Switched Mode Power Supplies Questions and Answers

Answer: D

The weight falls because an SMPS steps the voltage down at high frequency.

A conventional adapter steps the 50 Hz mains down directly, which needs a heavy iron-core transformer. An SMPS first rectifies the mains, then chops the DC at tens of kilohertz and steps it down at that frequency. For a given voltage the transformer equation \(V = 4.44\,f N A B\) shows that the core area and turns needed fall as the frequency rises, so the transformer, the heaviest part, becomes small and light.

Ferrite transformers (A) are the kind of core used at that frequency, so they are a consequence of the high-frequency design rather than the underlying reason. Power MOSFETs (B) are the switches that make the chopping possible but do not themselves cut the weight. Rectifiers in the first stage (C) do not distinguish the two designs, because a conventional adapter also rectifies.

Answer: B

The advantage of an on-line UPS is that there is no phase change or break in the output when the mains fails.

An on-line (double-conversion) UPS rectifies the mains into the battery and DC bus and always feeds the load from its inverter. When the mains fails nothing needs to switch, so the load sees no interruption or jump in phase. An off-line UPS feeds the load from the mains and switches to the inverter after it detects the failure, which takes a few milliseconds.

A true sine wave (A) depends on how the inverter is built; the topology does not fix it, so it is not the defining advantage. Power and backup time (C) are set by the inverter rating and battery bank, not by whether the UPS is on-line. Conversion efficiency (D) is actually lower in an on-line UPS, because all the power is converted twice.