EE 330 Lecture 27. Thyristors SCR TRIAC

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1 EE 330 ecture 27 Thyristors SC TIAC

2 eview from ast ecture S The JFET D With S =0, channel exists under gate between D and S S D Under sufficiently large reverse bias (depletion region widens and channel disappears - pinches off )

3 eview from ast ecture The JFET D D S D S n-channel S p-channel n-channel JFET (not available in this process) Square-law model of n-channel JFET 0 S P 2IDSS DS I S < - P 2 2 S IDSS S P DS > S-P P D S P DS S P DS S P Functionally identical to the square-law model of MOSFET Parameters I DSS and P characterize the device I DSS proportional to W/ where W and are width and length of n+ diff P is negative for n-channel device, positive for p-channel device thus JFET is depletion mode device Must not forward bias S junction by over about 300m or excessive base current will flow (red constraint) Widely used as input stage for bipolar op amps

4 eview from ast ecture The Thyristor A bipolar device in CMOS Processes Consider a Bulk-CMOS Process S D S D p n p n Have formed a lateral pnpn device! Will spend some time studying pnpn devices

5 eview from ast ecture The SC Silicon Controlled ectifier 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 ate C Cathode Usually made by diffusions in silicon p n p n A A A C C C Symbols Consider first how this 4-layer 3-junction device operates

6 eview from ast ecture ariation of Current ain (β) with Bias for BJT Note that current gain gets very small at low base current levels

7 eview from ast ecture Operation of the SC Consider a modified application by adding a load (depicted as ) I A C p n p n I C1 Q 1 I B2 C A Q 2 I C2 I B1 I I All operation is as before, but now, after the triggering occurs, the voltage will drop to approximately 0.8 and the voltage -.8 will appear across If is very large, the SC has effectively served as a switch putting across the load and after triggering occurs, I can be removed! But, how can we turn it off? Will discuss that later

8 Operation of the SC SC model I f, F 1 F I f 2 I A C As for MOSFET, Diode, and BJT, several models for SC can be developed The Ideal SC Model 1, IF f1 IA F, or 2 I f I f I F I f I 2I

9 Operation of the SC Consider the Ideal SC Model I f I F 1, I f 2I I A C I =0 I H is very small I H BF0 I =I 1 >0 I 1 is small (but not too small) I H BF1

10 Operation of the SC Operation with the Ideal SC oad ine: Analysis: = IF +F = IF +F I f I F 1, I F The solution of these two equations is at the intersection of the load line and the device characteristics I H I =0 oad ine when I =0 Note three intersection points Two (upper and lower) are stable equilibrium points, one is not When operating at upper point, =0 so appears across We say SC is ON When operating at lower point, approx 0 so no signal across We say SC is OFF When I =0, will stay in whatever state it was in

11 Operation of the SC Operation with the Ideal SC A I f I F 1, I C On State I =0 Off State For notational convenience will drop subscript unless emphasis is needed I f, I I f I F 1, F F

12 Operation of the SC Operation with the Ideal SC Now assume it was initially in the OFF state and then a gate current was applied I F I H I =0 oad ine = IF +F I f, I F F oad ine Now there is a single intersection point so a unique solution The SC is now ON I H I =I 1 >0 emoving the gate current will return to the previous solution (which has 3 intersection points) but it will remain in the ON state

13 Operation of the SC Operation with the Ideal SC Turning SC off when I =0 oad ine I F I H I =0 BF0 educe so that / goes below I H This will provide a single intersection point can then be increased again and SC will stay off Must not increase much above BF0 else will turn on

14 Operation of the SC Operation with the Ideal SC Turning SC off when I =0 F I oad ine I H I =0 BF0

15 Operation of the SC Operation with the Ideal SC Often is an AC signal (often 110) F SC will turn off whenever AC signal goes negative I oad ine I H I =0 BF0

16 Operation of the SC Operation with the Ideal SC Often is an AC signal (often 110) F SC will turn off whenever AC signal goes negative I oad ine I H I =0 BF0

17 Operation of the SC Operation with the Ideal SC Turning SC off when I >0 F oad ine I I H I =I 1 >0 educe so that / goes below I H This will provide a single intersection point But when is then increased SC will again turn on Will not turn off if I is very large

