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Power Electronics: Power semiconductor switches · The Ideal Switch and Power Losses

Why power electronics runs on switches, and where real switches lose energy through conduction, switching and heat.

Power Electronics · 25 questions

Try 3 questions

1. Why do power converters drive transistors fully on or fully off instead of partly on?

  1. An ideal switch dissipates almost no power
  2. Fully on transistors need no cooling at all
  3. Partly on transistors cannot carry current
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Answer: An ideal switch dissipates almost no power

A switch that is on has nearly zero voltage across it and one that is off has nearly zero current, so the product V × I, the lost power, stays small.

2. A linear regulator drops 12 V down to 5 V for a 1 A load. Roughly what is its efficiency?

  1. About 58%
  2. About 42%
  3. About 95%
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Answer: About 42%

The load gets 5 W while the source supplies 12 W, so efficiency is 5/12, about 42%; the rest becomes heat in the regulator.

3. In that same 12 V to 5 V linear regulator at 1 A, how much power heats the pass transistor?

  1. 12 W
  2. 7 W
  3. 5 W
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Answer: 7 W

The transistor carries the full 1 A while dropping the leftover 7 V, so it dissipates 7 V × 1 A = 7 W.

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