Hi,
One more thing you can try is add all the content of the popup div inside an update panel. Like below-
<div id="yourPopupDivId">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="DropDownList1" runat="server">
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</div>