Unit I: Study of Power Devices#
Latching and Holding Current (Questions 1, 2, 3)#
Question: What are latching and holding currents? What is the difference?#
Latching Current (IL):
Minimum anode current needed during turn-on for SCR to remain ON after gate signal removal.
Related to turn-on process.
Holding Current (IH):
Minimum anode current needed to keep SCR in ON state.
Falling below IH turns SCR OFF.
Related to maintaining conduction / turn-off condition.
Difference:
IL > IH.
IL associated with turning ON, IH associated with staying ON.
Power Device Comparison (Questions 4, 12, 23)#
Question: Difference between SCR, MOSFET, IGBT.#
SCR (Silicon Controlled Rectifier):
Type: Thyristor, current-controlled latching switch.
Structure: PNPN.
Control: Gate pulse turns ON; turns OFF when anode current < IH or reverse biased.
Speed: Slow.
Ratings: High V, High I.
Loss: Low conduction loss.
Use: Phase control, high power rectification.
Power MOSFET:
Type: Field Effect Transistor, voltage-controlled.
Structure: Unipolar (N or P channel).
Control: Gate-Source Voltage (VGS) controls ON/OFF.
Speed: Very Fast.
Ratings: Medium V, Medium I (Si); Higher with SiC/GaN.
Loss: Higher conduction loss (RDS(on)), low switching loss.
Use: High-frequency SMPS, low/medium power.
IGBT (Insulated Gate Bipolar Transistor):
Type: Hybrid (MOSFET input, BJT output), voltage-controlled.
Structure: Combines MOS gate and Bipolar structure.
Control: Gate-Emitter Voltage (VGE) controls ON/OFF.
Speed: Medium (faster than SCR, slower than MOSFET).
Ratings: High V, High I.
Loss: Low conduction loss (like BJT), higher switching loss than MOSFET.
Use: Motor drives, inverters, medium/high power.
Question: Difference between BJT & MOSFET.#
BJT (Bipolar Junction Transistor):
Control: Current controlled (Base current).
Input Impedance: Low.
Structure: Bipolar (uses electrons and holes).
Speed: Slower than MOSFET.
Conduction Loss: Low Vce(sat).
Thermal Stability: Prone to thermal runaway (negative temp coefficient).
MOSFET:
Control: Voltage controlled (Gate voltage).
Input Impedance: Very High.
Structure: Unipolar (uses majority carriers).
Speed: Faster than BJT.
Conduction Loss: Resistive (RDS(on)).
Thermal Stability: More stable (positive temp coefficient).
Question: Use / Application of each device.#
SCR: High power AC voltage controllers (dimmers, heating), controlled rectifiers, HVDC transmission.
MOSFET: High-frequency switching power supplies (SMPS), DC-DC converters, low voltage motor control, automotive.
IGBT: Medium-to-high power motor drives (AC drives, servo drives), UPS, induction heating, traction control, renewable energy inverters (solar, wind).
BJT: Power amplifiers, linear power supplies (less common in modern high-power switching).
DIAC: Triggering device for TRIACs.
TRIAC: AC power control (light dimmers, small motor speed control, appliance control).
Turning ON Power Devices (Question 5)#
Question: How do you turn on SCR / MOSFET / IGBT?#
SCR:
Apply positive Anode-Cathode voltage (VAK > 0).
Inject positive current pulse into Gate terminal (relative to Cathode).
Anode current must exceed Latching Current (IL).
MOSFET (N-Channel Enhancement):
Apply positive Drain-Source voltage (VDS > 0).
Apply Gate-Source voltage (VGS) greater than Threshold Voltage (Vth).
IGBT (N-Channel):
Apply positive Collector-Emitter voltage (VCE > 0).
Apply Gate-Emitter voltage (VGE) greater than Threshold Voltage (Vth).
SCR Family Comparison (Question 6)#
Question: Compare SCR, DIAC, TRIAC.#
SCR:
Terminals: 3 (Anode, Cathode, Gate).
Conduction: Unidirectional (A to K).
