Buck converters- No Load condition
A synchronous buck converter has no problem because it has two low impedance states in the push-pull output - it is either switch hard to the incoming supply voltage or switched hard to 0V. In other words it's a voltage waveform generator and, a simple LC low pass filter then behaves as an averager: -



However, it's different for the non-synchronous buck regulator because it only uses a single pass transistor and a flyback diode. Under these circumstances, and assuming a perfectly efficient scenario, the duty cycle MUST drop when the load current decreases. If the duty cycle doesn't drop, the output voltage will rise beyond acceptable limits.

Under near-no-load conditions a non-synchronous buck regulator must go into a different mode of operation where instead of operating at a fixed frequency of (say) 100 kHz, it pulses at a much lower rate. This is quite common to happen in many buck circuits.