Lecture Notes POWER ELECTRONICS & MOTION CONTROL I.

Lecture Notes POWER ELECTRONICS & MOTION CONTROL I.

Injection moulded plastic case

Copper electrodes stamped from copper ribbon

Heatsink and anode Planar silicon pellet

Cathode Anode Gate

Gold connecting wires

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Index INTRODUCTION _________________________________________________________________________________ 4 DEFINITION OF POWER ELECTRONICS __________________________________________________________________ 4 INTERDISCIPLINARY CHARACTER OF PE ________________________________________________________________ 4 HISTORICAL BACKGROUND OF DEVICES _________________________________________________________________ 5 CONTRIBUTION OF HUNGARIAN SCIENTISTS IN THE FIELD OF PE ______________________________________________ 6 ECONOMICAL SIGNIFICANCE:_________________________________________________________________________ 6 APPLICATION OF PE________________________________________________________________________________ 6 Industrial applications ___________________________________________________________________________ 6 Residential applications __________________________________________________________________________ 8 Commercial Applications _________________________________________________________________________ 8 Transportation _________________________________________________________________________________ 8 Utility systems__________________________________________________________________________________ 9 Aerospace _____________________________________________________________________________________ 9 Telecommunications _____________________________________________________________________________ 9 Military _______________________________________________________________________________________ 9 Computers_____________________________________________________________________________________ 9 Medical ______________________________________________________________________________________ 10 CIRCUITS WITH SWITCHES AND DIODES_________________________________________________________ 11 SWITCHED DC SOURCE ____________________________________________________________________________ 11 Resistive Load Circuit___________________________________________________________________________ 11 RL Load Circuit _______________________________________________________________________________ 13 Inductive Load Circuit __________________________________________________________________________ 14 POWER SEMICONDUCTOR SWITCHES ___________________________________________________________ 16 DIODES ________________________________________________________________________________________ 16 Simplified V-A characteristics ____________________________________________________________________ 16 Schottky diode_________________________________________________________________________________ 17 Fast recovery diode ____________________________________________________________________________ 17 Line frequency diode ___________________________________________________________________________ 17 THYRISTORS ____________________________________________________________________________________ 18 Thyristor characteristics_________________________________________________________________________ 18 Ways of turning on a thyristor ____________________________________________________________________ 18 Gate triggering of thyristors ______________________________________________________________________ 19 DC Gate Triggering Characteristics _______________________________________________________________________ 20 Parameters, Specifications_______________________________________________________________________________ 20 Load Lines___________________________________________________________________________________________ 21

Thyristor Turn-Off Characteristics and Methods ______________________________________________________ 23 Turn Off Methods _____________________________________________________________________________________ 23

Application of Thyristors: ON-OFF Control (Burst control) ____________________________________________ 25 Half-wave variable phase control configuration ______________________________________________________________ 25 Thyristor in the diagonal of a diode bridge __________________________________________________________________ 26 Back-to-back pair of thyristors ___________________________________________________________________________ 27

Chopper Circuits ______________________________________________________________________________ 28 Chopper circuit with auxiliary thyristor ____________________________________________________________________ 28 Buck Converter _______________________________________________________________________________________ 30

Parameters, specifications _______________________________________________________________________ 32 Thyristor encapsulations ________________________________________________________________________ 33 CONTROLLABLE SWITCHES _________________________________________________________________________ 34 BJT (Bipolar Junction Transistor) _________________________________________________________________ 34 MOSFET ( Metal Oxide Semiconductor Field Effect Transistor)__________________________________________ 35 GTO (Gate Turn-Off Thyristor) ___________________________________________________________________ 35 IGBT (Insulated Gate Bipolar Transistor) ___________________________________________________________ 36 Desirable characteristics of a controllable switch _____________________________________________________ 36 Comparison of controllable switches _______________________________________________________________ 37 POWER DISSIPATION OF A POWER SEMICONDUCTOR SWITCH ________________________________________________ 37 DC- DC SWITCH MODE CONVERTERS ____________________________________________________________ 38 2

3 INTRODUCTION __________________________________________________________________________________ 38 CONTROL OF DC-DC CONVERTERS ____________________________________________________________________ 38 STEP-DOWN (BUCK) CONVERTER ____________________________________________________________________ 40 STEP-UP (BOOST) CONVERTER_______________________________________________________________________ 42 CONTINUOUS AND DISCONTINUOUS CONDUCTION MODE (CCM & DCM) _____________________________________ 43 BUCK-BOOST CONVERTER__________________________________________________________________________ 43 CÚK DC-DC CONVERTER ___________________________________________________________________________ 44 POSSIBLE WAYS TO CALCULATE THE OUTPUT VOLTAGE RIPPLE:______________________________________________ 45

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POWER ELECTRONICS & MOTION CONTROL I. Introduction Definition of Power Electronics Task of Power Electronics (PE) is to control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.

Power input vi

Power processor

ii

Power output vo

io Control signals

Controller

Load

Measurements Reference

Block diagram of a power electronic system. The power processors usually consist of more than one power conversion stage where the operation of these stages is decoupled on an instantaneous basis by means of energy storage elements such as capacitors and inductors. Power processor

Input

Output Converter 1

Energy storage element

Converter 2

Power processor block diagram. Interdisciplinary Character of PE The study of power electronics encompasses many fields within electrical engineering, as illustrated in the figure below. These include power systems, solid-state electronics, electrical machines, analog/digital control and signal processing, electromagnetic field calculations, and so on. Combining the knowledge of these diverse fields makes the study of power electronics challenging as well as interesting. There are many potential advances in all these fields that will improve the prospects for applying power electronics to new applications. 4

5 Circuit theory Solid-state physics

Simulation and computing

Systems and control theory

Signal processing

Power electronics

Electronics

Electric machines

Power systems

Electromagnetics

Interdisciplinary nature of power electronics.

Historical background of devices Thyratron tubes Controllable rectifier, basically gas-discharge tube. drawback: limited in current (approx. 50A), limited lifetime ( due to emitting cathode) and high forward voltage drop. Mercury-Arc Rectifiers A G Vacuum pump Mercury C

Current is conducted by the electric arc between the anode (graphite) and the cathode (mercury) when the anode is positive as compared to the cathode. Conduction is initiated by ignition through a gate pulse. M-A-Rs made it possible to build large rectifier systems by putting several anodes into a common mercury tank containing the cathode. Drawback: They need continuous vacuum pumping. Saturable Reactors Use the magnetic properties of the core inside a coil. Were replaced by thyristor circuits. Further components: Relays and Contactors Mechanical Speed Changers 5

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Rheostats e.g. Electric arc welding and motor starters (wet resistance) Constant voltage transformers Types of Converter systems: 1. AC voltage controllers: Fixed voltage AC to variable voltage AC 2. Rectifiers: (uncontrolled) Fixed voltage AC to fixed voltage DC 3. Rectifiers: (controlled) Fixed voltage AC to variable voltage DC 4. DC/DC converters Fixed voltage DC to variable DC (Choppers): Reduction: Buck Converter Increase: Boost Converter Reduce/inc.: Buck-Boost Converter 5. Inverters (uncontrolled.): Fixed DC to fixed AC voltage 6. Inverters (controlled): Fixed DC to variable AC voltage (Square/trapezoidal/sine wave output) 7. Cycloconverters: Fixed frequency and voltage AC to variable frequency (and voltage)AC output (frequency reduction) 8. Matrix Converters Fixed frequency and voltage AC to variable frequency (and voltage) AC output (frequency increase/reduction, changing the number of phases) Contribution of Hungarian Scientists in the field of PE M. Déry (1854-1938) O.Bláthy (1860-1939) Zipernowsky K. Kandó (1869-1931) I. Rácz (

)

F. Csáky (1921-1977)

- 1 phase comutating repulsion motor, AC transformer - parallel connection of ac generators, induction current meter, contribution in development of electric locomotive, AC transformer - AC transformer - development of the first 3 phase high-voltage locomotive. - Park vector theory application in PE, particularly in induction motor drives - control theory, automation, power electronics

Economical Significance: Consumption of electric energy in the world is largely in converted form. The next list will show examples about the application of Power Electronic systems. From the list it is evident that PE has penetrated almost all fields of technology and our everyday life. Application of PE Industrial applications  Drives Pumps, Compressors, Blowers and fans, Machine tools, Elevators, Cranes, Conveyors, Hoists. Approx. 2/3 of the generated electric energy in industrial countries is consumed by the various drives. A large portion of these drives is ac drive. 6

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The basic parts of the drives are the power supplies and the electromechanical power converter (electrical machine.) A functional block diagram of an ac electric drive is seen in the Figure below.

Power processor

+

ac Converter 1

dc

Utility

ac C

Converter 2

ac motor

_

Figure 3 - Block diagram of an ac motor drive.

 Electric heat generation Arc furnace, Resistive heating , Induction heating (high frequency current in a coil)  HV transmission: The technical and economical reason for application is that there are certain areas where high voltage transmission is needed between two systems with different frequencies (e.g.: Japan, where a 50Hz system is connected to a 60Hz system) or the HV transmission is more economical as it has higher efficiency (e.g.: HV transmission between Scandinavia and Europe). Used for transmitting high electric power to large distances, through DC line (parallel losses). The system consists of an AC/DC and a DC/AC converter as shown in the figure below. Power level of such a system is 200-600MW.

High Voltage Line Load

DC Link AC/DC converter

DC/AC converter

 Power Supplies Important parameters:  Power density [W/in3,W/cm3] present limits are around 30-50W/in3.  Tolerance  Ripple - input reflected ripple (current) - output ripple (voltage), EMC/EMI - noise levels [dB] 7

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 UPS Systems (Uninterruptable Power Supply) are used where high availability, and/or reliability of the power supply is required. Application: Computers: process control, banks, servers Energy transmission and distribution Communication Hospitals  Robots where motors, actuators are needed  Induction Heating melting, annealing, heat treatment  Electric Arc Welding Power Sources Residential applications  Alarms  Refrigeration: temperature control (bang-bang control, on-off control)  Space heating  Air conditioning  Washing machines drives control- program storage electric heating  Cooking: microwave ovens, induction heating  Lighting  Electronics (PCs, Home Entertainment)  Electric Door Openers  Hand Power Tools Commercial Applications  Heating,  Ventilating, Air Conditioning (Fan drives)  Central Air Conditioning  Computers and Office equipment  UPS  Elevators  Lighting electric discharge tubes CFL (Compact Fluorescent Lamp with integrated PE device) Transportation  Traction Control  Battery Chargers (onboard chargers)  Electric Locomotives (Super Conducting material, MAGnetic LEVitation vehicle : 500 km/h)  Trams, Trolley Buses  Subways  Automotive electronics including engine control  Electric cars (onboard chargers, regenerative braking)

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Utility systems  High voltage DC transmission (HVDC)  Static var generation (to compensate reactive power in the system to reduce power loss)  Nuclear reactor control rod  Supplemental energy storage systems (magnetic energy storage with super conducting coil)   1 2  Joule W stored  2 iL  L   With super conducting material the current can be raised by magnitudes, which will result in increased stored energy.  Remote control by audio frequency signals  Generator exciters  Induced-draft fans and pumps B i F (They deliver the fluid trough the pipe using the interaction of magnetic field and current in the fluid.)

 Active filters, for filtering higher harmonic components  time domain control  frequency domain control Aerospace  Space shuttle power supply system  Satellite power systems, solar power supplies  Aircraft power supplies: they are using 400Hz network system. Airports are also using 400Hz networks. Telecommunications  Battery chargers  Power Supplies (DC and UPS)  HF inverter radio transmitter for AM service Military  Gun elevation  Tracking systems  High power density power supplies Computers  Power Supplies (SMPS, e.g. 3.3V, 500A)  Floppy and hard disc drives  Driving circuits for printer heads 9

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Medical  Artificial internal organs  Artificial heart systems  HV power supplies for X-ray machines  Power supplies for diagnostic equipment (magnetic resonance, computer tomography, ultrasonic diagnostics, ECG)  HF heating (Short wave)  Pace makers

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Circuits with Switches and Diodes In this chapter some circuits made up of ideal elements are discussed. These elements include ideal switches that present either infinite or zero resistance to current and are capable of instantaneous transition from one state to the other. They also include ideal diodes. An ideal diode has zero resistance to positive anode current iA, but infinite resistance to current in the reverse direction. Thus the diode conducts if the source voltage v is positive, and the anode-to-cathode voltage vAC is then zero. The diode does not conduct if the source voltage is negative, when vAC also is negative. In the diagram of iA versus vAC shown below, the operating point of the diode may thus lie on the positive axis of iA in the range 0  iA  2  V / R or on the negative axis of vAC in the range 0  v AK  2  V for the circuit of the figure below.

iA v

2V sin  t 

vAC _ + D

iA 2V R

R

2V 0

vAC

The purpose of analysing such ideal circuits is to give the reader the ability to look at a circuit embodying power semiconductor devices and to envisage approximately by inspection how that circuit functions. This should also show what measures must be taken to protect the practical devices from destruction. The operation of a switch in a network may: 1. Apply an energy source. 2. Remove an energy source. 3. Change the configuration of the network in other ways. In the following sections of this chapter, simple switched circuits are first discussed. Switched DC source The effect of applying a step function of voltage by means of a switch to circuits of different parameters is discussed in this section. The purpose of doing this is to arrive at conclusions that may be applied to similar circuits when they are switched by means of power semiconductor devices. Resistive Load Circuit In the circuit below, when switch SW is closed at t=0, the current rises instantaneously to the value V  A i R When SW is opened at t=t1, the current falls instantaneously to zero as illustrated. The voltage across the open switch is vS=V. 11

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+

V

vR

SW

i

V

_

R

0 i

t1

t

t1

t

V/R 0

RC Load Circuit In the circuit below, when SW is closed at t=0, by Kirchhoff’s voltage law t

V  vC  v R 

1 1 idt  Ri C0



V 

2.1

Solution of this equation gives an expression for the time variation of current I, and hence also for the voltages vC and vR. Differentiation of the equation yields di 1  i  0  A / s dt RC so that 2.2 i  Ae t / RC  A where A is a constant of integration that must be determined from the initial conditions. SW i C i V/ _ + + _ R t vC R V V t The capacitor is initially uncharged and therefore has zero potential difference between its plates. This potential difference cannot change instantaneously, since q  vc C

where q is the charge on each plate. For vC to change instantaneously, q must change instantaneously, and this would call for an infinite current. Thus immediately after the switch is closed at t=0+, vC=0, and from equation 2.1 V= vR= Ri [V] so that at t=0+, I= V/R [A] substitution of t=0 and i= V/R in equation 2.2 yields A= V/R [A] so that t

