the voltage drop across an Inductor instead of a Res. for current adjust.

[Shahriar]

Hi

I want ot add a current limit to my SMPS. at the output, I have an
inductor that creates a LC filter to reduce ripple.
Can I measure the voltage drop across this inductor instead of putting
a low Value resistor?
Please do not care about little voltage drop across that inductor because
of low resistance! I will amplfy it if needed.
What problem should I encounter?
My SMPS is going to be a variable output.

ThanX in advance
Shahriar

 

[audioguru2]

Hi Shahriar,
Current builds slowly in an inductor then discharges slowly, making a triangular waveform. Therefore if you use its resistance and reactance as a current detector, then an overload at the output will give a delayed response because the overload will draw a massive current supplied by the filter cap before the current can build in the inductor to your set amount. ;D

 

[OutToLunch]

The use of inductor current sensing is actually becoming more and more the norm with multiphase buck regulators that supply power to a PC processor. It has been around for awhile but it's main use has been to create a droop in the output as the load is increased. There are a number of advantages to this, one of them being the output capacitor bank can be smaller.

Another lossless method of current sensing is to use Rds(on) sensing (I say lossless in the sense that you would not be adding a lossy element to the system - the sense element is part of a component that is already passing current). Basically, you know the Rds(on) of the MOSFET and once the voltage across it has exceeded a set value, the system is in overcurrent. The Rds(on) of a FET will change as the junction temp rises, but this can be accounted for since all you're doing is tripping on a fault condition.

To detect the DC current passing through the inductor, you need to create a low pass RC filter across it. The voltage across the capacitor will be the DC voltage across the inductor that is created from the DCR drop. All you have to do is to make sure that R*C = L/DCR and you're cool. One point, though: if this is for a one off project, then just measure the DCR of the inductor with an accurate LCR meter. If this is to go into production, then you're going to have to call for a max of 10% tolerance on the DCR of the inductor and design with tolerances in mind.