[ToolboxData("<{0}:ComboBox runat=server></{0}:ComboBox>")]

public class ComboBox : DropDownList, IScriptControl, IPostBackEventHandler 

{

 // Defines the Change event. 

  public event EventHandler Change;

  //Invoke delegates event Change. 

  protected virtual void OnChange(EventArgs e) {

     if (Change != null) {

        Change(this, e);

     }   

  }

  // Implements IPostBackEventHandler that raises the change events. 

  public void RaisePostBackEvent(string eventArgument){

     OnChange(new EventArgs());

  }

Then I had to postback to my control as the target like this:

myctrlPostBackEventReference = Page.ClientScript.GetPostBackEventReference(myCtrl, "");

ScriptManager.RegisterStartupScript(Page, Page.GetType(), "initMyClientVariable", "var postBackEventReference=\"" + myctrlPostBackEventReference + "\";", true);

and in my javascript file I've added:

eval(postBackEventReference)

to execute it.

If you want to prevent full page render, so I used the same method, but posted back to a 'dummy' updatepanel as the target. The update panel will raise the event on the desired control (send the desired control id as an __EVENTARGUMENT (the second parameter of GetPostBackEventReference) for the update panel to distinguish which event should actually be raised.