lecture 2 power electronics

What is an ideal switch?

An ideal switch has:

• Zero voltage when ON (v = 0)

• Zero current when OFF (i = 0)

• No power loss

• Instant switching

• No polarity or magnitude limits

Why are real switches not ideal?

Real devices have:

• Voltage drops

• Finite switching speeds

• Power limits

• Polarity restrictions

What is a restricted switch?

A switch that is ideal except for polarity constraints (e.g., only conducts in one direction).

What is an ideal diode (FCRB)?

A Forward-Conducting Reverse-Blocking switch:

• Conducts forward current

• Blocks reverse voltage

• No voltage drop or leakage

What are the main switching devices in power electronics?

• Diodes

• BJTs (Bipolar Junction Transistors)

• FETs (Field Effect Transistors)

• SCRs (Thyristors)

Key characteristics of power diodes?

• Forward drop ≈ 1 V

• High current (up to thousands of amps)

• High voltage ratings (up to kV range)

• Speed varies (ns to μs)

What is a key advantage of FETs?

• Fast switching

• Easy parallel operation

• Voltage-controlled

What is an SCR?

• A thyristor device

• Latches ON after triggering

• Requires external action to turn OFF

Why can’t diode circuits be solved directly?

Because diode states (ON/OFF) are unknown beforehand.

What is the trial method?

1. Assume diode states

2. Solve circuit

3. Check consistency:

   • KCL and KVL satisfied

   • ON → current > 0

   • OFF → voltage < 0

What happens if assumptions are wrong?

Reassign diode states and repeat.

What is Fourier analysis?

Representation of any periodic signal as a sum of sinusoids.

General Fourier series form?

• DC component + sum of sine and cosine harmonics

What is the fundamental frequency?

The lowest frequency component (n = 1).

What are harmonics?

Multiples of the fundamental frequency.

What is the DC component?

The average value of the waveform.

Why is Fourier analysis important in converters?

Switching creates multiple frequencies → need analysis of harmonics.

What determines average power transfer?

Only matching frequency components of voltage and current.

What is frequency matching?

Power transfer occurs only when voltage and current share the same frequency.

What is a switching function?

A periodic function describing ON/OFF behavior of switches.

What defines a switching function?

• Duty cycle (D)

• Frequency

• Phase