Industrial Electronics (EX-504 (A)) - Unit 1 Short Notes

UNIT 1: Power Semiconductor Devices, Power Supplies, OP-AMPs, and PLCs

I. Power Semiconductor Devices

A. Silicon Controlled Rectifier (SCR)

Structure & Basic Operation:

A four-layer (PNPN), three-terminal device (Anode A, Cathode K, Gate G). It acts as a switch that can be turned ON by a gate signal and requires a separate commutation circuit to turn OFF.

• Forward Blocking: Anode (+ve) w.r.t Cathode, no gate signal → high impedance.

• Forward Conducting: Anode (+ve), gate pulse applied → low impedance (latches ON).

• Reverse Blocking: Cathode (+ve) w.r.t Anode → high impedance (like a diode).

Key Parameters:

• Latching Current ($$\displaystyle I_L $$): Minimum anode current required to maintain conduction immediately after gate signal is removed.

• Holding Current ($$\displaystyle I_H $$): Minimum anode current required to keep the SCR in the ON state. $$\displaystyle I_L > I_H $$.

[!TIP] Exam Distinction: $$\displaystyle I_L $$ is about entering conduction, $$\displaystyle I_H $$ is about staying in conduction. $$\displaystyle I_L $$ is always greater.

Turn-On Methods:

1. Gate Triggering: Standard method (positive current pulse to gate).

2. Forward Voltage Triggering: Avalanche breakdown at high anode-cathode voltage (undesirable).

3. dv/dt Triggering: Rapid rise in anode-cathode voltage charges junction capacitance, causing turn-on.

4. Thermal Triggering: High temperature increases leakage current, leading to thermal runaway.

Turn-Off (Commutation) Methods:

1. Natural Commutation: AC circuit where current naturally goes to zero (e.g., in controlled rectifiers).

2. Forced Commutation: Forced reversal of anode current.

   • Self-Commutated: Auxiliary SCR in circuit (e.g., in inverters).

   • Auxiliary Commutated: Separate commutating circuit (LC network).

   • Complementary Commutated: Two SCRs in parallel with opposite polarity.

Transistor Model & Anode Current:

SCR modeled as two coupled transistors (PNP $$\displaystyle T_1 $$, NPN $$\displaystyle T_2 $$).

\[ \alpha_1 I_A + \alpha_2 I_G = I_A \quad \Rightarrow \quad I_A = \frac{\alpha_2 I_G}{1 - (\alpha_1 + \alpha_2)} \]

Where $$\displaystyle \alpha_1, \alpha_2 $$ are current gains of $$\displaystyle T_1, T_2 $$. When $$\displaystyle \alpha_1 + \alpha_2 \ge 1 $$, regenerative feedback latches the SCR ON.

Static Characteristics:

• Forward Voltage Drop ($$\displaystyle V_T $$): 1-2V in conduction (low loss).

• Reverse Breakdown Voltage ($$\displaystyle V_{BR} $$): High (50V to several kV).

Causes of Damage & Protection:

• Overcurrent: Fuse, CB, or fast-acting electronic crowbar.

• Overvoltage: Snubber Circuit (RC or RCD across SCR), Varistor (MOV).

• dv/dt: Snubber circuit (capacitor across SCR).

• di/dt: Inductor in series with anode.

• Thermal Runaway: Adequate heat sink with thermal sensor shutdown.

Applications: AC/DC motor control, controlled rectifiers, inverters, power switching.

B. Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

Structure & Operation:

Voltage-controlled device. Gate insulated by SiO₂ (high input impedance ~10⁹ Ω).

• Enhancement Mode: Channel forms only when $$\displaystyle V_{GS} > V_{th} $$.

• Depletion Mode: Channel exists at $$\displaystyle V_{GS}=0 $$; can be depleted by $$\displaystyle V_{GS} $$.

• Power MOSFET (VDMOS): Vertical structure for high voltage/current. On-resistance ($$\displaystyle R_{DS(on)} $$) is key parameter.

