Hi Mario,

I think my reply on PIC input protection will be relevant.

I'll just add that a simple diode across the shunt resistor can add a degre=
e of=20
protection. A 1A or more silicon diode is relatively robust and cheap, and =
is a good=20
fit if the knee of the conduction curve is above your analog input range. I=
f AC is=20
necessary, add another diode the other way. For higher voltages you might u=
se=20
multiple diodes in series, but pretty soon a zener or TVS diode becomes a b=
etter=20
choice, then add series R to analog input and...

On 26 Jan 2018 at 16:08, Mario wrote:

>=20
> Hello,
> I'm playing with my SMPS step up (boost) design, where the controller is =
a PIC.
>=20
> I thus have +12V power input, an inductor, a MOSFET, and a diode that cha=
rges a
> high voltage capacitor, and a voltage divider before the diode, to monito=
r voltage
> (via a first PIC ADC input) of the cap without actually discharging it co=
ntinuously.
>=20
> The gate of the MOSFET is connected to a +5V rail through a 1k resistor a=
nd I use
> a PIC output in open drain configuration to ~quickly stop the MOSFET from=
 conducting
> (the opposite doesn't need to be ultrafast, as the SMPS will be always us=
ed in
> discontinuos mode, thus current is zero whenever the MOSFET starts to con=
duct).
> Switching frequency is pretty low.
>=20
> While it's certainly not the most performant boost circuit, I like its si=
mplicity
> and low components count.
>=20
> Now, I also need to monitor current (for which I have devoted a second AD=
C input),
> and thus I have placed a shunt resistor between the source of the MOSFET =
and the
> 0V rail. I can be 100% sure that the voltage out of it will never exceed =
the PIC
> ADC max input voltage, also due to insufficient input current from the po=
wer supply.
> The shunt has also pretty low resistance so the voltage never reachs 0.5V=
 (not much
> resolution from the ADC, but enough to not want to add an OpAmp, as space=
 on board
> is very very limited).
>=20
> Question is: should I put a series resistor between the PIC ADC input and=
 the
> shunt? Theoretically the voltage will never go below 0V and will never go=
 above
> 0.5V (as explained above), but I fear some insidious aspects that maybe I=
 haven't
> been able to consider, maybe for some hard to grasp parasitic capacitance=
, etc..
> could the shunt voltage go below 0V and thus fry my PIC ADC input pin unl=
ess I put
> a series resistor? Is this series resistor necessary?
>=20
> If it's totally useless, I don't want to put it, it's a matter of princip=
le. :D
>=20
> Thank you very much for augmenting my design knowledge and experience.
>=20
> Kind regards,
> Mario
>=20
> --=20
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> http://mailman.mit.edu/mailman/listinfo/piclist


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