2 When Can Extreme Narrow Input Pulses Happen in a Power Stage?

AC/DC power supplies are widely used to transform grid side AC input voltage to stable DC voltage in personal electronics, industrial and automotive applications. A Power Factor Correction (PFC) stage is incorporated to reduce harmonics and protect the grid. In Totem Pole PFC configurations and three phase full bridge PFC designs, there are very short duty cycles with fast switching MOSFETs at every AC input zero crossing. Some engineers also implement a soft start at zero crossings to avoid large current spikes. In this type of design, the duty cycle of power switch can be commanded to a very low value upon restart after a zero crossing.

In hard switching DC/DC converter systems, the output voltage can fluctuate during load transients, either from no load to high load, or high load to low load. Under these conditions, primary driver can send extreme low or high duty cycles command to adjust to the outer voltage loop feedback signal.

Figure 2-1 Narrow On Pulse and Narrow OFF Pulse