I am new to electronics so please bear with me! I tried searching for help on this one, a lot of useful info on reverse polarity but I couldn't find much on this situation.
I am using a DC-DC converter that supplies 12 V and 30 A. The DC-DC converter has a soft start, whereby it ramps up from 0 V to 12 V over some time, when the system is turned on. During this ramp up, the system cannot draw more than 2.66 A until the DC-DC converter reaches 6 V.
I initially thought of using a time delay relay, of say 1 second, to ensure that the system started correctly. However, I then thought I could use a P-Channel MOSFET as it looks to be a more elegant solution.
The idea of have is shown in Image 1. The idea was that the MOSFET would conduct less than 2.66 A until 6 V is reached. The problem I have is that I cannot find a MOSFET with a high enough gate to source threshold and low enough resistance, as at 30 Amps the resistance needs to be as low as possible.
I can only find high Rds, high Vgs(th) or low Rds, low Vgs(th) MOSFETs. I can't find a suitable Rds high Vgs(th).
I therefore thought I could add a resistor to the circuit as shown in Image 2 to a low Rds, low Vgs(th) MOSFET. The thought being that this resistor would also take some voltage, thereby effectively increasing the Vgs(th). The problem is I don't have sufficient knowledge to know whether this would work, or the equations involved.
Best Answer
Vgsth is too variable from part to part, and the response of the FET to soft to the gate voltage around Vgsth, that you should split your circuit into a sense part, and a power part.
Use a proper comparator in the sense part.
Use a power FET in the powe part, that gets switched by the comparator.
More complicated I know, but you can make it work to your specifications.