AsyncPostBackTrigger in a Repeater outside UpdatePanel

I am trying to get a cbxSupplement trigger updatepanel refresh, but am not sure if I am using a wrong EventName or it is just impossible to do it with CheckBox. If I replace CheckBox with Button, it works fine.

<asp:Repeater ID="repSupplements" runat="server">
    <ItemTemplate>
        <asp:CheckBox runat="server" ID="cbxSupplement" />
    </ItemTemplate>
</asp:Repeater>
<asp:UpdatePanel runat="server" ID="up1">
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="repSupplements" EventName="CheckedChanged" />
    </Triggers>
    <ContentTemplate>
        //Get checked items
    </ContentTemplate>
</asp:UpdatePanel>

Since CheckBox controls inside repeater are not available at design time you should register them with ScriptManager.RegisterAsyncPostBackControl method. This method requires ScriptManager either on page or on master page.

Create a handler for Repeater.OnItemCreated event and there register newly created CheckBox. The code is following (note that CheckBox should have AutoPostBack property set to true):

<asp:Repeater ID="repSupplements" runat="server" 
    OnItemCreated="repSupplements_ItemCreated">
    <ItemTemplate>
        <asp:CheckBox runat="server" ID="cbxSupplement" AutoPostBack="True" />
    </ItemTemplate>
</asp:Repeater>

<asp:UpdatePanel runat="server" ID="up1">
    <Triggers>

    </Triggers>
    <ContentTemplate>
        //Get checked items
    </ContentTemplate>
</asp:UpdatePanel>

Codebehind:

protected void repSupplements_ItemCreated(object sender, RepeaterItemEventArgs e)
{
    var control = e.Item.FindControl("cbxSupplement");
    ScriptManager.GetCurrent(Page).RegisterAsyncPostBackControl(control);
}

This should do what you want.