Dropdownlist SelectedIndexChanged inside AJAX UpdatePanel refresh the entire page during postback

Hi all,

My ASP.NET designed many sections to allow the user input the values. One of section with dropdownlist to allow the user select an dropdown item then display prize for it. at refresh the entire page, so previous values entries were missing during post back. My question is why ASP.NET AJAX not working for update panel only?  

Here is my code

<%-- STUDENT FILE  --%><tr><td class="col-xs-1"><h5>STUDENT</h5></td><td class="col-xs-3"><asp:DropDownList runat="server" ID="ddlName" CssClass="selectpicker" data-style="btn-default"
                                      OnSelectedIndexChanged="ddlMake_SelectedIndexChanged" AutoPostBack="true" AppendDataBoundItems="true"><asp:ListItem></asp:ListItem></asp:DropDownList></td><td class="col-xs-3"><asp:UpdatePanel runat="server" ID="UpdatePanel7"><ContentTemplate><asp:Label runat="server" ID="lblConductGrade" CssClass="smallerText"></asp:Label></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="ddlName" EventName="SelectedIndexChanged" /></Triggers></asp:UpdatePanel></td></tr>

I tried to us UpdatePanel but the page refresh entirely? Any idea how to solve above issue?

Thanks in advance.