Characteristics:

• Transfer Curve: $$\displaystyle I_D \propto (V_{GS} - V_{th})^2 $$ (quadratic in saturation).

• Output Curve: Pinch-off, saturation regions.

• Threshold Voltage ($$\displaystyle V_{th} $$): Gate voltage to create channel.

• Switching Speed: Very fast (nanoseconds), limited by gate charge ($$\displaystyle Q_g $$).

Advantages over BJT:

• Voltage-controlled (high input impedance, low drive power).

• No minority carrier storage → fast switching.

• No secondary breakdown.

• Easier parallel operation.

Limitations:

• Lower current density than IGBT/BJT.

• On-resistance increases with voltage rating ($$\displaystyle R \propto V_{BR}^{2.5} $$).

• Susceptible to gate oxide rupture by ESD/overvoltage.

Applications: Switch-mode power supplies (SMPS), DC-DC converters, motor drives, amplifiers.

C. Insulated Gate Bipolar Transistor (IGBT)

Structure & Operation:

Hybrid of MOSFET (input) and BJT (output). MOSFET controls base current of PNP BJT.

• Operation: Positive $$\displaystyle V_{GE} $$ creates N-channel, injects electrons into P⁺ base → conductivity modulation lowers $$\displaystyle V_{CE(sat)} $$.

• Minority Carrier Injection: Key to low conduction loss.

Characteristics:

• Output: Similar to BJT (saturation region).

• Transfer: MOSFET-like (threshold $$\displaystyle V_{GE(th)} $$).

• Saturation Voltage ($$\displaystyle V_{CE(sat)} $$): 1-3V (lower than MOSFET at high current).

• Switching: Fast (µs), but has tail current during turn-off due to minority carrier recombination.

Transistor Model:

Modeled as MOSFET driving a BJT (Darlington-like). On-state voltage:

\[ V_{CE(on)} = V_{CE(sat)} + I_C \cdot R_{on} \]

Where $$\displaystyle R_{on} $$ includes MOSFET channel resistance and BJT base resistance.

Features:

• High input impedance (MOSFET gate).

• Low saturation voltage (BJT-like).

• Fast switching (µs range).

• Good trade-off: Higher current/voltage than MOSFET, faster than BJT.

Comparison:

Applications: AC motor drives, inverters, UPS, welding, traction.

D. Comparative Analysis of SCR, MOSFET, and IGBT

II. Power Supply Circuits and Converters

A. Rectifiers

1. Single-Phase Half-Wave Rectifier (RL Load):

• Circuit: Diode in series with RL load.

• Operation: Diode conducts only during positive half-cycle. Current $$\displaystyle i_o $$ flows, creating magnetic field in inductor. When $$\displaystyle v_s $$ goes negative, inductor discharges through diode (freewheeling), maintaining current until it reaches zero.

• Waveforms: $$\displaystyle v_o $$ follows $$\displaystyle v_s $$ during conduction, decays exponentially during negative half. $$\displaystyle i_o $$ is continuous.

• Average Output Voltage:

  \[ V_{dc} = \frac{V_m}{\pi} \quad \text{(for purely resistive load)} \]

  For RL load (continuous conduction), same as resistive.

2. Single-Phase Full-Wave Bridge Rectifier (RL Load):

• Circuit: Four diodes in bridge configuration.

• Operation: During positive half, D1 & D2 conduct; during negative half, D3 & D4 conduct. Load current always flows in same direction. Inductor smoothens current.

• Waveforms: $$\displaystyle v_o $$ is absolute value of $$\displaystyle v_s $$. $$\displaystyle i_o $$ is nearly continuous.

• Average Output Voltage:

  \[ V_{dc} = \frac{2V_m}{\pi} \]

Advantages of Full-Wave Rectifiers:

• Higher efficiency (80% vs 40.5% for half-wave).

• Lower ripple frequency (2f vs f).

• Better transformer utilization (secondary used full cycle).

