In a fixed-frequency switching power supply, when does the pulse width of the switching circuit increase?

In a fixed-frequency switching power supply, the pulse width of the switching circuit increases when the load impedance decreases. This relationship is rooted in the fundamental operation of voltage regulation in switching converters.

When the load impedance decreases, this typically results in an increase in load current. The power supply must respond to this change to maintain a stable output voltage. In a feedback-controlled system, when the load demands more current (due to lower impedance), the control circuit detects that the output voltage is starting to drop. To correct for this and bring the voltage back to the desired level, the pulse width modulation (PWM) controller increases the pulse width of the switching signal. A wider pulse means that the switch is on for a longer duration, allowing more energy to be transferred to the load per cycle, thus raising the output voltage to the target level.

Understanding the inverse relationship between load impedance and necessary pulse width helps clarify why decreasing impedance leads to an increase in pulse width in a fixed-frequency context, ensuring the output voltage remains stable under varying load conditions.