using jquery to call onchange function causing full page postback when using masterpage

$('#bn1').click(function () {
$('#textbox1').change();
})<asp:UpdatePanel runat="server" ID="up123" UpdateMode="Conditional"><ContentTemplate><div style="margin-top: 250px;"><asp:TextBox runat="server" ID="textbox1" ClientIDMode="Static" AutoPostBack="true" CssClass="DateTimePicker" /><asp:Button runat="server" ID="bn1" ClientIDMode="Static" Text="clickme" /></div></ContentTemplate></asp:UpdatePanel>

when using the above function with masterpage in my content page, it cause full page postback instead partial postback
if i use this without masterpage, it's perfectly fine, any ideas?