turn-off conditions for power scrsseagrant.mit.edu/publications/mitsg_13-50.pdf · turn-off...

Suffolk University

Turn-off Conditions for Power SCRs

Rodrigo Midea Coelho

Suffolk University

Summer 2013

[email protected]

2

Abstract

This report focus on the turn-off conditions for power SCRs using a class-c commutation

circuit. There is a significant difference of the turn-off time (𝑡 ) between standard SCRs,

commonly intended for phase control applications, and fast SCRs, meant for inverters. Typical

𝑡 for fast SCRs is from 15us to 35us, whereas the 𝑡 of standard ones is between 30us to 300us.

A dozen units of small, moderate and high power SCRs were used on the experiments.

Experimental results showed that the forward current did not represent a major factor on 𝑡 . As

key design factor, it was found that control of the turn-off time is more critical than design for

the turn-off charge available. Also, temperature effect increased 𝑡 from 15 to 30%, depending

on the device. At last, two techniques were studied in an attempt to reduce 𝑡 : reverse-biased

gate technique and multi-pulse triggering. The first one was found to be an effective way to

reduce 𝑡 for small SCRs and to impede the 𝑡 increase due to temperature effect on fast SCRs

of moderate power.

3

Table of Contents

Abstract ............................................................................................................................... 2

Table of Contents ................................................................................................................ 3

Introduction ......................................................................................................................... 5

SCR Timing Definitions ................................................................................................. 5

Forced Commutation Methods ....................................................................................... 5

Apparatus ............................................................................................................................ 7

Definitions....................................................................................................................... 7

“Class C” Circuit for Tests ............................................................................................. 8

Circuits ............................................................................................................................ 8

Measurement Procedures .............................................................................................. 13

Devices .......................................................................................................................... 14

Experimental Results ........................................................................................................ 16

I) Different Kinds of SCRs ........................................................................................... 16

II) Current Effect........................................................................................................... 21

III) Effects on 𝒕𝒒 when Overdriving Small SCRs ........................................................ 23

IV) Key Design Factors for Small SCRs ...................................................................... 25

V) Current and Voltage Timing .................................................................................... 28

VI) Temperature Effect ................................................................................................. 30

VII) Power Size Effect .................................................................................................. 33

4

VIII) Reverse-Biased Gate Technique .......................................................................... 35

IX) Multi-Pulse Attempt ............................................................................................... 38

X) Suggestion for Further Work ................................................................................... 40

Summary ........................................................................................................................... 41

Appendices ........................................................................................................................ 43

Appendix A: Commutation Classes Circuits and Waveforms ...................................... 43

Appendix B: Design Considerations ............................................................................. 45

Appendix C: Trigger Software Guidelines ................................................................... 52

References ......................................................................................................................... 57

5

Introduction

The SCR is a power device that can be turned on by applying a gate pulse if the SCR is

forward biased. However, its structure does not allow an easy turn off. The SCR can be turned

off by two methods: natural and forced commutation. The natural commutation occurs when the

anode current drops below the holding current 𝐼 , and it is often used in AC circuits. SCRs

operating in DC systems require additional circuit to turn-off, using forced commutation.

SCR Timing Definitions

a) 𝑡 – After this time, a positive voltage rate can be applied across Anode and Cathode

without causing any spurious firing. This parameter defines the maximum operating

frequency of the SCR.

b) 𝑡 – Time that the SCR stays reverse-biased during the turn-off. If 𝑡 > 𝑡 ,

the SCR turns off. Otherwise, the SCR will not turn-off and will drive the load again.

Forced Commutation Methods

The forced commutation methods are classified as the following: [See Appendix A for

typical circuit implementations.]

Class A Self commutated by a resonating load

Class B Self commutated by an L-C circuit

Class C C or L-C switched by another load carrying SCR

Class D C or L-C switched by an auxiliary SCR

Class E An external pulse source for commutation

6

Class F AC line commutation

This experiment will use the Class-C commutation method, which consists of using a

capacitor and a secondary SCR to apply a reverse voltage across the anode and cathode of the

main SCR, forcing it to turn-off. The circuit will force a reverse current through the SCR for a

short period to reduce the anode current to zero. It is necessary to maintain the reverse bias for a

certain time to complete the turnoff. The minimum time that ensures the turn-off is defined as 𝑡 .

7

Apparatus

Definitions

The measurement employed oscilloscope traces to determine specific time values.