i

V  RC e R

 A

2.3 12

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and this relationship is shown in the figure. As vR falls, vC rises, until in infinite time (in practical circuits often merely a fraction of a second) the capacitor is fully charged, so that I=0 A: vC= V [V] If the switch were opened at t=t1 before the capacitor was fully charged, then the voltage across the switch would be vS= V-vC [V] From equation 2.3 and from the curves of the figure it may be seen that if the resistance in the circuit is very low, then the initial current may be extremely high, and the flow of current will form a pulse of very short duration. If the switch were a power semiconductor, it would be liable to be destroyed by this high current. RL Load Circuit in the circuit below, when SW is closed at t=0 V  vL  vR  L

di  Ri dt

V 

or di R V  i dt L L

 A  s 

2.4

i V/R

SW

i +

V

_

vS

L + v _ L

vR

R

0 vL V

t

0

t To



The current in the circuit may be divided into two components. The first is the forced or steady-state component of the current, and this represents the condition of operation of the circuit reached after SW has been closed for an infinitely long time. It is determined by the applied excitation and is the particular integral solution of the differential equation describing the circuit. The second component is the natural or transient component of the current, and this represents a condition of operation of the circuit that has disappeared after an infinite time. It is determined by the circuit parameters and the initial conditions existing in the circuit at t=0 and is the complementary function of the solution to the differential equation. When the steady-state has been reached, the derivative in equation 2.4 is by definition equal to zero, so that from equation 2.4 the forced component of the current is iF= V/R [A] The natural component is obtained by solution of the homogeneous equation formed from equation 2.4 which is diN R  iN  0  A / s dt L of which the solution is 13

14

iN  Ae

( R / L ) t

 A

where A is a constant of integration that is to be determined. The complete solution of equation 2.4 is thus V 2.5 i  iF  iN   Ae( R / L )t  A R At t=0, i=0, and substitution in equation 2.5 yields A= V/R [A] thus V i  (1  e ( R / L ) t )  A R and this function is shown in the figure. The voltage across the inductance is di v L  L  Ve( R / L ) t V  dt and this function also is shown in the figure. If the switch is reopened, the stored energy in the inductance is released, inducing a voltage at the terminals of the inductance which tends to maintain the current that was flowing while the switch was closed. The opening of the switch tends to reduce the current instantaneously to zero, so that di/dt approaches a value of minus infinity. Since the voltage across the terminals of the inductance is vL= Ldi/dt, this voltage also approaches infinity as indicated in the figure. In the case of a simple mechanical switch operating in atmosphere, the air between the opening contacts is ionized by the field due to the high voltage vS and momentarily conducts, forming a high resistance arc in which much of the energy formerly stored in the inductor is dissipated as heat. A power transistor operating as a switch would be destroyed in such a situation. Inductive Load Circuit In practical circuits, resistance may be very small and inductance large, so that the result obtained by neglecting resistance in analyzing the circuit approximates closely to its actual behaviour. For such an approximate circuit shown below, equation 2.4 becomes di V 2.6  A / s  dt L i

SW

L + vL _

i (V/L)t1 t

V vL

t1

V t

and the time variation of current is that shown in the upper figure, where if SW is closed at t=0, then at t=t1 i=(V/L)t1 [A] 2.7 14

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The problem of how to open the switch without the appearance of an infinite voltage across its contacts remains, and one solution is shown below, where an ideal diode is connected in parallel with the inductance. i

SW

vL=vD V

iL V

+ L

vL _

D iD

+ v _ D i

t1

t

(V/L)t1

iD

t1

t

(V/L)t1

iL

t1

t

(V/L)t1

t1

t

The diode D earns its name “free-wheeling diode” by its ability to permit current iL to continue to flow when the energy source has been removed by the opening of the switch at t=t1. The operation of the circuit for the interval 0 5kV 17

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Thyristors Standard MSZ-EN-symbol (EuroNorm): A

G C

These devices are capable of blocking voltages in both directions. They have four layers and three junctions. G A C n p n p

J1

J2

J3

If vT>0 (forward): J1, J3 are forward biased J2 is reverse biased. Thyristor characteristics iT

Forward voltage-drop (conducting) Latching current Reverse breakdown voltage

Holding current

Gate triggered

Forward breakover voltage

iL iH vF,br

Reverse leakage current

vAC

Forward leakage current

Ways of turning on a thyristor  gating by positive gate current 18

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   

by light, in case of Light Activated Thyristors with vF,br (forward breakover voltage), it might destroy the thyristor thermal turn-on (thermal runaway), is normally avoided high dv/dt, fast increase of the voltage vT (it might destroy the thyristor)

The junction of a thyristor can be considered as non-linear capacitance Cj (depends on the voltage vj). iCj  iCj 

dq j dt d (C j v j ) dt

qj  Cjv j  Cj

dv j dt

 vj

dC j dt

tq: turn off time (very important factor of the thyristor)

Unwanted firing through the anode if

tqc< tq,device or

dv dv  dt dt

r a te d

Gate triggering of thyristors The figure below shows the triggering response of the thyristor device. vD, iD iSt: steady vS state current 90%

