advanced courses in power electronic

ADVANCED POWER ELECTRONICS ADVANCED POWER ELECTRONICS SEMICONDUCTOR SWITCHES: LOSSES & PROTECTION

Dr. Adel GastliEmail: [email protected] http://adel.gastli.net

CONTENTS1. 2. 3. 4. 5. 6. Introduction Switching Losses Snubbering: Protection of Switching Devices Zero-Current Switching Zero Voltage Switching Summary

Dr. Adel Gastli

Semiconductor Switches: Losses & Protection

2

Section 1 INTRODUCTIONSwitches are very important and crucial components in power electronic systems. They are a substitution of the mechanical switches, but they are severely limited by the properties of the semiconductor materials and the process of manufacturing. This chapter will examine the losses incurred during the switching process in a generic (standard) switch.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 3

Section 2 SWITCHING LOSSESLosses in switches are characterized as Switching losses. These losses are: On-state losses Off-state losses Losses in transition states.

Dr. Adel Gastli


4

ON-STATE LOSSESIn on state the electric switches have non zero voltage across them. The on-state losses are: on son f

P =v i

Switch voltage in on-state

Forward current through the switch

Typical power diodes and transistors have nearly 0.5 to 1 volt on-state voltage across them. The forward currents can be hundreds of amperes. amperes The on-state losses are very significant.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 5

OFF-STATE LOSSESIn off state the electric switches withstand high voltages and have nonzero leaking current through them. The off-state losses are:

Poff = vsoff ir

Switch reverse bias voltage in off-state

Reverse current through the switch

Typical power diodes and transistors have high reverse off-state voltage across them in hundreds to thousands of volts. The reverse volts currents can be microamps to milliamps. milliampsDr. Adel Gastli Semiconductor Switches: Losses & Protection 6

TRANSIENT-STATE LOSSESPractical switches have limited capabilities of rate of voltage transition and rate of current steering. These nonabrupt transition rates give rise to power losses in the switching devices. These losses will be examined for two types of loads; inductive and capacitive loads.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 7

Switching with Inductive Load

L >>Load inductance

I o = constantLoad currentD L

At t=0 S is off

+ IoFreewheel through diode

Io+ _

vs

S

Dr. Adel Gastli


8

When S is turned on:

D L

vD=0 (ideal)

Io+ _

vswitch = +Vs

vs

S

Switch current builds up to +Io linearly (assumption). Diode ceases to conduct Switch voltage ramps linearly down to zero (assumption)

vswitch = 0 (ideal)

Dr. Adel Gastli


9

D

When S is turned off:Io

L+ _

Switch voltage builds up to +Vs linearly while diode is off.

vs

S

I switch = + I oAfter Switch voltage builds up to +Vs the current starts decreasing. Diode starts to conduct Switch current ramps linearly down to zero (assumption)

I switch = 0 (ideal)Dr. Adel Gastli Semiconductor Switches: Losses & Protection 10

Switch on Vs vsw isw Psw ton1 ton2D turns off

DL

Io

Io

+ _ vs

S

Ts

toff1 toff2D turns on

1 1 f s max = Psw = Vs I o [ton1 + ton 2 + toff 1 + toff 2 ] f s ton1 + ton 2 + toff 1 + toff 2 2

Switching power lossDr. Adel Gastli

Maximum switching frequency


11

Switching with Capacitive Load

C >>

Vo = constantLoad voltageIo

Load capacitance

At t=0 S is on

vsw = 0

Io = I s

IsC

+ Vo

S

Diode is reverse biased

and does not conduct

Dr. Adel Gastli


12

When S is turned off Switch voltage ramps linearly up to +Vo While diode is still off and I switch = I sIsC+ Vo

Io

S

After build up is over, the diode begins to conduct. Current through switch ramps linearly down to zero.

Vsw = Vo

I switch = 0Switch turns offDr. Adel Gastli Semiconductor Switches: Losses & Protection 13

When S is closed Switch current builds up to Is linearly while diode is on.IsC+ Vo

Io

S

vsw = +VoAfter Switch current builds up to Is the diode turn off. Switch voltage ramps linearly down to zero.

vsw = 0 (ideal)Dr. Adel Gastli Semiconductor Switches: Losses & Protection 14

Io

Switch off vsw Io isw Psw ton1 ton2D turns on

VoIsC+

Vo

S

Ts

toff1 toff2D turns off

1 1 f s max = Psw = Vs I o [ton1 + ton 2 + toff 1 + toff 2 ] f s ton1 + ton 2 + toff 1 + toff 2 2

Switching power lossDr. Adel Gastli

Maximum switching frequency


15

Minimizing Switching LossesDivert the energy from the switch to a lossy or non-lossy circuit (Snubbering). Switch at either zero current or at zero voltage.

Dr. Adel Gastli


16

Section 3 SNUBBERING: PROTECTION OF SWITCHING DEVICESLimit stresses on the switch to safe values. Divert energy during switching transition from the switch to another circuit. Thus, reduce power losses in the switch.

Dr. Adel Gastli


17

Switch stresses: Maximum transient voltage Protection is required at turnMaximum transient currenton and turn-off of power and in overvoltage conditions.

current transition give rise to Rate of current change di/dt local hot spots in the switch that may permanently damage the devices. Special circuits are used to slow down the high rate of transition. These circuits are called snubbering circuits.Dr. Adel Gastli

Rate of voltage change dv/dt High rate of voltage and


18

TURN-OFF SNUBBERInductor L maintains a dc current Io. It reduces the switching losses by reducing the voltage across the Vs switch during the transition of current through the switch.

