EE 330 Lecture 26
Thyristors •SCR • TRIAC Review from Last Lecture Area Comparison between BJT and MOSFET
• BJT Area = 3600 l2 • n-channel MOSFET Area = 168 l2 • Area Ratio = 21:1 Review from Last Lecture The Thyristor
A bipolar device in CMOS Processes
Consider a Bulk-CMOS Process D S G D S G
p n p n
Have formed a lateral pnpn device !
Will spend some time studying pnpn devices Review from Last Lecture The SCR Silicon Controlled Rectifier
• Widely used to switch large resistive or inductive loads • Widely used in the power electronics field • Widely used in consumer electronic to interface between logic and power Anode A
p A A A G n Gate p G n G G C C C C Symbols Cathode Usually made by diffusions in silicon
Consider first how this 4-layer 3-junction device operates Review from Last Lecture Variation of Current Gain (β) with Bias for BJT
Note that current gain gets very small at low base current levels Review from Last Lecture Operation of the SCR
Consider a modified application by adding a load (depicted as RL)
IF A IF A IB2 p RL R Q2 L IC1 n IG G IC2 VF p V VF G VG CC Q1 n VG IB1 IG VCC IG C C
All operation is as before, but now, after the triggering occurs, the voltage VF will drop to approximately 0.8 V and the voltage VCC-.8 will appear across RL
If VCC is very large, the SCR has effectively served as a switch putting VCC across the load and after triggering occurs, IG can be removed!
But, how can we turn it off? Will discuss that later Review from Last Lecture IF Operation of the SCR A I G VF G The Ideal SCR IFFG = fVV , C VG
IH is very small IG1 is small (but not too small)
called the SCR model
As for MOSFET, Diode, and BJT, several models for SCR can be developed IF
IG=0
IH
VF VBGF0
IF
IG=IG1>0
IH
VF
VBGF1 Operation of the SCR VCC Operation with the Ideal SCR RL
IF VF Load Line: VCC = I F R L +V F Analysis: VG
IFI = f V F, V G
The solution of these two equations is at the intersection of the load line and the device characteristics Note three intersection points I F Two (upper and lower) are stable equilibrium VCC R L points, one is not Load Line When operating at upper point, VF=0 so VCC
IH appears across RL We say SCR is ON V CC VF When operating at lower point, IF approx 0 so IG=0 no signal across RL We say SCR is OFF when I =0 G When IG=0, will stay in whatever state it was in Operation of the SCR
Operation with the Ideal SCR IF A I G VF G C I = f V, V FI F G VG
IF
On State
VG1
VF
Off State Operation of the SCR VCC Operation with the Ideal SCR RL
IF Now assume it was initially in the OFF state and then VF a gate current was applied
IF VG VCC
RL Load Line VCCFLF = IR+V IH V CC VF IFIFG = fVV , IG=0
I F Now there is a single intersection point so a VCC R L unique solution Load Line The SCR is now ON
IH Removing the gate current will return to the V CC VF previous solution (which has 3 intersection points) but it IG=IG1>0 will remain in the ON state Operation of the SCR VCC Operation with the Ideal SCR RL
IF Turning SCR off when I =0 VF G IF
VCC
RL V Load G Line
IH V CC VF VBGF0 IG=0
