Set Focus not working in asp.net - asp.net

Question

I'm trying to set focus on a control after Postback but its not working. below are the details.
I have some code inside a Panel. Panel will be open closed using ajax.
There is a radio button inside panel ... Once I click the radio button, i want to focus on that control but it goes to start of the page due to post back.
I tried
1. SetFocus(controlID);
2. controlID.Focus();
3. set focus to control in javascript also
but no solution working.
This is a long page ... I have more panels in it. Not sure why my solutions are not working.
Attached my code below:
<Panel> first panel comes here...</Panel>
<div class="demoarea">
<asp:Panel ID="pnl_2_Header" runat="server" CssClass="collapsePanelHeader" Height="30px">
<div style="padding:5px; cursor: pointer; vertical-align: middle;">
<div style="float: left;">User's Information</div>
<div style="float: left; margin-left: 20px;">
<asp:Label ID="lbl_2_Header" runat="server" Visible="False">(Show Details...)</asp:Label>
</div>
<div style="float: right; vertical-align: middle;">
<asp:ImageButton ID="img_2_Header" runat="server" ImageUrl="~/images/expand_blue.jpg" AlternateText="(Show Details...)" Visible="False"/>
</div>
</div>
</asp:Panel>
<asp:Panel ID="pnl_2" runat="server" CssClass="collapsePanel" Height="0" BorderStyle="solid" BorderWidth="1">
<table width="100%" style="text-align:left;font-weight:300;" cellpadding="7">
<tr valign="top">
<td style="width:50%;">
School in Primary 6 :
<br /><br />
<asp:RadioButton ID="rdoPri1" runat="server"
Text="sometext" GroupName="pri" value="1"
AutoPostBack="True" oncheckedchanged="rdoPri1_CheckedChanged" /> <br />
<asp:RadioButton ID="rdoPriOther" runat="server" Text="Others"
GroupName="pri" value="2" AutoPostBack="True"
oncheckedchanged="rdoPriOther_CheckedChanged" />
<asp:TextBox ID="txtPriOther" CssClass="txt" Width="50%" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="reqPriOther" ValidationGroup="pnl_2" Visible="false" runat="server" ControlToValidate="txtPriOther" Text="*" ErrorMessage="*" SetFocusOnError="true"></asp:RequiredFieldValidator>
</td>
</tr>
</table>
</asp:Panel>
</div>
<ajaxToolkit:CollapsiblePanelExtender ID="cpe_2" runat="Server"
TargetControlID="pnl_2"
ExpandControlID="pnl_2_Header"
CollapseControlID="pnl_2_Header"
Collapsed="True"
TextLabelID="lbl_2_Header"
ImageControlID="img_2_Header"
ExpandedText="(Hide Details...)"
CollapsedText="(Show Details...)"
ExpandedImage="~/images/collapse_blue.jpg"
CollapsedImage="~/images/expand_blue.jpg"
SuppressPostBack="true"
SkinID="CollapsiblePanelDemo" />
<Panel> 3rd Panel </Panel>
...
Code behind:
protected void rdoPri1_CheckedChanged(object sender, EventArgs e)
{
-- Some code here --
//SetFocus(pnl_2);
rdoPri1.Focus();
}

Answer 1

Web form server control event will trigger postback when Autopostback is set to true. It means your Set focus will not be working expected after one of the controls triggered postback.
So, my personal opinion, it you need such a UI/UX design, I would suggest to use client script instead.

Answer 2

Add the same control.Focus() or setfocus in page load, and add conditions like if needed
//in page load
if (rdoButton1.Checked) {//setfocus here}