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**Dual Resonant Solid State Tesla coils / Re: Gate Resistor Calculation **

« **on:**May 12, 2019, 05:41:02 PM »

I will venture an answer to this...

The purpose of the gate resistor is to suppress oscillations of the series LC circuit formed by the IGBT gate capacitor about 3000 pF for the IGBT you have chosen) and the inductance of the connection to the gate. This is one reason why the connection to the gate should be kept as short as possible, and the signal is carried to the gate using a transmission line of some sort, for example, a twisted pair of a cat 5 cable which has a characteristic impedance of about 100 ohms. Many gate drive transformers already use cat 5 twisted pairs so this is convenient. The gate is a load on this transmission line that is very much like a short circuit being a large value capacitor, and the capacitor formed by the two conductors of the transmission line adds only a little extra capacitance to this. If the gate resistor resistance is too large, the RC circuit formed by the gate resistor and the gate itself has a rise time that is lengthened and therefore slows down the transition of the gate. If it is too small, you get the oscillations. So for example, lets say you have 3 cm of trace which has a inductance of perhaps 20 nH. The impedance of the LC circuit formed by the trace and the gate is sqrt[20 nH/3000 pF]=3 ohms. To critically damp this LC circuit, you would use a 3 ohm resistor. This would result in a RC time constant of (3 ohms)(3000 pF)=9 ns. This does not increase the transition time of the gate that much. Typically you err a little on the side of a higher resistance, so that 5 ohms is a typical series resistor value. The value of the resistor is not that critical because in practice it is difficult to measure some of these quantities in-circuit.

Dan

The purpose of the gate resistor is to suppress oscillations of the series LC circuit formed by the IGBT gate capacitor about 3000 pF for the IGBT you have chosen) and the inductance of the connection to the gate. This is one reason why the connection to the gate should be kept as short as possible, and the signal is carried to the gate using a transmission line of some sort, for example, a twisted pair of a cat 5 cable which has a characteristic impedance of about 100 ohms. Many gate drive transformers already use cat 5 twisted pairs so this is convenient. The gate is a load on this transmission line that is very much like a short circuit being a large value capacitor, and the capacitor formed by the two conductors of the transmission line adds only a little extra capacitance to this. If the gate resistor resistance is too large, the RC circuit formed by the gate resistor and the gate itself has a rise time that is lengthened and therefore slows down the transition of the gate. If it is too small, you get the oscillations. So for example, lets say you have 3 cm of trace which has a inductance of perhaps 20 nH. The impedance of the LC circuit formed by the trace and the gate is sqrt[20 nH/3000 pF]=3 ohms. To critically damp this LC circuit, you would use a 3 ohm resistor. This would result in a RC time constant of (3 ohms)(3000 pF)=9 ns. This does not increase the transition time of the gate that much. Typically you err a little on the side of a higher resistance, so that 5 ohms is a typical series resistor value. The value of the resistor is not that critical because in practice it is difficult to measure some of these quantities in-circuit.

Dan

Hi all I am new to the forum. I have built quite a few Spark gap style coils throughout the years and am now trying my hand at my first DRSSTC. I am going to use loneoceans UD2.7 c for the driver and his 80mm Full bridge board for the bridge. I will be using FGA60N65SMD for the igbt's. I am having trouble wrapping my head around calculating the gate resistors values. Could someone point me in the right direction to a good place to read up on this ? I tried using the calculator but I am at a loss as to how to figure out the maximum gate drive current the driver is capable of. Better stated what do I calculated it off of? My assumption is the current of the mosfets that drive the bridge and if so is it x1 since there is a gate resistor per igbt or total current x4?

Any help would be greatly appreciated. Thank you