为什么PopupControlExtender在UpdatePanel内生成完整的回发？

I have a button inside an updatepanel. I have a PopupControlExtender linked to the button so when the button is clicked a panel pops up. It works fine except it does a full postback and I can't figure out why. The button and the PopupControlExtender is inside an update panel which inside the ContentTemplate tag. When I take out the PopupControlExtender the button only does a partial postback. I'm having trouble finding any useful information on the PopupControlExtender. Do I have to declare a postback trigger or something?I have a button inside an updatepanel. I have a