Except during an extra point, DPI is exempt from half the distance restrictions.
Not actually true. DPI is 15 yards if the previous spot is B-17 or further out and the foul is more than 15 yards from the previous spot. Otherwise the ball is placed on B-2 or the spot of the foul, whichever is closer to the previous spot. If the previous spot is B-2 or closer in the penalty is half the distance.
During extra points the last part changes so that if the previous spot is B-3, half the distance applies. Interestingly if the PS is B-2.5 during a try, half the distance does not apply (holding in the end zone, two false starts).