Calculating A FET Gate Resistor Part 1 & The Basics of A FET Transistor (Read 'More')

hi welcome to another video today i'm going to see if we can work out the resistance of a gate resistor required for a specific circuit so this circuit in front of you an amplifier beyond economical repair i'm using a current mode pwm controller runs on 15 to 17 volts it's driving this fet via 150 ohm resistor what i'm hoping to do is give you some simple equations to find the value of your resistor so this circuit is actually switching this led high power led on and off 85 000 times a second so 85 kilohertz it's connected to my power supply with some long leads i'll go into that later and my oscilloscope right here's my schematic of what you've seen just now so i've got 15 to 17 volts high power led with a couple of inline resistors this is my uc 3842 current mode pwm controller this is my fet irf-840a they quote a total gate charge of 38 nano coulombs don't let the nano coulombs worry you i'll go into that in a second i'm keeping this simple the workings of a fet are far more complicated than i am going to describe today but i'll give you the basics for hobbyists and how to work out the value of a gate resistor so on my end general mosfet i've got the drain the source and the gate the led is connected to the drain so when the fet transistor is not turned on this drain will sit up near 15 volts minus the voltage drop across this led when the fed turns on it will pull this strain down to ground minus a few milliamps which is the rds on so to turn on effect you basically have two parasitics one capacitor connected to the gate and the source and another connected to the gate and the drain if you have a look at this top capacitor it's usually much greater than this one and because the value is greater and the way it performs i'll show you on the scope they also refer to its effect as the miller effect but i'll show you on the scope it'll make sense so i said i'll keep the maths easy basics as a guide only the real stuff is way more complicated and it can get more complicated so if the total gate charge is measured in nano coulombs and it is divided by a switching time in nanoseconds the result will be in amps so that's easy isn't it so if you've got a gate charge of 10 nano coulombs and you want it to switch on in 100 nano seconds you need to supply 100 milliamps show you on a calculator so 10 nano coulombs divided by 100 nano seconds equals 0.1 amp or 100 milliamps so one coulomb is the charge in a circuit passing one amp over one second so have a look on the internet a coulomb is the amount of electrons passing in a circuit at one amp over one second it's something like huge quintillion number these parasitics they're not external to the fat they're internal so only when the drain starts conducting does the capacitance between the gate and source charge up when the gate has reached a stable state the capacitance between the gate and drain starts to charge so capacitance between the gate and drain is also known as the millicapasance but it behaves in the same manner described for inverting amplifiers it will all make sense on the scope so here's some more maths let's say you've got a gate charge of 20 nano coulombs and you want to switch it on in 20 micro seconds i'll show the long way first so 20 nano coulomb so point one two three that's all the middle ease one two three that's all the micros one two oh that's twenty nano coulombs divided by 20 microseconds so 0.00020 equals 0.001 amps i said i'd keep the maths easy so 20 nano coulombs divided by have a look over here 20 micro seconds is 20 000 so 20 nano coulombs divided by 20 000 nano seconds equals one milliamp and finally down the bottom 20 nano coulombs if you wanted to switch on in 20 nano seconds 20 divided by 20 is one so you need to supply one amp to get the switching speed of that time right here's a look at my live scope view the contrast between those colors at the moment isn't very clear so what i'll do i'll go to screen image capture the screen and show you that way this circuit's running at 84 kilohertz my green signal which there is no earth connected to that scope probe hence the ribble the green is the gate the yellow is the led right so i've turned the yellow led which is the drain voltage i've turned that signal off for a minute this is my gate so zero volts is down the bottom where my mouse is moving the gate turns on stays on for this duration then turns off and stays off for this duration and going back to my circuit drawing when the gate turns on it pulls the drain down to ground and the led will be on so when the led is on we will have no voltage on the drain other than a few milli or micro volts because of the rds on resistance i have to add that otherwise people will shoot me down in flames so it won't be absolute zero but i'm calling it zero when this is on it's effectively zero when the fet is off this will rise up to the supply but so i don't get shot down in flames it will rise up to the supply minus the voltage drop across this diode led diode so back to the scope so this is the gate turns on stays on for this time turns off now if i turn the led back on and this is just the voltage we see on the drain of the fet when it's at its highest peak the fet is turned off when it's down here at the bottom the fet is turned on i've done the screen capture here just so i can zoom in make it bigger so this green line to remind you the green line is the gate the yellow line is the voltage on the drain so your fet driver turns on and the voltage rises until it gets to the fet gate threshold when it reaches the gate threshold the drain starts to conduct and at that time the gate to source capacitance starts charging so once the gate to source capacitor is fully charged you can see we get this plateau the miller plateau the gate voltage levels off all the time while the drive current starts charging the gate to drain capacitance and remembering from the datasheet the gate to drain capacitance is larger than the