Author Topic: High-Side N-Channel mosfet driver question  (Read 1838 times)

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Offline Spike101

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High-Side N-Channel mosfet driver question
« on: September 09, 2014, 05:14:34 pm »
Hello,

I'd like to use a n-channel mosfet in a high-side configuration for a project i'm working on.

I was thinking to use the LT1161 high side n-channel driver because it would suit my application very well, but i don't know if it will work with the MOSFET i was planning to use.
The datasheet of the LT1161 states, that if the input for a mosfet is high, it will drive the output of the gate driver to its supply voltage + at least 7V and up to 10V at 12V supply voltage.
The mosfet I wanted to use (CSD16340Q3) specifies an absolut maximum rating of Vgs = 10V. I understand, that as soon as the channel becomes conductive, the voltage between gate and source will be 7V-10V.
But at the beginning the channel will not be conducting, so doesn't that mean, that the voltage between gate and source will be 19V - 22V, which exceeds the absolute maximum rating for Vgs?

Is this a problem, or does the voltage between drain and source drop so fast, that it doesn't matter?

I noticed that in some of the application examples in the LT1161 datasheet there is the same situation - they drive it with a supply of 24V (at which point the gate driver will output up to 38V), but the RFD16N05 mosfet they are using specifies an absolute maximum of +/-20V for Vgs.
 

Offline David Hess

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Re: High-Side N-Channel mosfet driver question
« Reply #1 on: September 10, 2014, 07:12:54 am »
But at the beginning the channel will not be conducting, so doesn't that mean, that the voltage between gate and source will be 19V - 22V, which exceeds the absolute maximum rating for Vgs?

That is possible but pretty unlikely.  By the time Vgs reaches 10 volts, the channel current will be very high so the MOSFET should be fully saturated with the source only millivolts from the drain.  Either the source will be pulled up to the supply voltage or the massive short will bring the supply voltage down.

Quote
I noticed that in some of the application examples in the LT1161 datasheet there is the same situation - they drive it with a supply of 24V (at which point the gate driver will output up to 38V), but the RFD16N05 mosfet they are using specifies an absolute maximum of +/-20V for Vgs.

The LT1161 outputs should never get above 14 volts so I do not think there is a problem here.

Driving your CSD16340Q3 to its absolute maximum rating may compromise reliability however.
 

Offline Spike101

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Re: High-Side N-Channel mosfet driver question
« Reply #2 on: September 10, 2014, 04:32:11 pm »
Thanks for your reply. I'm using a different mosfet now with the same size footprint, which is rated for +20V Vgs and 30V Vds, so i should be safe in any case. :)
 


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