What We’ve Covered:

• Evolution of electric drives

• Three drive configurations

• Five functional blocks

• Motor types and selection

• Power sources (AC/DC)

Today’s Focus:

• Power semiconductor devices

• The heart of power converters

• Enable control and efficiency

• Device characteristics

• Selection for drives

Symbol and Structure:

Characteristics:

• Two-terminal device

• Uncontrolled (no gate)

• Conducts when forward biased (\(V_A > V_K\))

• Blocks when reverse biased

• Acts as automatic switch

I–V Characteristic:

Key Parameters:

• Forward voltage drop: \(V_F \approx 0.7\)–\(1.5\) V

• Reverse blocking voltage

• Forward current rating

What Happens:

1. Diode conducting forward current

2. Voltage reverses suddenly

3. Stored charge must be removed

4. Diode conducts reverse briefly

5. Reverse current peaks (\(I_{\mathrm{RR}}\))

6. Then diode blocks

Consequences:

• Switching losses

• EMI generation

• Limits switching frequency

• Voltage spikes in circuit

Recovery Time Components:

Types:

• Standard recovery: \(t_{rr} = 1\)–\(10\) \(\mu\)s

• Fast recovery: \(t_{rr} = 100\)–\(500\) ns

• Ultra-fast: \(t_{rr} < 100\) ns

1. Uncontrolled Rectifiers:

• Convert AC to fixed DC

• Single-phase and three-phase

• Simple DC motor drives

• Input stage of VFDs

2. Freewheeling Diodes:

• Provide path for inductive current

• Protect switching devices

• Energy recovery

• Essential in choppers and inverters

3. Voltage Clamping:

• Snubber circuits

• Overvoltage protection

• Transient suppression

4. Brake Choppers:

• Regenerative braking

• Dynamic braking resistors

• DC bus voltage control

Symbol and Structure:

Characteristics:

• Three-terminal device

• Semi-controlled (ON control only)

• Four-layer PNPN structure

• Latching behavior

• High power capability

Operating Principle:

1. Forward biased but OFF (blocking)

2. Gate pulse applied (\(I_G > I_{GT}\))

3. Device turns ON (latches)

4. Gate loses control

5. Remains ON while \(I_A > I_H\)

6. Turns OFF when current falls below holding current

Key Terms:

• \(I_{GT}\): Gate trigger current

• \(I_H\): Holding current

• \(I_L\): Latching current

1. Gate Triggering (Most Common)

   • Short positive pulse to gate

   • \(V_G \approx 1\)–\(2\) V, \(I_G > 10\)–\(200\) mA (device dependent)

   • Pulse width: \(10\)–\(100\) \(\mu\)s

   • Used in all controlled applications

2. Forward Voltage Triggering

   • \(V_{AK}\) exceeds breakover voltage \(V_{BO}\)

   • Uncontrolled, to be avoided

3. dv/dt Triggering

   • Rapid voltage rise can trigger

   • Undesired, prevented by snubbers

4. Temperature Triggering

   • High temperature can cause turn-on

   • Prevented by proper cooling

Natural (Line) Commutation

• AC supply voltage reverses naturally

• Current becomes zero automatically

• Used in phase-controlled rectifiers

• Simple, no extra circuitry needed

Forced Commutation

• External circuit forces current to zero

• Required for DC applications (choppers)

• Uses capacitors, inductors, auxiliary devices

• Complex and adds cost

Important Ratings:

• \(V_{DRM}\): Forward blocking voltage

• \(V_{RRM}\): Reverse blocking voltage

• \(I_{T(AV)}\): Average ON-state current

• \(I_{TSM}\): Surge current rating

• \(di/dt\) rating: Max current rise

• \(dv/dt\) rating: Max voltage rise

• \(t_q\): Turn-off time

Protection Requirements:

• Overvoltage:

  • Voltage rating 2–3\(\times\) normal

  • Crowbar circuits

  • Snubber circuits

• Overcurrent:

  • Fast-acting fuses

  • Current limiting reactors

• \(dv/dt\) protection:

  • RC snubber across device

  • Prevents false triggering

• \(di/dt\) protection:

  • Series inductors

  • Limits current rise rate

Primary Applications:

1. Phase-Controlled Rectifiers

   • DC motor drives (declining)

   • Controlled DC voltage output

   • 1-phase and 3-phase

   • Line frequency (50/60 Hz)

2. AC Voltage Controllers

   • Soft starters for induction motors

   • Reduced voltage starting

   • Simple speed control (limited)

3. Cycloconverters

   • Direct AC-AC conversion

   • Large low-speed drives

   • Ship propulsion, cement mills

Advantages:

• Very high power capability (MW range)

• High voltage ratings (to 10 kV)