Control: Gate pulse turns ON.
DIAC:
Terminals: 2 (MT1, MT2 / A1, A2).
Conduction: Bidirectional.
Control: No Gate. Turns ON when voltage exceeds Breakover Voltage (VBO) in either direction.
TRIAC:
Terminals: 3 (MT1, MT2, Gate).
Conduction: Bidirectional.
Control: Gate pulse (positive or negative) turns ON for either polarity of MT2 relative to MT1.
Device Limitations (Question 7)#
Question: What are the limitations of SCR / MOSFET / IGBT?#
SCR:
Slow switching speed (limits frequency).
No gate turn-off control (requires commutation).
dv/dt and di/dt limitations.
MOSFET:
Higher conduction loss (RDS(on)) at high currents compared to IGBT/SCR.
Body diode reverse recovery can be poor.
Susceptible to ESD.
Lower voltage ratings than SCR/IGBT (traditionally).
IGBT:
Slower than MOSFET.
Switching losses higher than MOSFET, esp. due to ’tail current'.
Potential for latch-up (less in modern devices).
SCR Gate Current Effect (Question 8)#
Question: What will be the effect if we increase gate current in SCR?#
Decreases turn-on time.
Decreases required forward breakover voltage to turn ON.
Increases noise immunity (less sensitive to spurious dv/dt triggering).
Increases gate power dissipation.
Does NOT significantly change ON-state voltage drop or holding current.
Excessive current damages the gate.
SCR Operating Modes (Question 9)#
Question: Forward blocking, forward conduction mode.#
Forward Blocking:
Anode positive, Cathode negative.
Gate signal absent or insufficient.
Device OFF, blocks forward voltage.
Small leakage current flows.
Forward Conduction:
Anode positive, Cathode negative.
Gate triggered (or VAK > VBO).
Device ON, conducts current freely.
Low forward voltage drop (1-2V).
Stays ON if anode current > IH.
VI Characteristics (Question 10)#
Question: Explain VI characteristics of SCR / MOSFET / IGBT.#
SCR:
Plots Anode Current (IA) vs Anode-Kathode Voltage (VAK).
Regions: Reverse Blocking, Forward Blocking, Forward Conduction.
Forward Blocking: High VAK, low IA (leakage).
Forward Conduction: Low VAK (1-2V), IA determined by load (must be > IH).
Gate current lowers VAK needed to enter conduction.
MOSFET (N-Channel Output):
Plots Drain Current (ID) vs Drain-Source Voltage (VDS) for different Gate-Source Voltages (VGS).
Cut-off: VGS < Vth, ID ≈ 0.
Ohmic/Linear Region: Low VDS, ID increases with VDS (acts like resistor RDS(on)).
Saturation Region: Higher VDS, ID levels off, controlled by VGS.
IGBT (Output):
Plots Collector Current (IC) vs Collector-Emitter Voltage (VCE) for different Gate-Emitter Voltages (VGE).
Similar shape to BJT curves but controlled by VGE.
Cut-off: VGE < Vth, IC ≈ 0.
Active Region: IC controlled by VGE, low ON-state voltage drop VCE(sat).
MOSFET Modes (Question 11)#
Question: Modes of MOSFET (CE/D)? Likely Enhancement/Depletion.#
Enhancement Mode (E-MOSFET):
Normally OFF (no channel at VGS=0).
Applying VGS > Vth creates (’enhances’) a channel.
Most common type in power electronics.
Depletion Mode (D-MOSFET):
Normally ON (channel exists at VGS=0).
Applying VGS can enhance (increase conductivity) or deplete (reduce/turn off conductivity) the channel.
Less common in power switching.
IGBT Control Type (Question 13)#
Question: Is IGBT current controlled or voltage controlled?#
Voltage controlled.
Gate-Emitter Voltage (VGE) controls the state (ON/OFF) and conductivity.
High input impedance (like MOSFET).
