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LMG1210: Can't fully turn on the GaNFET

Asfan Fiirizky
Prodigy 20 points
Part Number: LMG1210

Tool/software:

Hi everyone, 

I just recently build a half bridge for motor driver using LMG1210 and GAN7R0 as the switch. Here's our schematic : 

The issue was, the output driver was pretty much perfect, which can be seen on yellow signal (Vp = 4.8V). However, the measured voltage between switch node to GND (Red Signal), indicates that the FET isn't fully turned on, there's hundreds milliVolt across the LowSide FET. The dropvoltage increases proportional with Vbus. The dropvoltage also the same when I increase or decrease the frequency. 

The picture above captured when this condition:

  • Vbus = Vin = 12V
  • Load = 20 Ohm (Resistive)
  • Frequency = 20 kHz.

Hence, there's a suggestion that would be highly appreciated. 

Thanks, everyone.

  • 0
    TI__Genius 9790 points

    Hello,

    What is the current through the transistor and sense resistor? The sense resistor below the low side GaN FET could be raising the source voltage and lowering the VGS voltage that the transistor sees causing the FET to not fully turn on.

    VSS of the gate drive might need to be connected to the low side GaN FET source above the sense resistor.

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 20 points

    Hi Walter,

    Appreciate for your answer. I do agree with you that the VSS of the gate drive should be connected directly to the source, that's my fault. However, take a look at the gate signal (yellow line) the FET is fully charged by looking at the stable output after miller plateau. That's approximately ~4.8V which on datasheet on their test that lowest Rds ON can be achieved at 5V. 

     I just recently measured the Vgs with 2 different reference, 1st one at directly to the source, 2nd one to the GND. The result was the same with the picture above. 

  • 0 in reply to Asfan Fiirizky
    TI__Genius 9790 points

    Hi,

    The sense resistor could be affecting the measurements. What is the voltage across the low side current sense resistor and what is the size of the resistor? Is it 0.01 ohms? 

    Have you tried replacing the FET?

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 20 points

    Hi Walter,

    Yes, it's 0.01 ohm. I've replace with other like a new FET (still the same type GAN7R0). But the result was the same, nothing different. 

    At this moment, I would like to change the layout, to make the Vss directly connect to the source of the LowSide FET. Let me tell you soon, when it's done.