Resistor Gate

[guilerme]

hi! how do I calculate the resistor gate? i cant find it nowhere. sorry for the bad english.

[AndrewT]

ask the question in your own language and hopefully someone will be able to translate it for us.

Explain why you need the information.

[guilerme]

Originally Posted by AndrewT

ask the question in your own language and hopefully someone will be able to translate it for us.

Explain why you need the information.

Boa ideia!

Minha duvida é: como eu faço para calcular o resistor de gate do transistor mosfet, o que eu devo considerar, e se devo considerar a aplicação Ex: amplificação ou como chave.

[wakibaki]

This is what I get from babelfish:

----------------------------------------------------------

Mine it doubts é: as I faço to calculate the electrical resistance of gate of the transistor mosfet, what I must consider, and if I must consider Former aplicação: amplificação or as key.

----------------------------------------------------------

The electrical resistance of a MOSFET gate is very high. >>1M. It is an insulator. Zero current flows into the gate. The gate has capacitance, however, to both drain and source. Current flows into and out of the capacitances.

This affects how fast it can respond to switching changes in input voltage, and also its maximum frequency as an amplifier.

Sometimes we modify this with a series or parallel resistor.

Is your circuit a switch or an amplifier?

Post a picture of your circuit.

w

[Gordy]

I think our new friend is asking for the value of the gate-stopper resistor on a MOSFET.

If this is the case, the general answer is typically in the range of 100 - 470 Ohms.

[guilerme]

..............Post have been edited by tinitus

English only please

[Sch3mat1c]

Three methods.
1. Guess. Where speed is not critical, around 100 ohms will ensure the MOSFET will not oscillate.

2. If your gate driver is current-limited, you can set the resistor so it drops 50-100% of the supply voltage in the first instant. This is essentially the largest practical resistor value for such a gate driver. Example: a driver using 2N3904, supplied from 12V, will supply about 200mA. Therefore, the resistance should be 12V/0.2A = 60 ohms. 47 ohms will be fine.

3. In speed-critical applications, you can solve the driver-resistor-gate capactance system and design for a certain rise/fall time. Say you need to drive a 10nF gate in 100ns, from a 30ns gate driver. The RC time constant needs to be about 40ns, or a resistor of R = t/C = 4 ohms. (The driver must be able to supply peak currents around 2A to do this.) Mind that gate capacitance does not include miller effect: to include this, find maximum total gate charge Qg and divide it by the gate voltage to find the effective capacitance. A 10nF gate may have more like 250nC total gate charge at 10V, which is closer to 25nF for the full edge!

Tim

[Jen-B]

Originally Posted by guilerme

..............Post have been edited by tinitus

English only please

Mr tinitus please note that from post #1 it was clear that guilerme had some problems with the English language, so in post #2 Andrew suggested a native language post which we might then translate. Surely the idea was to both help him technically and then encourage the use of English 'slowly but with progress'.

Unfortunately you have just completely deleted his last post, which does not help or encourage! Obviously the forum language is English, but completely deleting the post is surely not the best way to progress. It seems as if it is punishing him for not speaking English, and certainly provides 0% encouragement.

Help is usually better than force!

Regards,
J.

[guilerme]

Originally Posted by Sch3mat1c

Three methods.
1. Guess. Where speed is not critical, around 100 ohms will ensure the MOSFET will not oscillate.

2. If your gate driver is current-limited, you can set the resistor so it drops 50-100% of the supply voltage in the first instant. This is essentially the largest practical resistor value for such a gate driver. Example: a driver using 2N3904, supplied from 12V, will supply about 200mA. Therefore, the resistance should be 12V/0.2A = 60 ohms. 47 ohms will be fine.

3. In speed-critical applications, you can solve the driver-resistor-gate capactance system and design for a certain rise/fall time. Say you need to drive a 10nF gate in 100ns, from a 30ns gate driver. The RC time constant needs to be about 40ns, or a resistor of R = t/C = 4 ohms. (The driver must be able to supply peak currents around 2A to do this.) Mind that gate capacitance does not include miller effect: to include this, find maximum total gate charge Qg and divide it by the gate voltage to find the effective capacitance. A 10nF gate may have more like 250nC total gate charge at 10V, which is closer to 25nF for the full edge!

Tim

hi! i understand some things, but can you explai again the:
3. In speed-critical applications, you can solve the driver-resistor-gate capactance system and design for a certain rise/fall time. Say you need to drive a 10nF gate in 100ns, from a 30ns gate driver. The RC time constant needs to be about 40ns, or a resistor of R = t/C = 4 ohms. (The driver must be able to supply peak currents around 2A to do this.) Mind that gate capacitance does not include miller effect: to include this, find maximum total gate charge Qg and divide it by the gate voltage to find the effective capacitance. A 10nF gate may have more like 250nC total gate charge at 10V, which is closer to 25nF for the full edge!

[guilerme]

Originally Posted by guilerme

..............Post have been edited by tinitus

English only please

i will try to rewrite my last post:
i need to know how to calculate the resistor used in gate of omsfet, what i need to considerate(VGS, gate capacitance...) i dont know where i read, the gate is an capacitor that charge and descharge, and the resistor controls this time(and the current o dreno). i am working now in a smps of Elliot, modified to get 2500W, using the same fc, but with irf3205. but i need the information, for aplication as a switch (EX: SMPS), and for use it as ampplifier (amplifier class AB).
I dont remember what i write in the other post, but was +- this.