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Power Electronics: Devices, Power Conversion and Converter Circuits

Power electronics changes electrical power from one form to another using switches that are either fully ON or fully OFF, so little energy is wasted. Rectifiers change AC to DC, choppers change DC to a different DC level (output = duty cycle × input), inverters change DC to AC. Common devices: diode, thyristor (SCR), power MOSFET and IGBT.

🎬 Step-by-step story

  1. Here is the mains: an AC sine wave. The top graph is the input. With nothing in the middle, the output is the same AC.
  2. Now put one diode in the middle. A diode lets current go one way only. The orange half is cut off. The lamp gets weak power. This is a half-wave rectifier.
  3. Use four diodes in a bridge. The orange half is flipped upright. Nothing is thrown away. The lamp is about twice as bright.
  4. Now the input is steady DC. A switch turns ON and OFF very fast. The longer it stays ON, the higher the average voltage. This is a chopper.
  5. Four switches flip in turn. The steady DC becomes a back-and-forth wave. This is an inverter, as in a home UPS.
  6. Free play: press each converter and slide the ON time. Watch the green average line and the lamp.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why is there nothing in the middle in step 0?

It is a plain wire so you can compare. The output equals the input.

Where does the cut half go?

In a half-wave rectifier the diode blocks it, so that energy is simply not delivered. That is why it is wasteful.

How does a bridge flip the negative half?

In each half cycle two of the four diodes conduct and always send current through the load in the same direction.

Why is a chopper efficient while a resistor divider is not?

A resistor wastes the extra voltage as heat. A switch just spends less time ON, so almost nothing is wasted.

How can switches make AC from DC?

By flipping the connection to the load back and forth, the load sees the voltage reverse. Watch the output swing in the 3D.

Does the lamp really get 64% with the full-wave?

The average of the humps is 63.7% of the peak. It is twice the half-wave average of 31.8%.

Power electronic devices: switches that waste little

A normal transistor amplifier holds a voltage and a current at the same time, and gets hot. A power-electronic switch is either fully ON (current flows, voltage across it is near zero) or fully OFF (voltage across it, no current). Power lost = voltage × current, so both cases lose very little. That is why converters are efficient (often above 90%).

Power conversion: four jobs

Input power is changed to the form the load needs. There are four basic jobs:

Efficiency = power out ÷ power in × 100%. The difference is lost as heat, so power electronics often need a heat sink.

Converter circuits

Half-wave rectifier: one diode. Only the positive half passes. Vavg = Vm / π ≈ 0.318 Vm.

Full-wave bridge rectifier: four diodes. Both halves become positive. Vavg = 2Vm / π ≈ 0.637 Vm, and Vrms = Vm / √2. A capacitor across the load smooths the humps into near-steady DC.

Chopper (buck): a switch ON for time ton out of every period T. The duty cycle D = ton / T. Average output Vout = D × Vin. An inductor and capacitor smooth the pulses.

Single-phase inverter: four switches in an H-shape. Switches 1 and 4 ON gives +V; switches 2 and 3 ON gives −V. Alternating them makes a square wave; a filter or finer switching makes it closer to a sine.

Try it: play with the converters

Open the 3D. Press Half and look at the lamp: it shows about 32%. Predict: what will Full show? (About 64%, twice.) Press Chopper. Move ON time to 25%. Predict the lamp, then check it. Then press Inverter and notice the output swings positive and negative.

At home: a table-fan regulator or a dimmer lamp is a power converter. Turn it slowly and see the brightness change smoothly, though the switch is flicking ON and OFF 100 times a second.

Key formulas and definitions

Worked examples

1. A half-wave rectifier gets a sine of peak 325 V. Find the average output.

V_avg = V_m / π = 325 / 3.1416 ≈ 103.5 V.

2. The same input goes to a full-wave rectifier. Find V_avg.

V_avg = 2 V_m / π = 650 / 3.1416 ≈ 206.9 V, twice the half-wave value.

3. A chopper has V_in = 48 V and duty cycle 25%. Find V_out.

V_out = D × V_in = 0.25 × 48 = 12 V.

4. A chopper runs from 12 V and must give 9 V. What duty cycle is needed?

D = V_out / V_in = 9 / 12 = 0.75 = 75%.

5. A converter takes 100 W and gives 95 W. Find its efficiency and the heat loss.

η = 95 / 100 = 95%. Heat lost = 100 − 95 = 5 W.

6. A half-wave rectifier with peak 100 V feeds a 50 Ω resistor. Find the average current.

V_avg = 100 / π = 31.8 V. I_avg = 31.8 / 50 ≈ 0.637 A.

Common mistakes

Practice quiz

1. A rectifier converts:
2. Which device is NOT controllable by a gate?
3. A chopper with D = 0.5 and V_in = 20 V gives:
4. An inverter changes:
5. Switching converters are efficient because the switch is:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is power electronics in simple words?

It is the use of electronic switches to control and change electrical power: AC to DC, DC to DC, DC to AC, or AC to AC.

What is duty cycle?

It is the fraction of each period for which a switch stays ON. 50% means ON half the time.

What is the difference between an SCR and a MOSFET?

An SCR is turned ON by a pulse and stays ON until current falls to zero, so it suits slower AC jobs. A MOSFET can be turned ON and OFF at will, very fast, so it suits high-frequency converters.

Where this is taught

Japan高校（専門学科）1〜3年Electrical Machines

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