10% iG

t tR

tD

v0: forward voltage drop t

tON: Turn-On time tON=tD+tR tD: Delay Time Time interval between the time the gate current pulse reaches 10% of its final value and the time when the resulting forward current reaches 10% of its maximum value during switching from the off-state to the on-state into a resistive load under stated conditions. tR: Rise Time Time interval between the time the forward current reaches 10% of its maximum value and the time the forward current reaches 90% of its maximum value during switching from the off-state to the on-state into a resistive load under stated conditions

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DC Gate Triggering Characteristics

vG

Rated peak allowable forward gate voltage

Instantaneous gate voltage

Max allowable instantaneous gate power dissipation Preferred gate drive area Locus of possible triggering points

Limit lines

iG Instantaneous gate current

Parameters, Specifications  Instantaneous Forward Gate Current Instantaneous current flowing between gate and cathode terminals in a direction to forward bias the gate junction.  Instantaneous Forward Gate Voltage Instantaneous forward voltage between gate and cathode terminals with anode terminal open.  DC Gate Trigger Voltage Gate voltage with IGT (DC gate trigger current) flowing but prior to start of anode conduction.  DC Gate Trigger Current Forward gate current required to trigger a thyristor at stated temperature conditions.  Peak Reverse Gate Voltage Maximum allowable peak reverse voltage between the gate terminal and the cathode terminal.  Peak Gate Power Dissipation Maximum instantaneous value of gate power dissipation.  Average Gate Power Dissipation Maximum allowable value of gate power dissipation averaged over a full cycle.  Holding Current (Gate drive) Value of Instantaneous Forward Current below which thyristor returns to forward blocking state after having been in forward conduction under stated temperature and gate termination conditions.

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 Latching Current (no Gate drive) Value of minimum anode current to remain in the on-state after removal of the gate trigger pulse under specified condition.  Instantaneous Reverse Blocking Current Instantaneous anode current at stated conditions of negative anode voltage, junction temperature, and gate termination.  Instantaneous Forward Blocking Current Instantaneous anode current at stated conditions of forward blocking voltage, junction temperature, and gate termination.

Load Lines The trigger circuit load line must intersect the individual thyristor gate characteristic in the region indicated as “preferred gate drive area”. The intersection, or maximum operating point, should furthermore be located as close to the maximum applicable (peak, average, etc.) gate power dissipation curve as possible. Gate current rise times should be in the order of several amperes per microsecond in the interest of minimizing anode turn-on time particularly when switching into high currents. This in turn results in minimum turn-on anode switching dissipation and minimum jitter.

Trigger circuit open circuit voltage

vOC

Maximum operating point

SCR characteristics

Preferred gate drive area

Load line

iSC

Trigger circuit short circuit current

Construction of a “load line” is a convenient means of placing the maximum operating point of the trigger circuit-thyristor gate combination into the preferred triggering area. A basic trigger circuit for driving an thyristor gate is out of a source voltage vS and an internal

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resistance RG. The load line is constructed by connecting a straight line between the trigger circuit open circuit voltage and the short circuit current. If the trigger circuit source voltage is a function of time, the load line sweeps across the graph, starting as a point at the origin and reaching its maximum position, the load line, at the peak trigger circuit output voltage. The applicable gate power curve is selected on the basis of whether average or peak allowable gate power dissipation is limiting. For example, if a DC trigger is used, the average maximum allowable gate dissipation must not be exceeded. If a trigger pulse is used the peak gate power curve is applicable. For intermediate gate trigger waveforms the limiting allowable gate power dissipation curve is determined by the duty cycle of the trigger signal according to: peak gate drive power  pulse width  pulse repetition rate  allowable average gate power

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Thyristor Turn-Off Characteristics and Methods When a thyristor is in conducting state, there are free moving charge carriers in the junctions. If we want to block the device (open the switch) we must reduce it’s current to zero. If we have a periodical sine wave current on the device, it will conduct negative current for a time (trr), until the junction region is fully depleted. iAC iAC trr R t + + 0 vAC _ iG vAC vS Qrr _ tq t 0

trr: Reverse Recovery Time The time interval between zero current and the time at which the reverse current through the device has reached a specified value (usually 10% of the peak reverse recovery current) under specified conditions after having been in the on-state. tq: Thyristor (Device) Turn-Off time, a time delay which is defined by the physical device itself. If we apply forward voltage to the thyristor, before tq is passed, then a current starts to flow (the device goes On-state). It is the time interval between zero current and the time of reapplication of positive forward blocking voltage under specified conditions with the device remaining in the off-state after having been in the on-state. tqC: Circuit Turn-Off time, the time provided by circuit before positive voltage will be reapplied to the device. tqC  tq , must be ensured for safe operation. iRM: Reverse Recovery Current, the current which depletes the junction of charge carriers fully after on-state. Turn Off Methods a.) Forced commutation circuit, i=0 R + SW vd _

G

i t v vF

tqC

vd t

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Closing the switch will result in iT=0. We must ensure the required tq time. b.) Reverse voltage supply in the forced commutation circuit i=0, v= -vt

