FET gate series and pull down resistors - Page 1
Forum section: electronics / projects

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 24, 2015, 03:45:27 pm
content: I am building a 136kHz amplifier that uses 4 FET's as two paralleled pairs in the output. The schematic and parts list make no reference to the wattage of any resistors and I am unsure what the demands on the gate series resistors in particular may be. I am also unsure about the pull down ones.Finally the notes state the the de coupling caps for the FET driver chip should be placed very close to the pins of the chip, yet I see no decoupling caps in the schematic. Any ideas as to value and which pins need de coupling please?The build notes contain these lines, of relevance:"The TC4426 chip IC5 is capable of driving 1.5A into the gate capacitances of the big FETs and the decoupling Cs must be fitted close to the chip with short leads. The 6R8 series resistors are actually mounted on the gate pins of the FETs, the resistor leads forming the connections to the strip board. It is probably best to use four resistors, one for each gate. The strip board should be grounded to the earth plane as near to the FETs as convenient via a short lead.The FETs are static sensitive devices and can be destroyed by careless handling.They should be mounted so that the source leads are soldered straight down onto the ground plane. It is good idea to solder the 47k resistors from each gate to ground first, to prevent static build-up during construction.Having applied a very small amount of heat-sink compound to both sides of the transistor insulating washers, the FETs should be clamped firmly down to the heat sink and a test made to check that the drains are not shorted to it. The two 4n7 Cs should be connected straight from drain to source of the pairs of fets with short wires, they must be at least 1kV pulse rated."Any advice most welcome, thank you.

author: ajb
author_rank: Super Contributor
timestamp: January 24, 2015, 08:45:08 pm
content: The pulldown resistors are there to ensure that the gates don't float high enough that the FET turns on when the gate is not driven.The series resistors limit the instantaneous current into the gate when the transistor is turned on and off. The gate of a FET acts like a capacitor, so when you apply a voltage to the gate you get a large current that tapers off as the gate charges. Adding the series resistance limits that current and as a result increases the time it takes for the gate to charge. This has the side effect of increasing the time it takes for the FET to turn on and off, which is generally better for EMC, but decreases efficiency somewhat as the FET spends more time in the unsaturated region where the channel resistance is neither very high nor very low. Everything in life is a tradeoff.As for wattages, the pulldowns are such high value they'll barely dissipate anything. The dissipation in the series resistors will depend on the characteristics of the FETs and your switching speed. You can model the gate driver + resistor+ gate as a square wave driving an RC network and average the power dissipated in the resistor over time. Or if you're lucky there's a spice model for your FET and you can very easily simulate it.As for decoupling caps, yeah, it doesn't look like those were included in the schematic. You'll need to add then for sure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2015, 08:54:57 pm
content: Well, it's not like 136kHz is anything scary. That's hardly RF!It should be possible to get disgustingly high efficiency, though it looks like that wasn't attempted here. A shame.What are the transistors? Gate resistor power can be found from the datasheet. Look for total gate charge Qg, maximum if given. Eg = Qg * Vg. It will say what voltage Qg was tested to (usually 10V). This is the energy if you're driving them at the same voltage, but it will be about proportionally different if you use a different voltage. Power is energy per second, so multiply energy by switching frequency. Pick a gate resistor with rating about twice this figure and you'll be fine.Do you actually have the mile long antenna for this stuff? (Well, ~third of a mile.)Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 24, 2015, 09:15:47 pm
content: Hi, thanks for helping me out again! The FET's are 4 off IRFP450 as a pair of two in parallel. The driver chip is a TC4426. I will try and work the required wattage out for myself, but would appreciate it if you could have a look yourself, as I don't want to blow the thing up before i even put a load on it! It runs 100V DC to the PA section.No, I don't have a very good aerial for 136kHz, not many hams do, it'll be very inefficient, with a relatively short vertical with quite extensive top hat loading. Here in the UK we are allowed 1 Watt ERP. Even 1kW will struggle to achieve that, as the antenna will be so electrically short. But it keeps me out of the pub!

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 25, 2015, 11:22:43 am
content: OK, I am less than useless at maths, but maybe the units I am using are wrong?Qg is 150, Vg is 12V and the frequency it runs at is 136kHz.So I multiplied 150 * 12 * 136000 and get 2.448Is this Watts for the gate resistor?A similar amp, with similar gate capacitance on the 4 FET's, with similar gate drive voltage is using 1/4W resistors with no problems, built by someone who appears very competent. Have I done this wrong? Thanks. I did try.....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2015, 11:41:27 am
content: Check your units, you slipped a decimal.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 25, 2015, 11:55:47 am
content: It's the units I am not sure of. nano Coulombs is not a unit I have ever had to use before, I don't know if nC needs converting to C or what. I am not even sure what units I should input the switching frequency as....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2015, 12:16:52 pm
content: Unit multipliers are all in thousands (all the important ones, that is), so 1 m = 1000 u, 1 u = 1000 n and so on. If it helps, think of the multiplier as another way to write zeroes in front/after the decimal point of the number. So 250 n = 0.25 u = 0.00025 m = 0.00000025 and so on. Multipliers go up in multiplication and down in division.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 25, 2015, 12:51:22 pm
content: OK, I think I have it:0.00000015 * 12 * 136000 = 0.2448So call that 1/4W. Double up for safety margin, a half watt should be more than adequate? Am I finally there?Thanks Tim!!

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 25, 2015, 01:07:07 pm
content: Quote from: ajb on January 24, 2015, 08:45:08 pmActually, series resistors are usually there to "quench" the parasitic inductance to stop ringing on the gate - without series resistance the gate trace and the gate capacitance will form a small tuned circuit which will "ring", causing instability, excess power dissipation and may lead to failure of the fet.