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When connecting a low-side FET gate-driver to a FET (N-channel MOSFET) that has a current sense resistor connected between its source and ground, where should I connect the ground of the gate driver?

I am of two minds on this. First, all of the data sheets that I have looked at do not show a current sense resistor on the FET's source. As a result, the gate-driver has its ground connected to the source. Because it is the capacitor between the gate and source that I am charging/discharging with the driver, I am inclined to connect the gate driver's source, as this way the sense resistor will not degrade di/dt into the gate. However, if I connect the gate driver's ground to high side of the sense resistor, that means that the reference for the input to the driver will move with source current. If the sense resistor is small, then I should not expect the driver's ground to move much and as long as I make sure my driving signals take this into account it should not be a problem.

Long story short, should I connect my low-side gate-driver's ground to the high side or low side of the current sense resistor that is between the FET's source and ground?

Edit: I was asked what this circuit does. This is an active dump load for a tether-operated remotely operated vehicle. In order to provide sufficient power to the vehicle, the operator side power supply will need to increase its output voltage at times to compensate for tether losses. This circuit monitors the voltage at the vehicle-end of the load and diverts power to a power resistor when the vehicle-side voltage exceeds a limit in order to keep the input voltage to the vehicle within an acceptable range. The circuit also measures current through the power resistor and communicates this (as well as vehicle input voltage) to the power supply's controller at the top end so that the power supply can be commanded to reduce its output voltage as necessary. Conversely, when vehicle input voltage drops too low, the power supply is commanded to increase output voltage. The FET controls current through the power resistor via PWM.

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  • \$\begingroup\$ The location of the current sense resistor determines what current you are sensing. Unless you have a compelling need to measure the current used by the gate driver, I recommend that you place the gate driver ground on the GND side of the current sense resistor, not the FET source side of the resistor. The routing and heat sinking of the FET will likely be more complicated if the source net has a lot of nodes. \$\endgroup\$
    – user57037
    Dec 20, 2019 at 0:05
  • \$\begingroup\$ Can you say a bit more about your application? What is the purpose of the current sense resistor? What does this circuit do? DC-DC conversion? Motor drive? Etc. \$\endgroup\$
    – user57037
    Dec 20, 2019 at 0:06
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    \$\begingroup\$ "Because it is the capacitor between the gate and source that I am charging/discharging with the driver --> no, the driver's job is to handle Cgd \$\endgroup\$
    – bobflux
    Dec 20, 2019 at 1:01
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    \$\begingroup\$ Given the edit, I would suggest that you put the driver circuit ground on the low side of the current sense resistor. I do not know if you are using SMT components. But if you are, their ability to dissipate power is greatly influenced by the size of the copper structure to which they are soldered. So the node between FET source terminal and current sense resistor deserves special attention. \$\endgroup\$
    – user57037
    Dec 20, 2019 at 19:48
  • \$\begingroup\$ I don't know why you cant put in a partial schematic here to understand that this problem is a non-issue if you try to understand my answer. Vs should not be more than 100mV worst case which is trivial compared to Vgs \$\endgroup\$
    – Tony Stewart EE75
    Dec 21, 2019 at 15:35

2 Answers 2

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Because it is the capacitor between the gate and source that I am charging/discharging with the driver, I am inclined to connect the gate driver's source, as this way the sense resistor will not degrade di/dt into the gate.

If the current sense resistor is small enough it won't have a significant effect. And it should be small, to reduce conduction losses. The changing Gate charge at turn-on will cause a spike in the sensed current, but this can easily be ignored. In most cases the Gate drive will already have some resistance (to limit Gate current and prevent ringing) which will probably have more effect on di/dt into the gate.

However, if I connect the gate driver's ground to high side of the sense resistor, that means that the reference for the input to the driver will move with source current.

That's not good. Many Gate drivers have a logic low input voltage of only 0.8V, which could be compromised by voltage drop across the sense resistor.

One way around this is to 'float' the entire control circuit, or (essentially the same) monitor current in the negative supply lead. This works when the power source is a battery or dedicated power supply, provided controller current is insignificant compared to motor current (or can be subtracted from the reading). However it may not be practicable if the negative supply ground is shared with other equipment. It also distorts the current waveform as the bulk capacitors smooth out supply current, so it might not be suitable for precise 'cycle by cycle' peak current limiting.

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The current sense voltage drop should be the same as most other current sensors in the 50mV range to prevent self heating with a suitable small size such as 1/4, 1/2W and also prevent the Vg shift issues you suggested.

You can use Op Amps with Pch or PNP inputs to sense voltage to 0V on single supply IC’s for low side current sensing and regulation.

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