Hi Alex,
sorry, but your circuit can not be a shunt regulator for 500mA!
I don`t know so "high-power" Zener diode they can work for that goal...
Other them, that a zener has generel relative gross 5V only (usually in 20% range), exactly 5,1V and +/-10-20%.
For 0.5A Load you must take some complexer circuits.
You must specify too; how much can be the hum(or rest of AC-signal) on your output/DC-stabilized signa and what is Uin (how much V)!...
Is your source really 100KHz and not only 100Hz?
K.