What is Mosfet Driver?? How Does it work??

[keathow87]

1) Anyone please explain to me why we can't just drive the gate of the Mosfet by using DC voltage instead of using Mosfet driver??

2) How does Mosfet driver work actually?

3) What input should we give to the Mosfet driver to get the desired output?? Please explain using this Mosfet driver for example, if I want to have a 13.56MHz 15Vp output to be used for Mosfet switching https://www.farnell.com/datasheets/32537.pdf

Thanks in advanced and your help are very needed and appreciated =)

 

[btbass]

A Mosfet driver allows a low current digital output signal from something like a Microprocessor to drive the gate of a Mosfet. A 5 volt digital signal can switch a high voltage mosfet using the driver. The driver has level shifting circuitry and sometimes a bootstrap circuit to allow the use of cheaper N type devices on the high side.

The turn on time of a mosfet is set by it's total gate charge and the current from the driver to charge the gate. The total gate charge can be got from the device data sheet and is in nana coulombs The turn on time can be controlled using a series gate resistor.
At high switching frequencies, the current can be quite large.
Most power dissipation in the device takes place during switching when the mosfet is in its linear mode, these are called switching losses. So the faster a Mosfet can switch, the less power it has to dissipate.
The faster the switching frequency, the more power required from mosfet device and the driver.
13.56MHz is fast, you may have some problems achieving it?

 

[alexan_e]

A MOSFET has a gate capacitance that you need to charge so that the MOSFET can turn on and discharge it to switch off,
the more current you can provide to the gate (sink/source) the faster you switching on/off the mosfet, that is why you use a driver.
The driver is there to provide the current and also drives the gates with an appropriate voltage level not high to avoid the risk of damaging the mosfer but also high enough to produce a low Rdson (mosfer drain source resistance when turned on).

The MOSFET datasheets have a very important factor called Total Gate Charge and is given in nC.
It is actually the time * current at which the MOSFET is fully on, for example when you read 100nC
it means that if you give 1A (limit the gate current to 1A is what i mean) the gate will be charged
after 100ns (100nC /1A = 100ns ) or if you give 10mA the gate will be charged after 10000ns (100nC /0.01A = 10000ns ) etc
So when you choose a MOSFET and driver you should consider the parameters for both of them (MOSFET with lower nC are faster)

For mode details you can read
https://www.fairchildsemi.com/an/AN/AN-9010.pdf
see page 17 for the different gate capacitance charge stages

Alex

 

[leo_o2]

If switching on/off is necessary, then you need a MOSFET driver.
A DC voltage can only drive it on, but not off. MOSFET driver input can be a pulse signal that you want.
Another advantage of MOSFET driver is slew rate controlling. It will lead to fast speed and small ground bounce.

 

[keathow87]

thanks for all the explanation, now I have better understanding of it.

By the way, can you guys help me determine if this op-amp connection correct?? From the circuit, usually op-amp need a bipolar supply which is +/-15V...but the problem is I don't have a bipolar supply and I don't want to use the single ended design, so in the end I plan to supply it with two independent DC power supply, both supplying 15V but I'll connect one of the 15V positive pin to ground and its negative pin to the op-amp for the purpose of -15V, will this work?? I wish to amplify the 13.56MHz 3Vp square wave signal to 15Vp using EL2045, is that possible?

note: I'm not using it to switch the Mosfet but its another part needed in my project.

 

[leo_o2]

It is OK for supply connection.

 

[Ahmed Hussein]

the mosfet driver like "EL7104" has maximum "Vs=16 volt" and my power source that my Mosfet works on is "48"

so if I used the driver with PWM when it is on the Vgs won't be the desired one

I need Vgs between 10 and 20 volt to achieve acceptable current >>>> using IRF9540N power Mosfet

so, If I used that driver will it work or no?

 

[goldsmith]

the mosfet driver like "EL7104" has maximum "Vs=16 volt" and my power source that my Mosfet works on is "48"

so if I used the driver with PWM when it is on the Vgs won't be the desired one

I need Vgs between 10 and 20 volt to achieve acceptable current >>>> using IRF9540N power Mosfet

so, If I used that driver will it work or no?

Hi Ahmed
Just a simple guidance through choosing a mosfet driver ( a suitable one ) :
1- paying attention to the acceptable voltage offset ( VDD+15 if VGS is 15 )
2- paying attention to the tr and tf and delay .
3- paying attention to the maximum allowed current in out put sides .
4- paying attention to the internal circuitry .

That's all .
i.e : for a usual mosfet the on voltage for VGS will be around 15 volts to guarantee that it has been turned on as well . ( but you should refer to the datasheet of each manufacturer ) .
so now what do you think ? is your driver fine or not ?

Best Wishes
Goldsmith

 

[Dale Gregg]

It is actually the time * current at which the MOSFET is fully on, for example when you read 100nC
it means that if you give 1A (limit the gate current to 1A is what i mean) the gate will be charged
after 100ns (100nC /1A = 100ns ) or if you give 10mA the gate will be charged after 10000ns (100nC /0.01A = 10000ns ) etc

Hi,

is this formula a rule of thumb or is it transposed from another? Just wondering how it was derived as the one I'm using requires driver voltage rate of change (dV/dt) and impedance, this one is a lot simpler.

Dale

 

[Dale Gregg]

Found it,

Q = CV and I = C dv/dt, the Q = Time x current

IR MOSFETs Basics

not sure of the steps to get there though

 

[keith1200rs]

Q=C.V and i=C dv/dt, as you say.

If you are charging a capacitor C with a constant current i, after time t will have reached a voltage of i.t/C.

Substitute V=i.t/C into Q=C.V and you get Q=C.i.t/C = i.t or t=Q/i. So, taking Alex's example, with 100nC and 1A you get a time of 100ns.

Keith

 

[Dale Gregg]

Understood, thanks for the explaination

 

[faeraki]

Hi!
I am trying to make an electronic switch for an ignition system.
I am about to use IRGSL14C40LPbF which is an ignition IGBT and for mosfet driver the IR4426.
Could you help me with the design of the driving circuit?