Power Electronics Circuits
In a power supply that makes DC from the AC mains, which of the following is the main reason isolated converters, which isolate input from output with a transformer, are used?
- ATo separate input and output electrically to prevent electric shock, and to obtain large voltage ratios from the turns ratio
- BBecause a transformer alone, without any switching, can convert a DC voltage directly into a DC voltage of a different magnitude
- CBecause inserting a transformer always reduces the switching loss to zero
Show answer and explanation
Answer: A. To separate input and output electrically to prevent electric shock, and to obtain large voltage ratios from the turns ratio
In a power supply connected to the AC mains, electrically isolating the output side, which people may touch, from the input side is a basic safety requirement. An isolated converter applies a voltage switched at high frequency to a small transformer and rectifies its output into DC. The turns ratio of the transformer makes large step-down ratios, such as from several hundred volts to 5 V, possible without extreme duty cycles. Typical topologies include the flyback converter for low power, and the forward converter and full-bridge converter for higher power.
- A. Correct. Isolation for safety and large conversion ratios from the turns ratio are the main reasons.
- B. A transformer cannot convert DC. The voltage is first turned into AC by switching and then applied to the transformer.
- C. Inserting a transformer does not eliminate switching loss.