Constant current loads

[Mosaic]

Hi all:

The last time i asked about this on ETO...I got advice to pursue using an Opamp driving an FET with a current sense resistor on it's source. Fairly straightforward. Under PIC control I fed the OPA a ref voltage from PWM, filtered via an 2nd order RC to control the current flow...
great.
Problem is the high power dissipated by the FET (linear mode) at certain loads.

Well how about this approach...

Have the PWM drive the FET directly. Place the 2nd order filter on the sense resistor / FET source node and then sample this smooth voltage by the PIC adc to derive actual current flow.

Thus the OPA is eliminated and the NFET operates in switched mode (7Khz) so fairly low switching losses.

Any pitfalls with this approach? I'd have to allow for 1 or 2 mSec between adc samples to permit the filter caps. delta V time constant to expire. Then derive a small increment to the PWM and cycle the loop.

 

[alec_t]

Any pitfalls with this approach?

Can't think of any, apart from the obvious one that the current is no longer constant (although the average current is)!