Device Symbols (Question 14)#
Question: Symbols of SCR / MOSFET / BJT / IGBT (N/P).#
SCR: Diode symbol with a gate terminal off the P-layer near the cathode.
MOSFET:
N-Channel Enhancement: Three terminals (Gate, Drain, Source), arrow on substrate points IN. Channel line is broken.
P-Channel Enhancement: Arrow on substrate points OUT. Channel line is broken.
N-Channel Depletion: Arrow points IN. Channel line is solid.
P-Channel Depletion: Arrow points OUT. Channel line is solid.
(Include body diode symbol between source and drain).
BJT:
NPN: Arrow on Emitter points OUT.
PNP: Arrow on Emitter points IN.
Terminals: Base, Collector, Emitter.
IGBT:
N-Channel: MOSFET-like gate, BJT-like Collector and Emitter. Arrow on Emitter like NPN BJT (points OUT).
P-Channel: Similar, but arrow on Emitter like PNP BJT (points IN).
Unit II: AC to DC Power Converters#
Converter Types (Question 15)#
Question: Difference between Full controlled converter and Semi controlled converter.#
Full Controlled Converter:
Uses only controllable switches (e.g., SCRs).
Output voltage can be positive or negative (two-quadrant or four-quadrant operation possible with appropriate configurations).
Allows regeneration (power flow from DC side back to AC side).
Example: Single-phase full bridge with 4 SCRs.
Semi Controlled Converter (Half Controlled):
Uses a mix of controllable switches (SCRs) and diodes.
Output voltage is typically only positive (one-quadrant operation).
No regeneration possible.
Simpler control, often better power factor than full converter at high firing angles.
Example: Single-phase bridge with 2 SCRs and 2 Diodes.
Firing Angle Control (Questions 16, 17)#
Question: What is purpose of controlling firing angle in converter?#
To control the average DC output voltage of the converter.
By delaying the point (angle) in the AC cycle at which the SCRs are triggered (fired), the start of conduction is delayed, reducing the average output voltage.
Question: Define firing angle (alpha).#
The angle (measured in degrees or radians) of the input AC voltage waveform at which the gate pulse is applied to the SCR to turn it ON.
Measured from the point where the device could start conducting if it were a diode.
Devices in Full Converters (Question 18)#
Question: Name of device used in full converters.#
Primarily Silicon Controlled Rectifiers (SCRs) are used in traditional line-commutated full converters.
IGBTs or MOSFETs can be used in modern PWM rectifiers (which are also AC-DC converters but operate differently).
Unit III: DC to AC Converters (Inverters)#
Bridge Inverters (Questions 20, 21, 22)#
Question: Function of bridge inverter.#
To convert DC input voltage/power to AC output voltage/power.
Allows control over output AC voltage magnitude and frequency.
Question: Half bridge and Full bridge inverter.#
Half Bridge:
Uses two switches (e.g., MOSFETs/IGBTs) and two capacitors (or split DC source).
Output voltage swings between +Vdc/2 and -Vdc/2 (relative to midpoint).
Requires fewer switches but needs a center-tapped DC source or large capacitors.
Full Bridge (H-Bridge):
Uses four switches.
Output voltage swings between +Vdc and -Vdc.
Can produce higher output voltage (twice the half-bridge for the same Vdc).
More complex but standard configuration.
Question: O/P of bridge inverter.#
Basic square wave inverter: Output voltage is a square wave.
PWM Inverter: Output is a series of pulses (Pulse Width Modulated) which, when filtered or averaged over a cycle, approximates a sine wave (or other desired waveform).
Output frequency and fundamental voltage magnitude are controllable.
Contains harmonics in addition to the fundamental frequency.
Unit IV: DC to DC Converters (Choppers / SMPS)#
Chopper Basics (Questions 29, 30, 31, 32, 33)#
Question: Why step down chopper called so?#
Because the average DC output voltage (Vo) is less than the DC input voltage (Vs).
It “chops” the input voltage, and by varying the ON time relative to the total period, the average output is controlled to be lower than the input.
