I think the positive energy burst isn't a bad idea, but I think it's still awkward, it's either ineffective at low levels, or too powerful (making undead obsolete). I've always viewed it as a mook killer, or a croud controller...
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How about this mechanic instead;
Turn Undead:
1. Determine what CR of undead you can affect by adding your Cleric Level + Charisma Bonus.
1a. divide by 2 (rounded down), within 30'.
1b. divide by 4 (rounded down), within 30' and 60'.
1A - CR 1/2 turn undead of that CR within 30' (6 squares).
1B - CR 1/4 destroy undead of that CR within 30' (6 squares) or turn them within 35'-60'.
This means a cleric of first level with a +4 wisdom bonus can turn CR2 undead within 30' (level + charisma /2), can turn CR1 undead up to 60' and destroy CR1 undead within 30' (level + charisma /4).
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if staying with the positive energy burst, might I suggest:
3d6+chr bonus +1d6 per 5 levels within 30'.
This gives lower level clerics a much better ability, and doesn't get overly powerful as the cleric levels up.