Question 8-26C3

A switching power supply (SPS) operates by rapidly turning a DC voltage on and off at a very high frequency, typically tens of kilohertz to megahertz, far exceeding the AC line frequency (50/60 Hz). This high-frequency switching allows for smaller, more efficient transformers and filter components. At the junction of CR6, CR7, and L1, you are measuring after the rectification stage (CR6, CR7) that converts the high-frequency pulses from the switching transformer or inductor. This rectification produces a rapidly fluctuating, unidirectional voltage – *pulsating DC*. The inductor L1 is designed to smooth these high-frequency pulses into a stable DC output, but at the point of measurement *before* L1, the pulses are still very evident. * **D) Pulsating DC much higher than line frequency:** This is correct. The diodes rectify the high-frequency switched waveform, creating pulsating DC. The frequency of these pulses is the power supply's switching frequency, which is intentionally much higher than the AC line frequency for efficiency and component size reduction. * **A) Filtered DC:** This would be the final output *after* L1 and the output capacitor have completed the filtering process, not at the input to the main filter inductor. * **B) Pulsating DC at line frequency:** This describes the output of a traditional linear power supply's rectifier, which operates directly on the 50/60 Hz AC mains. A switching power supply's operation is at much higher frequencies. * **C) AC at line frequency:** The diodes (CR6, CR7) are rectifying components; their output is unidirectional (DC), even if it's pulsating, not alternating current (AC). Also, the frequency is much higher than line frequency.