Power Electronics - Unit 2 Notes
Power Electronics Introduction
Power electronics combines power engineering (generation, transmission, distribution of electric energy) with electronics engineering (data and signal transmission at low power levels).
Power engineering uses electromagnetic principles, while electronics engineering uses physical phenomena in vacuum, gases/vapors, and semiconductors.
Power electronics applies electronic principles at power levels, using semiconductor power switches like thyristors and GTOs.
Relationship to Power, Electronics, and Control
Power electronics relies on switching power semiconductor devices.
Development in semiconductor technology has improved power-handling capabilities and switching speed.
Applications of Power Electronics
Aerospace: Space shuttle, satellite, and aircraft power supplies.
Commercial: Advertising, HVAC, computers, UPS, elevators, light dimmers.
Industrial: Furnaces, blowers, pumps, lasers, transformer-tap changers, welding.
Residential: Air conditioning, cooking, lighting, heating, refrigerators, door openers, dryers, fans, computers.
Telecommunication: Battery chargers, power supplies, UPS.
Transportation: Battery chargers, traction control in electric vehicles, locomotives, street cars, trolley buses, subways, automotive electronics.
Utility systems: HVDC transmission, excitation systems, VAR compensation, static circuit breakers, supplementary energy systems (solar, wind).
Advantages of Power-Electronic Systems
High efficiency due to low losses in power semiconductor devices.
High reliability.
Long life and low maintenance due to no moving parts.
Fast dynamic response compared to electromechanical systems.
Small size and less weight.
Lower cost due to mass production of power semiconductors.
Power Electronics System Block Diagram
Output may be variable DC or AC voltage, or variable voltage and frequency, depending on load requirements.
Feedback measures load parameters and compares them with command values, controlling semiconductor device turn-on.
Power Semiconductor Devices
Classified by:
Turn-on and turn-off characteristics
Gate signal requirements
Degree of controllability
Types:
(a) Diodes: Uncontrolled rectifying devices.
(b) Thyristors: Controlled turn-on via gate signal, latched-in on-state.
(c) Controllable Switches: Turn-on and turn-off controlled by signals (BJT, MOSFET, GTO, SITH, IGBT, SIT, MCT).
Abbreviations: SCR, LASCR, ASCR, RCT, GTO, SITH, MCT, BJT, MOSFET, SIT, IGBT.
Power Diodes
Consist of a heavily doped substrate with a lightly doped epitaxial layer and a heavily doped layer.
The layer absorbs the depletion layer of the reverse-biased junction.
Thickness of the layer determines breakdown voltage; thicker layer results in higher breakdown voltage.
The n-layer adds ohmic resistance, leading to power dissipation requiring cooling.
Diode i-v Characteristics
In forward bias, current increases linearly with voltage.
In reverse bias, a small reverse saturation current flows due to minority carriers.
Reverse saturation current is independent of reverse voltage.
Exceeding reverse breakdown voltage () causes a large current flow, potentially destroying the diode. (Repetitive Peak Reverse Voltage) is the maximum reverse voltage a diode can withstand repeatedly.
Applications: Freewheeling diodes, rectification, battery charging, electroplating, UPS, choppers, SMPS.
Gunn Diode
It's a transferred electron device, with only N-type semiconductor, used as a low-power oscillator to generate microwaves using the Gunn effect.
Made of three layers of N-type semiconductor (GaAs, GaN, CdTe, CdS, InP, InAs, InSb, ZnSe).
Topmost and bottommost layers are heavily doped, while the middle layer is lightly doped.
Working of Gunn Diode
Not a P-N junction diode; it has two electrodes. Voltage appears across the active (middle) region.
Current pulses traverse the active region; the active region's thickness modifies the working frequency.
Electrons move from the valence band to the conduction band; current decreases as electrons move to a band above the conduction band.
Negative resistance region allows amplification and oscillation, generating frequencies from 10 GHz to THz.
V-I Characteristics of Gunn Diode
Current increases with voltage until the threshold point, then decreases, creating a negative resistance region for amplification and oscillation.
