On Wed, Apr 17, 2013 at 11:56:37PM +0530, Manu Abraham wrote:
> Hi,
>=20
> As the subject states, how to implement cycle by cycle current limiting
> on the dsPIC33F ? (I have been eyeballing DS70323B High Speed PWM,
> for quite a while and unable to get a grasp on how it is supposed to work=
)
>=20
> I understand that one reads in the current through the MOSFET's, but I
> am not seeing any ADC related stuff working as described in the doc.
> (Maybe, I am missing something here ?)
>=20
> Can someone explain the basic idea ?

Sure. Current Limiting is a fault. Faults are covered in section 43.10 of
the document you listed. Each PWM can have faults from selectable sources.

The analog part of it is using a comparator. See section 43.10.1 and
Example 43.19 to see how it is done. You take the voltage that matches
your current limit and put it on one input of a comparator. You take the
voltage that measures your current (using a shunt resistor and possibly an
opamp or instrumentation amp to scale) on the other side of the comparator.
Then the shunt/amp voltage exceeds the current limit voltage, the
comparator changes state, causing a fault. So for example if you had a
10A/100mV shunt and a 10x opamp, the shunt would produce 0.5V at 5A. So if
you wanted the current limit to be 5A, you'd put a fixed 0.5V on on
comparaitor terminal and the scaled voltage across the shunt to the other.
As long as the current is less than 5A then the scaled voltage across the
shunt would be below 0.5V. Once it gets above that, the comparitor changes
state, indicating an overcurrent fault.

In 43.10.2.1 FAULT INPUT PIN MODES setting Cycle-by-Cycle mode will cause
the PWM to terminate early if a fault is detected. PWM restarts normally at
the next cycle. So you get full PWM as long as the current limit is not
exceeded. But once the current is exceeded, then the PWM cycle is
terminated. Presumably this will cause the current to drop.

Hope this helps,

BAJ



>=20
> Thanks,
> Manu
> --=20
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