Color dodge with transparency?

Hey guys!

I'm trying to simulate little flash when firing a gun by flashing white circle with soft transparent "edges". I'm using Looki's color dodge as effect. So it highlights the background with more brighter colors etc.
It would work perfectly, but it seems that it does not recognize the transparency. So now it only appears as white circle when I would like it to fade away from "edges" to make it smoother.

Any idea why it does not work? And how could I create this effect somehow? :)

Thank you!

- Jesse

EDIT: I don't know how I do it. I try to figure it out for some hours and then I'll post here to ask. And instantly I figure something out. :D
Instead of having the circle with transparent background .png I used black background for white transparent circle. Now it works quite well! :) Gotta do more tweaking.