Hi LtDel,
To achieve this you will need to insert an niFgen property node from the niFgen functions palette into your code flow somewhere in amongst the configuration vi steps. Wire up the reference and error inputs and outputs. Left click on the "property" and select >>output attributes>>sync duty cycle high. Right click and set to "write" then wire in a control or constant defining the % duty cycle for the high phase.
Hope this helps,
Nick