- 𝑫𝑼𝑻: Device Under Test.

- 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆: Time between the point that the IDUT cross the zero and the point where the

voltage turns positive.

- 𝒕𝒒: Minimum 𝑡 necessary to successfully turn-off the SCR.

IDUT

VDUT

8

“Class C” Circuit for Tests

The circuits are based on the following block diagram:

Figure 1-Block Diagram of the Class-C Circuit

Circuits

1) Standard Trigger

The function of the trigger circuit is to create two step signals. The first step turns on the

main SCR, and the second step, which is produced 1ms apart from the first one, turns on the

second SCR. The trigger circuit is based on the MSP430G2533 microcontroller, which has 14

general input/output pins. There was no special reason to use this specific microcontroller other

than the availability at the moment of the design.

DUT Turn-Off SCR

Pre-Load

Capacitor

Pre-Charge

9

The gate of both SCRs are capacitive coupled. A step is applied at the MOSFET’s gate of

the main SCR, which results in a pulse that turns on the main SCR. The main SCR (Device

Under Test, or DUT) remains on during 1ms, when the same turn-on procedure occurs at the

second SCR. With the second SCR on, the C1’s voltage is applied across the DUT, forcing it to

turn-off.

2) Reverse-Biased Gate Trigger

A third SCR is used on the reverse-biasing gate technique. It is triggered at the same time

as the second SCR. Referring to Circuit 2, the capacitor C1 applies the reverse voltage across the

anode, whereas the C5 applies the reverse voltage across the gate of the DUT.

3) Multi-Pulse Trigger

A third SCR is triggered after 𝑡 from the second SCR and applies reverse voltage

across the anode of the DUT. The minimum value of 𝑡 is around 4us, and it can be

increased/decreased by the switches.

10

Circuit 1- Regular trigger complete circuit

Circuit Schematic –

Standard Trigger

11

Circuit 2 - Reverse-biased gate trigger complete circuit


Reverse-Biased Gate Trigger

12

Circuit 3 – Multi-pulse trigger complete circuit


Multi-Pulse Trigger

13

Measurement Procedures

1) Regular procedure:

2) Reverse-biased gate procedure: (apply at the end of Step 2)

3d) Turn on SCR3.

4) C1 will now be charged with Vr, much higher

than Vr-Vs. Press S1 again to turn off the DUT.

5) Increase C1 and repeat procedure until 𝑡 is

found.

4) Decrease C1 and repeat procedure until

𝑡 is found.

Did DUT shut off?

1) Limit Vs current to 0.3A, to prevent

temperature effect if DUT doesn’t turn off.

2) Set R1, Vr and Vs

Step 1

1) Press S1.

2) Trigger circuit tasks:

3a) Turn on DUT;

3b) Wait 1ms;

3c) Turn on SCR2.

Step 2

Increase Reverse Time Decrease Reverse Time

Minimum Reverse Time ≡ 𝑡

14

3) Temperature effect initial tasks:

Devices

The following table lists all the devices and their main parameters.

Part # Manufacturer Package

S6015L TECCOR TO220 S6020L TECCOR TO220 TYN1225 ST MICROELECTRONICS TO220 NTE5558 NTE TO220 25TTS12 INTERNATIONAL RECTIFIER TO220 CS19-08H01 IXYS TO220 MMO62-16IO6 IXYS SOT-227 B NTE5372 NTE TO94 T507 POWEREX TO94 MCC95-16IO1B IXYS TO240 C430PB POWEREX POW-R-DISC

Table 1 – SCRs and their manufacturers and packages.

1) Limit Vs current to 3.5A.

2) Set R1, Vr and Vs.

3) Reduce C1 to a value that does not turn-off the

DUT.

4) Press S1.

5) Leave DUT on for 5 minutes.

15

Part # Irms (A) Itsm (A) VT (V) VDRM (V) tq specified (us)[1]

S6015L 15 225 1.6 600 35 S6020L 20 300 1.6 600 35

TYN1225 25 260 1.6 1225 70

NTE5558 25 300 1.8 800 15 25TTS12 25 300 1.25 1200 110

CS19-08H01 31 180 1.31 800 150

MMO62-16IO6 54 400 1.57 1600 150

NTE5372 135 2450 2.15 1200 30

T507 125 1400 3.2 800 15

MCC95-16IO1B 180 2250 1.5 1600 185

C430PB 1070 8000 2.4 1200 125 Table 2 – SCRs main parameters.

