p-channel mosfet ndp6020p question


I'm trying to make a little test with a simple resistor and an NDP6020P p-channel mosfet, you can find here: https://www.circuitlab.com/circuit/bcc7d46ym8qc/p-channel-mosfet-ndp6020p-test/

Any idea on why i get only 60mA of current while i think have to get at least 160mA ?

thanks !

by L0cutus
October 18, 2020

I checked the element parameters of the NDP6020P in your circuit and found that they are identical to those of CL's default p-channel MOSFET IRF9530. The IRF9530 is designed for higher voltages e.g switch-on at Vgate of -10V.

I tried these 2 CL models in your circuit and they both pulled more than 160mA (DC Solver):- FDC640P, FDV304P. Perhaps they will pass the test.

by EF82
October 19, 2020

sorry for my bad description, i mean i get 60mA in a REAL circuit and i do not know why... Thanks !

by L0cutus
October 19, 2020

A real circuit problem sounds much more interesting! According to a datasheet I found for the ESP01, the difference between 160 and 60mA current demand is similar to that between RF Tx and Rx - or am I stating the obvious?

Can you post what your real circuit is?

by EF82
October 19, 2020

leave out for now the esp01, my test are only with the NDP6020P and a simple resistor to see what is the max current i can get with this circuit, but while my calculations say that i must have 169mA (3.3v / 19.5ohm), what in reality i get is only 60mA and i do not know why... thanks!

by L0cutus
October 20, 2020

Thanks for clarification! Great that you have updated the sim circuit with FDC640P. I'll come back to the sim in a moment to show how it can be used to bridge between theory and practice.

Okay, so you have a real circuit in 6 parts, 5 of which are straightforwardly checkable: battery (or psu?), proto board, switch, 2 resistors. That leaves the more complicated mosfet which is also susceptible to ESD (that leaves no external trace of non-fatal damage to, say, the gate oxide layer). Here are a few investigations which I would try.

1] Does the mosfet turn off? (that's a drain current of less than -1uA with 0v gate bias, from the NDP6020P datasheet).

2] Is there a reasonably linear (if quite sharp) turn on with gate voltage? The sim (DC Sweep) will give qualitative results as to what to expect, as this quasi-linear behaviour is not explicitly in datasheets for high current power mosfets. Your real circuit will have slightly different voltages in practice.

On your simulator circuit I replaced the switch with a resistor and found (with DC Sweep) that the FDC640P turned on with "Rswitch" between 260k (few mA drain) and 200k (drain just saturating). Vdrain-gate approx -0.9v to -1.1v. What I have just described is convenient for the CL simulator; in practice there are other ways in which to apply variable gate voltage. Also, in both practice and sim, there is a small possibility that the mosfet might oscillate when in the linear region. A small parasitic gate capacitor will help to dampen.

In my experience, faults/problems are often to be found hiding behind "sure-fire" assumptions. I've not discussed the other real circuit parts - there may yet be something there which is not "obviously" correct...

Good luck!

by EF82
October 21, 2020

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