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Calculating Mosfet Gate Current

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Stardom

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I'm trying to determine what the gate current will be when i turn on a mosfet. The only current way i know to do this is to use the gate charge that the datasheet gives, say 1000pf. Since i know I = dQ/dT = gate charge/start-time i could calculate average current in that time if i knew how fast the start up time was. But i don't know this time! I have also heard that a gate resistance increases the start up time but nowhere have i seen where they tell how to calculate how much it slows the start up time.

In addition, since i'm using a mosfet driver to turn the mosfet on/off at a certain frequency, i imagine the fact that the current isn't constant also plays a part into determining how much pulsed current the mosfet driver can handle. Any ideas? Pointers to tutorials which talk about it?
Thanks!:D
 

eem2am

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FIRST IT IS GATE-source capacitance that you charge up

then when you reach VTH, then you charge up the miller capacitance = DG capacitance.....

miller is the bigger one that you have to charge up.

so its kind of complicated cuzz you first charge up one then the other one.
 

Stardom

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K, but how am i supposed to choose the appropriate mosfet driver chip if i don't know at least an approximation of the gate current? Is there some general rule I don't know about...
 

eem2am

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average gate current is very low and almost any driver can supply it.....it is Qg*switchingFrequency Amps.

Just look at peak current pulse rating of driver.....as long as its say at least 1Amp peak then you should be ok.....unless it is a really high power ciruit you do.

rem the fet gate current charge-up pulse is very very short say 200ns
 
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    ismu

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Stardom

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Thank you, this was helpful, especially in searching google for key terms. Just another quick question. For internal switching full bridge drivers, specifically the L298 (https://www.st.com/stonline/books/pdf/docs/1773.pdf) , when they say it can take up to 3A dc current, do they mean without a heat sink or with one? If so, how do i calculate the necessary heat sink?
 

eem2am

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is that for stepper motor?

if its 3A constantly then it will need heetsink.

you calculate power and use thermal resistances as from datasheet and calc heetsink thermal res needed...knowing yor ambient temp.
 

    Stardom

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K. So i got it. Thanks!
 

ismu

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average gate current is very low and almost any driver can supply it.....it is Qg*switchingFrequency Amps.

Just look at peak current pulse rating of driver.....as long as its say at least 1Amp peak then you should be ok.....unless it is a really high power ciruit you do.

rem the fet gate current charge-up pulse is very very short say 200ns



Can u give me calculation by using below example:
----------------------------------------------------
Vds=12v ,
Id=30A ,
Switching freq: 50Hz square ,
Vgs=12V .
Number of MOSFET= 4no: (Paralleled)
Mosfet : STB80NF55
---------------------------------------------------
I want to find required Ig (max)=?
 

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