How does this emitter follower FET driver work?

[ACharnley]

A common voltage regulator. The transistor won't pass more than the voltage at it's base minus the base-emitter drop.

Yet in the common FET driver, if 3.3v is presented at the base the FET gets 10v minus base-emitter drop.

How does that work?

 

[Ylli]

No it doesn't. If you input a square wave of 0 - 3.3 volts at point mpptB, you can expect an output at pins 1,4 of 0.6V - 2.7V. Now 2.7 volts may be enough to turn the MOSFET on fully, depends on the MOSFET.

 

[ACharnley]

Exactly what I thought, so it's not great as a high side driver!

 

[ronsimpson]

How does that work?

It works if "mpptB" has 10V of p-p signal.

 

[ACharnley]

It's now made sense why bucks offering 100% duty cycle (aka bypass) can't use bootstrapping so use a PFET on the upper.

I used the above circuit plus a similar one to drive the top. [dc] is limited to 16v. Both drivers are 600mA rated which should be plenty for the small fet's I intend to use.

 

[ronsimpson]

dc=16V?
Will the bottom MOFET turn on well at 4V on the gate?
What is the voltage of mpptB? 5V?
Bottom MOSFET pin1 at gnd?

 

[ACharnley]

dc=16V? -> yes, no more to protect the gate. May lower a bit.
Will the bottom MOFET turn on well at 4V on the gate? -> Yes, they lower dc helps to get fast low Vgs fets. Won't be 4 though, more like 2.5v!
What is the voltage of mpptB? 5V? -> 3.3v (the 2.5v is a bit of a worry but it'll still be "mostly on" and there's a schkotty just out of shot parallel to the fet)
Bottom MOSFET pin1 at gnd? -> correct

 

[ronsimpson]

A number if companies make "gate drivers". This one needs a supply in the 5 to 30V range. (you have 16V)
You can choose inverting or not-inverting or one of each.
Input signal is "ttl levels" and the output is ground to Vcc.
Just a thought. RonS.

 

[ACharnley]

I feel a better solution, because I'm having a similar limitation with some other areas is to use a level shifter.

This one looks perfect -
74ALVC164245

 

[ACharnley]

I've made another mistake, the PFET is swinging to ground.

I think a better solution would be to keep it as NFET and drive it with a higher voltage of say 15v. I know when directly connected to the load this would give a healthy 10Vgs but what about when there's an inductor in the path?