LD1

Io

RD2

iswvsw+

C

Assumptions: In the following we assume ideal diodes and ideal switch. The switch is also assumed to have limited maximum rates of rise and fall transitions of voltage and current.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 19

Without Snubber CapacitorPrior to turnoff:

LD1

Io

isw = I o

vsw = 0

D1 is off

iswv sw+

Vs

During turnoff: vsw : 0 Vs (linearly) in time t f 1

isw = I o until t = t f 1 D1 remains off until vsw = Vs t > t f 1 isw : I o 0 (linearly) for duration t f 2 t isw = I o 1 t f2 I o isw = I o t through D1 for t f 2 freewheeling through L. Current diverted

Dr. Adel Gastli


20

Waveforms of voltage current trough switch (without C)15

Io10

isw vsw Vs

5

tf10 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 x 10Dr. Adel Gastli Semiconductor Switches: Losses & Protection-6

2 f1

t +tf221

Power loss in the switch120 100 80 60 40 20

Without C

With C0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -6 x 1022

Dr. Adel Gastli


Switching trajectory15

Without C10

isw5

With C

0 0Dr. Adel Gastli

1

2

3

4

5

vsw

6

7

8

9

Vs

1023


With Snubber Capacitor LPrior to turnoff:

Io

RD2

isw = I o

vsw = 0

D1 is offVs

D1

iswvsw+

During turnoff:

C

isw : I o 0 (linearly) in time t f 2 1 t isw = I o t f2 Current diverted through I o isw = I o t D and C and charges tf2 2 capacitor.

1 t 1 Io t 2 vsw = vc = ( I 0 isw )dt = C 0 2 C tf2Dr. Adel Gastli

vc = vsw : 0 Vs24


Waveforms of voltage current trough switch (with C)15

Io10

isw

iC Vs vsw

5

tf 20 0Dr. Adel Gastli

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8 2 -6 x 1025


tf2

is determined by the value of the capacitor.

C=

I ot f 2 2Vs

vsw rises with a small slope.Note that the initial holding time tf1 for the switch current at Io is absent.

I o2 1 t t 2 psw = vswisw = 2Ct f 2 t f 2 Dr. Adel Gastli Semiconductor Switches: Losses & Protection 26

120 100 80 60 40 20 0 0

Power loss in the switch

Without C

With C0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -6 x 10

Note the very significant reduction in the power loss through the switch in comparison with the case without C.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 27

The maximum rate of dvsw/dt occurs when the current has dropped to a low or zero value.

dvsw dt

max

Io t = C tf 2 t tf2 1

t tf2

dvsw dt

max

=

Io C

This equation can be used to calculate the capacitance required for a switch with a specified maximum dv/dt (slew) rating.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 28

Circuit behavior during turn onL

First, lets consider R=0. Prior to turnon:D1

Io

RD2

isw+ v sw

I o freewheels in D1 and L D2 is short - circuited and vc = VsDuring turnon:

Vs

C

D1 stays on until isw=Io vsw = Vs until isw = I o

When isw = I o vsw : Vs 0

(voltage decrease rate is a function of snubber circuit capacitor)

isw : I o I rr + I o until vsw = 0 isw : I rr + I o I oDr. Adel Gastli Semiconductor Switches: Losses & Protection 29

Vs isw vsw

I rr

Io

tr

t

I rr Current overshoot is determined by the amount of thecharge of C and by the characteristic of diode D1.

Dr. Adel Gastli


30

A non-zero resistance R takes away the capacitor energy in the form of heat out of the system thus retarding the current rise beyond Io. Diode D2 isolates the rate of fall of the switch voltage from the snubber capacitor. Designers restrict the amount of current overshoot to 20% of Io by selecting R from the following equation:

Vs R= 0.2 I oDr. Adel Gastli Semiconductor Switches: Losses & Protection 31

TURN-ON SNUBBERReduces switching losses by reducing vsw during current transition through switch.Assumptions: In the following we assume ideal diodes and ideal switch. The switch is also assumed to have limited maximum rates of rise and fall transitions of voltage and current.Dr. Adel Gastli Semiconductor Switches: Losses & Protection

D1

Io

Vs

+ L vsw+

D

R

isw

32

Prior to turnon:

D1

Io

vsw = Vs isw = 0

D1 is on

Vs

During turnon: t isw : 0 I o (linearly) in time tr isw = I o tr

+ L v sw +

D2

R

i sw

isw overshoots beyond Io with an amount Irr which depends on energy stored in snubber inductor and characteristic of D1. isw flows also through the snubber inductor, hence, the inductor voltage instantly reduces vsw to zero.

disw LI o = Vs vsw = Vs L dt trDr. Adel Gastli Semiconductor Switches: Losses & Protection 33

Io VsL

I rrdisw dt

Io

isw vswtr t

LI o t psw = vswisw = Vs I o 1 trVs tr Dr. Adel Gastli Semiconductor Switches: Losses & Protection 34

Circuit behavior during turn offFirst, lets consider R=. Prior to turnoff:

D1

Io

isw = I o , vsw = 0

Vs

+ L v sw +

D2

R

During turnoff: isw = I o vsw : 0 Vs(linearly at rated dv/dt)

i swD1

V

+ v sw

When vsw = Vs D1 turns on current I o freewheels.Inductor energy (1/2LIo2) is dumped over to the switch. vsw overshoots and falling rate of isw decreases. A finite resistance R takes over the inductor energy as heat, thus retarding voltage overshoot.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 35

isw : I o 0

OVERVOLTAGE PROTECTION SNUBBERSo far the role of parasitic inductance of the conductors in the switching process has been ignored. This inductance must be added to the circuit in series with the source.LsD1

TurnoffIo

Voltage spike

Drawback

VsIo

Vs

i swv sw +

vswtr

isw

t36

Dr. Adel Gastli


The following circuit is able to protect the switch from over voltage.LD1

Io

R

vsw Io isw

Vs

iswVsv sw+

D2

C

tr

t

Prior to turnoff, the snubber capacitor is charged to Vs through R. Diode D2 is reverse-biased. During turnoff, Diode D2 clamps the switch to Vs.Dr. Adel Gastli Semiconductor Switches: Losses & Protection 37