18 Operation of the SC Operation with the Ideal SC Duty cycle control of F I AC t OAD I ATE

19 Operation of the SC Operation with the Ideal SC Duty cycle control of F I OAD I ATE OAD I ATE

20 Operation of the SC Operation with the Ideal SC Duty cycle control of F I AC t OAD I ATE

21 Operation of the SC Operation with the actual SC I A C Δ I =0 B I H BF0

22 Operation of the SC Operation with the actual SC I A C B I H BF0 I 4 >I 3 >I 2 >I 1 =0

23 Operation of the SC Operation with the actual SC Δ F I =0 I B I H BF0 Still two stable equilibrium points and one unstable point

24 Operation of the SC Operation with the actual SC I F B I H I 4 >I 3 >I 2 >I 1 =0 BF0 To turn on, must make I large enough to have single intersection point

25 SC Terminology I F I I =0 B I H BF0 I H is the holding current I is the latching current (current immediately after turn-on) BF0 is the forward break-over voltage B is the reverse break-down voltage I T is the gate trigger current T is the gate trigger voltage

26 SC Terminology Issues and Observations I F I I =0 B I H BF0 Trigger parameters (T and I T ) highly temperature dependent Want gate sensitive but not too sensitive (to avoid undesired triggering) SCs 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 F Exceeding B will usually destroy the device Exceeding BF0 will destroy some devices ack 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

27 Thyristors The good SCs Triacs The bad Parasitic Device that can destroy integrated circuits

28 imitations of the SC A I C I =0 I H BF0 1. Only conducts in one direction 2. Can t easily turn off (though not major problem in AC switching) I =I 1 >0 I H BF1

29 Operation of the SC Performance imitations with the SC Assume is an AC signal (often 110) and is static F AC t oad ine SC is always off

30 Operation of the SC Performance imitations with the SC Assume is an AC signal (often 110) and is static F AC t oad ine SC is ON about 50% of the time

31 Operation of the SC Performance imitations with the SC Assume is an AC signal (often 110) and is static F AC oad ine t BF0 SC is ON less than 50% of the time (duty cycle depends upon ) Often use electronic circuit to generate

32 Alarm Application eset Switch (NC) S 1 Buzzer 6 DUT 1 2 S 2 NC Foil/ Widow Switch

33 Bi-directional switching MT MT 2 Use two cross-coupled SCs imitations Size and cost overhead with this solution Inconvenient triggering since 1 and 2 WT different terminals

34 Bi-directional switching with the Triac MT 2 MT 2 n p n n p n MT 1 MT 1 Has two cross-coupled SCs! Manufactured by diffusions Single ate Control

35 The Triac n Q4 MT 2 Q2 MT 2 I Can define two cross-coupled transistor pairs in each side I C3 Q 3 I B4 Q 4 Q 2 I B3 2 I C4 MT 2 I 3 MT 1 MT2 I T I 1 MT1 I C2 I B1 I B2 Model for Quadrants 1 and 4 (n-diffusion for gate not shown) Q 1 I C1 nn Q3 Q1 MT 1 As for SC, both circuits have regenerative feedback MT 1 Can turn ON in either direction with either positive or negative current Defines 4 quadrants (in MT21 --MT1 plane) for operation > >0 Quadrant 1 MT2 MT1 -MT1 > <0 Quadrant 2 MT2 MT1 -MT1 < <0 Quadrant 3 MT2 MT1 -MT1 < >0 Quadrant 4 MT2 MT1 -MT1 Usually use only one : MT for control p n p n Different voltage, duration strategies exist for triggering Can t have single : MT control with two SCs

36 End of ecture 28

EE 330 Lecture 26. Thyristors SCR TRIAC

EE 330 Lecture 26. Thyristors SCR TRIAC EE 330 ecture 26 Thyristors SC TIAC eview from ast ecture Area Comparison between BJT and MOSFET BJT Area = 3600 l 2 n-channel MOSFET Area = 168 l 2 Area atio = 21:1 eview from ast ecture The Thyristor