[1] Each manufacturer defines 𝑡 according to their own distinct test conditions. Typical variation is with the amount and duration of the current pulse. For further information, consult the datasheet.

16

Experimental Results

I) Different Kinds of SCRs

There are two kind of SCRs: the “standard” and “fast” devices. In general, the standard

SCRs are classified for phase control applications, whereas the fast devices are for inverters.

The typical 𝑡 from datasheet is from 15us to 35us for fast devices, and 35us to 300us for

standard SCRs.

The following waveforms illustrate the typical current and voltage behavior during the

turn-off for these two kinds of SCRs. The horizontal scale of 25us applies for all the waveforms

below.

1) Standard SCRs

Settings for the experiment: DUT = MCC95, IDUT = 40A, 𝑉 =65V, 𝑉 = 45V.

a) Successful Turn-off: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 ≥ 𝒕𝒒

- DUT Current (Vert. 20A/div) - Voltage across DUT (Vert. 1V/div)

17

- DUT Current and Voltage

b) Failure: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 < 𝒕𝒒 (near tq)



18

c) Failure: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 ≪ 𝒕𝒒



19

2) Fast SCRs

Settings for the experiment: DUT = T507, IDUT = 40A, 𝑉 = 70V, 𝑉 = 45V.

a) Successful Turn-off: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 ≥ 𝒕𝒒



20

b) Failure: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 < 𝒕𝒒 (near tq)



21

c) Failure: 𝒕𝒓𝒆𝒗𝒆𝒓𝒔𝒆 ≪ 𝒕𝒒



II) Current Effect

With the data obtained, it is possible to affirm that as the current increases the turn-off

time also increases. In contrast to literature, which says that forward current is a major parameter

for 𝑡 , the 𝑡 increase due to forward current was small. The experiment showed that 𝑡 does not

have a linear or exponential response, but more like a flat one.

Settings of this experiment: Vs = 13V, Vr =37V

22

1) Standard SCR

The 𝑡 slope is more pronounced at lower currents for small SCRs. The following chart

shows 𝑡 for IDUT up to 10A for the TYN1225 device, which nominal current is 20A.

Chart 1 - Turn-Off time vs Current for TYN1225 (standard SCR)

0

5

10

15

20

25

30

0 2 4 6 8 10

tq (us)

IDUT (A)

Turn-off Time vs SCR Current

TYN1225

23

2) Fast SCR

Chart 2 - Turn-Off Time vs Current for NTE5558 (fast SCR)

III) Effects on 𝑡 when Overdriving Small SCRs

In this test, both standard and fast SCRs were driven with forward current up to 250% of

nominal current. The maximum surge current 𝐼 defined by the manufacturer is around ten

times higher than the maximum steady current 𝐼 , which allows this test to be done without

damaging the SCR.

Even when overdriven, small SCRs showed a consistent pattern of minor increase on 𝑡

as the current increases.

Settings: Vr = 65V, Vs = 45V

0

5

10

15

20

25

30

0 2 4 6 8 10

tq (us)

IDUT (A)


NTE5558

24

Chart 3 - Turn-off time vs current when overdriving small SCRs, up to 2.5x steady current rating

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

tq (us)

IDUT (A)


TYN1225 (Standard)

NTE5558 (Fast)

25

IV) Key Design Factors for Small SCRs

1) Total capacitor charge available

In the beginning, there was a suspicion that the available charge at the capacitor might be

critical to turn-off the SCR. To measure the capacitor charge effect, the SCR was set at a specific

current and C was increased to higher values. The minimum 𝑉 necessary to turn-off would

determine the minimum charge necessary 𝑄 = 𝐶 ∗ 𝑉 to turn-off the SCR for a given

capacitance. The tests were done using small SCRs.

Settings: DUT = TYN1225, Vs = 12V, IDUT = 2A

C (uF) Minimum Vc to turn-off (V)

Capacitor Charge (uC)

Q relative (%)

Turned-Off?

21.9 35 766.5 100.00 Yes 35.4 22.6 800.04 104.38 Yes 50.8 18.1 919.48 119.96 Yes 65 16.5 1072.5 139.92 Yes

86.7 14.7 1274.49 166.27 Yes Table 3 – Charge effect on the turn-off for the TYN1225.