SIMULINK SIMULATIONRun psbswitch Simulink demo example of ideal switch with series R-C snubber circuit. (Matlab 6).VC1 g 2 m + + v -

Timer

Ideal Switchi I_load

120 Vrms 60 HzI_switch Demux V_switch

V_load

R C L Scope

Demux

-

I_load

powergui

Ideal Switch in switching circuit Double click on the More Info button (?) button for details

? More Info

Dr. Adel Gastli


38

Section 4 ZERO-CURRENT SWITCHINGTurning on and off switches at zero current is the surest way of minimizing switching losses. An inductor in series with the switch will ensure zero current turn on because current through inductor cannot build instantaneously. However, turn off at zero current is impossible. Adding one capacitor in parallel with the inductor enables both switchings at zero current.C LDr. Adel Gastli

Sw

During turn off L dumps its energy on C through D enabling switch to turn off at zero current.39


Section 5 ZERO-VOLTAGE SWITCHINGTurning on and off switches at zero voltage is also the surest way of minimizing switching losses. An capacitor in parallel with the switch will ensure zero voltage turn off because voltage on capacitor cannot build instantaneously. However, turn on at zero voltage is not possible. Adding a diode in series with the capacitor enables both switchings at zero voltage. (see also turn-off snubber)C

SwDr. Adel Gastli

During turnoff, D conducts and C permits slow voltage buildup on switch. During turnon, D prevents C to discharge through Sw.Semiconductor Switches: Losses & Protection 40

SUMMARYThis chapter has covered the power switch losses involved during on-off switching. It has also presented the means of minimizing those losses and protecting the switches using snubbing circuits, zerocurrent switching, or zero-voltage switching.

Dr. Adel Gastli


41

ADVANCED POWER ELECTRONICS ADVANCED POWER ELECTRONICS

FIRING CIRCUITSC1 1n Rs 0.1 V1 = 0 V2 = 15 TD = 0 TR = 0 TF = 0 PW = 2.5u PER = 5u V2 RG 1k I R1 100 RD 10 VDD IRF740 80Vdc V M1

0


CONTENTS1. 2. 3. 4. 5. 6. MOSFET Gate Drive BJT Base Drive Isolation of Gate & Base Drives Thyristor Firing Circuit Thyristor Converter Gating Circuit Summary(Textbook: Sections 17.2, 17.3, 17.4, 17.5, 17.8)Dr. Adel Gastli Firing Circuits 2

MOSFET Gate DriveThe turn on time of a MOSFET depends on the charging time of the input or gate capacitance.

The turn-on time can be reduced by connecting an RC circuit as shown in the figure to charge the gate capacitance fasterDr. Adel Gastli

Firing Circuits

3

C1 1n Rs 0.1 V1 = 0 V2 = 15 TD = 0 TR = 0 TF = 0 PW = 2.5u PER = 5u V2 RG 1kI

RD 10 VDD IRF740 80Vdc

R1 100

0

OrCAD circuit simulationDr. Adel Gastli Firing Circuits 4

V

M1

C1=1pF88.4

75.0

50.0

25.0

0 2.6us ID(M1)*5 3.0us V(M1:d) 3.5us 4.0us Time 4.5us 5.0us 5.4us

Dr. Adel Gastli

Firing Circuits

5

C1=1nF95

75

50

25

0 2.51us V(M1:d) 3.00us ID(M1)*5 3.50us 4.00us Time 4.50us 5.00us 5.27us

Dr. Adel Gastli

Firing Circuits

6

C1=100nF87.6

75.0

50.0

25.0

0 2.475us 2.800us ID(M1)*5 V(M1:d) 3.200us 3.600us Time 4.000us 4.400us 4.800us 5.150us

Dr. Adel Gastli

Firing Circuits

7

When the gate voltage is turned on, the initial charging current of the capacitance is

VG IG = RS + RGSVGS RGVG = RS + R1 + RG

IG RGS

and the steady state value of gate voltage isTypically between 10 and 20V in on-state

where the steady-state gate-source current is considered negligible.Dr. Adel Gastli Firing Circuits

IG

8

In order to achieve switching speeds of the order of 100 ns or less, the gate-drive circuit should have: a low output impedance and the ability to sink and source relatively large currents.

Dr. Adel Gastli

Firing Circuits

9

Turn off100

Turn on

50

0

Gate current-50

Sink

-100 0s -I(VDD)*5

1.0us V(M1:d) IG(M1)*500

2.0us

3.0us Time

4.0us

5.0us

6.0us

Source

Dr. Adel Gastli

Firing Circuits

10

A totem-pole arrangement that is capable of sourcing and sinking a large current is shown in below.The PNP- and NPNtransistors act as emitter followers and offer a low impedance. These transistors operate in the linear region rather than in saturation mode, thereby minimizing the delay time.Dr. Adel Gastli Firing Circuits 11

The gate signal for the power MOSFET may be generated by an opamp. Feedback via the capacitor C regulates the rate of rise and fall of the gate voltage, thereby controlling the rate of rise and fall of the MOSFET drain current. A diode across the capacitor C allows the gate voltage to change rapidly in one direction only. There are a number of integrated drive circuits on the market that are designed to drive transistors and are capable of sourcing and sinking large currents for most converters.Dr. Adel Gastli Firing Circuits 12

VG

0Q1

-15VdcV

RD 10

VDD R1 1k V2 IRF740 Q3 Q2N5226 Q2N5223 M1I

V1 = 0 V2 = 10 TD = 0 TR = 0 TF = 0 PW = 2.5us PER = 5us

D1 80Vdc Dbreak

0

0

Dr. Adel Gastli

Firing Circuits

13

87 75

50

25

0 2.54us ID(M1)*5 3.00us V(M1:d) 3.50us 4.00us Time 4.50us 5.00us

Dr. Adel Gastli

Firing Circuits

14

Key PointsA MOSFET is a voltage-controlled device. Applying a gate voltage turns it on and it draws negligible gate current. The gate drive circuit should have low impedance for fast turn-on.Dr. Adel Gastli Firing Circuits 15

