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MALLA REDDY COLLEGE OF ENGINEERING &
TECHNOLOGY
(Autonomous Institution – UGC, Govt. of India)
(Affiliated to JNTUH, Hyderabad, Approved by AICTE - Accredited by NBA & NAAC – ‘A’ Grade - ISO 9001: Certified) Maisammaguda, Dhulapally (Post Via. Hakim pet), Secunderabad– 500100, Telangana State, INDIA.
DEPARTMENT OF ELECTRICAL & ELECTRONICS
ENGINEERING
DIGITAL NOTES
ON
POWER ELECTRONICS
For
B.TECH III YEAR - I SEM
Prepared by
Dr. M. SHARANYA,
Professor, EEE Department
MALLA REDDY COLLEGE OF ENGINEERING AND TECHNOLOGY
III B.Tech EEE I Sem L T/P/D C
(R18A0210) POWER ELECTRONICS
COURSE OBJECTIVES:
To get an understanding of semiconductor devices and systems
which have a large application in the power generation, transmission,
distribution and utilization.
To get an in depth understanding of the basic concepts of different
types of power semiconductor devices and their applications in
converters, inverters, choppers, cycloconverter, dual converters, etc.
UNIT - I
POWER SEMI CONDUCTOR DEVICES & COMMUTATION
CIRCUITS: Thyristors – Silicon Controlled Rectifiers (SCR’s) - Two
transistor analogy - Static and Dynamic characteristics - Turn on and turn off
methods - Series and parallel connections of SCR’s – Snubber
circuit details – Line Commutation and Forced Commutation
circuits – Power MOSFET, Power IGBT, their characteristics.
UNIT - II
AC - DC CONVERTERS (1-PHASE & 3 - PHASE CONTROLLED
RECTIFIERS):
Phase control technique – Single phase Line commutated converters – Bridge
connections – Half controlled converters with R, RL loads – Derivation of average
load voltage and current – Numerical problems. Fully controlled converters, Bridge
connections with R, RL loads – Derivation of average load voltage and current.
Three phase converters – Three pulse and six pulse converters – Bridge
connections average load voltage with R and RL loads – Effect of Source inductance
- Single phase Dual converters.
UNIT - III
DC - DC CONVERTERS (CHOPPERS):
Choppers – Time ratio control and Current limit control strategies – Step
down choppers Derivation of load voltage and currents with R, RL loads-
Step up Chopper – load voltage expression, DC Jones Chopper.
UNIT – I
Power semiconductor devices and commutation
circuits
Introduction to power electronics: Power Electronics is a field which combines Power (electric power), Electronics and Control systems. Power engineering deals with the static and rotating power equipment for the generation, transmission and distribution of electric power. Electronics deals with the study of solid state semiconductor power devices and circuits for Power conversion to meet the desired control objectives (to control the output voltage and output power). Power electronics may be defined as the subject of applications of solid state power semiconductor devices (Thyristors) for the control and conversion of electric power. Power electronics deals with the study and design of Thyristorised power controllers for variety of application like Heat control, Light/Illumination control, Motor control - AC/DC motor drives used in industries, High voltage power supplies, Vehicle propulsion systems, High voltage direct current (HVDC) transmission.
Power Electronics refers to the process of controlling the flow of current and voltage and converting it to a form that is suitable for user loads. The most desirable power electronic system is one whose efficiency and reliability is 100%.
Take a look at the following block diagram. It shows the components of a Power Electronic system and how they are interlinked.
Figure: 1.1. Block diagram of DC power supply
A power electronic system converts electrical energy from one form to another and ensures the following is achieved −
Maximum efficiency Maximum reliability Maximum availability Minimum cost Least weight Small size Applications of Power Electronics are classified into two types − Static Applications and Drive Applications.
Static Applications
This utilizes moving and/or rotating mechanical parts such as welding, heating, cooling, and electro- plating and DC power.
DC Power Supply
Figure: 1.2. Block diagram of DC power supply
Drive Applications
Drive applications have rotating parts such as motors. Examples include compressors, pumps, conveyer belts and air conditioning systems.
Air Conditioning System
Power electronics is extensively used in air conditioners to control elements such as compressors. A schematic diagram that shows how power electronics is used in air conditioners is shown below.
Types of power electronic converters
- Rectifiers (AC to DC converters): These converters convert constant ac voltage to variable dc output voltage.
- Choppers (DC to DC converters): Dc chopper converts fixed dc voltage to a controllable dc output voltage.
- Inverters (DC to AC converters): An inverter converts fixed dc voltage to a variable ac output voltage.