Settings: DUT = S6015L, Vs = 12V, IDUT = 2A

C (uF) Minimum Vc to turn-off (V)

Capacitor Charge (uC)

Q relative (%)

Turned-Off?

33.3 30.8 1025.64 100.00 Yes 25.2 43.1 1086.12 105.90 Yes 22.7 50.4 1144.08 111.55 Yes 18.9 67.9 1283.31 125.12 Yes 16.3 92.9 1514.27 147.64 Yes

Table 4 – Charge effect on the turn-off for the S6015L

The total available capacitor charge is not critical to turn-off the SCR, since different Q

were needed to turn-off the SCR at a given capacitance for small SCRs.

26

2) Capacitor time constant

Empirically, it was found that the capacitor time constant can be assumed as 𝜏 = 𝑅 ∗

𝐶1 without much deviance. During the regular measurements, for example, if the 𝑅 was

decreased to get higher currents, the C1 capacitance had to be increased to achieve the same

𝑡 . This was a clue that the turn-off was somewhat related to C1’s time constant.

This table illustrates 𝑡 and 𝜏 for a small SCR.

Settings: DUT = TYN1225, Vs = 13V, Vr = 37V

IT1 (A) 𝒕𝒒 (us) Rload (ohms) Capacitance

(uF) 𝝉 (us) 𝒕𝒒/𝝉

0.85 18.00 20.00 1.07 21.40 0.84 1.32 19.40 10.90 2.05 22.35 0.87 1.69 20.00 8.00 2.88 23.04 0.87 3.10 22.00 4.00 5.87 23.48 0.94 6.20 22.00 1.60 15.00 24.00 0.92 8.55 22.00 1.00 26.00 26.00 0.85 9.96 21.60 0.80 34.40 27.52 0.78

Table 5 – 𝑡 and time constant necessary to turn-off an SCR at a given current.

27

The following chart is the result of plotting the 𝑡 /𝜏 ratio for small SCRs.

Settings: Vs = 13V, Vr = 37V

Chart 4 - 𝑡 /𝜏 ratio versus SCR current

The 𝑡 /𝜏 ratio is kept close to a constant value for the current range of 0 to 10A for small

SCRs. It can be inferred from this result that leaving the SCR reverse-biased for at least 𝑡 is

critical to achieve a successfully turn-off , as the C1’s time constant defines 𝑡 .

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.00 2.00 4.00 6.00 8.00 10.00

tq/R1C1

IDUT (A)

tq and C1's Time Constant ratio vs SCR Current

NTE5558

CS19-08H1

S6015L

25TTS12

TYN1225

28

V) Current and Voltage Timing

According to the literature, the turn-off time 𝑡 can be divided into two parts. During the

𝑡 period the SCR will display a “reverse recovery current,” which is the charge clearing away

from the junctions. A further waiting time 𝑡 must then elapse while charges recombine at the

gate junction. Usually, 𝑡 is much smaller than 𝑡 .

The typical value for 𝑡 is less than 8𝜇𝑠. Measurements showed that the time constant

𝑅 ∗ 𝐶 is valid after 𝑡 and can be used into the capacitor charge equation if the initial voltage

is known empirically. An equation that helps to choose the C1 value based on 𝑡 and the

necessary 𝑡 to turn off the SCR was initially the goal for this part of the experiment.

However, the attempt to obtain a function for 𝑡 was not successful due to divergence from

the expected theoretical initial value and the one obtained from the experiments. This divergence

might be related to what occurs during 𝑡 .

Current and voltage timing:

Settings: DUT = CS19-08h01, IDUT = 24A, Vs = 40V, Vr = 65V, 𝑎𝑡 𝑡 .

𝑡

𝑡 𝐼𝐷𝑈𝑇 (20A/Div)

𝑉𝐷𝑈𝑇 (5V/Div)

29

Different time scale:

𝑡 𝑡 𝐼𝐷𝑈𝑇 (20A/Div)

𝑉𝐷𝑈𝑇 (5V/Div)

30

VI) Temperature Effect

Most manufacturers specify the turn-off time for a junction temperature equals to 125°C,

as worst case scenario. To study the temperature effect, the SCR was warmed up with 3.5A for 5

minutes before the test. Below are the case temperatures for both devices during the test.

