Electronics > Projects, Designs, and Technical Stuff
Opamp based current limiter oscillates with inductive load
brumbarchris:
Hi Treez,
The schematic that you propose is OK, but:
1. It has the exact same problem as my own circuit with the current overshoot that appears when turning on the current source. This does not help and it is an un-wanted effect.
2. It has a considerably slower turn-off time compared to my circuit, I measured something in the range of 10us (vs the <100ns in my schematic)
Turn-off time was a goal I mentioned in my original post, although not explicitly mentioning a hard figure, sorry for not being clear enough:
--- Quote ---Just to explain the V3 and M1 function: this has nothing to do with regulating the current, I just use that to turn OFF the main MOSFET very fast, as this is another requirement that I have.
--- End quote ---
Best regards,
Cristian
ocset:
OK , the attached has no overshoot.
...ill work on the turn-off, i am sure you appreciate, the only way to reduce the current in an inductor quickly is to quickly put a big voltage on it.....so eg make the inductor discharge through a high voltage zener, etc.
The problem is that the voltage then on the VGS of the pfet may go above max limits.
brumbarchris:
--- Quote ---.so eg make the inductor discharge through a high voltage zener, etc.
--- End quote ---
We will have a 1k in parallel to the inductor. And we decided to also reduce the current through the inductor to 100mA, instead of 130mA. That will give us 100V negative spike, and the selected MOSFET is rated at that. In practice, we will probably keep the current even lower, at 90mA or so, to give us some margin, but for the sake of numbers, we'll keep it at 100mA in simulation.
Apart from these details, I am analyzing the changes you made to remove the overshoot. I must admit, I have quite some idfficulties in correlating the differences between the two schematics and the result (i.e. dissapearance of the overshoot).
Regards,
Cristian
ocset:
i removed the overshoot by pre-conditioning the opamp so it didnt rail its output just before it is brought into regulation......as such, when it is called upon to get into regulation, its output voltage has less far to slew...and thereofre the overhsoot does not hapen......its more of a trick than a feedback loop mathematical thing....but it does the job.
There is a resistor there, and by the bjt conducting through it, the opamp gets fooled into a fake type of regulation, so its output doesnt get railed.
This is often a problem, with many circuits.......at first the regulation opamp output is railed, and has a long way to slew to get into regulation, so you just trick it into haveing its output near the reguilation point....then it has less far to go, and so gets into regulation quicker...and so no overshoot.
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