gate to source it requires more charge so you can see we've got a longer waiting period here once the gate to drain capacitance is fully charged then the gate voltage continues to rise well i've changed the scale to make it easy we are now on 100 nanoseconds of division the drain is starting to conduct and it's starting to pull that voltage down to ground and since i had a nice 150 ohm on 15 volts that's a nice tidy 100 milliamps and we have got roughly 100 nanosecond switching time you see it starts nearly at this left hand edge and stops down at this edge so that switching times roughly 100 nanoseconds you can see once the gate to joint capacitance has finished charging the gate then continues to charge up to whatever you put on the gate and that takes significantly longer you can see so starting from down here we've got 100 nanoseconds here on this left hand corner so one two three four five six seven roughly 800 nanoseconds before the gate voltage has come up to its maximum but we are concerned with this threshold down here we've got a delay of 100 nanoseconds before we reach the threshold of the gate and once we've reached the threshold of the gate the transistor then switches on in its roughly 100 nanoseconds and that was with 100 milliamps and 150 ohm resistor now what's important about this gate threshold although everything's happening here it's important that you keep applying the gate voltage above this threshold for the fet to function correctly so back to my transistor so knowing i had a gate to source capacitance to fulfill nine nano coulombs i rounded up to ten so ten nano coulombs divided by one hundred nanoseconds equals one hundred milliamps i've now put in a 100 ohm resistor from the 150 so if we work that out i've now got 19 volts on the drain not 15 so 19 volts divided by 100 ohms equals 190 milliamps so i've nearly doubled the current and have a look at the scope 50 nanoseconds of division the drain is coming down to nothing in just over 50 nanoseconds just by reducing the 150 ohm down to 100 but turning the voltage up on the drain to 19 volts but you can see we've still got this delay on the gate if you actually look at where that drain starts conducting it's roughly here so we've got so 50 nanoseconds of division we've got roughly 25 nanoseconds there plus 50 nanoseconds there and a small bit there so really the answer to the value to your gate resistor is a big question because hey what fet transistor are you using how quickly do you want it to switch on but with the formula i've shown you it put you in the ballpark so now i've shown you this plateau also known as the miller plateau you know what you're up against but what i will leave you with is the video i did many years ago or september 2017 where i was fixing an amplifier the original part had the number of the fet engraved off thank you to pv i replaced an unknown vet with an infineon vet it had a large gate capacitance a large total capacitance and when i fitted it to the amplifier the fats were getting red hot in seconds and i knew that because the fans were coming on i kept quickly turning the amplifier off before those transistors destroyed themselves now i explained in the very beginning of this video the phenomenon which i described in the video happened first in an amplifier on a quality pcb and i was then able to reproduce it on the bench the phenomenon is gate loop ringing so because of the parasitic capacitance within the fet or maybe parasitic inductance on your pcb you can get into a situation where as you're turning the transistor on it will start to ring or another word is oscillate and i was able to reproduce that phenomenon gate loop ringing on a bench that infineon fed transistor was ringing at 2.3 mix just in this view but what was important with that infinion phet is that ringing caused it not to switch off it would stay turned on even when the gate was pulled down to zero volts now i'd heard of gate loop ringing never experienced it in all years of servicing electronic equipment even back to the 80s probably because the equipment was designed with specific parts so if a part went wrong you would replace it with a genuine part when you have parts and the numbers are engraved you have to make do with what you got that's when i fitted a fet with a large total capacitance a large gate capacitance into that position and then experienced this ringing gate loop ringing the phenomenon happened in an amplifier and i was able to reproduce it on the bench so hopefully my basic maths has given you an idea of how to roughly work out what a gate resistor you need but if you're prototyping something you can find the resistor you need connect it to your fet connect it to a scope and watch the results if you need it to be faster make it faster but and depending on effect you have if your pcb has long traces long thin traces you'll have a large amount of inductance you could possibly have a large amount of stray capacitance you could end up with gate loop ringing to remedy gate loop ringing you would take your gate resistor and increase its value so have a go at calculating the value of your own resistor and what switch on time you require it's all easy if you keep the times in nanoseconds if you're prototyping something put an oscilloscope on your device if you see it's ringing you've got stray capacitance and stray inductance somewhere and you can mitigate that by making the tracks shorter and wider little tip for you so this video is quite lengthy but and i've only touched the surface of field effect transistors and selecting a gate resistor so hopefully you learned something give me a thumbs up if you liked it thanks for watching

Channel: John B

Views: 4,645

Keywords: FET, fet gate resistor, calculating fet gate resistor, fet operation, fet gates, miller effect, pwm controller uc3842, coulomb, fet gate charge, Millar Plateau, miller capacitance, gate to drain capacitance, Field effect transistors

Id: oynwrZ_Hp7I

Length: 15min 38sec (938 seconds)

Published: Sun Nov 07 2021