place all the dropdownlist which are firing a post back event in updatepanel's content tempalte field control of ajax.
for example refer
<asp:scriptmanager enablepartialrendering="true" xmlns:asp="#unknown">
ID="ScriptManager1" runat="server"></asp:scriptmanager>
<div>
<asp:updatepanel id="UpdatePanel1" runat="server" xmlns:asp="#unknown">
UpdateMode="Conditional"> <contenttemplate>
<asp:dropdownlist id="NetworkCodeDropDown" autopostback="true" onselectedindexchanged="DropDown_Changed" runat="server"> </asp:dropdownlist>
</contenttemplate>
</asp:updatepanel>