• Continuous load current (with sufficient inductance).

Disadvantages:

• Requires 4 diodes (or center-tapped transformer with 2 diodes).

• Higher Peak Inverse Voltage (PIV) across each diode ($$\displaystyle V_{PIV} = V_m $$ for bridge, $$\displaystyle 2V_m $$ for center-tap).

• Slightly higher cost.

B. Switch Mode Power Supplies (SMPS)

Principle:

Switch (transistor) operates in cutoff/saturation (not linear region). Energy stored in inductor/capacitor during ON time, delivered to load during OFF time. High efficiency (80-90%) vs linear regulator (40-60%).

Types of Switching Regulators:

Switched-Mode Voltage Regulator (General Block):

Vin → Switch (MOSFET) → Inductor → Diode → Capacitor → Load

              ↑          ↑

            Driver     Control IC (PWM)

• Continuous Conduction Mode (CCM): Inductor current never zero. Easier control, higher efficiency at high load.

• Discontinuous Conduction Mode (DCM): Inductor current reaches zero. Simpler, but higher ripple.

Design Example (Buck Regulator Inductor):

Given: $$\displaystyle V_i = 110V $$, $$\displaystyle V_o = 60V $$, $$\displaystyle I_o = 30A $$, $$\displaystyle f_s = 25kHz $$, $$\displaystyle \Delta I_L = 1.2A $$ (peak-to-peak).

\[ D = \frac{V_o}{V_i} = \frac{60}{110} = 0.545 \]

\[ L = \frac{V_i \cdot D}{\Delta I_L \cdot f_s} = \frac{110 \times 0.545}{1.2 \times 25 \times 10^3} = \frac{59.95}{30,000} \approx 2.0 \, \text{mH} \]

\[ I_{L(peak)} = I_o + \frac{\Delta I_L}{2} = 30 + 0.6 = 30.6 \, \text{A} \]

Full Bridge Regulator from Buck:

Use four switches (Q1-Q4) in bridge configuration across input. By switching diagonally (Q1,Q4 ON then Q2,Q3 ON), the load sees a polarity-reversing voltage with duty cycle D. Average output $$\displaystyle V_o = 2DV_i $$. This is a full-bridge converter.

Advantages of SMPS:

• High efficiency, small size/weight (high frequency).

• Wide input range, good regulation.

• Multiple output voltages possible.

Disadvantages:

• EMI/RFI noise (needs filtering, shielding).

• Complexity, need for careful PCB layout.

• Output ripple higher than linear regulator.

C. Linear Voltage Regulators (Brief)

• Series Regulator: Pass transistor in series, feedback controls conduction. Simple, low noise, but poor efficiency ($$\displaystyle P_{loss} = (V_i - V_o)I_o $$).

• Shunt Regulator: Zener diode + series resistor. Very inefficient for high current.

• Limitation: Dropout Voltage ($$\displaystyle V_{do} $$) minimum $$\displaystyle V_i - V_o $$ to maintain regulation. Heat dissipation major issue.

D. Uninterruptible Power Supplies (UPS)

On-Line UPS (Double-Conversion):

     ┌─────────┐     ┌─────────┐     ┌─────────┐     ┌─────────┐

AC ─▶│ Rectifier│───▶│  DC Bus │───▶│ Inverter│───▶│   AC    │───▶ Load

     │  + Battery│     │ (Battery)│     │         │     │         │

     └─────────┘     └─────────┘     └─────────┘     └─────────┘

                          │

                     Static Switch (bypass)

• Operation: AC → Rectifier → DC bus (charges battery). DC → Inverter → AC to load. Continuous power processing. No transfer time.

• Functions: Backup (battery), surge protection, voltage regulation (inverter controls output).

Types Comparison:

• Off-Line (Standby): AC → Load directly. Battery/inverter only on failure. Transfer time (ms). Cheaper.

• Line-Interactive: Adds transformer with tap switching or buck-boost converter for voltage regulation. Better protection, moderate cost.

