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Author Topic: Current sensing for buck boost converter  (Read 3040 times)
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[ The hall sensor mentioned above is ACS712, not ACH712 BTW. ]

If you have a SMPS like this you have to be really careful to shutdown the switch fast if the current goes too high - or else
use a large inductor (slower switching required, more safety margin for saturation).

If the current rises high enough to saturate the inductor then the inductor then just becomes a short-circuit and the current
will slam up to perhaps 10's of amps in a few microseconds or so and blow the MOSFET before the opamp can respond, and certainly
far faster than analogRead()'s 110us execution time.

You would normally use a fast comparator to monitor for this severe overcurrent condition - sub-microsecond response is
good here.  Once you have that you might as well let it do the regulation too...
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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See attached peak detector schematic. R1 is the current sense resistor. (R2 + R3)/R2 defines the op amp gain, and C1 * (R2 + R3) defines the time constant, which should be large compared to 1/f where f is the PWM frequency.

Ok !
So if I want to get the voltage value of Rsense I have to integrate the diode drop voltage  ?

Basically :
Vc1 = Vr1 * gain - Vdrop
Otherwise my Ir1 evaluation will be lowered by a 0,7v factor !

Is that  right ?

No, the circuit is designed (i.e. op amp feedback resistor taken from the cathode of the diode) so that the diode voltage drop does not enter the equation.
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hi !

I finally read a lot on OPAMP because many values seen on tech-doc were completely unknown for me.

My choice for the op-amp is now Maxim MAX4167, this one is a rail to rail input and output op-amp ! While the input can vary from 0.0165V to 0.33V the rail to rail input aspect is very important. The MAX4167 have a 0.25mV-0.85mV Input Offset Voltage so my input voltage will be correctly processed. The Gain-Bandwith is around 36Mhz (input is 170Khz)

My problem is the op-amp gain. I use a non inverting amplifier so Vout = (1+(R2/R1))Vin
With a digital multimeter (and with arduino ADC), I sense a  ~0.010V voltage. On the opamp output, voltage is "only" ~0.016V... Something is wrong somewhere because i expect somewhat x10 ? I double check R1 & R2 value everything seem to be okay.

What wrong with this ?
I can attach a photo of the breadboard if you need !

Thanks for your help.
BR


* Arduino01.png (10.98 KB, 700x365 - viewed 12 times.)
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