BJT BASE DRIVEThe transistor turning-on time (ton) can be reduced by allowing base current peaking during turn-on, resulting in low forced gain (F) at the beginning. After turn on, F can be increased to a sufficiently high value to maintain the transistor in quasi-saturation region.Dr. Adel Gastli Firing Circuits 16

The turn off time (toff) can be reduced by reversing base current and allowing base current peaking during turn-off. Increasing the reverse base current IB2 decreases the storage time. iB Commonly used techniques for optimizing IB1 IBs the base drive are: Turn-on control Turn-off control Proportional base control Antisaturation control t -IB2

Dr. Adel Gastli

Firing Circuits

17

Turn-on controlV V I B = 1 BE R1V V = 1 BE R1 + R2Limits base current when VB turns on (VC1=0) Final base current value (VC1=IBSR2)+ VB _

C1 R1 R2 + VC1 _ IB C IC + _ RC

E IE

VCC

V1

vB t1 t2 t

I BS

0

R2 Final C1 charge up VC1 V1 R1 + R2 voltage

vB=0 Base-emitter junction becomes reverse biased and C1 discharges through R2.18

1 =

R1 R2C1 R1 + R2

Capacitor charging time constantFiring Circuits

Dr. Adel Gastli

2.0A

Base Current0A

-2.0A IB(Q1) 100

Collector Current0

Collector-Emitter Voltage

-100 V(RC:2) 2.0A IC(Q1)*5

Current in R20A SEL>> -2.0A 0s

Current in C1I(R2) 20us I(C1) 40us 60us Time 80us 100us 120us

Turn offDr. Adel Gastli Firing Circuits

Turn on19

Zooming on turn-off transition2.0A

0A SEL>> -2.0A IB(Q1) 100

Negative base current

0

-100 V(RC:2) 1.0A 0A -1.0A -2.0A 50.00us I(R2) 51.00us I(C1) 52.00us 53.00us IC(Q1)*5

Capacitor discharge current

Negative currentTime R2 inFiring Circuits 20

Dr. Adel Gastli

Zooming on turn-on transition1.0A 0.5A SEL>> -0.1A 100

Positive base current (fast current increase)IB(Q1)

0

-100 V(RC:2) IC(Q1)*5

Capacitor charge current (fast current increase)1.0A 0A 100us I(R2) 101us I(C1) 102us 103us

Positive current in R2 (slow current increase) TimeFiring Circuits 21

Dr. Adel Gastli

1 =

R1 R2C1 Capacitor charging time constant R1 + R2Capacitor discharging time constant

2 = R2C1

To allow sufficient charging and discharging times, the width of the pulse must be t1 of the pulse must be t 2

5 1

and the off period

5 2

The maximum switching frequency is:

f s maxDr. Adel Gastli

1 1 0.2 = = = Tmin t1 + t 2 1 + 222

Firing Circuits

C1 100n R1 5I

R2 15

I

Q1I

V

RC 10 Vcc

V1 = 0 V2 = 15V TD = 0 TR = 0 TF = 0 PW = 50us PER = 100us

V2I

MRH1240N/125CV I

80Vdc

0

PSpice Simulation

Dr. Adel Gastli

Firing Circuits

23

0.66A

0A

-1.00A

-1.56A 50.0us IB(Q1) 52.0us I(R2) I(C1) Time 54.0us 56.0us

Capacitor provides a negative current spike as the base charge is removed. Then, the capacitor continues discharging through R2.Dr. Adel Gastli Firing Circuits 24

Turn-off controlvB = V2VBE = (VC + V2 )Reverse voltage across transistor base-emitter junction As C1 discharges, reverse voltage is reduced to steady-state value -V2Dr. Adel Gastli Firing Circuits

C1 R1 + VB _ R2 + VC1 _ IB C IC + _ RC

E IE

VCC

V1

vB t1 t2 t

-V2

vB VCE (sat ) RCDr. Adel Gastli Firing Circuits

Soft

Hard

IB

29

Without clamping, the base current is adequate to drive transistor hard.

I B = I1 =

VB Vd 1 VBE RBRB + VB _I1

I2=IC-IL + Vd2 _ IB + Vd1 _ C2 C IC

IL RC + _

After IC rises, transistor turns on, and clamping takes place (due to the fact that diode D2 gets forward biased and conducts)

I C = I B

VCE VCC

I VBE E E

Collector clamping circuit (Bakers Clamp)

VCE = VBE + Vd 1 Vd 2

IL =

VCC VCE VCC VBE Vd 1 + Vd 2 = RC RC

I C = I B = ( I1 I C + I L ) =Dr. Adel Gastli Firing Circuits

1+

( I1 + I L )30

For clamping, Vd1>Vd2 and this can be accomplished by connecting two or more diodes in place of D1. Load resistance RC should satisfy Since I L =

I B > I L

VCC VBE Vd 1 + Vd 2 RC

I B RC > VCC VBE Vd 1 + Vd 2

Clamping action results in a reduced collector current and almost elimination of storage time. At the same time, a fast turn-on is accomplished. However, due to increased VCE, the on-state power loss is increased, whereas the switching power loss is decreased.Dr. Adel Gastli Firing Circuits 31

Example 17.1: Finding the transistor voltage and current with clamping The base drive has VCC=100V, RC=1.5, Vd1=2.1V, Vd2=0.9V, VBE=15V, RB=2.5, and =16. Calculate: Collector current without clamping Collector-emitter voltage VCE Collector current with clamping

Dr. Adel Gastli

Firing Circuits

32

SolutionWithout clamping V Vd 1 VBE 15 2.1 0.7 I B = I1 = B = = 4.88 A RB 2.5

I C = I B = 16 4.88 = 78.08 AClamping voltage

VCE = VBE + Vd 1 Vd 2 = 0.7 + 2.1 0.9 = 1.9VWith clamping

VCC VCE 100 1.9 = = 65.4 A RC 1. 5 (I1 + I L ) = 16 (4.88 + 65.4) = 66.15 A IC = 1+ 16 + 1 IL =Dr. Adel Gastli Firing Circuits 33

Key pointsA BJT is a current controlled device Base current peaking can reduce the turn-on time and reversing the base current can reduce the turn-off time The storage time of a BJT increases with the amount of base drive current, and overdrive should be avoided.

Dr. Adel Gastli

Firing Circuits

34

ISOLATION OF GATE & BASE DRIVES For power transistors, the control voltage should be applied between the gate and the source terminals or between the base and emitter terminals. Power converters generally require multiple transistors and each transistor must be gated individually.Dr. Adel Gastli Firing Circuits 35

Common terminal for pulses Ground terminal

Generates 4 pulses

Dr. Adel Gastli

Firing Circuits

36

Terminal g1, which has a voltage of Vg1 with respect to C cannot be connected directly to gate terminal G1. Vg1 should be applied between gate terminal G1 and source terminal S1 of transistor M1. There is a need for isolation and interfacing circuits between the logic circuit and power transistors. However, M2 and M4 can be gated directly without isolation or interfacing circuits if logic signals are compatible with gate drive requirements of the transistors.Dr. Adel Gastli Firing Circuits 37

The importance of gating a transistor between its gate and source rather than applying gating voltage between the gate and common ground can be demonstrated with the following figure.

VGS = VG RL I D (VGS )ID(VGS) varies with VGSVGS when transistor turns on and reaches a steadystate value (requiredto balance the load drain current)

Load resistance38

Dr. Adel Gastli

Firing Circuits

The effective value of VGS is thus unpredictable and such an arrangement is not suitable. There are basically two ways of floating or isolating the control or gate signal with respect to ground: Pulse transformers Optocouplers

Dr. Adel Gastli

Firing Circuits

39

Dr. Adel Gastli

Firing Circuits

40

Key pointThe low-level gate circuit must be isolated from the high level power circuit through isolation devices or techniques such as optocouplers and pulse transformers.

Dr. Adel Gastli

Firing Circuits

41

THYRISTOR FIRING CIRCUITIn thyristor converters, different potentials exist at various terminals. The power circuit is subjected to a high voltage, usually greater than 100 V, and the gate circuit is held at a low voltage, typically 12 to 30 V. An isolation circuit is required between an individual thyristor and its gate-pulse generating circuit.Dr. Adel Gastli Firing Circuits 42

The isolation can he accomplished by either pulse transformers or optocouplers. An optocoupler could be a phototransistor or photosilicon-controlled rectifier (SCR). A short pulse lo the input of a LED, D1 turns on the photo-SCR T1; and the power thyristor TL is triggered. This type of isolation requires a separate power supply Vcc and increases the cost and weight of the firing circuit.

Dr. Adel Gastli

Firing Circuits

43

Short-Pulse: When a pulse of adequate voltage is applied to the base of Q1, the transistor saturates and the dc voltage Vcc appears across the transformer primary, inducing a pulsed voltage on the transformer secondary. When the pulse is removed, Q1 turns off and a voltage of opposite polarity is induced across the primary and the freewheeling diode Dm conducts. The current due to the transformer magnetic energy decays through Dm to zero. During this transient decay a corresponding reverse voltage is induced in the secondary.Dr. Adel Gastli Firing Circuits 44

Long-Pulse: The pulse width can be made longer by connecting a capacitor C cross the resistor R. The transformer carries unidirectional current and the magnetic core can saturate, thereby limiting the pulse width. This type of pulse isolation is suitable for pulses of typically 50 to 100 s.Dr. Adel Gastli Firing Circuits 45

Pulse train: In many power converters with inductive loads, the conduction period of a thyristor depends on the load power factor (PF); therefore, the beginning of thyristor conduction is not well defined. In this situation, it is often necessary to trigger the thyristors continuously. However, a continuous gating increases thyristor losses. A pulse train that is preferable can be obtained with an auxiliary windingDr. Adel Gastli Firing Circuits 46

When transistor Q1 is turned on, a voltage is also induced in the auxiliary winding N3 at the base of transistor Q1, such that diode D1 is reverse biased and Q1 turns off. In the meantime, capacitor C1 charges up through R1 and turns on Q1 again. This process of turn-on and turn-off continuous as long as there is an input signal v1 to the isolator.

Dr. Adel Gastli

Firing Circuits

47

In-stead of using the auxiliary winding as a blocking oscillator, an AND-logic gale with an oscillator (or a timer) could generate a pulse train. In practice, the AND gate cannot drive transistor Q1 directly, and a buffer stage is normally connected before the transistor.

Dr. Adel Gastli

Firing Circuits

48

The output of gate circuits is normally connected between the gate and cathode along with other gate-protecting components. The resistor Rg in (a) increases the dvldt capability of the thyristor, reduces the turn-off time, and increases the holding and latching currents. The capacitor Cg in (b) removes high-frequency noise components and increases dv/dt capability and gate delay lime.Dr. Adel Gastli Firing Circuits 49

The diode D1 (c) protects the gate from negative voltage. However, for asymmetric silicon-controlled rectifiers, SCRs, it is desirable to have some amount of negative gate voltage to improve the dvldt capability and also to reduce the turn-off time. All these features can be combined as shown in (d), where diode D1 allows only the positive pulses and R1 damps out any transient oscillation and limits the gate current.Dr. Adel Gastli Firing Circuits 50

Key pointsApplying a pulse signal turns on a thyristor The low-level gate circuit must be isolated from the high-level power circuit through isolation techniques The gate should be protected from triggering by a high frequency or interference signal.Dr. Adel Gastli Firing Circuits 51