E. Active Power Line Conditioners

Concept: Use power electronics to dynamically correct power quality issues (sags, swells, harmonics, flicker).

1. Dynamic Voltage Restorer (DVR):

     ┌─────────┐     ┌─────────┐     ┌─────────┐

AC ─▶│   DVR   │───▶│  Load   │

     │ (Series)│     │         │

     └─────────┘     └─────────┘

• Operation: Series-connected voltage source converter injects voltage $$\displaystyle V_{inj} $$ in phase with sag to boost load voltage. Uses DC capacitor source.

• Function: Compensates voltage sags/swells.

2. Active Power Filter (APF):

     ┌─────────┐     ┌─────────┐     ┌─────────┐

AC ─▶│   APF   │───▶│  Load   │

     │(Shunt)  │     │         │

     └─────────┘     └─────────┘

• Operation: Shunt-connected current source converter injects harmonic currents (180° out of phase with load harmonics) to cancel them. Also compensates reactive power.

• Function: Harmonic mitigation, power factor correction.

III. Operational Amplifiers (OP-AMPs) and Applications

A. Basic OP-AMP Characteristics

Ideal OP-AMP Assumptions:

• $$\displaystyle A_{OL} \to \infty $$ (Open-loop gain)

• $$\displaystyle Z_{in} \to \infty $$ (Input impedance)

• $$\displaystyle Z_{out} = 0 $$ (Output impedance)

• Bandwidth $\to \infty$, slew rate $\to \infty$, CMRR $\to \infty$, offset = 0.

Key Practical Parameters:

• CMRR (Common Mode Rejection Ratio): $$\displaystyle CMRR = \frac{A_d}{A_{cm}} $$ (differential gain / common-mode gain). High CMRR rejects noise on both inputs.

• Slew Rate (SR): Maximum rate of output voltage change ($V/\mu s$). Limits large-signal bandwidth.

• Gain-Bandwidth Product (GBW): Constant for a given OP-AMP. $$\displaystyle f_T = A_{OL} \times f_{3dB} $$.

• Input Offset Voltage ($$\displaystyle V_{io} $$): Voltage needed to make output zero.

• Input Bias Current ($$\displaystyle I_B $$): Average current into inputs.

B. Basic Configurations

1. Inverting Amplifier:

          Rf

Vin ──┬───╱╱╱───┬─── Vout

      │         │

      └───╱╱╱──┘

          Rin

      │

     (-) OP-AMP

      │

     GND

• Voltage Gain: $$\displaystyle A_v = -\frac{R_f}{R_{in}} $$

• Features: Virtual ground at (-) input, high input impedance ($$\displaystyle R_{in} $$), inverts signal.

• Applications: Inverting scaling, summing amplifier ($$\displaystyle V_{out} = -R_f (\frac{V_1}{R_1} + \frac{V_2}{R_2} + ...) $$).

2. Non-Inverting Amplifier:

Vin ────┬───(-) OP-AMP ──── Vout

        │      │

        └───╱╱╱──┘

             Rf

        │

       GND

        │

       (+) OP-AMP

        │

       GND (via Rin to GND)

• Voltage Gain: $$\displaystyle A_v = 1 + \frac{R_f}{R_{in}} $$

• Features: Very high input impedance ($$\displaystyle Z_{in} \to \infty $$), no phase inversion.

• Voltage Follower (Unity Gain Buffer): $$\displaystyle R_f=0 $$, $$\displaystyle R_{in}=\infty $$. $$\displaystyle A_v=1 $$. Used for impedance buffering.

C. OP-AMP Based Circuits

1. Comparator:

• Basic: No feedback. Output saturates to $$\displaystyle +V_{sat} $$ if $$\displaystyle V_+ > V_- $$, else $$\displaystyle -V_{sat} $$.

• Window Comparator: Two comparators (upper/lower thresholds) with logic gate (AND for inside band, OR for outside).

