I'm building a programmable current load. The big picture is : A microcontroler (stm32f303) drives (through its dac) some n-mos (tested with PSMN034 and IRL2203) that drawn current. The microcontroler also measure current and adjust the gate voltage until the current reach the target.
My problem is I have some strange behaviour while controling the mos' gate with the DAC.
For this test, I only output a fix value on the DAC. With nothing connected to DAC, the output is stable. When connecting the gate to the DAC, I measure voltage oscillations on the DAC output. I supposed that the DAC was not able to provide enough current, so I added a AOP but the result is the same. With a fix DAC output, the oscillations' frequency is slowing down while the current is raising. The speed is "exponential". Current is growing from 20mA up to 1A, when I stop the experience.
I expected everything to be stable : DAC voltage and current drawn in mos.
Do you know what it can be ?
Here's a schematic:
Here's a screenshot of the measures. The top curve is the DAC output. The bottom curve is the output of the AOP (tied to mos' gate).
and a video to see the frequency changing: Video