MOSFET Driver question

[dyjguilin]

Hello all,

currently I need to design a driver for the external power noms (high side, using boostrap structure)
while as you know, the NMOS has a threashold voltage (around 2.5V), and I need to charge gate to this value very fast (around 5us, and the capacitor of gate is around 1nF),
but the threashold voltage will vary with the temp and case, and if I overcharge the gate, the NMOS will turn on and a big current can broken the Power MOS
So, how to solve this problem?
Thanks

 

[alexan_e]

The usual voltage to drive the gate is about 10-12v and the mosfet usually has no problem up to 20v, check your datasheet for the max Vgs.
The threshold voltage is the lowers gate voltage at which the mosfet turns on but Rds-on (drain-source on resistance) is worse compared to a higher bias and since you want the best possible efficiency (and less power as heat on the mosfet) you should use a higher bias.
You can't overcharge the gate, the mosfet will only be damaged if you use a higher Vgs than the rated max (or Vds or Id of course).

Alex

 

[DenisMark]

Hi dyjguilin,

What's you are going to achieve by "I need to charge gate to this value very fast"? Do you need to decrease turn-on delay?
For last purpose as I know there is no an elegant solution.

You can charge gate until Vth threshold with high current than switch to low current to obtain desired slew-rate.
You can use dedicated voltage comparator (comparing gate voltage and some threshold in the simplest case) to switch gate current.

If you have some extra sense nmos close to external power nmo you can build circuit to measure Vth of the sense nmos and use it as the threshold.
Or you can detect when output starts to change (not a good way). Or you can detect dv(gate)/dt change )))

Or make the threshold equal to Vth or to min. Vth across possible process cases of external nmos. Additionally you can make temperature dependence
of the threshold the same as temperature dependence of Vth
(e.g. you can shift gate voltage by using string of diodes, K*Vbe, K=2..3 and compare it with some 0TC reference voltage).

Regards

 

[Teddy]

a) Vt of the top fet is surely changing but for you the real requirement is what Rdson of the mosfet you need. At Vt it will be quite high so to go to mOhms you need to be cca 3xVt.
b) if you are afraid about overcharging you can clamp the voltage.
c) you can try tio use charge pump to make reference for switching the top fet

 

[leo_o2]

A safe way is to use a LDO to supply the driver.
The LDO output should be lower than the maximum rating of NMOS Vgs.