  
  Vin ──┬───[+]───┬─── AND ── Vout (within band)
  
        │        │
  
        └───[-]──┘
  
        │        │
  
        └───[+]──┘
  
  

2. Filters (Active RC):

• Low-Pass Filter (1st Order):

  
        Rf
  
  Vin ──┬───╱╱╱───┬─── Vout
  
        │         │
  
        C         │
  
        │         │
  
       (-)       GND
  
        │
  
       GND (via Rin)
  
  

  Transfer: $$\displaystyle H(s) = \frac{V_{out}}{V_{in}} = \frac{-1}{1 + sR_f C} $$ (inverting). Cutoff: $$\displaystyle f_c = \frac{1}{2\pi R_f C} $$.

  Non-inverting version: $$\displaystyle H(s) = \frac{1}{1 + sR_f C} $$.

3. Oscillators:

• Wien Bridge Oscillator:

  
        Rf
  
  Vout ──┬───╱╱╱───┬───(-) OP-AMP
  
         │         │
  
         C         │
  
         │         │
  
        (+)───╱╱╱──┘
  
         │      R
  
         C      │
  
         │      │
  
        GND    GND
  
  

  • Frequency: $$\displaystyle f_o = \frac{1}{2\pi RC} $$

  • Condition: $$\displaystyle A_v \ge 3 $$ for oscillation (Barkhausen: $$\displaystyle |A\beta|=1 $$, phase shift 0°).

  • Amplitude Stabilization: Use diodes/NTC thermistor in feedback path to limit gain.

  • Application: Audio frequency generation (1kHz-1MHz).

• Relaxation Oscillator (Square Wave):

  OP-AMP with positive feedback (hysteresis) and RC integrator. Output switches between rails when capacitor voltage crosses thresholds. Frequency: $$\displaystyle f \approx \frac{1}{2RC \ln(\frac{1+\beta}{1-\beta})} $$.

4. Function Generator:

• Sine: Wien Bridge Oscillator.

• Square: Comparator with integrator (Schmitt trigger + RC).

• Triangle: Integrator with square wave input (from comparator).

D. General Applications of OP-AMPs

• Instrumentation: Instrumentation amplifiers (high CMRR, high $$\displaystyle Z_{in} $$), active filters.

• Signal Conditioning: Voltage/current conversion, level shifting, scaling.

• Control Systems: Error amplifier (compare ref vs feedback), PID controller (using integrator/differentiator).

IV. Programmable Logic Controllers (PLCs)

A. Introduction & Fundamentals

Definition: Industrial digital computer for logic control, sequencing, timing, data handling in harsh environments. vs Relay Controllers:

Functions: Logic (AND/OR/NOT), timers, counters, data manipulation, communication, PID, motion control.

B. PLC Hardware Architecture

┌─────────────┐

│ Power Supply│

│ (AC/DC)     │

└──────┬──────┘

       │

┌──────▼──────┐     ┌─────────────┐

│    CPU      │────▶│   Memory    │

│ (Processor) │     │ (ROM/RAM)   │

└──────┬──────┘     └─────────────┘

       │

┌──────▼─────────────────────────────┐

│          I/O System                │

│  ┌─────────┐          ┌─────────┐ │

│  │ Digital │          │ Analog  │ │

│  │ (DI/DO) │          │ (AI/AO) │ │

│  └─────────┘          └─────────┘ │

└────────────────────────────────────┘

       │

┌──────▼──────┐

│Communication│

│ Ports (RS232│

│ /485, Eth)  │

└─────────────┘

• Types:

  • Compact (Fixed I/O): All in one unit. Small applications.

  • Modular (Rack-Based): CPU, power, I/O modules separate. Scalable.

• I/O Modules:

  • Digital: Sinking (NPN) vs Sourcing (PNP) configurations.

  • Analog: Voltage (±10V) or current (4-20mA) input/output.

  • Specialty: High-speed counter, thermocouple, RTD, motion.

