如何在AJAX UpdatePanel中触发ASP.NET TextBox，使其触发onTextChanged事件？

我试图让一个TextBox在每次击键时触发它的ontextChanged事件，而不是仅在失去焦点时才触发。我以为添加AsyncPostBACktrigger可以做到这一点，但仍然无法正常工作。我要做什么甚至有可能吗？代码如下：

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Items.aspx.cs" MasterPageFile="~/MMPAdmin.Master" Inherits="MMPAdmin.Items" %>
<asp:Content ID="content1" runat="server" ContentPlaceHolderID="ContentPlaceHolder1">
<asp:ScriptManager ID="sm_Main" runat="server" />
    <div style="left:10px;position:relative;width:100%;overflow:hidden">
        <asp:updatePanel ID="up_SearchText" runat="server">
            <triggers>
                 <asp:AsyncPostBACktrigger ControlID="tb_Search" EventName="TextChanged" />
            </triggers>
            <ContentTemplate>
                <div style="position:relative;float:left">
                    <b style="font-size:xx-large">Items</b>(<a href="Item.aspx">Add New</a>)
                </div>
                <div style="right:25px;position:absolute; top:30px">
                    Search: <asp:TextBox ID="tb_Search" runat="server" Width="200" OntextChanged="updateGrid" AutoPostBACk="true" />
                </div>
                <br />
                <div>
                    <asp:GridView runat="server" AutoGeneratecolumns="true" ID="gv_Items" AutoGenerateEditButton="true" AutoGeneratedeleteButton="true" />
                </div>
            </ContentTemplate>
        </asp:updatePanel>
    </div>
</asp:Content>