page is refreshed while i am using update panel

i am using update panel . a textbox is inside the update panel , still page is refreshed when text of textbox is chaned .

    

        
        
        

        
                    
                        

            

                

                            Name
                        
                        

                            
                        
                        

                            

                                
                                
                            
                        
                    
                
            
            

                

                    LastName
                
                

                    
                
                

                
            
           
        
    
    

code behind is 

 protected void chkName(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(TextBox1.Text))
        {
            con = new SqlConnection(str);
            con.Open();
            cmd = new SqlCommand("select Name from info where Name=@1", con);
            cmd.Parameters.AddWithValue("@1", TextBox1.Text);
            SqlDataReader dbr = cmd.ExecuteReader();
            if (dbr.HasRows)
            {
                checkusername.Visible = true;
                imgstatus.ImageUrl = "del.gif";
                lblStatus.Text = "UserName Already Taken";
                System.Threading.Thread.Sleep(2000);
            }
            else
            {
                checkusername.Visible = true;
                imgstatus.ImageUrl = "available.gif";
                lblStatus.Text = "UserName Available";
                System.Threading.Thread.Sleep(2000);
            }
        }
        else
        {
            checkusername.Visible = false;
        }
    }