What are the main reasons for using mosfet driver?

[deskwarmer]

What are the main reasons for using mosfet driver?

Assuming the mosfet is an n-channel em, w/ Vgs threshold of 5V. Why can't you just use the 5V output of a ucontroller or otherwise?

Thanks

 

[IanP]

Youe question is about using a driver to control MOSFET gate, is it not?

You want the mosfet to be switched on and of very fast so it is in the linear stage for a very short period of time.
The mosfet gate represents quite substantial capacitance. A microcontroller's output is not designed to drive capacitive loads and therefore you use a driver..

 

[nicleo]

If the Vgs threshold is 5V, the output voltage of microcontroller might not be high enough than the Vgs threshold.

 

[deskwarmer]

Thanks for your replies,

So from what you've said, its seems that the slew rate on most ucontrollers are not high enough to reduce time spent in the linear stage of amplification. So then a driver is most useful for high frequency applications?

 

[IanP]

Not exactly.
Slave rate on microcontrollers is all right, howevr they were not designed to drive capacitive load such as MOSFET gate. To fast cherge and discherge the capacitance (gate) you need a specially designed driver and not TTL/CMOS pin with weak pull-up inside.
Also, this has not much to do with high or low frequences. Whether it is 5Hz or 5MHz you always want a MOSFET to be switched ON an OFF as fast as possible thus preventig generation of heat during switching (linear stage).

 

[nicleo]

So from what you've said, its seems that the slew rate on most ucontrollers are not high enough to reduce time spent in the linear stage of amplification. So then a driver is most useful for high frequency applications?

Not really. If you wish to switch the mosfet very fast, the I/O pins of ucontroller is not capable of sourcing sufficient PEAK current to turn on the mosfet properly.

For example:
I = C dv/dt

i = 450pF 5V/dt = 2.25xE-9 / dt
(450pF is gate capacitance, 5V is Vgs, dt is the time required by Vgs to reach 5V)

If the dt is 10ns, then
i = 2.25x10-9 / 10x10-9 = 0.225A

If the dt is 1ns, then
i = 2.25A

In my opinion, ucontroller is not capable of supplying 225mA.

 

[deskwarmer]

Awwww, I see.

Thank you very much, your replies were very helpful.

 

[HINDI]

REMEMBER THAT THERE ARE 2 MODES OF OPERATION FOR THE MOSFET+VE TEMP COIEFFIENT & -VE TEMP COIEFFIENT , AND U HAVE TO KEEP IT IN THE +VE TEMP COIEF TO AVOID THE MOSFET DAMAGE . GATE DRIVER ENSURES THAT BY KEEPING THE Vgs MORE THAN 10V NEARLY 15 V.

ALSO, TO AVOID MORE POWER LOSSES DURING SWITCHING U HAVE TO MOVE FROM ON /OFF , OFF/ON AS QUICKLY AS POSSIBLE , BECAUSE THE MOSFET GOES FROM SHORT TO OPEN OR OPEN TO SHORT .

I.E. IT PASSES DURING THIS TRANSATION THROUGH A DIFFRENT VALUES OF R
INFINITY /ZERO OR THE OPPOSITE SO IT 'LL APPEARE AS A RESISTANCE .

SO WE HAVE TO USE THE GATE DRIVER TO GET A FAST SWITCHING.
MOSFET DRIVER WORKES AS A CHARGE PUMP CIRCUIT TO CHARGE OR DISCHARGE "ON/OFF" THE PARASITIC BETWEEN THE SOURCE AND THE GATE .

 

[seyyah]

In addition, high side mosfet driving is more difficult than driving low side. Also you may need to drive more than one mosfet. In this cases to reduce complexity, saving board space and ensuring proper operation, a mosfet driver can be used.

 

[HINDI]

Hi


Also u have to note that the mosfet switching mainly depends on the voltage between the gate and source , so u have to distinguish between the high side switching and the low side switching , because the source in the low side is tied to GROUND , while in the high side the source is FLOATING , so theit is more complicated to drive high more than low.
when the high side mosfet is turned on the drain will CONDUCT its voltage to the source .So it will distort the switching condition , but when u use a high side driver the driver compansate the source voltage to satisfy the switching condition.

best regards
hindi