• Robust and reliable

• Low ON-state losses and simple gate drive

Disadvantages:

• No turn-OFF control

• Poor power factor and high harmonic content

• Limited to line frequency

• Being replaced by IGBTs

Symbol:

Characteristics:

• Current-controlled device

• \(I_C = \beta \times I_B\)

• Low ON-state voltage (0.5–2 V)

• Requires continuous base current

• Second breakdown issue

Advantages:

• Low saturation voltage

• High current capability

• Good for linear applications

Disadvantages:

• Current-driven (complex drive)

• Slow switching speed and high switching losses

• Second breakdown risk

• Negative temperature coefficient

• Being phased out

Symbol (N-channel):

Characteristics:

• Voltage-controlled device

• Very high input impedance

• \(I_D\) controlled by \(V_{GS}\)

• Fast switching (\(<\)100 ns)

• Positive temp coefficient

Advantages:

• Very fast switching and simple gate drive

• No second breakdown

• Positive temp coefficient (parallel operation safe)

• Low switching losses

Disadvantages:

• Higher ON-state resistance

• Limited voltage (\(<\)1 kV) and current rating

• More expensive at high power

Symbol:

Hybrid Device:

• Combines MOSFET input + BJT output

• MOSFET: High impedance gate

• BJT: Low ON-state drop

• Best of both worlds!

Advantages:

• Voltage-controlled (simple drive)

• Low ON-state voltage (like BJT)

• High voltage/current ratings

• Medium switching speed (good balance)

• Large Safe Operating Area and easy to parallel

Disadvantages:

• Slower than MOSFET

• Tail current during turn-off

• More expensive than thyristors

Voltage Ratings:

• Standard: 600 V, 1200 V, 1700 V

• High voltage: 3.3 kV, 4.5 kV, 6.5 kV

• Suitable for industrial drives

Current Ratings:

• Low power: 10–50 A

• Medium power: 50–300 A

• High power: 300–3600 A

• Modular design for higher

Switching Frequency:

• Typical: 2–20 kHz

• Low voltage: up to 50 kHz

• Higher than thyristors

• Lower than MOSFETs

Performance Features:

• \(V_{CE(sat)}\) = 1.5–3 V (low loss)

• Turn-on time: 0.5–2 \(\mu\)s and Turn-off time: 1–5 \(\mu\)s

• Tail current increases losses

• Junction temp: up to \(175^\circ\)C

Module Configurations:

• Single switch

• Half/Full-bridge (2/4 IGBTs + diodes)

• Six-pack (3-phase inverter), Seven-pack (inverter + brake)

Power Loss Components:

1. Conduction losses \[P_{cond} = V_{CE(sat)} \times I_{avg}\]

2. Switching losses \[P_{sw} = \frac{1}{2}V_{DC} \times I \times (t_{on}+t_{off}) \times f_{sw}\]

3. Gate drive losses (small)

Total Power Loss: \[P_{total} = P_{cond} + P_{sw}\]

Cooling Methods:

• Natural convection

  • \(<\)10 W ; Simple heatsinks

• Forced air cooling

  • 10 W – 5 kW; Fans + heatsinks

  • Most common in drives

• Liquid cooling

  • \(>\)5 kW ; Water/glycol for high power density

Thermal Design:

• Junction temperature \(<125-150^\circ\)C

• Thermal resistance calculation

• Heatsink sizing

• Temperature monitoring

Overcurrent Protection:

• Fast-acting fuses

• Electronic current limiting

• Desaturation detection (IGBT)

• Response time \(<\)10 \(\mu\)s

Overvoltage Protection:

• Snubber circuits (RC, RCD)

• Voltage clamping diodes

• Active voltage control

• Proper PCB layout

Overtemperature Protection:

• Thermistor/thermocouple

• Thermal shutdown

• Derating at high temp

• Cooling system monitoring

Gate Drive Protection:

• Isolated power supplies

• Undervoltage lockout (UVLO)

• Shoot-through prevention

• Dead-time insertion

SiC MOSFETs:

• Voltage: 650 V – 3.3 kV

• Very fast switching (\(<\)50 ns)

• High temperature (\(200^\circ\)C)

• Lower losses than Si IGBT

• Higher frequency possible

• Smaller passive components

• Cost: 3–5\(\times\) Si devices

Applications:

• EV inverters (Tesla, others)

• High-efficiency drives

• Aerospace

• Renewable energy

Advantages over Silicon:

• 10\(\times\) breakdown field strength

• 3\(\times\) thermal conductivity

• Higher switching frequency

• Lower switching losses

• Smaller heatsinks

• Higher power density

• Better efficiency (98–99%)

Challenges:

• High cost (decreasing)

• Limited suppliers

• Gate drive design critical

• EMI considerations