Gate resistor from mosfet. Buck converter

Thread Starter

Andu2508

Joined Aug 21, 2021
5
Hello
I have a buck converter with external transistors. I do not know how to calculate the value of these resistors ( R210 & R211) . If anyone could guide me, I would be grateful.

IC its TPS40195 (for buck)
And the transients are RQ3L050GN / RQ3E180AJ

Thanks !
1629538394470.png
 

DickCappels

Joined Aug 21, 2008
7,953
The values of those two resistors are not critical, the usually don't do mucl. I would try 20 or 22 ohms, which are typical values for gate resistors.
 

Irving

Joined Jan 30, 2016
2,308
The values of those two resistors are not critical, they usually don't do much. I would try 20 or 22 ohms, which are typical values for gate resistors.
Sorry, but no... In a high-power circuit they can be critical. TI says:

" External gate drive resistors play a crucial part in
limiting noise and ringing in the gate drive path.
Parasitic inductances and capacitances, high dV/dt
and di/dt, and body-diode reverse recovery can cause
unwanted behaviour without an appropriately sized gate
resistor "

Though you may get away with a random low value... too high a value can cause MOSFET overheating...

@Andu2508 More info on the subject here
 

Irving

Joined Jan 30, 2016
2,308
Also
R212/R214 aren't needed.

Is there a reason for using 2 different MOSFETs? - normally you would use identical ones.
 

Juhahoo

Joined Jun 3, 2019
249
Why do you add resistors to gate when Texas doesn't even have them on their circuits? If I was you, I would add a zero ohm resistor and IF you are having trouble and you source the problem to MOSFET, then you can at least do something.
Gate pull down is useless and none of the circuits have them since gate drivers are always push pull type.
 

Ian0

Joined Aug 7, 2020
3,750
Why do you add resistors to gate when Texas doesn't even have them on their circuits?
Because you never know if it is just a representative diagram from which they have been omitted for clarity, or whether the damping resistance is being provided by a particular sort of driver IC or elsewhere in the driving circuit.
The inescapable fact is that there must be some damping resistance in the circuit somewhere or it will oscillate at frequencies too high to be seen on an average scope, and the result will be a number of unexplained failures.
 

Irving

Joined Jan 30, 2016
2,308
Why do you add resistors to gate when Texas doesn't even have them on their circuits?
The example on the front page of the data-sheet is labelled " Simplified Application Diagram "! The need is dependent on many factors including board layout. The example layout in the data-sheet shows R6 and R8 clearly as gate resistors; they clearly listened to you and made adjustments to their board ...:D:rolleyes:
1629823063269.png

If you look in the supporting "using the TPS40195 evaluation module" its mentioned and the worked example shows gate resistors as per spec below.1629822553721.png
 

Juhahoo

Joined Jun 3, 2019
249
Because you never know if it is just a representative diagram from which they have been omitted for clarity, or whether the damping resistance is being provided by a particular sort of driver IC or elsewhere in the driving circuit.
The inescapable fact is that there must be some damping resistance in the circuit somewhere or it will oscillate at frequencies too high to be seen on an average scope, and the result will be a number of unexplained failures.
That's what I was typing, "add zero ohm resistors and if you have problems, then at least you can do something" No need for resistors unless you have a reason, which you don't know before testing it out.
 

Juhahoo

Joined Jun 3, 2019
249
The example on the front page of the data-sheet is labelled " Simplified Application Diagram "! The need is dependent on many factors including board layout. The example layout in the data-sheet shows R6 and R8 clearly as gate resistors; they clearly listened to you and made adjustments to their board ...:D:rolleyes:


If you look in the supporting "using the TPS40195 evaluation module" its mentioned and the worked example shows gate resistors as per spec below.
You left out much in your quote. I specially advised to add zero ohm resistors as option if you need to rework your circuit.
 
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