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I am trying to choose a series resistor for a MOSFET (PSMN1R5-25YL) in order to reduce ringing, and I wish to know the guidelines for choosing the right resistance.

I'm not sure if it's relevant, but the application is two DC-DC converters, one is switched in 200KHz with 1A current from the gate drivers, and the other in 1MHz with 4A current from the gate drivers.

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Anything in the few Ohms to 10 Ohms range will work somewhat. A "nice" cure is to place a reverse biased Schottky between gate and source close to the FET and with minimum length leads. Any ground symmetric ringing will turn on the diode on negative going gate excursions and dissipate energy. Very high drive currents may give you faster switching edges than you need and increase overall power dissipation. You can tolerate a gate resistor that allows the gate capacitor to be charged or discharged in comfortably less than the needed gate switching speed. –  Russell McMahon May 7 at 16:05
    
Is the Schottky diode supposed to replace the pulldown resistor (which is connected between the source and the gate)? About the drive currents, I think they are high enough, considering the total gate charge, the rise time is below 5% of the pulse durartion in both converters. –  Edwin May 7 at 19:25
    
Would that be the Rgs that you don't show on the circuit diagram that you haven't provided nor mentioned heretofore neither? :-). ie as you can see, what may sound like an utterly trivial circuit becomes anything but in real world conditions and what you are REALLY intending to use might just possibly affect what REALLY happens. We haven't yet got to eg the ferrite bead on the source lead against the MOSFET body, but it will do no great use getting to such things if we don't have even all the resistors in place. | So ... -> –  Russell McMahon May 9 at 1:14
    
... If your driver is sinking and sourcing at the ~~ 1A level then do you need an Rgs as well. If Rdrive_gate is say 4R7 and if the driver output is low impedance is Rgs going to help? A: It MAY as the driver may tend to ring and Rdrive_gate isolates driver and gate and then Rgs damps the ringing. Maybe :-). If so, what is the value of Rgs and does it interact with the value of Rdrive_gate? As Rdrive_gate and Rgs form a voltage divider which reduces gate drive voltage and 'steals' drive current then if Rgs ~= Rdrive-g it has major bad effects and if >> Rdrive-g what does it achieve? –  Russell McMahon May 9 at 1:21
    
The reversed gs Schottky has no intended effect when Vgs is + ve whereas Rgs affects both half cycles. If Rgs has an intended effect on +ve half cycles it is not replaced by the Schottky. [Schottky adds reverse leakage and capacitance on +ve drive cycle] –  Russell McMahon May 9 at 1:23

2 Answers 2

up vote 4 down vote accepted

If you model the gate drive circuit as so:

schematic

simulate this circuit – Schematic created using CircuitLab

Where C1 is the gate capacitance of the MOSFET, L1 is the total inductance of the drive loop including package inductance, and R1 is the gate resistor plus resistance of the driver, then you can solve for the minimum value of the gate resistor that will result in a damped response that does not ring.

R1 \$\ge2\sqrt{\frac {L1}{C1}}\$

So if your input capacitance is 5nF, the loop inductance is 100nH,then R1 > 9\$\Omega\$ (subtract the minimum of the high/low driver resistances to get the minimum gate resistor).

In practice, you may not be able to predict the loop inductance all that accurately and you may have to select the exact value of the gate resistor empirically.

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Thanks for the detailed response! –  Edwin May 7 at 14:11

I assume by ringing you mean oscillations in the power path. These are caused by stray inductances in the power path and capacitances present in the converter (like MOSFET output capacitance).

Gate resistor current will impact the switching speed. The higher the switching speed the more voltage will be induced in the stray inductances. This voltage energizes the LC circuit causing it to oscillate after the MOSFET has switched (typically after the turn-off). To minimize these oscillations you either need to slow down the switching speed by increasing the gate resistor or optimize your converter (minimize stray inductances). Normally you would increase the inductance until your product complies with EMC standards but not too high not to cause more switching loss.

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