C. PLC Programming (IEC 61131-3)

1. Ladder Logic (LD): Most common. Relay logic analogy.

• Symbols: ──| |── (NO contact), ──|/|── (NC contact), ──( )── (coil/ output).

• Rung: Logic from left power rail to right. Evaluated left-to-right, top-to-bottom.

• Example: Motor Start-Stop with Seal-in

  
  Ladder Rung 1:  [ Start PB (NO) ]──[ Stop PB (NC) ]──[ Motor Coil (SEAL) ]──( Motor )
  
  Ladder Rung 2:  [ Motor Coil (SEAL) ]──────────────────────────────────────( )
  
  

  PB = Push Button. Seal-in contact is parallel to Start PB.

2. Other Languages:

• Instruction List (IL): Text-based (like assembly).

• Function Block Diagram (FBD): Graphical blocks (like electronic schematics).

• Structured Text (ST): High-level text (Pascal-like).

• Sequential Function Chart (SFC): Steps and transitions (for sequential processes).

Basic Programming Examples:

• Timer (TON - On-Delay): [Input]──[TON T1, PT=5s]──(Output after 5s)

• Counter (CTU - Up): [Count Pulse]──[CTU C1, PV=10]──(Done when C1=10)

• Traffic Light: Sequential control using timers and internal bits.

D. PLC Operating Modes

• Program Mode: Edit/download program. CPU stops scanning. Used by programmer.

• Run Mode: Execute program cyclically (scan → input read → logic solve → output update). Controls process.

• Test/Monitor Mode: Run mode but with forced I/O, watch variables. No effect on controlled process if done carefully.

E. PLC Communication & Networking

Standard Protocols:

1. Modbus (RTU/TCP): Simple master-slave, widely used.

2. Profibus (DP/PA): Fast deterministic, industrial standard (Siemens).

3. DeviceNet: CAN-based, device-level network.

4. Ethernet/IP: Industrial Ethernet (CIP protocol).

5. CANopen: CAN-based, motion control.

Role: Plant-wide integration (PLC ↔ SCADA/HMI ↔ other PLCs/servers/variables).

F. PLC Applications & System Integration

Applications:

• Assembly lines (pick-and-place, sorting).

• Material handling (conveyors, AS/RS).

• Packaging (filling, labeling, capping).

• Process control (batch, discrete, continuous).

Interface with Power Electronics:

• Control: PLC digital outputs → gate drivers → SCR/IGBT/MOSFET in drives/inverters/soft starters.

• Feedback: PLC analog inputs ← sensors (encoders, CTs, PTs, RTDs) for closed-loop control.

• Example: PLC controls VFD (Voltage Frequency Drive) via analog output (0-10V/4-20mA) or communication (Modbus).

Event-Driven Devices:

• Concept: High-priority tasks triggered by external events (interrupts), not by cyclic scan.

• Implementation: Hardware interrupt modules (high-speed counter, encoder) or software interrupts (communication message).

• Example: Emergency stop (hardwired safety input) triggers immediate output shutdown, bypassing normal scan.

V. Special Topics and Protection Strategies

A. Protection Circuits for Power Devices

1. Overvoltage Protection:

• Snubber (RC/RCD): Across device. Limits dv/dt, absorbs transient energy.

  
  ┌───R───┐
  
  │       │
  
  ┌───| |──┴─── Device (SCR/IGBT)
  
  │   C     │
  
  └─────────┘
  
  

• Varistor (MOV): Voltage-dependent resistor. Clamps overvoltage.

• Crowbar Circuit: SCR/TSCR across supply. On overvoltage, SCR fires, shorts supply → fuse blows. Fast protection.

2. Overcurrent Protection:

• Current Sensing: Shunt resistor, CT, Hall effect sensor.

• Current Limiting: Series resistor, active current control (feedback to gate drive).

• Electronic Crowbar: Fast detection circuit → triggers SCR across supply → immediate shutdown.

3. Thermal Protection:

• Heat sink with thermal interface material.

