MOSFET Gate Drive Resistor Selection - Part 1: Turn On - YouTube
https://www.youtube.com/watch?v=fsgKpAq2gd0

Transcript:
(00:00) hi my name is Alicia from breacher digital and in this video we're going to talk about why we need a Gate Drive resistor when we design our power supply and how to choose a good value [Music] so first let us consider case whereby we have zero ohm as our gate driver resistor here I have got a mosfet internally there is some gate resistance let's call that RG and then here is my gate driver internally there is some resistance but it changes whether the Gate Drive Chip is driving high or low so let's call this R high and let's call
(00:43) this R low and then here I have my Gate Drive resistance so let's call this r g d as my my Gate Drive initially let's assume that we put in zero ohms here as you can see now this is the fastest way that we can we can charge the input capacitance of the fet in order to turn it on the advantage is that the switching transition will be very fast and your switching losses will be lower however there are several disadvantages with this first of all you can see that there will always be some parasitic inductance and other capacitances and
(01:21) and if the damping our resistance is near zero this is going to ring so when we send the signal in order to turn the fet on for example it is going to look like this that is time and that is the V out of your Gate Drive now this has several disadvantages first of all this spike is going to appear on your noises Spectrum when you go to the EMC test and that is why often we try to slow down the fets secondly the amount of power that is being dissipated because this resistance is low is entirely almost entirely in the
(02:05) Gate Drive Chip and a little bit in the mosfet resistance so the Gate Drive will get hot so we would like to put some resistance here so that we dissipate the heat not all of it inside of the chip but some of it in the Gate Drive resistance so now let us consider K Square by instead of zero ohms we actually put a lot of resistors 100 ohms here in this case now the turn on of the mosfets become very very very slow and of course the switching losses will be very large and we've got another video about the Miller plateau and switching losses
(02:40) um but that talks about this but in our case now this is clearly too big so there has got to be a nice sweet spot between zero ohms and 100 ohms whereby we have got a Optimum value of the Gate Drive resistor now there is an excellent paper by Laszlo ballock whereby he gives you an equation to calculate RG Bay or rgd your Gate Drive resistance based on RG which you can measure or high on our Arlo which you can also measure under Trace inductance which you can try and measure however these measurements are hot if you can measure these then you
(03:17) can use this equation 15 the link the URL is in the description uh you can you can use that equation and the paper if you you cannot measure these then we're gonna have to do it empirically and find a nice compromise we know that at zero ohms we're gonna get a spike and at 100 ohms it's going to be really slow and in fact we're going to see the um the the Miller Plateau so um all we have to do is try different values of rgd in order to damp this so that it's slightly under that so that would give us a q of one so basically
(03:53) what I would like is something that will look like that yeah we start with value of 10 ohms and we look whether the spike has done gone up or down and we change the value until we get a nice value and that is the value of your Gate Drive resistance so now let us go to the lab and we're going to show you in real life how it works out okay so here we are in the lab uh here I've got a flyback converter this is a type of board that we use um in some of our workshops and what we do here is that we've put some uh jumpers and we
(04:30) can change the value of the gate resistor and then on the oscilloscope we can have a look at the impact of this so what we see right now is zero ohms worth of gate resistance and you can see that there is this big spike here however the rise time is really quite low so it is turning on in around 30 nanoseconds now what I'm going to do next is I'm going to replace that gate resistor with a 100 ohm resistor and you can immediately see how slow it actually becomes now here is your Miller Plateau it's very very slow
(05:03) the rise time has gone to 600 nanoseconds and you can clearly see the shape of this capacitor charging now that is the gate Source capacitance of the mosfet charging through a very big resistor let me put that back in we're going back to zero ohms you can see this Spike that clearly is not a good thing and as we said earlier on what we try to do is get to a sweet spot whereby the value of the resistance just gives you a slight overshoot not too much and and and not to that okay so now let us change the value to 100 ohms set a big
(05:38) opponent to 10 ohms now this is 10 ohms and you can see here that is a slightly overdamped our rise Time Has Fallen to 600 so a bigger pardon to 80 nanoseconds from compared to the 100 ohm one this is fast but compared to the zero on one this is very slow the bigger issue is that it is clearly overdamped uh if I zoom in a little oh the other way you see this shape here shows the Overland okay so now let's go down to perhaps uh seven and a half ohms let me put that there and take that out okay this is now seven and a half ohms you
(06:30) see the Miller Plateau there Rise time has become even faster at 56 Nanos compared to the 10 ohm and but it still is a little bit overdamp let me zoom in just a little bit more you can see that and finally let's go to around 5 ohms and you can see now that the rise time is 45 nanoseconds it's the slower than zero ohms it's faster than all the other resistors it is slightly under damped that in my opinion is a good thing and it is uh turning on and turning off beautifully without any ringing at the optimum value
(07:13) obviously you could go and try to measure the gate resistance and on and off resistance of the of the Gate Drive and so on but if you need to get something working very quickly this will be a way that uh you get to the to the good value of your gate resistor so uh I hope you have enjoyed the video and I hope to meet you in one of our workshops
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