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series resistance selection for mosfets.

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nxt_

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Dear all,
How to determine the exact series gate resistance for any irf (based on the data read from the devicedatasheet) what i have to look for to determine the gate resistance, and how it effects the source drain flow speed?

kindly support me with the equations needed.
many thank
 

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yeah that was my though too, but i clicked the "Helped me button anyway: cuz i found some useful information on the link he post. but no relation manieceindia actually with what i already asked for. :)
 

Dear nxt
Hi
It depends on your mosfet specifications. you will can select that resistance , according to the Ciss and your frequency as simple as.
Best wishes
Goldsmith
 
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Dear GoldSmith. could you please tell more about the relation between the frequency and the ciss and the series resistance with the gate?.. i.e for irf640 how i can calculate the perfect series resistance with maximum frequency 100kHz
 

Generally the gate resistor is there to reduce EMI by slowing down the rise and fall times on the gate and drain, prevent overshoot on the gate, and to limit the peak current from the gate driver. Deciding the value is usually a matter of trial and error, but a good starting point is to set the time constant Rgate * Cgate be your desired switching rise/fall time. Cgate is a combination of Ciss and Crss; usually near Cgate=Ciss + (Vd/Vgate+1)*Crss. Or you could look at the given plot of Vgate vs Qgate. Either way, for the IRF640 you end up with something like 4.7nF. So if you want to limit the rise and fall times to 30ns, then you would guess 6.4ohms for Rgate. Or if you want to critically damp the waveform and your parasitic inductance is 50nH, then you would use 3.3ohms. Again it's usually something you fool around with before making a decision.
 
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Cgate=Ciss + (Vd/Vgate+1)*Crss <--- what is the Vd? drian voltage?
 

Yeah, drain voltage (while off). Vd/Vgate is essentially the "gain" of the switched circuit.
 
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Dear nxt_
Again Hi
When you have ciss and frequency (value) you can predict the current of ciss as simple as . you have the current and the voltage across that , and also you can calculate the value of resistor as well as and as simple as possible.(according to the ohm's law)
Best Wishes
Goldsmith
 
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In all my circuits, I mostly used 22ohms, 10ohms and 4.7ohms. Never had any problem with any of these resistors all the way from DC to ~100kHz. One thing is, you may want to connect an anti-parallel diode across the resistor to discharge the MOSFET quicker.

Hope this helps.
Tahmid.
 
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Dear Tahmid
Hi
It is important to know that at frequencies , over than 180KHZ , the value of this resistor , should be calculate , accurately . because of that , at high frequencies , due to Ciss , and high value of resistance , (such as 22 ohms) your mosfet can be drive at linear region and it will be dangerous for your mosfet. ( it probably will destroy) .
Best Regards
Goldsmith
 
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    Tahmid

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Dear Mtwieg:
Yeah, drain voltage (while off). Vd/Vgate is essentially the "gain" of the switched circuit. <--- how can i know the drain voltage while off?
what if my source is ground?! so no voltage across the drain?, so in the Equation " cgate =ciss+[vd/vg+1]*crss will leads to cgate=ciss+crss only? is that right? i mean if the source was ground !? kindly tell me more about it.

and dear all, if my frequency is X hz and the information written on the datasheet said that these information are made under certian conditions. for example
input capacitance "Ciss" is 1234pf tested under 1MHZ, VDS=25v, vgs=0, so how can imanuplate this data to suit my circuit conditions,
 

Yes. I was talking about frequencies upto 100kHz. But yes, you are right.
 
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nxt_ said:
Dear Mtwieg:
Yeah, drain voltage (while off). Vd/Vgate is essentially the "gain" of the switched circuit. <--- how can i know the drain voltage while off?
Click to expand...
Depends on the circuit it's used in. But you should have a good idea what it is.
what if my source is ground?! so no voltage across the drain?, so in the Equation " cgate =ciss+[vd/vg+1]*crss will leads to cgate=ciss+crss only? is that right? i mean if the source was ground !?
Click to expand...
Sure, but if both the drain and source of the FET are just tied to ground all the time, then the switch isn't doing anything anyways...
 
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I think there is a simple way:
Put a resistor(Rs) in series with the mosfets gate. apply a very sharp pulse tarin to the gate with proper frequency.
Now the current amplitude should be Vp/(Rs+Rg). so by measuring the current amplitude you can calculate the Rg.
 
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Dear gold smith,
can i apply same rule "Cgate=Ciss + (Vd/Vgate+1)*Crss" for the IGBT? by placing the Vd by Collector voltage?
thanks
 

Dear nxt
Hi
The IGBT is similar with mosfet. (with some little differences). and the calculations of igbt are like mosfet . but about dissipation you can not say this.
Best wishes
Goldsmith
 

Hi All,
It has been discussed optimum gate resistance in this topic. But, I think, this is not useful method in bridge type mosfet circuits. Because the reverse recovery of upper (or lower) mosfet is very slow and restricts the maximum turn-on frequency.

When the gate series resistance is very low, turn-on is very fast and so the reverse recovery current will be so high. If the there is not a proper layout, this high recovery current may create a serious problems. The gate resistance should be increased to a safe level until the recovery current do not cause a problem. Apparently, we can say that, the gate resistor value depends also on layout.

At this point, I have a question. Increasing the gate resistance beyond the 100 ohm value is safe? One of my circuit uses a 150 ohm gate resistor. What is your oppinion?

Regards,
 
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