THYRISTOR CONVERTER GATING CIRCUITSThe generation of gating signals for thyristors of ac-dc converters requires: Detecting zero crossing of the input voltage Appropriate phase shifting of signals Pulse shaping to generate pulses of short duration Pulse isolation through pulse transformer or optocouplers

Dr. Adel Gastli

Firing Circuits

52

Dr. Adel Gastli

Firing Circuits

53

DRIVE IC FOR CONVERTERSThere are numerous IC gate drives that are commercially available for gating power converters. These include Pulse-width-modulation (PWM) control, Power factor correction (PFC) control, Combined PWM and PFC control Current mode control Bridge driver Servo driver, half-bridge drivers, stepper motor driver, thyristor gate driver EtcDr. Adel Gastli Firing Circuits 54

These ICs can be used for applications such as: Buck converters for battery chargers Dual forward converters for switched reluctance motor drives Full-bridge inverter with current-mode control Three-phase inverter for brushless and induction motor drives Push-pull bridge converter for power supplies Synchronous PWM control of switched-mode power supplies (SMPSs)Dr. Adel Gastli Firing Circuits 55

An IC gate drive integrates most of the control functions including some protection functions to operate under overload and fault conditions. The especial purpose ICs for motor drives include many features such as gate driving with protection, soft start charging of DC bus, linear current sensing of motor phase current, and control algorithms for V/Hz to sensorless vector or servo control.Dr. Adel Gastli Firing Circuits 56

SUMMARYMOSFETs are voltage-controlled devices requiring low gating power. Gate signals can be isolated from the power circuit by pulse transformers or optocouplers. BJTs are current-controlled devices requiring reverse base current during turn-off to reduce storage time, but they have low on state or saturation voltage. A mean of isolation between power circuit and gate circuit is necessary.

Dr. Adel Gastli

Firing Circuits

57

The pulse transformers are simple, but leakage inductance should be very small. The transformers may be saturated at low frequency and long pulse. Optocouplers require separate power supply. For inductive loads, a pulse train reduces thyristor loss and is normally used for gating thyristors, instead of a continuous pulse. There are numerous drive ICs for drives that are commercially available for gating power converters. These ICs integrate logic, gate isolation, protection, and control functions. As a result, discrete gate circuits have become obsolete.Dr. Adel Gastli Firing Circuits 58


STATIC SWITCHESPreferred source Alternate source

Static transfer Switch

Critical Load


CONTENTS1. 2. 3. 4. 5. 6. 7. 8. 9.Dr. Adel Gastli

Introduction Single-Phase AC Switch Three-Phase AC Switches Three-Phase reversing Switches AC Switches for Bus Transfer DC Switches Solid-State Relays Design of Static Switches Summary(Textbook: Sections 12.1-12.9)Static Switches 2

INTRODUCTIONThyristors that can be turned on and off within a few microseconds may be operated as fast-acting switches to replace mechanical and electromechanical circuit breakers. For low power dc applications power transistors can also be used as switches.

Dr. Adel Gastli

Static Switches

3

Static switches have many advantages: Very high switching speeds No moving parts No contact bounce on closing

In addition to applications as static switches, the thyristor (or transistor) circuits can be designed to provide time-delay, latching, over- and undercurrent protection, and voltage detectionDr. Adel Gastli Static Switches 4

Transducers for detecting mechanical, electrical, position, proximity, and so on, can generate the gating or control signals for the switches. Static switches can be classified into two mainly types: AC Switches (line commutated, switching speed limited by supply frequency and turn-off time) Single-phase Three-phase

DC switches (forced commutated, switching speed limited by turn-on and off times of devices)

Dr. Adel Gastli

Static Switches

5

SINGLE-PHASE AC SWITCHESis i T1 iT 2 T1 T2

i0 v0RL

Vsm 0

vs , v0

Vsm2

vs , v0

vs

t

0

2

t

i0 mis TR1

i0

i0 m2

i0

i0 0 v0RL

t

0

2

t

vs

g1 , g 21 12

g1 , g 2

0

Bidirectional switchesDr. Adel Gastli Static Switches

t 0

2

t

Resistive load

Inductive load6

ApplicationsTransformers Static Tap-Changing Control. Controls the output voltage of a power transformer by selecting appropriate taps.

Static VAR Controller (SVC) Controls power to an inductor Controls power to a capacitor bank Controls power to a mixed inductor and capacitor elements.Dr. Adel Gastli Static Switches 7

Key PointsThe switches are turned at the zero crossing of the input voltage (resistive load) or output current (inductive load). The circuit operation is similar to the single-phase ac voltage controller with a delay angle =0.

Dr. Adel Gastli

Static Switches

8

THREE-PHASE AC SWITCHEST1

AT3 T4

v a b T5 T6

vab1

vbc3

vca5 1

B

ZL ZL ZL4 6 2 4 6

= t

c

g10

CT2

g20

AB

a

g3 g40 0

TR1b

ZL ZL ZL

g5 g60 0

TR2c

C

2 TR3 Similar to three-phase ac voltage controller with a delay angle =0

Dr. Adel Gastli

Static Switches

9

THREE-PHASE REVERSING SWITCHEST1

Five thyristor pairs can be used and gated to produce a phase reversal to a threephase load. Common applications in ac (induction & synchronous) motor rotation reversal.Dr. Adel Gastli Static Switches

AT3 T4

a b

BT5 T6

ZL ZL ZL

c

CT9 T2

T7

T10

T810

AC SWITCHES FOR BUS TRANSFERT1 T2 T2 T1

v1

v0

RL

v2

Two or more thyristor pairs can be connected in back-toback for bus transfer from on supply source to another.T1

AT3 T4

T1 aZL

A B C

T4

BT5 T6 T2Dr. Adel Gastli

T3 T5

b ZL c

ZL

T6 T2

C

Static Switches

11

DC SWITCHES In the case of dc switches, the input voltage is dc and power transistors or fastswitching thyristors or GTOs can be used.+ Dc Vs supply Base drive voltage vg Q10

+ RL v0 _

_

t

Dr. Adel Gastli

Static Switches

12

Automotive powering systems with 42V batteries

DC AC + 42V

To 42V load Distribution boxes containing switches and fuses

A

sDC 14V

A s

Alternator Starter motor

DC 42V

To 14V loads

Dc/dc converter

+ 14V

Dr. Adel Gastli

Static Switches

13

SOLID-STATE RELAYSStatic switches can be used as solid-state relays (SSRs), which are used for the control of ac and dc power. SSRs are used for many applications in industrial control such as the control of: Motors Transformers Resistance heating

SSRs are normally isolated electrically between the control circuit and the load circuit by reed relay, transformer, or optocoupler.