Advantages: Portable, small, low cost, better noise to signal ratio, reliable and stable at higher frequencies, high bandwidth.
Disadvantages: Poor temperature stability, higher operating current and power dissipation, low efficiency below 10GHz.
Applications: Oscillators, amplifiers, radar sources, fast controlling equipment, tachometers, microwave relay data link transmitters.
Schottky Diode
Also known as hot-carrier diode or Schottky barrier diode.
Formed by a junction of a semiconductor with a metal (e.g., aluminum), replacing the p-type semiconductor of a PN junction diode.
Construction of Schottky Diode
Metal contact (Pt, W, Al) connected directly to n-type silicon.
Thinner depletion region than normal diodes due to absence of p-type region.
A Schottky barrier is created at the junction due to the difference in work functions between the metal and semiconductor.
Working of Schottky Diode
Formed by joining a metal with a semiconductor material, creating a metal-semiconductor junction.
Forward bias reduces the Schottky barrier, allowing electrons to flow easily from the metal into the semiconductor, resulting in a low forward voltage drop (0.2 to 0.4 volts).
V-I Characteristic of Schottky Diode
When a forward bias voltage is applied across a Schottky diode, the V-I characteristics show that it conducts current very quickly with a relatively low forward voltage drop.
In the reverse bias condition, the V-I characteristics of a Schottky diode display a small reverse current, often referred to as the leakage current.
Applications of Schottky Diode
Power rectifier: When having high power supply, Schottky diode acts as a rectifier.
RF mixer: Schottky diodes are widely used in RF and microwave circuits due to their fast switching speed and low noise characteristics.
Electronic appliances: Schottky diodes finds its use in logic gates, digital circuits, and memory devices.
Battery charging circuit
Switching power supplies
LED drivers
Audio amplifiers
Voltage clamps.
Power OR circuits
Solar Cell Applications
Advantages of Schottky Diode
Low Forward Voltage Drop & Fast Switching Speed & Negligible Reverse Recovery Time & High Temperature Operation.
Disadvantages of Schottky Diode
Temperature Sensitivity, Lower Reverse Recovery Time, High Reverse Leakage Current, Noise Generation, Higher Cost.
IMPATT Diode
Impact Ionization Avalanche Transit-Time diode used for microwave applications (3 to 100 GHz).
Used as oscillator and amplifier.
Construction
Consists of 4 regions: .
Operates on a high voltage gradient (400KV/cm) to produce avalanche current.
Materials used: GaAs (preferred for low noise), Si, Ge, or InP.
Working of IMPATT Diode
Operates on avalanche breakdown and transit time delay. High-velocity carriers collide with other atoms and generate electron-hole pairs.
The moving charges generate high current inside the device. This is known as avalanche condition or impact ionization and is utilized in IMPATT diodes.
Applications of IMPATT Diode
Microwave Oscillators
Amplifiers
Frequency Multipliers
Radar Systems
Satellite Communication
Scientific Research
Advantages of IMPATT Diode
Wide frequency range (GHz to THz), compact dimensions, simplified circuitry, continuous waveform, silicon technology compatibility, adaptability, potential of producing significant electricity.
Disadvantages of IMPATT Diode
Elevated Noise Levels, Inadequate Performance, Inappropriate Harmonics, Limited Capability to Tune Frequency, Environmental Factor Sensitivity
Introduction to Thyristor
Thyristor is a four-layer semiconductor device, consisting of alternating P-type and N-type materials (PNPN).
Thyristor Applications
Power switches in factories and similar industrial settings, Vehicle ignition switches, Controlling the speed of electric motors, Liquid level regulators
Pressure control systems, Surge protectors
PNPN Diode
PNPN Diode or Shockley Diode is not widely available commercially.
✓ The PNPN diode is a four-layer (P-N-P-N), two terminals (namely anode and cathode) semiconductor switching device
Silicon Control Rectifier (SCR)
A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solid-state current-controlling device
Construction
➢ An SCR conducts when a gate pulse is applied to it, just like a diode.