Question: Types of chopper.#
Classification by Quadrant Operation:
Class A: Step-down, First quadrant (Vo > 0, Io > 0).
Class B: Step-up, Second quadrant (Vo > 0, Io < 0).
Class C: Two-quadrant (Class A + Class B).
Class D: Two-quadrant.
Class E: Four-quadrant.
Classification by Circuit Topology (SMPS types):
Buck (Step-down).
Boost (Step-up).
Buck-Boost (Step-down or Step-up, inverting).
Cuk, SEPIC, Zeta (variations).
Isolated: Flyback, Forward, Push-Pull, Half-Bridge, Full-Bridge.
Question: Another name of chopper.#
DC-to-DC converter.
Switching regulator.
Question: Components of chopper.#
Basic step-down (Buck) chopper:
Power semiconductor switch (MOSFET, IGBT, BJT).
Diode (Freewheeling diode).
Inductor (Energy storage).
Capacitor (Output filter).
Control circuit (generates switching signal, e.g., PWM controller).
Other topologies have variations in component arrangement.
Question: Duty cycle & O/P Vtg relation.#
Duty Cycle (D): Ratio of the ON time (Ton) of the switch to the total switching period (T = Ton + Toff). D = Ton / T.
Step-Down (Buck): Vo = D * Vs (assuming continuous conduction).
Step-Up (Boost): Vo = Vs / (1 - D).
Buck-Boost: Vo = -Vs * D / (1 - D).
Switched Mode Power Supplies (SMPS) (Questions 24, 25, 26, 27, 28)#
Question: Use, application of SMPS.#
Powering virtually all modern electronics: Computers, laptops, TVs, phone chargers, servers.
Industrial power supplies, DC-DC converters in vehicles, LED drivers.
Advantages: High efficiency, small size, light weight compared to linear supplies.
Question: Types of SMPS.#
Non-Isolated: Buck, Boost, Buck-Boost.
Isolated (use transformer): Flyback, Forward, Push-Pull, Half-Bridge, Full-Bridge.
Question: Difference between SMPS & linear power supply.#
Linear Power Supply:
Uses a transformer (usually 50/60Hz), rectifier, filter, and a linear regulator (transistor in active region or LDO).
Regulation achieved by dissipating excess power as heat in the regulator.
Low efficiency, bulky, heavy.
Low output ripple and noise.
SMPS:
Uses a switch operating at high frequency (kHz to MHz), energy storage elements (inductor, capacitor), rectifier/filter, and often a high-frequency transformer for isolation/voltage scaling.
Regulation achieved by changing the duty cycle of the switch.
High efficiency, compact, lightweight.
Can generate more output noise/ripple (EMI).
Question: Meaning of Switch mode in SMPS.#
Refers to the power semiconductor (MOSFET/BJT) acting as a controlled switch: either fully ON (saturation) or fully OFF (cut-off).
Minimizes power dissipation in the switching element because either voltage or current is near zero (ideally).
This contrasts with a linear regulator where the transistor operates in the active region with significant voltage and current simultaneously, leading to power loss (heat).
Question: Block diagram of SMPS.#
Basic AC-DC SMPS:
Input Rectifier & Filter: Converts AC input to unregulated DC.
Inverter/Chopper Stage: High-frequency switch(es) chop the DC. May include high-frequency transformer for isolation and voltage scaling.
Output Rectifier & Filter: Converts chopped high-frequency AC back to DC and filters it.
Control Circuit: Senses output voltage/current and adjusts the duty cycle of the switch(es) via feedback (often using PWM) to maintain regulation.
Basic DC-DC SMPS (e.g., Buck):
Input DC Source.
Switch (e.g., MOSFET).
Energy Storage (Inductor, Diode).
Output Filter (Capacitor).
Control Circuit (PWM controller with feedback).
General#
SMPS: Switched-Mode Power Supply
IGBT: Insulated Gate Bipolar Transistor
BJT: Bipolar Junction Transistor
SCR: Silicon Controlled Rectifier
UPS: Uninterruptible Power Supply
MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor