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EE4532, POWER ELECTRONICS & DRIVES

Part [A]: Power Electronic Systems & Devices

• Section 1: Power Electronic Systems • Section 2: Power Devices • Section 3: Uncontrolled & Controlled Rectifiers

Total Lecture Hours: 12 Tutorial Hours: 6

Instructor: Dr. Ali I. Maswood Associate Professor, EAMASWOOD@NTU.EDU.SG

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TYPES OF POWER ELECTRONIC CIRCUITS For the control of electric power or power conditioning, the conversion of electric power from one form to another is necessary and the switching characteristics of the power devices permit these conversions. The static power converters perform these functions of power conversions. A converter may be considered as a switching matrix. The power electronics circuits can be classified into 5 types: 1. Diode rectifiers 2. Ac-dc converters (controlled rectifiers) 3. Ac-ac converters (ac voltage controllers) 4. Dc-dc converters (choppers) 5. Dc-ac converters (inverters) Diode Rectifiers: A diode rectifier circuit converts ac voltage into a fixed dc voltage and is shown Figure. The input voltage to the rectifier vi could be either single phase or three phase.

Thyristor (controlled) rectifier: A single-phase converter with two natural commutated thyristors is shown. The average value of the

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output voltage vo can be controlled by varying the conduction time of thyristors or delay angle α. The input could be a single- or three-phase source.

Ac-ac converters: These converters are used to obtain a variable ac output voltage vo from a fixed ac source and a single-phase converter with a TRIAC is shown. The output voltage is controlled by varying the conduction time of a TRIAC or firing delay angle α. These types of converters are also known as ac voltage controllers.

Dc-dc converters: A dc-dc converter is also known as a chopper or switching regulator, and a transistor chopper is shown in following

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figure. The average output vo voltage is controlled by varying the conduction time t1, of transistor Q1. If T is the chopping period, then t1 = δT. Where; δ is called as the duty cycle of the chopper.

Dc-ac converters: A dc-ac converter is also known as an inverter. A single phase transistor inverter is shown in following figure. If transistors M1, and M2 conduct for negative half of a period and M3 and M4 conduct for the other half, the output voltage is of he alternating form. The output voltage can be controlled by varying the conduction time of transistors.

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DESIGN OF POWER ELECTRONICS -EQUIPMENT The design of a power electronic equipment can be divided into four parts:

1. Design of power circuits 2. Protection of power devices 3. Determination of control strategy. 4. Design of logic and gating circuits

Fig: An UPS Circuit PERIPHERAL EFFECTS The operations of the power converters are based mainly on the switching of power semiconductor devices; and as a result the converters introduce current and voltage harmonics into the supply system and on the output of the converters. These can cause problems of distortion of the output voltage, harmonic generation into the supply system, and interference with the communication and signaling circuits. It is normally necessary to introduce filters on the input and output of a converter system to reduce the harmonic level to an acceptable magnitude.

Load AC mains

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Figure above shows the block diagram of a generalized power converter. The application of power electronics to supply the sensitive electronic loads poses a challenge on the power quality issues and raises problems and concerns to be resolved by researchers. The input and output quantities of converters could be either ac or dc. Factors such as total harmonic distortion (THD), displacement factor (DF), and input power factor (IPF) are measures of the quality of a waveform. To determine these factors, finding the harmonic content of the waveforms is required. To evaluate the performance of a converter, the input and output voltages and currents of a converter are expressed in a Fourier series. The quality of a power converter is judged by the quality of its voltage and current waveforms. The control strategy for the power converters plays an important part on the harmonic generation and output waveform distortion, and can be aimed to minimize or reduce these problems. The power converters can cause radio-frequency interference due to electromagnetic radiation, and the gating circuits may generate erroneous signals. This interference can be avoided by grounded shielding. POWER MODULES Power devices are available as a single unit or in a module. A power converter often requires two, four, or six devices, depending on its

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topology. Power modules with dual (in half-bridge configuration) or quad (in full bridge) or six (in three phase) are available for almost all types of power devices. The modules offer the advantages of lower on-state losses, high voltage and current switching characteristics, and higher speed than that of conventional devices. Some modules even include transient protection and gate drive circuitry. INTELLIGENT MODULES Gate drive circuits are commercially available to drive individual devices or modules. Intelligent modules, which are the state-of-the-art power electronics, integrate the power module and the peripheral circuit.

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VOLT AMPERE (VA) RANGES OF POWER DEVICES

Your Notes:

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CLASSIFICATION OF POWER DEVICES (L2) 1. Uncontrolled turn on and off (e.g., diode); 2. Controlled turn on and uncontrolled turn off (e.g., SCR); 3. Controlled turn-on and -off characteristics (e.g., BJT, MOSFET, GTO); 4. Continuous gate signal requirement (BJT, MOSFET, IGBT, SIT); 5. Pulse gate requirement (e.g., SCR, GTO, MCT); 6. Bipolar voltage-withstanding capability (SCR, GTO); 7. Unipolar voltage-withstanding capability (BJT, MOSFET, GTO, IGBT); 8. Bidirectional current capability (TRIAC); 9. Unidirectional current capability (SCR, BJT, MOSFET,IGBT, diode) Figure below shows the applications and frequency range of power devices. A super power device should: (1) have a zero on-state voltage, (2) withstand an infinite off-state voltage, (3) handle an infinite current, and (4) turn on and off in zero time, thereby having infinite switching speed.

Your Notes:

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Fig. V-I Characteristics of various power Devices

CONTROL CHARACTERISTICS OF POWER DEVICES

• Power semiconductor devices can be operated as switches by applying control signals to the gate terminal of thyristors, and to the base of bipolar transistors.

• The required output is obtained by varying the conduction time of these

switching devices. Figure below shows the output voltages and control characteristics of commonly used power switching devices. Once a thyristor is in a conduction mode, the gate signal of either positive or negative magnitude has no effect.

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Fig. V-I ratings & Operating Frequencies of Power devices

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Fig. Device Symbols & their V/I Characteristics

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POWER SEMICONDUCTOR DIODES AND CIRCUITS A diode acts as a switch to perform various functions such as:

• switches in rectifiers, • freewheeling in switching regulators, • charge reversal of capacitor and energy transfer between components,

Power diodes can be assumed as ideal switches for most applications, but practical diodes differ from the ideal characteristics and have certain limitations. The power diodes are similar to pn-junction signal diodes. However, the power diodes have larger power, voltage, and current-handling capabilities than those of ordinary signal diodes. The frequency response (or switching speed) is low compared with that of signal diodes. Semiconductor basics Power semiconductor devices are based on high-purity, single-crystal silicon. A pure silicon material is known as an intrinsic semiconductor with resistivity that is too low to be an insulator and too high to be a conductor. It has high resistivity and very high dielectric strength (over 200 kV/cm). n-Type silicon material: If pure silicon is doped with a small amount of a Group V element, such as phosphorus, arsenic, or antimony, each atom of the dopant forms a covalent bond within the silicon lattice, leaving a loose electron. These loose electrons greatly increase the conductivity of the material. p-Type material: If pure silicon is doped with a small amount of a Group III element, such as boron, gallium, or indium, a vacant location called a hole is introduced into the silicon lattice. These holes greatly increase the conductivity of the material. In a p-type material the holes are called the majority carriers and electrons are called the minority carriers. In the n-type material, the electrons are called the majority carriers, and holes are called the minority carriers.

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DIODE CHARACTERISTICS A power diode is a two-terminal pn-junction device and a pn-junction is normally formed by alloying, diffusion.

• When the anode potential is positive with respect to the cathode, the diode is said to be forward biased and the diode conducts.

• When the cathode potential is positive with respect to the anode, the

diode is said to be reverse biased. Under reverse-biased conditions, a small reverse current (also known as leakage current) in the range of micro or milliampere flows. The v-i characteristics shown in following figure can be expressed by an equation known as Schockley diode equation, and it is given under dc steady-state operation by

Fig. Diode Practical & Ideal V/I Characteristics

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VD >0…Forward biased, VD <0…Reverse biased, VD <-VBR…Breakdown

REVERSE RECOVERY CHARACTERISTICS

Once a diode is in a forward conduction mode and then its forward current is reduced to zero due to the natural behavior of the diode circuit or application of a reverse voltage, the diode continues to conduct due to charges that remain stored in the pn-junction and the bulk semiconductor material. The charges require a certain time to recombine with opposite charges and to be neutralized. This time is called the reverse recovery time of the diode.

Fig. Reverse Recovery (RR) Characteristic

The reverse recovery time is denoted as trr and is measured from the initial zero crossing of the diode current to 25% of maximum (or peak) reverse current IRR.

ttt barr += trr consists of two components, ta and tb. The ratio tb/ta, is known as the softness factor (SF). The peak reverse current can be expressed in reverse di/dt as

dtdi

tI arr =

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Reverse recovery charge QRR, is the amount of charge carriers that flows across the diode in the reverse direction due to changeover from forward conduction to reverse blocking condition. RR charge:

tititiQ rrrrbrrarrrr 21

21

21

=+≈

If tb is negligible compared ta to as usually is the case, then the following eqns. Are valid:

RR time, dtdiQrr

trr/

2= , RR current dt

diQrrI rr 2=

Example: Reverse Recovery Current

Your Notes: FIN

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POWER DIODE TYPES (L3) Depending on the recovery characteristics and manufacturing techniques, the power diodes can be classified into the following three categories:

1. Standard or general-purpose diodes 2. Fast-recovery diodes 3. Schottky diodes

General-Purpose Diodes • The general-purpose rectifier diodes have relatively high reverse recovery time, typically 25 μs; and are used in low-speed applications, where recovery time is not critical, e.g., diode rectifiers and converters for a low-input frequency up to 1-kHz applications and line-commutated converters.

• current ratings from less than 1 A to several thousands of amperes, with voltage ratings from 50 V to around 5 kv Following Figure shows various configurations of general-purpose diodes, which basically fall into two types.

o One is called a stud, or stud-mounted type; o Other one is called a disk, or hockey-puck type.

Fast-Recovery Diodes • The fast-recovery diodes have low recovery time, normally less than 5 μs. • They are used in dc-dc and dc-ac converter circuits, where the speed of recovery is often of critical importance. • Ratings of voltage from 50 V to around 3 kV, and current from less than 1 A to hundreds of amperes.

Fig. Stud & Hockey Puck configuration of power diodes.

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Schottky Diodes • The charge storage problem of a pn-junction can be eliminated (or minimized) in a Schottky diode. • A Schottky diode has a relatively low forward voltage drop. • The leakage current of a Schottky diode is higher than that of a pn-junction diode. The maximum allowable voltage of this diode is generally limited to 100 V The current ratings of Schottky diodes vary from 1 to 400 A. • The Schottky diodes are ideal for high-current and low-voltage dc power supplies. However, these diodes are also used in low-current power supplies for increased efficiency.

The zener diode

• The Zener diode or reference diode, whose symbol is shown in following Fig., finds primary usage voltage regulator or reference.

• The forward conduction characteristic of a Zener diode is much the as that of a rectifier diode;

1. The reverse voltage breakdown is rather sharp. The breakdown voltage can be controlled through the manufacturing process so it has a reasonably predictable value. 2. When a Zener diode is in reverse breakdown, its voltage remains extremely close to the breakdown value while the current varies from rated current to 10 percent or less of rated current. Zener regulator is designed so that iz ≥ O.l*IZ to ensure the constancy of vz.

Sharp Reverse V Breakdown

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Example: Find the voltage vz across the Zener diode of above Fig. if iz = lO mA and it is known that Vz(Rated) = 5.6 V, Iz(rated) = 25 mA, and Rz = 10 Ω. Since 0.1*Iz< iz< Iz, operation is along the safe and predictable region of Zener operation.

vz≈ Vz + iz* Rz =5.6+(10x10-3)(10)=5.7V Rz is frequently neglected in the design of Zener regulators. SERIES-CONNECTED DIODES In many high-voltage applications, e.g., high-voltage direct current HVDC transmission lines, one commercially available diode cannot meet the required voltage rating, and diodes are connected in series to increase the reverse blocking capabilities. In the following Fig., iD and VD are the current and voltage, respectively.

Fig. Series connection of diodes

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In the forward direction; vD1, and vD2 are the sharing reverse voltages of diodes D1, and D2, respectively. In practice, the v-i characteristics for the same type of diodes differ due to tolerances in their production process. In the forward-biased condition, both diodes conduct the same amount of current, and the forward voltage drop of each diode would be almost equal. However, in the reverse blocking condition, each diode has to carry the same leakage current, and as a result the blocking voltages may differ significantly.

IIIII RSRSS 2211 +=+= However, IR1 = VD1/R1, and IR2 = VD2/R2= VD1/R2. For equal voltage sharing:

R

vI

R

vI

DS

DS

2

12

1

11 +=+

If R1=R2, two diode voltages can be slightly different because of their V-I characteristic. A simple solution to this problem is shown in following Figure:

Fig. Equal voltage sharing among series diodes

• Force equal voltage sharing by connecting a resistor across each diode. • Due to equal voltage sharing the leakage current of each diode would be different. The total leakage current must be shared by a diode and its resistor, Values of VD1 and VD2 can be determined from:

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R

vI

R

vI

DS

DS

12

11 +=+

; assuming R1=R2

VD1+ VD2 = VS Voltage sharing under transient condition

PARALLEL-CONNECTED DIODES • In high-power applications, diodes are connected in parallel to increase the current carrying capability • The current sharings of diodes would be in accord with their respective forward voltage drops. • Uniform current sharing can be achieved by providing equal inductances (e.g., in the leads) or by connecting current-sharing resistors (not practical due to power losses); • It is possible to minimize this problem by selecting diodes with equal forward voltage drops or diodes of the same type. Because the diodes are connected in parallel, the reverse blocking voltages of each diode would be the same.

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• The resistors R1, R2 help current sharing under steady-state conditions. • Current sharing under dynamic conditions can be accomplished by connecting coupled inductors as shown in Figure b. If the current through D1, rises, the L*di/dt across L1, increases, and a corresponding voltage of opposite polarity is induced across inductor L2. The result is a low-impedance path through diode D2 and the, current is shifted to D2.

Fig. Current sharing networks

Your Notes: FIN

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DIODES WITH RC AND RL LOADS Following figure shows a diode circuit with an RC load. For the sake of simplicity the diodes are considered to be ideal. By ideal we mean that the reverse recovery time and the forward voltage drop VD are negligible. The source voltage Vs is a dc constant voltage. When the switch S1, is closed at t=0, the charging current i that flows through the capacitor can be found from:

Observation: • In a RC circuit, voltage rises exponentially • In a RC circuit, current falls exponentially On the other hand: • In a RL circuit, Current ???? exponentially • In a RL circuit, voltage ???? exponentially

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FREEWHEELING DIODE • If switch S1, in following Fig. is closed for time t1, a current is established through the load, and then if the switch is opened, a path must be provided for the current in the inductive load. • This is normally done by connecting a diode Dm, as shown in Fig. a, and this diode is usually called a freewheeling diode.

Operation in two modes: • Mode 1 begins when the switch is closed at t = 0, and mode 2 begins when the switch is then opened. • The equivalent circuits for the modes are shown in b. i1 and if are defined as the instantaneous currents for mode 1 and mode 2, respectively. t1 and t2 are the corresponding durations of these modes.

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UNCONTROLLED DIODE RECTIFIERS (L4)

• A rectifier is a circuit that converts an ac signal into a unidirectional (DC) signal. the rectifiers are classified into two types:

• single phase and • three phase. • Diodes are considered to be ideal. By ideal we mean that the reverse

recovery time trr and the forward voltage drop VD are negligible. That is, trr, = 0 and VD = 0.

Single-phase half-wave rectifiers

• During the positive half-cycle of the input voltage, diode D, conducts and the input voltage appears across the load.

• During the negative half-cycle of the input voltage, the diode is in a blocking condition and the output voltage is zero.

Fig. Rectifier with resistive load

rms value DC value

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PERFORMANCE PARAMETERS

• The output voltage is discontinuous and contains harmonics. • A rectifier is a power processor that should give a dc output voltage with

a minimum amount of harmonic contents. • At the same time, it should maintain the input current as sinusoidal as

possible and in phase with the input voltage so that the power factor is near unity.

• The power-processing quality of a rectifier requires the determination of harmonic contents of the input current, the output voltage and current.

• We can use Fourier series expansions to find the harmonic contents of voltages and currents,

The average value of the output (load) voltage Vdc. The average value of the output (load) current, Idc. The output dc power,

Pdc =Vdc*Idc Output rms power:

Pd(rms) = Vd(rms)* Id(rms) Rectifier Efficiency:

P

P

rmsD

dc

)(=η

Ripple factor (RF) measuring the DC ripple content (for DC side only):

12

)( −

=

VVRF

DC

rmsD

Transformer utilization factor (TUF):

IV

P

SS

DCTUF =

Where VS and IS are the transformer secondary (ac) rms voltage & current respectively.

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Displacement factor (DF) is defined as:

);(θCosDF = where: θ is the phase angle between voltage and fundamental current. Total harmonic distortion (THD) on the AC side:

2/12

1

1

−

=

II STHD

Where: I1 is the rms fundamental input current & I S is the net rms input current. Rectifier input power factor (PF) is defined as:

)()( 11 θθ CosCosPFI

I

IV

IV

S

S

SS

SS ==

• THD is a measure of the distortion of an AC waveform.

• If the input current is purely sinusoidal, IS = IS1, and the power factor (PF) equals the displacement factor (DF) '

• Displacement factor DF is often known as displacement power factor

(DPF).

• An ideal rectifier should have: η = 100%, RF = 0, TUF = 1, THD = 0, PF = DPF = 1.

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WAVEFORMS AND THEIR HARMONICS

Waveforms Harmonics (FFT)

Pure ac

Square wave ac Pure dc Pulsating dc

50

Hz 50

50*3

0 Hz

2*50

Fundamental

Dc Component

3rd Harmonic

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Fig. Input (ac) voltage, current and its fundamental component (IS1) of a 1-phsae full rectifier under highly inductive load. Example: Performance Parameters of a 1-P half-wave rectifier Note: In examples, reference to certain equations, pls see Text book by M. H. Rashid

A Half Wave Rectifier

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Half Wave Rectifier with R-L load and Battery

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Fig. Half wave rectifier with a battery at the load.

Diode Conduction found from: ESinV m =α , Hence, )(1

VmESin−=α

Average charging current: )()(21

.)( tdR

EtVmSinI avO ωωπ

β

α∫

−=

FIN

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SINGLE-PHASE FULL-WAVE RECTIFIERS (L5)

• Each half of the transformer with its associated diode acts as a half-wave rectifier and the output of a full-wave rectifier is shown in Figure b. • Because there is no dc current flowing through the transformer, there is no dc saturation problem of transformer core. The average output voltage is:

Vv

VV mT m

mdc dttT

6366.02)sin(2 2/

0=== ∫

πω

Where: Vm – Input peak voltage

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• Instead of using a center-tapped transformer, we could use four diodes

• This is known as BRIDGE TOPOLOGY

As shown in the following Fig. Next

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THREE-PHASE BRIDGE RECTIFIERS

• Used in high-power applications • Can operate with or without a transformer and gives six-pulse ripples on the output voltage • The diodes are numbered in order of conduction sequences and each one conducts for 1200 • The pair of diodes which are connected between that pair of supply lines having the highest amount of instantaneous line-to-line voltage will conduct

The average output voltage (DC):

VV

VV mm

mdc tdt 654.133)cos(36/2

2 6/

0=== ∫

π

πωω

π Where: Vm – Input line–neutral peak voltage

The rms output voltage:

=

= ∫

2/16/

0

22 )(cos36/2

2 πωω

πtdtvV mrms

= VV mm 6554.14

3923

2/1

=

+

π

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Rms diode current:

III mmr tdt 5518.0)(cos24 2/1

6/

0

22 =

= ∫

πωω

π Where: Im- peak value of the input/load/diode current

Rms input (transformer secondary) current=

IIII mmmS tdt 78.06

2sin21

62)(cos

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2/12/16/

0

22 =

+=

= ∫

πππ

ωωπ

π

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TUF= PDC /3VS*IS

VDC

VDRMS

RF = SQRT(VDRMS/VD)2-1=4%

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Diode/Thyristor Protection

T charging < T discharging Lower IRR

above

below

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EXAMPLE

R=5 Ohm

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Your Notes: FIN

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RECTIFIER CIRCUIT DESIGN (L6) Design of a rectifier involves: • determining the ratings of semiconductor diodes. • ratings of diodes are normally in terms of:

• average current, rms current, peak current, peak inverse voltage. Other parameters involve: • Supply transformer ratings (kVA), • Input/output filters • Switch protection circuit (snubbers) • thyristor gating signal generator

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FILTERS

Input (top) & Output filter Topologies

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Your Notes:

As shown in the following

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From the above eqn.

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EFFECTS OF SOURCE AND LOAD INDUCTANCES • In a practical circuit due to transformer leakage and supply inductances

(source) inductances are always present and the performances of rectifiers are changed

• Consider point ωt = Π where voltages vac and vbc are equal. • Due to L1, the current cannot fall to zero immediately and the transfer of

current cannot be instantaneous • Current through D1 id1 decreases resulting in an induced voltage across L1 • Current through D3, id3 increases from zero, inducing an equal negative

voltage across L2

Voltage across L2 is:

dtdi

LV L 22 = , with a linear rise of current, it LV L ∆=∆ 22

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Average DC voltage drop:

( ) ILvvvV dcSLLLX fiT

621321 =∆++=

Your Notes: FIN

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THYRISTORS (L7)

• It’s a bistable switch, operating from non-conducting state to conducting state • Compared to transistors, thyristors have lower on-state conduction losses and

higher power handling capability. • On the other hand, transistors generally have superior switching performances

in terms of faster switching speed and lower switching losses. A thyristor is a four-layer semiconductor device of pn-pn structure.

• When the anode voltage is made positive with respect to the cathode, the junctions J1, and J3 are forward biased. • The junction J2 is reverse biased, and only a small leakage current flows from anode to cathode. • The thyristor is then said to be in the forward blocking or off-state condition • If the anode-to-cathode voltage VAK is increased to a sufficiently large value, the reverse-biased junction J2 breaks. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage VBO. Because the other junctions J1, and J3 are already forward biased, there is free movement of carriers across all three junctions, resulting in a large forward anode current. The device is then in a conducting state, or on-state. • The voltage drop would be due to the Ohmic drop in the four layers and it is small, typically 1 V In the on-state, the anode current is limited by an external impedance. • The anode current must be more than a value known as latching current IL to maintain the required amount of carrier flow across the junction; otherwise, the device reverts to the blocking.

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• Once a thyristor conducts, it behaves like a conducting diode and there is no control over the device. • If the forward anode current is reduced below a level known as the holding current IH, a depletion region develops around junction J2 due to the reduced number of carriers and the thyristor is in the blocking state. • The holding current is on the order of milli amperes and is less than the latching current IL. That is, IL > IH. • Holding current IH is the minimum anode current to maintain the thyristor in the on-state. • In practice, the forward voltage is maintained below VBO and the thyristor is turned on by applying a positive voltage between its gate and cathode

Switching Characteristics

• Thyristor latches into conduction when its anode is positive with respect to the cathode and only when a voltage pulse is applied to its gate terminal.

• The forward anode current of a thyristor must be more than its latching current to latch into the conduction state

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TWO-TRANSISTOR MODEL OF THYRISTOR The latching action can be demonstrated by using a two-transistor model of thyristor. A thyristor can be considered as two complementary transistors, one pnp-transistor Q1, and other npn-transistor Q2, as shown. Equivalent circuit model is shown in Figure b.

The thyristor load (anode current) is linked to the gate current, the gains of the two transistors (α1, α2) and the leakage currents of the collector-base junctions of respective transistors ICBO, as:

)(1 21

212

ααα

+−++

= IIII

CBOCBOGA

If (α1+α2) ≈1, anode current can be very large, thyristor can be turned on with a small gate current.

IE, mA

α -Current Gain, α=α1+α2 <1

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THYRISTOR TURN-ON • During the turn-on process of a thyristor, there is a regenerative or positive feedback effect. As a result, a thyristor can turn on with a small gate current and latch into conduction carrying a large value of anode current. • In the blocking mode, the applied dv/dt must be less than the rated voltage

A thyristor is turned on by increasing the anode current. This can be accomplished in one of the following ways:

• Light: The light-activated thyristors are turned on by allowing light to strike the silicon wafers.

• dv/dt: The manufacturers specify the maximum allowable dvldt of thyristors. • Gate current: If a thyristor is forward biased, the injection of gate current by

applying positive gate voltage between the gate and cathode terminals turns on the thyristor.

As the gate current is increased, the forward blocking voltage is decreased, as shown following figure.

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Designing gate circuit:

1. The gate signal should be removed after the thyristor is turned on. A continuous gating signal would increase the power loss in the gate junction.

2. A gating signal must be wide enough so that IT > IL.

Where: td – delay time tr – Rise time

On time, tON = td + tr THYRISTOR TURN-OFF

• Turn-off time tq is the minimum value of time interval between the instant when the on-state current has decreased to zero and the instant when the thyristor is capable of withstanding forward voltage without turning on.

• Reverse recovered charge QRR is the amount of charge that has to be recovered during the turn-off process.

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A thyristor that is in the on-state can be turned off by reducing the forward current to a level below the holding current IH.

Off time, tq = trr + tr

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THYRISTOR TYPES

• Manufacturers use various gate structures to control the dildt, turn-on time, and turn-off time.

• Thyristors can easily be turned on with a short pulse

• For turning off, they require special drive circuitry or special internal structures to aid in the turning-off process.

• There are several versions of thyristors with turn-off capability

Depending on the physical construction, and turn-on and turn-off behavior, thyristors can be broadly classified into many categories:

• Phase-controlled thyristors (or SCRs)

• Fast switching thyristors (or SCRs) ,

• Light-activated silicon-controlled rectifiers (LASCRs)

• Bidirectional triode thyristors (TRIACs)

• Gate turn-off thyristors (GTOs) Your Notes: FIN

55

CONTROLLED RECTIFIERS (L8)

• Diode rectifiers provide a fixed output voltage only • For controlled output voltages, thyristors are used instead of diodes. • The output voltage of thyristor rectifiersis varied by controlling the delay or

firing angle of thyristors. Used in variable-speed drives, ranging from fractional horsepower to megawatt power level. Phase-control converters can be classified into two types:

(1) single-phase converters, and (2) three-phase converters

Each type can be a:

(a) semiconverter, (b) full converter, and (c) dual converter.

• A semiconverter is a one-quadrant converter and it has one polarity of output

voltage and current. • A full converter is a two-quadrant converter • A dual converter can operate in four quadrants; and both the output voltage

and current can be either positive or negative

• In the converter of the following Fig., during the positive half-cycle of input voltage, the thyristor anode is positive with respect to its cathode and the thyristor is said to be forward biased

• When thyristor T1 is fired at ωt = α, T1, conducts and the input voltage appears across the load.

• When the input voltage starts to be negative at ωt =Π, the thyristor anode is negative with respect to its cathode and thyristor T, is said to be reverse biased; and it is turned off.

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The average output voltage (Vm is the peak input voltage):

[ ]

)cos1(2

cos2

)(*sin21

απ

ωπ

ωωπ

πα

π

α

+=

−== ∫

V

VvV

m

mmdc ttdt

Rms output voltage:

2/1

2/122/122

22sin1

2

)()2cos1(4

)(*sin21

+−=

−=

= ∫∫

ααππ

ωωπ

ωωπ

π

α

π

α

V

VvV

m

mmrms tdttdt

57

SINGLE-PHASE FULL CONVERTERS

• Converter is shown in Figure with a highly inductive load so that the load current is continuous and ripple free

• During the positive half-cycle, thyristors T1, and T2 are forward biased

• Due to the inductive load, thyristors T1, and T2 continue to conduct beyond ωt=Π, even though the input voltage is already negative.

• During the negative half-cycle of the input voltage, thyristors T3 and T4 are forward biased; and firing of thyristors T3 and T4 applies the supply voltage across thyristors T1, and T2 as reverse voltage

• Provides two quadrant operation, vo= + or – • However, io is always positive • With a resistive load, vo is positive only

58

The average output voltage (Vm is the peak input voltage):

[ ]

απ

ωπ

ωωπ

απα

απ

α

cos

cos2

)(*sin22

2

2

V

VvV

m

mmdc ttdt

=

−== ++

∫

Rms output voltage:

VVV

vV Smm

mrms tdttdt ===

−=

= ∫∫

+

2)()cos(1(

2)(*sin

22 2/122/1

22π

α

απ

αωω

πωω

π

Previous Fig.

In Previous Fig.

59

60

DUAL CONVERTER

• If two full converters are connected back to back, as shown, both the output voltage and the load current flow can be reversed

• The system provides a four-quadrant operation and is called a dual converter. Dual converters are normally used in high-power variable-speed drives

• If α1 and α2 are the delay angles of converters 1 and 2, respectively, the corresponding average output voltages are Vdc1 and Vdc2

• The delay angles are controlled such that one converter operates as a rectifier and the other converter operates as an inverter, but both converters produce the same average output voltage.

)cos(211 α

πV

Vm

dc = )cos(222 α

πV

Vm

dc =

Where: α2 = π- α1

61

62

3-phase half wave Controlled Rectifiers (L9)

• Higher average output voltage compared to a 1-phase • Higher DC side pulses resulting in higher harmonic orders • Lower filtering requirement

63

For resistive load at α < 300& Highly imductive load at any α If the input L-N peak voltage is Vm, then the:

Average output voltage: απ

ωωπ

απ

απcos

2)(*sin

23 336/5

6/

VvV m

mdc tdt == ∫+

+

Rms output voltage;

2/12/16/5

6/

22 2cos8

3613)(*sin

23

+=

= ∫

+

+α

πωω

παπ

απVvV mmdrms tdt

For resistive load & α ≥ 300: DC output voltage:

++== ∫

+

αππ

ωωπ

π

απ 6cos1

2)(*sin

23 3

6/

VvV mmdc tdt

Rms output voltage:

2/1

2/1

6/

22

23

sin81

42453

)(*sin23

++−

=

= ∫

+

απππ

α

ωωπ

π

απ

V

vV

m

mdrms tdt

Disadvantages:

• Input currents are pulsating DC, can saturate the transformer • Only limited to low power applications

64

THREE-PHASE FULL CONVERTERS

• Extensively used in industrial applications up to the 120-kW level, where a two-quadrant operation is required.

• The thyristors are fired at an interval of 600

• The frequency of output ripple voltage is 6f, and the filtering requirement is less than that of half-wave converters

• When thyristors T1 and T2 conduct, the line-to-line voltage va appears across the load

• If the thyristors are numbered, as shown in Figure, the firing sequence is 12, 23, 34, 45, 56, and 61

If the line-to-neutral voltages are defined as:

Corresponding line-to-line voltages are:

65

RMS Average

66

Average output voltage:

απ

ωπωπ

ωπ

απ

απ

απ

απ

cos

)(*6

sin33)(*26

33

2/

6/

2/

6/

V

vvV

m

mabdc tdttd

=

== ∫∫

+

+

+

+

Rms output voltage:

2/1

2/12/

6/

22

2cos4

33213

)(*6

sin26

3

+=

+= ∫

+

+

απ

ωπωπ

απ

απ

V

vV

m

mdrms tdt

Finding the Performances of a three-phase Full-wave Converter A three-phase full-wave converter is operated from a three-phase Y-connected, (Vab(rms))208-V, 60-Hz supply and the load resistance is R=10Ω. If it is required to obtain an output voltage of 50% of the maximum possible output voltage, calculate: (a) the delay (α), (b) the rms and average output currents, (c) the average and rms thyristor currents. (d) rectification efficiency, (e) the TUF, and (f) the input PF

Solution:

67

68

For Highly Inductive Load and for the same delay angle, PF is Lower. FIN

Previous example The load type, i.e. R or L.

69

Twelve-pulse Rectifiers (L10)

AФ

Vo

Voy, VoΔ 6-pulses

12-pulses

70

• One of the bridges is supplied through a Y-Y connected transformer, • The other is supplied through a Y-Δ (or Δ-Y) transformer • The purpose of the Y-Δ transformer connection is to introduce a 300

phase shift between the source and the bridge. • This results in input voltage-current phase shift, which are 300 apart. • The two bridge outputs are similar, but also shifted by 300. • The dc output is the sum of the dc output of each bridge:

Net output voltage:

απ

απ

απ

coscoscos 633 )(2

)(1

)(0

VVVVVV

LLmLLmLLmooy

−−−∆ =+=+=

• Since a transition between conducting SCRs occurs every300, there are a

total of 12 such transitions for each period of the ac source. • The output has harmonic frequencies which are multiples of 12 times the

source frequency (12k, where: k = 1, 2,3 ... ). • Filtering to produce a relatively pure dc output is less costly than that

required for the six-pulse rectifier.

• Another advantage of using a twelve-pulse converter rather than a six-pulse converter is the reduced harmonics that occur in the ac system.

The current in the ac lines supplying the Y- Y transformer in Fourier series:

+−+−= ...cos

131cos

111cos

71cos

51cos32)( 131175 tttttt ooooooY Ii ωωωωω

π

The current in the ac lines supplying the Y-Δ transformer is in Fourier series:

+−−+=∆ ...cos

131cos

111cos

71cos

51cos32)( 131175 tttttt ooooooIi ωωωωω

π

71

The ac systern current, which is the sum of 2 transformer currents in Fourier series:

+−=+= ∆ ...cos

131cos

111cos34)()()( 1311 tttttt ooooYac Iiii ωωω

π

(No 5th and 7th harmonics) Some of the harmonics (5th & 7th) on the ac side are canceled by using the twelve-pulse scheme. The harmonics that remain in the ac system are of order l2k±1. This principle can be expanded to an-arrangements of higher pulse numbers by incorporating increased pulse Nos. of six/pulse converters with transformers with appropriate phase shifts. lower-voltage industrial systems commonly have converters with up to 48 pulses. POWER FACTOR IMPROVEMENTS

• The PF of phase-controlled converters depends on delay angle α, and is in general low, especially at the low output voltage range. These converters generate harmonics into the supply.

• Forced commutations can improve the input PF and reduce the harmonics levels.

1. Extinction angle control 2. Symmetric angle control 3. Pulse-width modulation (PWM) 4. Single-phase sinusoidal PWM 5. Three-phase PWM control

72

Extinction Angle Control

• The switching actions of S1, and S2 can be performed by GTO or IGBT. An IGBT remains on as long as a gate voltage is applied to its gate terminal.

• The output voltage is controlled by varying the extinction angle β. • The fundamental component of input current leads the input voltage, and

the displacement factor (and PF) is leading. • In some applications, this feature may be desirable to simulate a capacitive

load and to compensate for line voltage drops.

73

Average output voltage:

( )βπ

ωωπ

βπcos1)(*sin

22

0+== ∫

− VvV

mmdc tdt

Vdc can be varied from 2Vm /Π to 0 by varying β from 0 to Π. The rms output voltage:

2/12/1

0

22)(

22sin1

2)(*sin

22

+−=

= ∫

− ββππ

ωωπ

βπ vvV

mmrmsd tdt

Symmetric Angle control Provides an unity Displacement factor Average and rms output voltage can be controlled by angle β. The average output voltage:

2sin)(*sin

22 22/)(

2/)(

βπ

ωωπ

βπ

βπ

VvV

mmdc tdt == ∫

+

−

The rms output voltage:

( )2/12/12/)(

2/)(

22)( sin1

2)(*sin

22

+=

= ∫

+

−ββ

πωω

π

βπ

βπ

vvV

mmrmsd tdt

74

75

76

77

PWM CONTROL

• In PWM control, the converter switches are turned on and off several times during a half-cycle and the output voltage is controlled by varying the width of pulses.

• The gate signals are generated by

comparing a triangular wave with a dc signal

78

The average output voltage due to p number of pulses per half cycle:

[ ]∑∑ ∫==

++−=

=

p

mmmm

mp

mmdc

vvV

mm

m

tdt11

)cos(cos)(*sin22

δααπ

ωωπ

δα

α

Input Current can be expressed as:

79

Single-Phase Sinusoidal PWM

• It is possible to choose the widths of pulses in such a way that certain harmonics could be eliminated

• Most common one is the sinusoidal pulse-width modulation (SPWM) • Pulse widths are generated by comparing a triangular reference vr with a

carrier half-sinusoidal voltage vc of variable amplitude and fixed frequency 2fs

• The widths of the pulses (and the average/rms output voltage) are varied

by changing the amplitude of vc, or the modulation index M from 0 to 1. The modulation index is defined as:

A

A

r

CM =

80

• In a sinusoidal PWM control, the DF is unity and the PF is improved • The lower order harmonics are eliminated or reduced • For example, with four pulses per half-cycle the lowest order harmonic

is the fifth; and • with six pulses per half-cycle, the lowest order harmonic is the seventh.

Your Notes: FIN

81

PF IMPROVEMENT BY FILETRS (L11)

82

83

Your Notes: