.
    3. Check the MOSFETs. The MOSFET symbols show an n-channel FET which is correct. However, the part number is for a p-channel FET which is obviously not correct.

    Best regards,

    Mike

  • Thanks Mike! I ended up driving the Square wave at 6v (vs. 3.5v it was at previously) and it worked! Thank you so much I really appreciate it!

    Additional Questions:

    1. I am running the LTC7060 board at 1.76MHz and want to go up to 2.65MHz (although I believe the specifications only go up to 1MHz. Do you have any recommendations for running the LTC7060 board at high frequencies?

    2. Additionally when powering the board with 7v and running a 6.4 volt 1.76MHz square wave as input to the two LTC7060 boards the power consumption is .106A and .762 watts. Do you have recommendations for lowering the power consumption

  • Hello,

    For most cases, optimal performance and efficiency can be achieved with a switching frequency below 1.0MHz. Operation above 1.0MHz is possible, however keep in mind the power losses will be higher. As shown in the curves below, the power consumption of the LTC7060 is a function of the switching frequency and gate charge.

             

    The power losses are dominated by the losses in the top FET and bottom FET gate drivers. The sum of these losses can be calculated as follows:

    PLOSS(driver) = (QGtop + QGbottom) * Fin * BGVcc

    To keep the switching frequency at 1.76MHz but with a lower power consumption in the LTC7060, then lower Qg MOSFETs will be required. The trade-off is typically higher Rds(on) which results in higher conduction losses. To achieve your overall efficiency goals, the switching frequency may need to be reduced.

    You mentioned that a 6.4V square wave will be the input to the two LTC7060 boards. Are you referring to the PWM signal? If yes, keep in mind that the PWM inputs have a maximum rating of 6V and are typically driven with a 5V signal.

     

    Best regards,

     

    Mike