R

+

i SW _

vd

G

t v

vt +

_

vd

tqC

vF

t -vt

The opposite direction voltage vt helps clear out the minority carriers of the junction section. Below the effect of increasing the voltage vt can be seen. tq 1 1/3 vt

c.) Line commutation vS R

t vS

G

i

G 50Hz tqC1-10ms (ensured by the power circuit) tq=10-100s (device turn-off time) In rectifier operation  tqc>tq

t

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Application of Thyristors: ON-OFF Control (Burst control) In this chapter we are going to discuss examples of using thyristors as static switches in line commutated circuits (turn onelectronic circuit, turn offpower circuit). With varying of the pulse width (pw) pw=tON/TS the output voltage can be controlled. In this case pulse width (pw) is similar to the duty ratio (D). t D:  duty ratio  ON TS OFF

ON tON

t

TS

VO , RMS  VI , RMS

tON T

 VO2, RMS   POUT   the output power RL  

Half-wave variable phase control configuration Operation: The gate circuit is driven by the power circuit. A variable resistor is used to adjust the gate current (iG), and so the firing angle (). With increasing the resistor (RG), the gate current will decrease and the firing angle will grow until a maximum angle of max=/2. The reason for this is that the falling current gets below the minimal gate current, which is the minimal current required to trigger the thyristor.

RL iL

iG

A T

vi vG

C

G

D RG

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vi, iG

vi iG iG,mi

G

t

vi  vG v  as vi  vG and RG  RL  iG  i RG  RL RG v  vF v iL  i  i RL RL

iG 

iG 

V m  sin( t ) RG

Turn-on results when iG=iGmin at the firing angle G iG ,min 

G,max

Vm sin( RG

G

)



G

R i   arcsin G G ,min   Vm 



2 The output voltage (Vo,RMS): Vo, RMS 

1 2

 V

 sin( t ) d t 2

m

G

and the power dissipation on RL: Pout 

Vo2, RMS RL

Full wave operation can be realised by using another (inverse-parallel) thyristor with variable resistance. The gate resistances are adjusted together. Drawback: High di/dt by firing the thyristor Causes large RFI (Radio Frequency Interference) Thyristor in the diagonal of a diode bridge

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RL D2

D1

iL

T

~ vi D4

D3

Advantage: It provides a full wave rectification with one thyristor. Back-to-back pair of thyristors RL A vi

iL

G

C T2

T1 SW C

A

When a thyristor has a reverse voltage on its terminals (like T2 on the figure), then its collector-gate terminals can be considered as the terminals of a Zener diode. By closing the switch (SW), the “Zener diode” between C-G of the right (T2) thyristor operates in the Zener field, and provides the gating signal to T1. Advantage: average RFI is low in burst-control mode of operation.

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Chopper Circuits Classification: Configurations, properties: Application : Chopper circuit with auxiliary thyristor

R1 Vd =

R2

C

A2

A1 T1 i1

T2

vc

T1: Main thyristor T2: Commutating (auxiliary) thyristor C: Commutating Condenser R1iB iB0=0

vCE

vCE(sat)

MOSFET ( Metal Oxide Semiconductor Field Effect Transistor) It is a voltage controlled device. In steady state base current is not required. Standard symbol: D + G

+

 S Switching frequency range: 100kHz3MHz Characteristics: iD iDS iDSS

vGS=0 vGS

vT

vGS15kA, Vr=>7kV Characteristics: Standard symbol: iA A G

Turn-Off

C

Turn-On vAK

The GTO is used when a switch is needed for high voltages and large currents in a switching frequency range of a few 100 Hz to 10 kHz. Application: traction drives in locomotives IGBT (Insulated Gate Bipolar Transistor) Similar to the MOSFET, the IGBT has a high impedance gate, which requires only a small amount of energy to switch the device. Like the BJT, the IGBT has a small on-state voltage even in devices with large blocking voltage ratings (for example, VON is 2-5 V in a 1000-V device). Similar to the GTO, IGBTs can also be designed to block negative voltages. IGBTs have turn-on and turn-off times on the order of 1 s and are available in module ratings as large as 1700 V and 1200 A. Power requirements for the control are very small. Forward voltage drop is much smaller than that of the MOSFET (for high voltage applications). Characteristics: Symbol: C

iC

G

vGE

E 0

vCE

Desirable characteristics of a controllable switch 1. 2. 3. 4.

Block arbitrarily large forward and reverse voltage. Conduct arbitrarily large current with zero voltage drop when on. Switch on and off instantaneously when triggered. Small power to control. 36

37

Comparison of controllable switches Device BJT MOSFET GTO IGBT

Power Capability Medium Low High Medium

Switching Speed Medium Fast Slow Medium

Power dissipation of a power semiconductor switch The power dissipation results of several components. By the thyristor:  Forward conduction loss (calculated from the forward voltage drop and the forward current) PF  v F i RMS

 Switching loss (turn-on, turn-off time) 1 1  PSW  WD f SW  f SW  V max I max t C ( on )  V max I max t C ( off )  2  2

 Gate loss (on higher frequencies it is important to consider) PG  v G iG

Other loss can be for example the reverse loss (calculated from the leakage current and the reverse blocking voltage), but in most cases it is negligible.