Dr. Adel Gastli

Static Switches

14

RL vg0

+ v0 _ Q1

+

+

+ v0 _ Q1

RL

RB

Vs t Optocoupler

Vs dc supply _

_

Reed relay isolation

Dc solid-state relays+ RL v0 _ TR

vg0

R t Reed relay

Vs Vs

v0 TRControl signal

ac supply

Transformer isolation

Ac solid-state relaysDr. Adel Gastli Static Switches 15

DESIGN OF STATIC SWITCHESSolid-state switches are available commercially with limited voltage and current rating ranging up to 440V and from 1 to 50A. The design of an SSR to meet specific requirements is simple and requires determining the voltage and current ratings of power semiconductor devices.Study examples 12.1 and 12.2Dr. Adel Gastli Static Switches 16

SummarySolid-state ac and dc switches have number of advantages over conventional electromechanical switches and relays. With the developments of power semiconductors devices and integrated circuits, static switches are used in a wide range of applications in industrial control. Static switches can be interfaced with digital or computer control systems.Dr. Adel Gastli Static Switches 17


PWM INVERTERS


CONTENTS CONTENTS1. Single-Phase Half-Bridge Inverter 2. Single-Phase Bridge Inverter 3. Three-Phase Inverter 4. Three-Phase PWM Inverter 5. Sinusoidal PWM 6. Modified Sinusoidal PWM 7. Sinusoidal PWM 3-Phase 8. 60-Degree Modulation 9. Transformer Connection 10. Single-Phase Current Source 11. Three-Phase Current Source 12. Variable DC Link Inverter 13. AC Filters 14. Summary

Textbook: Chapter 6Dr. Adel Gastli PWM Inverters 2

Single-Phase Half-Bridge Invertervo (t ) = 2VS n sin nt n =1,3,5,. for n = 2, 4,..

=0

Vo ( rms ) = Vo1( rms ) =

2

0

VS VS d = 2 2 = 0.45 VSPWM Inverters 3

2

2VS 2

Dr. Adel Gastli

Performance ParametersVon HFn = for n > 1 Vo1 1 THD = Vo1 1 DF = Vo1n = 2,3,..

Harmonic factor of nth harmonic

2 Von

Total Harmonic Distortion factor2

Von n2 n = 2,3,..

Distortion factor

Von DFn = for n > 1 2 Vo1n LOH 3% Vo1Dr. Adel Gastli

Distortion factor of nth harmonic- Frequency is closest to fundamental - Amplitude is greater than or equal to 3% the fundamental PWM Inverters 4

Lowest Order Harmonic

Example 6.1 (Homework)Study the example by yourself. Simulate the circuit and check the results. (Use any software) (Life-long learning)

Dr. Adel Gastli

PWM Inverters

5

Single-Phase Bridge Inverter4VS vo (t ) = sin nt n =1,3,5,. n =0 for n = 2, 4,..

Vo ( rms ) = Vo1( rms ) =Dr. Adel Gastli

2

0

VS2 d = VS

4VS = 0.90 VS 2PWM Inverters 6

R = 10, L = 31.5mH , C = 112uF , f o = 60 Hz, Vs = 220V , = 2 f = 377 rad / s X L = jn L = j11.87 n , X c =2 2

Example 6.3j j 23.68 = nC n

1 23.68 2 Z n = R + n L = 10 + 11.87 n nC n 11.87 n 23.68 n = tan 1 10n 10 vo (t ) = 4VS n sin nt n =1,3,5,. for n = 2, 4,.. 4VS 1 n R 2 + n L nC 2

2

=0

vo (t ) io (t ) = = Z n n n =1,3,5,.

sin(nt n )

Dr. Adel Gastli

PWM Inverters

7

a. The instantaneous output voltage

v0 (t ) = 280.1sin(377t ) + 93.4 sin(3 377t ) + 56.02 sin(5 377t ) + 40.2 sin(7 377t ) + 31.12 sin(9 377t ) + LDividing the output voltage by the load impedance and considering the appropriate delay due to the load impedance angles, we can obtain the instantaneous load current as

i0 (t ) = 18.1sin(377t + 49.72) + 3.17 sin(3 377t 70.17 o ) + sin(5 377t 79.63o ) + 0.5 sin(7 377t 82.85o ) + 0.3 sin(9 377t 84.52o ) + LDr. Adel Gastli PWM Inverters 8

b. The peak fundamental load current is Im1=18.1A. The rms current at fundamental frequency is I01=12.8A c. Considering up to 9th harmonic, the peak load current,2 2 2 2 2 I m = I m1 + I m 3 + I m 5 + I m 7 + I m 9

= 18.12 + 3.17 2 + 1.0 2 + 0.52 + 0.32 = 18.41AThe rms harmonic load current is

Ih =

2 2 I m I m1

2

18.412 18.12 = = 2.38 A 22 2 I m I m1 = 18.59% I m1PWM Inverters 9

THD =Dr. Adel Gastli

d. The rms load current is The total load power is

I0

I m 18.42 = = 13.02 A 2 2

P0 = I 02 R = 13.02 2 10 = 1695W

The fundamental output 2 P01 = I 01 R = 12.82 10 = 1638.4W power is e. The average supply current f. The peak transistor current

