LM5113: Negative Output Bias with the LM5113

Part Number: LM5113

One good solution to mitigate ring/oscillation with GaN FETs is using a negative gate bias (-3V) for turn-off. You have devices that accomplish this (LM5110), however, I strongly desire the half-bridge driving capability of the LM5113

Is it possible to configure the LM5113 to drive with a negative gate bias? For example, could I simply just lower the ground potential of the gate driver 3V relative to the half-bridge ground, essentially treating the "GND" pin on the LM5113 as a "VEE"

Appreciate the support!

16 Replies

  • Hi Joel,

    I have a couple of points I would like to draw your attention to before attempting to separate the grounds:

    1. Separating the two voltages, it would make the gate loop inductance a lot larger, which might defeat the purpose as you would most likely experience an increase in the ground bounce.

    2.Whatever negative ringing you would get on the gate , is pushed further negative by this bias,which leaves a smaller margin to the negative abs-max for the Vgs.

    The preferred ways to mitigate  gate ringing/ accidental turn-on are:

    1. Design the gate loop as small as possible, by using microvias and a layer2 return with a layer-to-layer spacing of less than 5mils, you can get 3-5nH of inductance, which reduces the ringing

    2.add some resistance to dampen out the LC and slow down the slew rates slightly

    3.if the miller ratio is unfavorable, you can add some small capacitance from gate to source (generally this is not a preferred option!)

    Best regards,

    Alberto

  • In reply to Alberto Doronzo:

    Joel,

    another consideration is that the LM5113 ratings are referenced to ground. If you put a negative voltage on its ground, you will be lowering the positive Vgs voltage available to the FET.

    Best regards,
    Alberto
  • In reply to Alberto Doronzo:

    Alberto,

    Both are very good considerations to make. My hopes that there was a half-bridge driver equivalent to the LM5110

    Regards,

    Joel

  • In reply to Joel Barker:

    Hi Joel,
    I understand you were looking for a GaN version of LM5110, unfortunately this is not available.

    Following the guidelines I've mentioned in my previous response you should be able to mitigate ringing issues.

    If you are seeing something in your design that is not behaving as expected I would be happy to have a look at it.

    Best regards,
    Alberto
  • In reply to Alberto Doronzo:

    Hey Alberto, 

    The GaN devices we are using has an Rds(on) value that is dependent on the gate drive voltage. So, when driving it at 6V we get a better Rds(on) value (~7% better than 5V). 

    When the LM5113 is supplied with 6V, the lowside outputs 6V to the gate, but the bootstrap clamps the high side driver at 5.2V.. is there a way around this?

    Thanks again for all your help!

    -Joel Barker

  • In reply to Joel Barker:

    Hi Joel,

    I want to start with reminding you that connecting 6V to Vdd is above the recommended operating condition maximum of 5.5V.

    Operating at higher voltages might affect operation and lifetime and any assurance on the part (i.e. you are operating it outside specifications).

    Now, if you want to experiment with a higher voltage my recommendation would be to use an external schottky diode to bootsrap Vdd to HB, and add an LDO clamp in series after the diode, to limit the voltage seen by the HB cap to whichever level you decide to select.

    If you do not want to add an LDO, a zener clamp across HB would provide similar protection. In this case you would have to add a 2-5ohm resistor in series with the bootstrap diode to limit peaks.

    Best regards,

    Alberto

  • In reply to Joel Barker:

    Doesn't this conflict with the internal clamping of the LM5113? Are you able to provide a diagram showing the method that you had in mind?

    -Joel
  • In reply to Joel Barker:

    Hi Joel,

    please see below. In red is what you would need to add. Either the LDO or the zener (and add a resistor in series with the schottky).

    Best regards,

    Alberto

  • In reply to Alberto Doronzo:

    Alberto,

    I've tried your recommendation, however, despite my efforts I can not get the high side to drive above 5.2V.. I attached an image of one of the simulations I have done. 

  • In reply to Joel Barker:

    Hi Joel,

    I will look into this and answer to you shortly.

    Best regards,

    Alberto