37

38

dc- dc switch mode converters Introduction The dc-dc converters are widely used in regulated switch-mode dc power supplies and in dc motor drive applications. As shown in the figure below, often the input to these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage, and therefore it will fluctuate due to changes in the line-voltage magnitude. Switch mode dc-to-dc converters are used to convert the unregulated dc input into a controlled dc output at a desired voltage level. These converters are very often used with an electrical isolation transformer in the switchmode dc power supplies and almost always without an isolation transformer in case of dc motor drives. To discuss these circuits in a generic manner, only the nonisolated converters are considered in this chapter, since electrical isolation is an added modification. Battery

AC line voltage (1-phase or 3-phase)

Uncontrolled DC DC Filter DC-DC DC Diode (unregulated) Capacitor (unregulated) Converter (regulated) Rectifier

Load

vcontrol

In this chapter, the converters are analysed in steady state. The switches are treated as being ideal, and the losses in the inductive and the capacitive elements are neglected. Such losses can limit the operational capacity of some of these converters and are discussed separately. The dc input voltage to the converters is assumed to have zero internal impedance. It could be a battery source; however, in most cases, the input is a diode rectified ac line voltage witch a large filter capacitance, as shown in the above figure to provide a low internal impedance and a low-ripple dc voltage source. In the output stage of the converter, a small filter is treated as an integral part of the dc-todc converter. The output is assumed to supply a load that can be represented by an equivalent resistance, as is usually the case in switch-mode dc power supplies. A dc motor load ( the other application of these converters) can be represented by a dc voltage in series with the motor winding resistance and inductance. Control of dc-dc converters In dc-dc converters, the average dc output voltage must be controlled to equal a desired level, though the input voltage and the output load may fluctuate. Switch-mode dc-dc converters utilize one or more switches to transform dc from one level to another. In a dc-dc converter with a given input voltage, the average output voltage is controlled by controlling the switch on and off durations (tON and tOFF ). To illustrate the switch-mode conversion concept, consider a basic dc-dc converter shown in the figure below. The average value Vo of the output voltage vo depends on ton and toff. One of the methods for controlling the output voltage employs switching at a constant frequency (hence, a constant switching time 38

39

period TS=ton+toff) and adjusting the on duration of the switch to control the average output voltage. In this method, called pulse-width modulation (PWM) switching, the switch duty ratio D, which is defined as the ratio of the on duration to the switching time period, is varied. The other control method is more general, where both the switching frequency (and hence the time period) and the on duration of the switch are varied. Variation in the switching frequency makes it difficult to filter the ripple components in the input and the output waveforms of the converter. vo

+

Vd

+

Vd

Vo

R 

Vo



ton

toff TS

In the PWM switching at a constant switching frequency, the switch control signal, which controls the state (on or off) of the switch, is generated by comparing a signal-level control voltage vcontrol with a repetitive waveform as shown below in the block diagram and the comparator signals of the PWM. Vo (desired) Vo (actual)

+ Amplifier 

vcontrol

Switch control signal

Comparator vcontrol

Repetitive waveform vst = sawtooth voltage vcontrol (amplified error)

 V st

0

t vcontrol > vst

Switch control signal

On

On

ton

toff

On

Off

Off vcontrol < vst

TS

(switching frequency fS= 1/TS)

39

40

The control voltage signal generally is obtained by amplifying the error, or the difference between the actual output voltage and its desired value. The frequency of the repetitive waveform with a constant peak, which is shown to be a sawtooth, establishes the switching frequency. This frequency is kept constant in a PWM control and is chosen to be in a few kilohertz to few hundred kilohertz range. When the amplified error signal, which varies very slowly with time relative to the switching frequency, is greater than the sawtooth waveform, the switch control signal becomes high, causing the switch to turn on. Otherwise, the switch is off. In terms of vcontrol and the peak of the sawtooth waveform V st in the figure above, the switch duty ratio can be expressed as t v D  on  control  T V S st

Step-down (Buck) converter As the name implies, a step-down converter produces a lower average output voltage than the dc input voltage Vd. Its main application is in regulated dc power supplies and dc motor speed control. Conceptually, the basic circuit of the figure below constitutes a step-down converter for a purely resistive load. Assuming an ideal switch, a constant instantaneous input voltage Vd and a purely resistive load, the instantaneous output voltage waveform is also shown in the figure below as a function of the switch position. The average output voltage can be calculated in terms of the switch duty ratio: Vo 

1 TS

TS

 0

vo ( t )dt 

1 TS

TS  t on   Vd dt  0  dt   t on Vd  D  Vd   TS  0  t on





Substituting for D in the above equation yields V V  d v  k v  o V control control st

where k

V d  constant  V st

40

41

id + iL Vd

io

+ voi



+

L vL

+ 

C



vo=Vo

R (load)



voi Vd

Vo 0 ton

t

toff TS=1/fS Vfs Frequency spectrum of voi

V2fs V3fs

0

fS

2fS

3fS

f

(=1/TS)