Modes of Operation in SCR
1) Forward Blocking Mode (Off State)
In this mode of operation, the positive voltage (+) is given to anode A (+), negative voltage (-) is given to cathode K (-), and gate G is open circuited.
2) Forward Conducting Mode (On State)
The Silicon Controlled Rectifier can be made to conduct in two ways:, By increasing the forward bias voltage applied between anode and cathode beyond the breakdown voltage, ➢ By applying positive voltage at gate terminal.
3) Reverse Blocking Mode (On State)
In this mode of operation, the negative voltage (-) is given to anode (+), positive voltage (+) is given to cathode (-), and gate is open circuited
V-I Characteristics of SCR
The forward breakover voltage (Vbo): This is the maximum forward voltage that can be applied between anode and cathode, without initiating forward conduction
The forward leakage current: The small forward current flowing in the forward blocking state of the device
The holding current (IH): It represents the minimum current that can flow through SCR and still “hold” it in the on state.The voltage associated with the holding current is termed as holding voltage VH
ON-state voltage:
Note that the voltage across SCR in its on state is very low as compared to the off-state voltage.
Latching current (IL): It is the minimum anode current that must flow through SCR to latch it into the on-state
THYRISTOR RATINGS
Thyristor ratings indicate voltage, current, power and temperature limits within which a thyristor can be used without damage or malfunction
POWER TRANSISTORS
Power diodes are uncontrolled devices. In other words, their turn-on and turn –off characteristics are not under control.
Power transistors, however, possess controlled characteristics. These are turned on when a current signal is given to base, or control, terminal.
i ) Bipolar junction transisors (BJ Ts) (i i) Metal-oxide-semiconductor field-effect transistors (MOSFETs) (iii ) Insulated gate bipolar transistors (IGBTs) and (iv ) Static induction transistors (SITs)
Transistor Switch
Transistor operation as a switch means that transistor operates either in the saturation region or in the cut-off region and now where else on the load line
When the control, or base, signal is reduced to zero, the transistor is turned off and its operation shifts to B' in the cut-off region
BJT Switching Performance
When base current is applied, a transistor does not turn on instantly because of the presence of internal capacitances
After some time delay td , called delay time, the collector current rises to 0.1 ICS, VCE falls from VCC to 0.9 VCC and VBE reaches VBES =0.7 V.
After delay time td , collector current rises from 0.1 ICS to 0.9 ICS and VCE falls from 0.9 VCC to 0.1 VCC in time tr .
After ts collector current begins to fall and collector-emitter voltage starts building up
Power MOSFET
A power MOSFET has three terminals called drain (D), source (8) and gate (G)
A BJT is a current controlled device whereas a power MOSFET is a voltage- controlled device. As its operation depends upon the flow of majority carriers only, MOSFET is a unipolar device.
Power MOSFETs are of two types; n-channel enhancement MOSFET and p- channel enhancement MOSFET.
PMOSFET Characteristics
➢ The switching characteristics of a power MOSFET are influenced to a large extent by the- internal capacitance of the device and the internal impedance of the gate drive circuit.
➢ At turn-on, there is an initial delay tdn during which input capacitance charges to gate threshold voltage VGST. Here tdn is called turn-on delay time. There is further delay tr called rise time, during which gate voltage rises to VGSP, a voltage sufficient to drive the MOSFET into on state. During tr, drain current rises from zero to full-on current ID. Thus, the total turn-on-time is ton =tdn + tr.
➢ As MOSFET is a majority carrier device, turn-off process is initiated soon after removal of gate voltage at time t1. The turn-off delay time, tdf, is the time during which input capacitance discharges from overdrive gate voltage VI to VGSP. The fall time, tf is the time during which input capacitance discharges from VGSP to threshold voltage. During tf drain current falls from ID to zero. So when VGS≤ VGST, PMOSFET turn- off is complete. Switching waveforms for a power MOSFET are shown in Fig. (d).