You seem to be describing this backwards - If the FET is off then you expect to see 9V (ie. the full supply voltage) between drain and source; if the FET is on you hope for that voltage to reduce close to zero.
Another issue in your diagram: you show the DC-DC converter as having separate input and output terminals, with the FET connected on the input side. This circuit will only work if the input and output negative terminals on the DC-DC converter are connected together giving you a common 0V reference - if not, the gate (driven by the Pico) will not be at 3V relative to the separate battery negative. It may well be that your DC-DC converter does connect those pins together and there is no problem, but something to check.
Finally, what current are you aiming for? This FET (like many) isn't fully turned on at 4V Vgs. If you look at the curves in fig1 of the datasheet , with a 3V Vgs then the maximum current you can have is a bit less than 20A and that would be consistent with your measured 2V Vds.
So if you need more than 20A then you need a different FET (though may be hard to find one), a more complicated drive circuit (drive the gate with a 74HCT logic gate powered from 5V?), or something else - although FETs sometimes seem like 'ideal' components with infinite gain, their various other limitations sometimes mean that bipolar transistors (eg. a darlington arrangement) are easier/cheaper/better for a given application.
Another issue in your diagram: you show the DC-DC converter as having separate input and output terminals, with the FET connected on the input side. This circuit will only work if the input and output negative terminals on the DC-DC converter are connected together giving you a common 0V reference - if not, the gate (driven by the Pico) will not be at 3V relative to the separate battery negative. It may well be that your DC-DC converter does connect those pins together and there is no problem, but something to check.
Finally, what current are you aiming for? This FET (like many) isn't fully turned on at 4V Vgs. If you look at the curves in fig1 of the datasheet , with a 3V Vgs then the maximum current you can have is a bit less than 20A and that would be consistent with your measured 2V Vds.
So if you need more than 20A then you need a different FET (though may be hard to find one), a more complicated drive circuit (drive the gate with a 74HCT logic gate powered from 5V?), or something else - although FETs sometimes seem like 'ideal' components with infinite gain, their various other limitations sometimes mean that bipolar transistors (eg. a darlington arrangement) are easier/cheaper/better for a given application.
Statistics: Posted by arg001 — Fri Jan 03, 2025 7:11 pm