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I've seen a couple application notes suggesting a gate to source resistor being added to help with depleting the gate charge when trying to turn off MOSFETs.

At the moment I have an H-bridge that is controlled by two gate drivers and I am having some issues (which I think are just connection / solder joint issues) that end up killing a particular top-side MOSFET. The top-side MOSFETs are driven with a boot strap circuit which performs well in 13/15 of the PCBs I have had manufactured.

RGS circled in blue:

enter image description here

I don't think it is a good idea to add the gate to source resistor on the top-side MOSFETs because then there would be a path for current back to the other H-bridge half top-side gate. See this article for suggestion of adding this resistor.

I have also seen application notes suggesting the use of a diode in series with a resistor roughly half the value of the gate resistor like so (circled in blue) to help with turning off MOSFETs and am wondering if this might improve my circuit design and possibly prevent the failures I am seeing in the top-side MOSFET given my particular layout with gate drivers driving the MOSFETs with a boot strap circuit. App note I saw using this method.

enter image description here

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  • \$\begingroup\$ Rgs is there only to ensure M1 is off when power is applied. What is L2? Where is the rest of the H-bridge? Adding this info to your question should attract better answers. I'll delete this comment later. \$\endgroup\$
    – rdtsc
    Oct 19, 2022 at 18:03

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The gate-source resistors are good practice even if they are silent most of the operating time. They are usually not there to help switching but they keep the MOSFET off in case the gate connection is left open during the manufacturing process. Assume a daughter-board hosts the drivers and the MOSFETs are accessed via connectors. If a solder joint is bad during assembly, having both MOSFETs off by default via their respective gate-source resistors prevents a failure at power-on during the test. No need to go down as low as 10k as too low a resistance obviously affects driving losses. A 22-47-kOhm value is usually adequate:

enter image description here

For the other resistors, their values depend on the application. In a half-bridge configuration driving a motor for instance, hard-switching is very common and you want to avoid stressing the conducting body diode by reducing the di/dt imposed by the other MOSFET turning on abruptly: the series resistor helps slowing down the transition. It also helps damp the resonating drive path and must be located closely to the gate.

Same philosophy for the turn-off resistor in series with the diode. Unlike the turn-on sequence, you want to quickly turn the transistor off and limit the losses during the transition. A low-value resistance and the diode make this happen quite easily. The whole circuit you've shown in a classic in switching converters.

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