By varying the duty ratio ton/TS of the switch, Vo can be controlled. Another important observation is that the average output voltage Vo varies linearly with the control voltage, as is the case in linear amplifiers. In actual applications, the foregoing circuit has two drawbacks: (1) In practice the load would be inductive. Even with resistive load, there would always be certain associated stray inductance. This means that the switch would have to absorb (or dissipate) the inductive energy and therefore it may be destroyed. (2) The output voltage fluctuates between zero and Vd, which is not acceptable in most applications. The problem of stored inductive energy is overcome by using a diode as shown in the figure above. The output voltage fluctuations are very much diminished by using a low-pass filter, consisting of an inductor and a capacitor. The above figure also shows the waveform of the input voi to the low-pass filter , which consists of a dc component Vo, and the harmonics at the switching frequency fS and its multiples, as shown in the figure. The damping for the low-pass filter is provided by the load resistor. The corner frequency fc of this low-pass filter is selected to be much lower than the switching frequency, thus essentially eliminating the switching frequency ripple in the output voltage.

41

42

During the interval when the switch is on, the diode becomes reverse biased and the input provides energy to the load as well as to the inductor. During the interval when the switch is off, the inductor current flows through the diode, transferring some of its stored energy to the load. In the steady-state analysis presented here, the filter capacitor at the output is assumed to be very large, as is normally the case in applications requiring a nearly constant instantaneous output voltage vo(t)Vo. The ripple in the capacitor voltage (ouput voltage) is calculated later. From the above figure we observe that in a step-down converter, the average inductor current is equal to the average output current Io, since the average capacitor current in steady state is zero. Step-up (Boost) converter As the name implies, the output voltage is always greater than the input voltage. + L D Vd

io

on

+ off



R (load)

C

Vo

 VL Vd

0

t Vd-Vo ton

iL

toff TS=1/fS

t

When the switch is on, the diode is reverse biased, thus isolating the output stage. The input supplies energy to the inductor. When the switch is off, the output stage receives energy from the inductor as well as from the input. If D=0 (the duty ratio) then Vo=Vd 42

43

If D is near to 1, then Vo will be very large Main application of this type is regulated dc power supplies and the regenerative braking of dc motors. Continuous and Discontinuous Conduction Mode (CCM & DCM) In these devices there are this two types of operation. In the CCM the output current flows continuously, therefore the output voltage is a linear function of the duty ratio. If we are in DCM this means that we are below the boundary current iL,min and the output voltage isn't anymore a linear function of the duty ratio. +

iL

iRL

+ Vd

+ 



L vL

+ 

C

Vo

R (load)



vL Vd-Vo 0

ton

t

toff TS=1/fS

iL CCM Boundary between the two modes DCM

There are several applications where it is necessary to avoid the operation in DCM, because the output voltage isn't anymore a linear function of the duty ratio and a control circuit is necessary. Buck-Boost converter The main application of a step-down/step-up or buck-boost converter is in regulated dc power supplies, where a negative-polarity output may be desired with respect to the 43

44

common terminal of the input voltage, and the output voltage can be either higher or lower than the input voltage. A buck- boost converter can be obtained by the cascade connection of the two basic converters: the step-down converter and the step-up converter. In steady-state, the outputto-input voltage conversion ratio is the product of the conversion ratios of the two converters in cascade (assuming that switches in both converters have the same duty ratio): Vo 1 D Vd 1 D

This allows the output to be higher or lower than the input voltage, based on the duty ratio D. The cascade connection of the step-down and the step-up converters can be combined into the single buck-boost converter shown in the figure below. + D Vd + L

iL

Vo

C



io

R (load)



VL Vd 0 t -Vo ton iL

toff TS=1/fS

iL=(iD+io)

CCM

0 t

CÚK dc-dc converter Named after its inventor, the Cúk converter is shown in the figure below. This converter is obtained by using the duality principle on the circuit of a buck-boost converter, discussed in the previous section. Similar to the buck-boost converter, the Cúk converter provides a negative-polarity regulated output voltage with respect to the common terminal of the input voltage. 44

45

Its advantage is that because of the two inductors both the input and output currents are continuous. Possible ways to calculate the output voltage ripple: 1.) If we need the vo(t) : - Fourier components are calculated - Y()  Attenuated output calculation - summation of the higher harmonic components 2.) If we need only the value of the peak output ripple vo.

vo(t)

vo Vd-Vo toff ton

TS

t

iL -Vo

t

Q I L 2

t TS/2 t

1 Q iC  dt   C0 C

t off  (1  D )TS

vo (t ) 

1 1 1 I T  Q    L  S C C 2 2 2 V I L  o (1  D )TS L

V o T  S (1  D ) Vo 8 LC

Q  C  Vo

Vo 

2

The resonance frequency of the filter: f C 

1 2

LC



2  f  Vo  (1  D )   C  Vo 2  f SW 

2

45