We previously saw the FQP7N20 and IRFP150MPBF mosfets exhibit a current discontinuity vs gate voltage as the gate voltage crossed the turn-on threshold. This is not desirable in a load bank, since we want to be able to control current smoothly all the way down to 0 amps.
As VDS increases, the mosfet will spend less time in the linear and triode region, and instead just turn on hard as VGS passes the threshold voltage. Our IRFP150MPBF with VDS set to 20V happily jumps right up to 5 amps and current limits my bench supply as VGS increases.
Adding a base resistor below the mosfet makes a big difference. The transition region is much smoother, since the resistor helps to reduce the VDS voltage as the current increases. This results in a nice smooth transition from fully off to passing moderate amounts of current.
Here’s the chart, with a few resistance values.
Of course, adding a resistor puts a limit on the max current that can be passed for a given input voltage, as seen in the 10 and 7.5 ohm cases. This will need to be tested with smaller resistances, perhaps down to an ohm or so. Hopefully this resistor can be low value without making the FET exhibit the sharp turn on characteristics, but this is likely a trade off with VGS stability, VGS threshold stability, and how much VDS is reduced by the resistor. If the resistor is too small, the FET will be out of the triode region to soon, and will not be controllable at low currents.
Next step will be finding the limits of how small that source / ground resistor can be, while still keeping the slope in the VGS / IDS smooth.
