Oh well, I found myself one case when the voltage at the shunt can go negat=
ive:
after the MOSFET is turned OFF and the zener stops conducting as well, the =
body
diode of the MOSFET (or the zener itself, depends if it's connected to the =
source
of the MOSFET instead of to the ground, and I prefer the former solution as=
 it
lets me monitor current also during the decay phase) then there won't be an=
ything
to absorb the (small) remaining energy of the coil, but the parasitic capac=
itance,
resistance, etc.. and the circuit will thus exhibit ringing, which can inde=
ed go
negative, if the Q is high.

So, in answer to my own question, a resistor is really needed to protect th=
e PIC
ADC input.

Cheers,
Mario


At 17:41 2018-01-27, Mario wrote:
>
>My question really wasn't about overloading a PIC input (I even specified =
it
>won't happen, at least from the shunt normal operation) but if there could=
 be
>some "parasitic" or anyhow difficult to understand phenomena that will mak=
e
>the shunt go negative. I'm thinking about overshoot, etc.. although=20
>theoretically
>this should not happen.
>
>Anybody that can elaborate on it please?
>
>
>
>>Hello,
>>I'm playing with my SMPS step up (boost) design, where the controller=20
>is a PIC.
>>
>>I thus have +12V power input, an inductor, a MOSFET, and a diode that=20
>charges a
>>high voltage capacitor, and a voltage divider before the diode, to=20
>>monitor voltage
>>(via a first PIC ADC input) of the cap without actually discharging it=20
>>continuously.
>>
>>The gate of the MOSFET is connected to a +5V rail through a 1k=20
>>resistor and I use
>>a PIC output in open drain configuration to ~quickly stop the MOSFET=20
>>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=20
>conduct).
>>Switching frequency is pretty low.
>>
>>While it's certainly not the most performant boost circuit, I like its=20
>>simplicity
>>and low components count.
>>
>>Now, I also need to monitor current (for which I have devoted a second=20
>>ADC input),
>>and thus I have placed a shunt resistor between the source of the=20
>>MOSFET and the
>>0V rail. I can be 100% sure that the voltage out of it will never=20
>>exceed the PIC
>>ADC max input voltage, also due to insufficient input current from the=20
>>power supply.
>>The shunt has also pretty low resistance so the voltage never reachs=20
>>0.5V (not much
>>resolution from the ADC, but enough to not want to add an OpAmp, as=20
>>space on board
>>is very very limited).
>>
>>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=20
>never go above
>>0.5V (as explained above), but I fear some insidious aspects that=20
>>maybe I haven't
>>been able to consider, maybe for some hard to grasp parasitic=20
>>capacitance, etc..
>>could the shunt voltage go below 0V and thus fry my PIC ADC input pin=20
>>unless I put
>>a series resistor? Is this series resistor necessary?
>>
>>If it's totally useless, I don't want to put it, it's a matter of=20
>principle. :D
>>
>>Thank you very much for augmenting my design knowledge and experience.
>>
>>Kind regards,
>>Mario
>
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