I have dropdown on my page, I am changing selected value of dropdown from popup window using javascript. I have some logic in dropdown SelectedIndexChanged event, so I need
document.getElementById('<%= yourDropdown.ClientID %>').onchange();
This should work, if you are still getting some error, you can try like this:
setTimeout('__doPostBack(\'yourcontrolClientSideID\',\'\')', 0);
yourcontrolClientSideID is the ID of Rendered Client ID.
yourcontrolClientSideID