我想在后台手动刷新updatepanel 但是无目的. 求教是怎么回事

JScript code

<script language="javascript" type="text/javascript">
        function couponTypeChanged() {
            xmlHttp = GetXmlHttpObject();
            var url = "Default.aspx?type=couponTypeChanged";
            xmlHttp.open("GET", url, true);
            xmlHttp.onreadystatechange = function () {
                if (xmlHttp.readyState == 4) {
                    if (xmlHttp.responseText == "success") {
                        $("#cph_btnHideTypeChanged").click();
                    }
                }
            }
            xmlHttp.setRequestHeader("If-Modified-Since", "0");
            xmlHttp.send(null);
        }
    </script>

HTML code

<table width="50%" border="0" cellpadding="0" cellspacing="0" class="data-table-3"
        align="center">
        <%--优惠券发送部分 --%>
        <tr>
            <td class="v14-header-3" colspan="2">
                123            </td>
        </tr>
        <tr>
            <td width="100px" align="center">
                主类型:
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlCouponType" Width="150px" onchange="couponTypeChanged()">
                    <asp:ListItem Text="代金" Value="LOC"></asp:ListItem>
                    <asp:ListItem Text="实物" Value="GOODS"></asp:ListItem>
                </asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td width="100px" align="center">
                具体分类:
            </td>
            <td>
                <asp:UpdatePanel runat="server" ID="UP3" UpdateMode="Conditional" ChildrenAsTriggers="false">
                    <ContentTemplate>
                        <asp:DropDownList runat="server" ID="ddlCTD" Width="150px">
                            <asp:ListItem Text="--请选择主类型--" Value="0"></asp:ListItem>
                        </asp:DropDownList>
                    </ContentTemplate>
                    <Triggers>
                        <asp:AsyncPostBackTrigger ControlID="btnHideTypeChanged" EventName="Click" />
                    </Triggers>
                </asp:UpdatePanel>
            </td>
        </tr>
        <tr id="trCouponTips" style="display: none">
            <td colspan="2" align="center">
                处理中,请稍候……
            </td>
        </tr>
      </table>
    <asp:Button runat="server" ID="btnHideTypeChanged" OnClick="btnHideTypeChanged_Click"
        Style="display: none" />

C# code

protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.QueryString["type"] == "couponTypeChanged")
            {
                couponTypeReload();
            }
        }

        protected void btnHideTypeChanged_Click(object sender, EventArgs e)
        {
            UP3.Update();
            //ScriptManager.RegisterClientScriptBlock(UP3, GetType(), "123", "alert('click')", true);
        }

        private void couponTypeReload()
        {
            try
            {
                ddlCTD.Items.Clear();
                for (int i = 0; i < 3; i++)
                {
                    ddlCTD.Items.Add(i.ToString());
                }
                Response.Write("success");
            }
            catch (Exception ex) { Response.Write(ex.Message.Replace("\r\n", "").Replace("'", "")); }
            finally { Response.End(); }
        }

很惆怅啊 这到底是为什么呢