Hi,
edit: sorry for the wrong order of posts..
some issues:
* you need a current limiting resistor between TTL input an base of bjt
* you don´t need the base-emitter resistor, but it doesn´t harm
* you need a current limiting resistor between collector of bjt and gate of mosfet.
* the 20V zener is not suitable, because V_GS_absoulteMaximum is 20V --> use a lower value one.
* in case the load is very low ohmic you will blow the mosfet.
* the output current is not constant. It varies in time and magnitude with load impedance. But maybe you don´t want it to be constant.
* you can´t draw all of the capacitor´s energy. Only down to 3..4V. But maybe you don´t need this.
* the 1k resistor needs to be rated for at least 2.5W
Why not low side switching?
Klaus
*************************************
Added:
I wanted to pull it low incase of uncertanty or partially charged gate
If you had a
gate, then yes, but it is a bjt with
base..
But as said: it will not harm...
Was this in reference to the zenner sizing or just a general reference? The load is what I would call low ohmic in any case, do I need to alter the design here or do something to protect the FET. There is a possiblility of a short on the load also.
It has nothing to do with zener.
You need to protect the MOSFET according it´s SOA
Imagine the load is 1mOhms...How much current do you expect?
The current does not need to be constant but does need to be above the 2/4A region for the 20ms
You know the discharge curve of an RC?
2200uF, 2Ohms:
peak current: 25A
time constant = 4.4ms. During this time the current drops to about 37% of the initial current.
Safety:
I don´t think that high side switching is more safe than low side switching.
Klaus
- - - Updated - - -
Current limit:
use a 1Ohms, 2W (pulse rating is much higher) resistor in the source line,
and use a 10V zener instead of your 15V (20V) zener.
This should limit the current to about 3..4A.
Klaus