Try,
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True"
style="z-index: 1; left: 548px; top: 366px; position: absolute">
<asp:ListItem>---Select---</asp:ListItem>
<asp:ListItem>True</asp:ListItem>
<asp:ListItem>False</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="your control which postback" EventName="event name" />
</Triggers>
</asp:UpdatePanel>
Hope it helps.