Set focus after a textbox is updated

Group, I have an asp textbox for "quantity" and an asp textbox for "product code."  When I change the quantity I want the focus to shift to textbox "product code."  Instead I change the quantity and the cursor doesn't appear anywhere.  I have to click in that textbox after the postback is finished.
Basically what I want to have happen is when I change a quantity, I want it to do the postback and after the postback I want the focus to be set to txtProductCode.
Any suggestions? 
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