P0 1695 Is = = = 7.7 A Vs 220I p I m = 18.41A

The maximum I p 18.41 I0 permissible rms I Q max = = = = 9.2 A 2 2 2 transistor current isDr. Adel Gastli PWM Inverters 10

g. The waveforms of the output voltage and current and their fundamental components are shown below.30 v /100

v01/10 20 Load Voltage (V) and Current (A) i0 i 1001

Q1,Q20

D1,D2

Q3,Q4

D3,D4

-10

-20

-30

0

0.002

0.004

0.006

Dr. Adel Gastli

0.008 0.01 0.012 0.014 Time, (sec) PWM Inverters

0.016

0.01811

h. The conduction time of each transistor is found approximately from the previous waveforms as

tQ = 180 49.72 = 130.28o or tQ =

130.28 = 6.031ms 180 377

i. The conduction time for each diode is approximately

tD =

1 T tQ = 6.031 10 3 2 120 = (8.333 6.031) 10 3 = 2.302ms 49.72 180 377PWM Inverters 12

=

Dr. Adel Gastli

Notes:This example can be repeated for different types of loads (R, RL, RLC) with an appropriate change in load impedance ZL and load angle n Gating sequence is as follows: Generate two square-wave gating signals vg1 and vg2 at an output frequency f0. The gating signals vg3 and vg4 should be the logic invert of vg2 and vg1 respectively. Signals vg1 and vg3 drive Q1 and Q3, respectively, through gate isolation circuits. Signals vg2 and vg4 drive Q2 and Q4, respectively, without any gate isolation circuits.

Dr. Adel Gastli

PWM Inverters

13

THREE-PHASE BRIDGE INVERTERThree Single-Phase Inverter Three-phase Bridge Inverter180o Conduction 120o Conduction

Dr. Adel Gastli

PWM Inverters

14

Three Single-Phase Inverter12 transistors 12 diodes 3 transformers Risk of voltage unbalance Transformer secondary windings can be connected in Y or . connection eliminates triplen harmonics (3, 6, 9,..)Dr. Adel Gastli PWM Inverters

Figure 6.415

Three-Phase Bridge Inverter

Dr. Adel Gastli

PWM Inverters

16

180o Conduction

Vs 3 2V vbn = s 3 V vcn = s 3 van =

2Vs 3 V vbn = s 3 Vs vcn = 3 van =

Vs 3 V vbn = s 3 2V vcn = s 3 van =

Dr. Adel Gastli

PWM Inverters

17

4Vs n cos sin n t + vab = 6 6 n =1, 3, 5,K n

4Vs n cos sin n t vbc = 6 2 n =1, 3, 5,K n

4Vs 7 n cos sin n t vca = 6 6 n =1, 3, 5,K n

Note that for n=3,9,15,21,...Dr. Adel Gastli

vab=vbc=vca=0PWM Inverters 18

Line-to-line rms voltage

2 vL = 2

2 / 3

V d (t ) 0 2 s

1/ 2

2 = Vs = 0.8165Vs 3

Dr. Adel Gastli

PWM Inverters

19

Line-to-line rms harmonic voltage

4Vs 4Vs n vLn = cos cos = 0.7797Vs vL1 = 6 6 2n 2Phase rms voltage

2Vs vL vp = = = 0.4714Vs 3 3Dr. Adel Gastli PWM Inverters 20

/ 3 2 / 3

2

Only two transistors remain on at any time.

120o Conduction

vab

vbc

vca

Vs 2 V vbn = s 2 vcn = 0 van =

Vs 2 vbn = 0 van = vcn = Vs 2

van = 0 Vs 2 V vcn = s 2 vbn =

Note: The waveforms of phase voltages are the same as the waveforms of line voltages with the only difference in the amplitudes (Vs/2 instead of Vs)Dr. Adel Gastli PWM Inverters 21

van =

2Vs n cos sin n t + 5,K n 6 6 n =1, 3,

2Vs n vbn = cos sin n t 6 2 n =1, 3, 5,K n 2Vs n 7 vcn = cos sin n t 6 6 n =1, 3, 5,K n

vline = 3v phDr. Adel Gastli PWM Inverters 22

Voltage Control of Single-Phase InvertersSingle-Pulse-Width modulation Multiple-Pulse-Width Modulation Sinusoidal-Pulse-Width Modulation Modified Sinusoidal-Pulse-Width Modulation Phase Displacement control

Dr. Adel Gastli

PWM Inverters

23

Single-Pulse Width Modulationvo (t ) = 4VS n sin n 2 sin nt n =1,3,5,.

d=

= t t 2 1T 2

= MTs = MModulation index M=Ar/Ac

Switching Period

V0 ( rms ) =

2 2

( + ) / 2

( )/2

Vs2 d = Vs

PWM Inverters

Figure 6.1124

Dr. Adel Gastli

Pulse width T t1 = 1 = (1 M ) s 2t2 =

2 T = (1 + M ) s 2Prove these two t1 and t2 equations

d = = t2 t1 = MTs Ts = T 2

T is the desired period of the output voltage

Dr. Adel Gastli

PWM Inverters

25

Harmonic Profile for p =1vo (t ) = 4VS n sin sin nt 2 n =1,3,5,. n

The dominant harmonic is the third. DF increases significantly at a low output voltage (small M). Figure 6.12Dr. Adel Gastli PWM Inverters 26

Start Gating Signals Algorithm Generate a triangular carrier signal v

cr cr

(Magnitude Vc, Switching Period Tss=T/2) c Change frequency27 28

Change vr to change the modulation index and hence the output voltage rms

Compare vcr with a dc reference signal vrr cr ve=vcr-vrr>0 gate signal vg=0 e cr g ve=vcr-vrr

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