How to refresh ASP.Net UpdatePanel on DropDownList change in ASP.Net

Hi,

I am trying to refresh updatepanel using dropdownlist but facing problems. Please Help!

<asp:DropDownList ID="ddlClass" EnableViewState="true" AutoPostBack="false" class="input-large span10" runat="server" onchange="loadAllDefaults();" Style="width: auto;">

</asp:DropDownList>

Above is the code of dropdownlist

<asp:ScriptManager ID="ScriptManager1" runat="server" />
            <asp:UpdatePanel ID="UpdatePanel1" UpdateMode="Conditional" runat="server">
                <Triggers>
                    <asp:AsyncPostBackTrigger ControlID="ddlClass"  />
                </Triggers>
                <ContentTemplate>
                    <asp:Repeater ID="rep1" runat="server">
                        <HeaderTemplate>
                            <div id="videolist" style="width: 380px; margin-bottom: 20px; margin-left: 640px; margin-top: -385px; padding: 0 30px 10px 10px; position: relative; overflow: auto; max-height: 300px;">
                                <ul style="list-style-type: none">
                        </HeaderTemplate>
                        <ItemTemplate>
                            <li style="height: 90px; margin: 0 8px 0 0; margin-bottom: 15px;">
                                <a href='javascript:refreshIframe("<%# Eval("key") %>","<%# Eval("description") %>")' id='<%# Eval("id") %>' style="width: 200px;" onclick="refreshIframe();">
                                    <span style="float: left;">
                                        <img width="120" alt="" src="<%# Eval("thumbnail_path") %>" />
                                    </span>
                                    <span style="height: 90px; margin-left: 2px;"><%# Eval("title") %></span></a>
                            </li>
                        </ItemTemplate>
                        <FooterTemplate>
                            </ul>
                    </div>
                        </FooterTemplate>
                    </asp:Repeater>
                </ContentTemplate>
            </asp:UpdatePanel>

This is updatepanel code. as shown in the code, i am referring to ddlClass in triggers which already calls a javascript function loadAllDefaults();

The problem is, since ddlClass is calling javascript function 'onchange' property, i cannot call any other function of backend code and I want to refresh the updatepanel on the basis of selected value of dropdownlist.

How can i do this?

P.S I have created a function in c# code in backend but I am not able to call both functions together!

Thanks in Advance :)