MULTIPLE UPDATE PANEL

I AM USING 2 UPDATE PANEL AND SET UPDATE MODE AS CONDITIONAL BUT TXTPWD TEXTBOX CLEARED EVERY TIME WHEN I CLICK BUTTON1

<asp:ScriptManager ID="ScriptManager1" runat="server">

</asp:ScriptManager>   <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><asp:Button ID="Button1" runat="server" Text="Button" /><asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>           <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:TextBox ID="txtpwd" runat="server" TextMode="Password"></asp:TextBox><asp:PasswordStrength ID="PasswordStrength1" runat="server" 
                        TargetControlID="txtpwd"></asp:PasswordStrength></ContentTemplate></asp:UpdatePanel></ContentTemplate></asp:UpdatePanel>

 Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
        Label1.Text = TextBox1.Text & txtpwd.Text
    End Sub