- AC voltage controllers: These converters converts fixed ac voltage to a variable ac output voltage at same frequency.
- Cycloconverters: These circuits convert input power at one frequency to output power at a different frequency through one stage conversion.
Power semiconductor devices
i. Power Diodes. ii. Power transistors (BJT's). iii. Power MOSFETS. iv. IGBT's. v. Thyristors
Thyristors are a family of p-n-p-n structured power semiconductor switching devices
Power diodes
Power diodes are made of silicon p-n junction with two terminals, anode and cathode. P-N junction is formed by alloying, diffusion and epitaxial growth. Modern techniques in diffusion and epitaxial processes permit desired device characteristics. The diodes have the following advantages High mechanical and thermal reliability High peak inverse voltage Low reverse current Low forward voltage drop High efficiency Compactness.
Power transistors
Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices are used a switching devices and are operated in the saturation region resulting in low on-state voltage drop. They are turned on when a current signal is given to base or control terminal. The transistor remains on so long as the control signal is present. The switching speed of modern transistors is much higher than that of thyristors and is used extensively in dc-dc and dc-ac converters. However their
voltage and current ratings are lower than those of thyristors and are therefore used in low to medium power applications. Power transistors are classified as follows o Bipolar junction transistors(BJTs) o Metal-oxide semiconductor filed-effect transistors(MOSFETs) o Static Induction transistors(SITs) o Insulated-gate bipolar transistors(IGBTs)
Advantages of BJT’S
i. BJT’s have high switching frequencies since their turn-on and turn-off time are low. ii. The turn-on losses of a BJT are small. iii. BJT has controlled turn-on and turn-off characteristics since base drive control is possible. iv. BJT does not require commutation circuits
Demerits of BJT
i. Drive circuit of BJT is complex. ii. It has the problem of charge storage which sets a limit on switching frequencies. iii. It cannot be used in parallel operation due to problems of negative temperature coefficient.
Thyristors – Silicon Controlled Rectifiers (SCR’s)
A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate current- controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. SCRs are mainly used in electronic devices that require control of high voltage and power. This makes them applicable in medium and high AC power operations such as motor control function. An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three junctions labeled as J1, J2 and J3 and three terminals(anode, cathode and a gate). An SCR is diagramatically represented as shown below.
Figure: 1.4. Symbol of thyristor
Characteristics of Thyristor A thyristor is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n junctions, functioning as an electrical switch for high power operations. It has three basic terminals, namely the anode, cathode and the gate mounted on the semiconductor layers of the device. The symbolic diagram and the basic circuit diagram for determining the characteristics of thyristor is shown in the figure below,
V-I Characteristics of a Thyristor
Figure: 1.6. Circuit diagram for characteristics of SCR
From the circuit diagram above we can see the anode and cathode are connected to the supply voltage through the load. Another secondary supply Es is applied between the gate and the cathode terminal which supplies for the positive gate current when the switch S is closed. On giving the supply we get the required V-I characteristics of a thyristor show in the figure below for anode to cathode voltage Vaand anode current Ia as we can see from the circuit diagram. A detailed study of the characteristics reveal that the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. Which are discussed in great details below, to understand the overall characteristics of a thyristor.
Reverse Blocking Mode of Thyristor
Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S open. For understanding this mode we should look into the fourth quadrant where the thyristor is reverse biased.
Figure: 1.7. Reverse blocking mode of SCR
Here Junctions J 1 and J 3 are reverse biased whereas the junction J 2 is forward biased. The behavior of the thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across them. As a result only a small leakage current of the order of a few μAmps flows. This is the reverse blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular voltage, known as the critical breakdown voltage VBR, an avalanche occurs at J 1 and J 3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR, which results in heating. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit.
Figure: 1.8. V- I characteristics of SCR
voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the thyristor.
Two transistor analogy of SCR
Basic operating principle of SCR , can be easily understood by the two transistor model of SCR or analogy of silicon controlled rectifier, as it is also a combination of P and N layers, shown in figure below
Figure: 1.10. Two transistor structure of SCR
This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp transistor with J 1 and J 2 junctions and another is with J 2 and J 3 junctions as shown in figure below.
Figure: 1. 11. Two transistor structure of SCR
When the transistors are in off state, the relation between the collector current and emitter current is shown below
Figure: 1. 12. Two transistors connection of SCR
Here, IC is collector current, IE is emitter current, ICBO is forward leakage current, α is common base
forward current gain and relationship between IC and IB is Where, IB is base current and β is common emitter forward current gain. Let’s for transistor T 1 this relation holds
And that for transistor T 2
Now, by the analysis of two transistors model we can get anode current,
From equation (i) and (ii), we get,
and as a result α 2 also increases and this further increases
.