Reduce VCC so that VCC/RL goes below IH This will provide a single intersection point
VCC can then be increased again and SCR will stay off
Must not increase VCC much above VBGF0 else will turn on Operation of the SCR VCC Operation with the Ideal SCR RL
IF Turning SCR off when IG=0 VF
IF VG VCC
RL Load Line
IH V CC VF VBGF0 IG=0 Operation of the SCR VCC Operation with the Ideal SCR RL
Often V is an AC signal (often 110V) IF CC VF SCR will turn off whenever AC signal goes negative
VG
IF
VCC
RL Load Line
IH V CC VF VBGF0 IG=0
V CC RL Operation of the SCR VCC Operation with the Ideal SCR RL
Often V is an AC signal (often 110V) IF CC VF SCR will turn off whenever AC signal goes negative
VG
IF
VCC
RL Load Line
IH V CC VF VBGF0 IG=0
V CC RL Operation of the SCR VCC Operation with the Ideal SCR RL
IF Turning SCR off when IG>0 VF
IF V CC VG RL Load Line
IH V CC VF IG=IG1>0
V CC RL
Reduce VCC so that VCC/RL goes below IH This will provide a single intersection point
But when VCC is then increased SCR will again turn on
Not practical to turn it off if IG is very large Operation of the SCR VAC Operation with the Ideal SCR
RL Duty cycle control of RL IF VF VAC
t VG
VLOAD
IGATE Operation of the SCR VAC Operation with the Ideal SCR
RL Duty cycle control of RL IF VF
VLOAD
VG
IGATE
VLOAD
IGATE Operation of the SCR VAC Operation with the Ideal SCR
RL Duty cycle control of RL IF VF VAC
t VG
VLOAD
IGATE Operation of the SCR VAC Operation with the Ideal SCR
RL Duty cycle control of RL IF VF
VLOAD
VG
IGATE
VLOAD
IGATE Operation of the SCR
Operation with the actual SCR
IF I A F I G VF G C
VG ΔVF
IG=0
VBRR IH
VF VBRF0 Operation of the SCR
Operation with the actual SCR IF A I I F G VF G C
VG
IH VBRR VF VBRF0 IG4>IG3>IG2>IG1=0 Operation of the SCR
Operation with the actual SCR
IF VCC VCC
RL ΔVF RL
IG=0 IF VF VBGR IH
VCC VF VG VBGF0
Still two stable equilibrium points and one unstable point Operation of the SCR
Operation with the actual SCR VCC
I F RL VCC IF RL VF
VG
VBRR IH VF VCC
IG4>IG3>IG2>IG1=0
VBGF0
To turn on, must make IG large enough to have single intersection point VCC
SCR Terminology RL IF VF IF
VCC
RL VG
IG=0 IL
VBRR IH VF
VBGF0
IH is the holding current IL is the latching current (current immediately after turn-on) VBGF0 is the forward break-over voltage VBRR is the reverse break-down voltage IGT is the gate trigger current VGT is the gate trigger voltage SCR Terminology VCC IF V R Issues and Observations CC L RL IF VF
IG=0 IL VG VBRR IH VF
VBGF0
• Trigger parameters (VGT and IGT) highly temperature dependent • Want gate “sensitive” but not too sensitive (to avoid undesired triggering) • SCRs can switch very large currents but power dissipation is large • Heat sinks widely used to manage power • Trigger parameters affected by both environment and application • Trigger parameters generally dependent upon VF
• Exceeding VBRR will usually destroy the device • Exceeding VBGF0 will destroy some devices • Lack of electronic turn-off unattractive in some applications • Can be used in alarm circuits to attain forced reset • Maximum 50% duty cycle in AC applications is often not attractive Thyristors
