Jan 03, 2008 12:59 AM|kaushalparik27|LINK
check out small example at >
AsyncPostBackTrigger vs PostBackTrigger
AsyncPostBackTrigger: Refreshing an UpdatePanel Control with an External Button
By default, a postback control (such as a button) inside an UpdatePanel control causes a partial-page update. By default, a button or other control outside an UpdatePanel control causes the whole page to be refreshed, as you have seen.
You can also configure a control outside the update panel to be a trigger that refreshes just the update panel.
check out the last section "Refreshing an UpdatePanel Control with an External Button" at below link,
ASP.NET AJAX > The UpdatePanel Control > Introduction to the UpdatePanel Control
hope it helps./.