As
,
α 1 again increases. This continuous positive feedback effect increases towards unity and anode current tends to flow at a very large value. The value current then can only be controlled by external resistance of the circuit.
Turn on methods of SCR
The turning on Process of the SCR is known as Triggering. In other words, turning the SCR from Forward-Blocking state to Forward-Conduction state is known as Triggering. The various methods of SCR triggering are discussed here.
The various SCR triggering methods are Forward Voltage Triggering Thermal or Temperature Triggering Radiation or Light triggering dv/dt Triggering Gate Triggering
(a) Forward Voltage Triggering:- In this mode, an additional forward voltage is applied between anode and cathode. When the anode terminal is positive with respect to cathode (VAK), Junction J1 and J3 is forward biased and junction J2 is reverse biased. No current flow due to depletion region in J2 is reverse biased (except leakage current). As VAK is further increased, at a voltage VBO (Forward Break Over Voltage) the junction J undergoes avalanche breakdown and so a current flows and the device tends to turn ON(even when gate is open)
(b) Thermal (or) Temperature Triggering:- The width of depletion layer of SCR decreases with increase in junction temperature. Therefore in SCR when VAR is very near its breakdown voltage, the device is triggered by increasing the junction temperature. By increasing the junction temperature the reverse biased junction collapses thus the device starts to conduct. (c) Radiation Triggering (or) Light Triggering:- For light triggered SCRs a special terminal niche is made inside the inner P layer instead of gate terminal. When light is allowed to strike this terminal, free charge carriers are generated. When intensity of light becomes more than a normal value, the thyristor starts conducting. This type of SCRs are called as LASCR (d) dv/dt Triggering:- When the device is forward biased, J1 and J3 are forward biased, J2 is reverse biased. Junction J2 behaves as a capacitor, due to the charges existing across the junction. If voltage across the device is V, the charge by Q and capacitance by C then, ic =dQ/dt Q=CV ic =d(CV)/dt =CdV/dt+VdC/dt as dC/dt = 0 ic = CdV/dt Therefore when the rate of change of voltage across the device becomes large, the device may turn ON, even if the voltage across the device is small. (e) Gate Triggering:- This is most widely used SCR triggering method. Applying a positive voltage between gate and cathode can Turn ON a forward biased thyristor. When a positive voltage is applied at the gate terminal, charge carriers are injected in the inner P- layer, thereby reducing the depletion layer thickness. As the applied voltage increases, the carrier injection increases, therefore the voltage at which forward break-over occurs decreases.
(i) Resistance triggering:
The following circuit shows the resistance triggering.
Figure: 1. 14. Resistance triggering circuit of SCR
In this method, the variable resistance R is used to control the gate current. Depending upon the value of R, when the magnitude of the gate current reaches the sufficient value(latching current of the device) the SCR starts to conduct. The diode D is called as blocking diode. It prevents the gate cathode junction from getting damaged in the negative half cycle. By considering that the gate circuit is purely resistive, the gate current is in phase with the applied voltage. By using this method we can achieve maximum firing angle up to 90°.
(ii) RC Triggering
The following circuit shows the resistance-capacitance triggering.
Figure: 1. 15. Resistance Capacitance triggering circuit of SCR
By using this method we can achieve firing angle more than 90°. In the positive half cycle, the capacitor is charged through the variable resistance R up to the peak value of the applied voltage. The variable resistor R controls the charging time of the capacitor. Depends upon the voltage across the capacitor, when sufficient amount of gate current will flow in the circuit, the SCR starts to conduct. In the negative half cycle, the capacitor C is charged up to the negative peak value through the diode D2. Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half cycle.
3. Pulse Gate Triggering:-
In this method the gate drive consists of a single pulse appearing periodically (or) a sequence of high frequency pulses. This is known as carrier frequency gating. A pulse transformer is used for isolation. The main advantage is that there is no need of applying continuous signals, so the gate losses are reduced.
Advantages of pulse train triggering:
Low gate dissipation at higher gate current. Small gate isolating pulse transformer Low dissipation in reverse biased condition is possible. So simple trigger circuits are possible in some cases When the first trigger pulse fails to trigger the SCR, the following pulses can succeed in latching SCR. This important while Triggering inductive circuits and circuits having back emf's.
Turn off methods of SCR:
SCR can be turned ON by applying appropriate positive gate voltage between the gate and cathode terminals, but it cannot be turned OFF through the gate terminal. The SCR can be brought back to the