• Thermal sensor (thermistor) → shutdown circuit.

B. Power Losses in Semiconductor Devices

1. Conduction Losses ($$\displaystyle P_{cond} $$):

• MOSFET: $$\displaystyle P_{cond} = I_{rms}^2 \cdot R_{DS(on)} $$

• IGBT/BJT: $$\displaystyle P_{cond} = V_{CE(sat)} \cdot I_{avg} $$ (approx)

• SCR: $$\displaystyle P_{cond} = V_T \cdot I_{avg} $$

2. Switching Losses ($$\displaystyle P_{sw} $$):

During $$\displaystyle t_{on} $$ and $$\displaystyle t_{off} $$, voltage and current overlap.

\[ P_{sw} = \frac{1}{T} \left( \int_{t_{on}} v(t)i(t)dt + \int_{t_{off}} v(t)i(t)dt \right) \approx \frac{1}{2} (V \cdot I_{avg}) (t_{on} + t_{off}) \cdot f_{sw} \]

• Increases with switching frequency $$\displaystyle f_{sw} $$.

Loss Reduction Methods:

• Soft Switching: Zero-Voltage Switching (ZVS) or Zero-Current Switching (ZCS) to eliminate overlap.

• Proper Device Selection: Match $$\displaystyle V_{BR} $$ and current rating to application.

• Synchronous Rectification: Use MOSFET instead of diode (lower forward drop).

• Optimize Gate Drive: Strong gate drive reduces $$\displaystyle t_{on/off} $$.

C. Integrated Short Notes (From Past Exams)

Switch Mode Power Supply (SMPS)

Principle: Switching regulator operates transistor in saturation/cutoff (not linear). Energy stored in inductor/capacitor during ON time, delivered to load during OFF time.

Types: Buck (step-down), Boost (step-up), Buck-Boost (inverting), Flyback, Forward, Half/Full Bridge.

Advantages: High efficiency (80-90%), small size/weight (high frequency), wide input range.

Disadvantages: EMI, complexity, output ripple.

Applications: Computer power supplies, telecom, industrial controls.

Power MOSFET

Structure: Vertical DMOS (double-diffused). Gate oxide (SiO₂) insulated.

Operation: Voltage-controlled. Enhancement mode most common. $$\displaystyle I_D \propto (V_{GS}-V_{th})^2 $$.

Characteristics: High input impedance (~10⁹ Ω), fast switching (ns), no secondary breakdown.

Limitations: On-resistance increases with voltage rating, vulnerable to ESD/gate overvoltage.

Applications: SMPS, DC-DC converters, motor drives, amplifiers.

Wien Bridge Oscillator

Circuit: OP-AMP with positive feedback through Wien network (series RC || parallel RC).

Frequency of Oscillation: $$\displaystyle \boxed{f_o = \dfrac{1}{2\pi RC}} $$

Condition: $$\displaystyle A_v \ge 3 $$ (for sustained oscillation).

Amplitude Stabilization: Diodes or thermistor in feedback path to limit gain.

Application: Audio frequency generation (1Hz-1MHz).

Event-Driven Device

Definition: A device or task that executes in response to a specific external or internal event (interrupt), rather than at fixed intervals.

Role in Control Systems: Provides immediate response to critical conditions (emergency stop, high-speed pulse counting, communication messages). Breaks cyclic scan for high-priority actions.

Example in PLC: Hardware interrupt module for encoder feedback, communication interrupt for Modbus message.

Final Exam Strategy: Focus on derivations (rectifier $$\displaystyle V_{dc} $$, SCR transistor model, SMPS inductor calc), comparisons (SCR/MOSFET/IGBT, rectifiers, PLC vs relay), and diagrams (SCR VI char, SMPS topologies, OP-AMP circuits, PLC block diagram). Practice past numericals (buck inductor, Wien freq). Always mark holding/latching current, CMRR/slew rate, duty cycle, conduction/switching losses clearly.