The good SCRs Triacs
The bad
Parasitic Device that can destroy integrated circuits Limitations of the SCR
IF IF A I G VF G C
IG=0 VG
IH
VF VBGF0
IF 1. Only conducts in one direction 2. Can’t easily turn off (though not major problem in AC switching)
IG=IG1>0
IH
VF
VBGF1 Operation of the SCR VCC
Performance Limitations with the SCR RL IF V Assume VCC is an AC signal (often 110V) and VG is static F
VAC
t VG IF
VCC
RL Load Line
V CC VF
V CC RL
SCR is always off Operation of the SCR VCC
Performance Limitations with the SCR RL IF V Assume VCC is an AC signal (often 110V) and VG is static F
VAC
t VG IF
VCC
RL Load Line
V CC VF
V CC VRL RL
SCR is ON about 50% of the time Operation of the SCR VCC
Performance Limitations with the SCR RL IF V Assume VCC is an AC signal (often 110V) and VG is static F
VAC
IF t VG VCC
RL Load Line
V CC VF VBGF0
VRL
V CC RL
SCR is ON less than 50% of the time (duty cycle depends upon VG)
Often use electronic circuit to generate VG Alarm Application
Reset Switch (NC)
S1
r e
z R
z
u B
V 6V 1 S2 DUT NC Foil/ Widow Switch V2 Bi-directional switching
MT1
G2
G1
MT2
Use two cross-coupled SCRs
Limitations
Size and cost overhead with this solution
Inconvenient triggering since G1 and G2 WRT different terminals Bi-directional switching with the Triac
MT2 MT2 n p n G p G nn n
MT1 MT1
• Has two cross-coupled SCRs ! • Manufactured by diffusions • Single Gate Control MT2 Q4 Q2 The Triac MT2 n p G n p G MT1 nn n Q3 Q1 MT1 • Can define two cross-coupled transistor pairs in each side VMT2 IT As for SCR, both circuits have regenerative MT 2 feedback Can turn ON in either direction with either I F2 positive or negative current
I I B4 B2 Defines 4 quadrants (in VMT21-VG-MT1 plane) for operation Q4 Q2 I IC3 C1 VMT2 >V MT1 V G-MT1 >0 Quadrant 1 IC4 IC2 VMT2 >V MT1 V G-MT1 <0 Quadrant 2 Q3 Q1 I B3 IG3 IG1 IB1 VMT2
IT IG VTR
-V IG=0 G BGF MT I 1 H VGT1 VTR VBGF Consider the basic Triac circuit
VAC
IT RL
IT IG=IG1<0 VTR or IG=IG1>0 -VBGF
IH VGT1 VTR VBGF VBGF1 VAC Assume ideal Triac The Basic Triac Circuit RL IT Load Line: V = I R +V VAC CC T L TR IT VTR RL Analysis: VAC = I T R L +V TR -VBGF IG=0 -V VGT1 AC IH V IFI = f V TR, V GT1 AC VTR VBGF
V The solution of these two equations is at the intersection of AC R L the load line and the device characteristics
IT VAC R L Two stable operating points for both positive and negative VAC
IG=0 -VBGF -V AC IH V AC VTR VBGF
V AC RL Assume ideal Triac The Basic Triac Circuit VAC
IT V Load Line: AC VCC = I T R L +V TR RL RL
IT IG=IG1<0 Analysis: VTR or VAC = I T R L +V TR -VBGF IG=IG1>0 -V AC IH IFI = f V TR, V GT2 V V AC VTR GT1 VBGF VBGF1
V AC RL Single solution for both positive and negative VAC
IT V AC Thus this state turns on the Triac for any input RL
IG=IG1<0 or -VBGF IG=IG1>0 -V AC IH V AC VTR VBGF VBGF1
V AC RL Assume ideal Triac The Basic Triac Circuit VAC
RL IT IT VAC VTR
RL
VGT1
-VAC
VAC VTR VBGF1
V AC RL The Actual Triac MT2
IT G
MT1
VTR
IG4>IG3>IG2>IG1=0 The Actual Triac in Basic Circuit
VAC IT
VAC RL
RL ΔV IT F VTR
IG=0 V BGR VGT1 I VTR - VAC HF V IHR AC VBGF0
V AC R L Two stable operating points
IG=0 State The Actual Triac in Basic Circuit
VAC
RL
IT VTR
VGT1 IF IF
VAC VAC
RL RL
- V - VAC AC VF VF V VAC AC I >I >I >I =0 IG4>IG3>IG2>IG1=0 G4 G3 G2 G1
V AC VAC RL RL
Can turn on for either positive or negative VAC with single gate signal Phase controlled bidirectional switching with Triacs
VAC
t
VLOAD
IGATE
VLOAD
IGATE
VLOAD
IGATE Quadrants of Operation Defined in VM21-VGT1 plane
(not in the IT-VM21 plane)
MT2 IT V I M21 T IG VM21 G MT1 VGT1 Quadrant 2 Quadrant 1
VGT1 VM21
IG4>IG3>IG2>IG1=0 Quadrant 3 Quadrant 4
But for any specific circuit, can map quadrants from the VM21-VGT1 plane to IT-VM21 plane Identification of Quadrants of Operation in IT -VM21 plane VM21
MT2 IT
IG VM21 Quadrant 2 Quadrant 1 G MT V 1 GT1 VGT1
Quadrant 3 Quadrant 4 Identification of Quadrants of Operation in IT-VM21 plane
VM21
MT2 IT
Quadrant 2 Quadrant 1 IG VM21 G MT1 VGT1 VGT1
Quadrant 3 Quadrant 4
Curves may not be symmetric between Q1 and Q3 in the IT-VM21 plane
Turn on current may be large and variable in Q4 (of the VM21-VGT1 )
Generally avoid operation in Q4 (of the VM21-VGT1 plane)
Most common to operate in Q2-Q3 quadrants or Q1-Q3 quadrants (of the VM21-VGT1 plane) Quadrants of Operation Defined in VM21-VGT1 plane
(not in the IT-VM21 plane)
MT2 IT V I M21 T IG VM21 G MT1 VGT1 Quadrant 2 Quadrant 1
VGT1 VM21
IG4>IG3>IG2>IG1=0 Quadrant 3 Quadrant 4
But for any specific circuit, can map quadrants from the VM21-VGT1 plane to IT-VM21 plane Identification of Quadrants of Operation in IT -VM21 plane VM21
MT2 IT
IG VM21 Quadrant 2 Quadrant 1 G MT V 1 GT1 VGT1
Quadrant 3 Quadrant 4 Identification of Quadrants of Operation in IT-VM21 plane
VM21
MT2 IT
Quadrant 2 Quadrant 1 IG VM21 G MT1 VGT1 VGT1
Quadrant 3 Quadrant 4
Curves may not be symmetric between Q1 and Q3 in the IT-VM21 plane
Turn on current may be large and variable in Q4 (of the VM21-VGT1 )
Generally avoid operation in Q4 (of the VM21-VGT1 plane)
Most common to operate in Q2-Q3 quadrants or Q1-Q3 quadrants (of the VM21-VGT1 plane) VM21
Some Basic Triac Application Circuits Quadrant 2 Quadrant 1
VGT1
Quadrant 3 Quadrant 4
VAC VAC
RL RL
IT IT VTR VTR
MT MT1 1
VGG VGG
(V often from logic/control circuit) GG (VGG often from logic/control circuit)
Quad 1 : Quad 4 Quad 2 : Quad 3
(not attractive because of Quad 4) VM21
Some Basic Triac Application Circuits Quadrant 2 Quadrant 1
VGT1 VAC
Quadrant 3 Quadrant 4
RL
IT VTR Quad 2 : Quad 3 MT1
VGG
Limitations ?
If VAC is the standard 120VAC line voltage, where do we get the dc power supply?
10K VOUT
120VAC CFILTER 1K
Direct digital control of trigger voltage/current with dedicated IC VM21
Some Basic Triac Application Circuits Quadrant 2 Quadrant 1 VGT1
Quadrant 3 Quadrant 4 VAC
VAC VAC RL
RL RL
IT IT VTR VTR
IT
MT1 MT1 VTR
MT1
Quad 1 : Quad 3 Quad 1 : Quad 3 Quad 1 : Quad 3 Some Basic Triac Application Circuits VM21
Quadrant 2 Quadrant 1
VGT1
VAC Quadrant 3 Quadrant 4
IT
VTR
MT1
RL
Quad 1/ Quad 2 : Quad 3/Quad 4
Not real popular End of Lecture 26