Device Tcase (°C) C430PB 32

T507088074AQ 45

1) Standard SCR

Settings: DUT = C430PB, Vs = 45V, Vr = 65V

IDUT (A) Regular 𝒕𝒒(us) Warmed up 𝒕𝒒 (us)

𝒕𝒒increase (%)

7.2 22 23.6 7.27 12 22.8 28.4 24.56 16 26 30 15.38

24.8 26 30 15.38 32.8 26 30.8 18.46 38.4 26.8 31.6 17.91 45.6 26.8 32.4 20.90 50.4 27.2 32.8 20.59

Table 6- 𝑡 increase with the temperature for standard SCR

31

Considering the junction temperature as 40% more than the case temperature, the

following chart provides an exponential estimation for the 𝑡 vs 𝑇 , based on 𝑡 given by the

manufacturer.

Chart 5 - Estimative for the turn-off time versus junction temperature for standard SCRs

2) Fast SCR

Settings: DUT = T507, Vs = 45V, Vr = 65V

IDUT (A)

Regular 𝒕𝒒 (us)

Warmed up 𝒕𝒒 (us)

𝒕𝒒increase (%)

7.2 4.4 5.8 31.82 12 4.7 6.2 31.91

19.2 5.6 6.4 14.29 25.6 5.8 6.6 13.79 29.6 5.8 7.8 34.48 36 5.2 8.6 65.38

41.6 6.4 7.8 21.88 50.4 6.2 7.6 22.58

Table 7 - 𝑡 increase with the temperature for fast SCR

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

tq (us)

Tj (°C)

Turn-Off Time vs Junction Temperature

32

Chart 6 - Estimative for the turn-off time versus junction temperature for fast SCRs

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120 140

tq (us)

Tj (°C)

Turn-Off Time vs Junction Temperature

33

VII) Power Size Effect

The following table defines the classes for SCRs which turn-off times were measured.

Power Class Steady Current Range Small Less than 25A

Moderate Between 25A and 150A High More than 150A

Table 8 - Power classification

1) Small SCRs

Chart 7 - 𝑡 vs current for small SCRs

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10

tq (us)

IDUT (A)

Turn-Off Time vs SCR Current

S6015L (Standard)

CS19-08H1 (Standard)

25TTS12 (Standard)

TYN1225 (Standard)

NTE5558 (Fast)

34

2) Moderate SCRs

Chart 8 - 𝑡 vs current for small SCRs

3) High Power SCRs

Chart 9 - tq vs current for high power SCRs

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

tq (us)

IDUT (A)


MCC95 (Standard)

T507 (Fast)

NTE5387 (Fast)

05

1015202530354045

0 10 20 30 40 50

tq (us)

IDUT (A)


C430PB (Standard)

35

VIII) Reverse-Biased Gate Technique

This technique consists of applying a negative pulse at the DUT’s gate during the turn-

off. It is applied at the same time as the C1 reverse voltage.

1) Small SCRs

The reverse-biased gate technique is very effective in reducing 𝑡 for small devices. For

some of them, the reduction is more than 30%. An interesting data is that the 𝑡 obtained through

this method is always better or slightly equal to the regular 𝑡 .

Chart 10 - 𝑡 reduction using the reverse-biased gate technique for small SCRs

-10

10

30

50

70

90

0 2 4 6 8 10

Tq Reduction (%)

IDUT (A)

Tq Reduction vs SCR current

S6020L (Standard)

CS19-08H0 (Standard)

NTE5558 (Fast)

TYN1225 (Standard)

36

Typical turn-off waveforms:

a) Standard SCR (DUT = CS19-08H0, Vr = 65V, Vs = 45V)

Regular Turn-Off Reverse-Biasing Gate

b) Fast SCR (DUT = NTE5558, Vr = 65V, Vs = 45V)

Regular Turn-Off Reverse-Biasing Gate

Legend:

1)DUT Voltage (1V/div)

2)DUT Current (20A/div)

3)DUT Gate Voltage (10V/div)

1 2

3

Legend:

1)DUT Voltage (1V/div)

2)DUT Current (20A/div)

3)DUT Gate Voltage (10V/div)

1 2

3

37

2) Moderate and High Power SCRs

The results showed that moderate and high power SCRs are not susceptible to the

improvements of the reverse-biased gate technique at room temperature. In the other hand, this

technique reduced the 𝑡 increase of fast SCRs due to temperature effect, although such

arrangement has not been studied in depth.

Settings: DUT = T507, Vs = 45V, Vr = 65V

Room Temperature Warmed Up

Regular Turn-off Regular Turn-off Reverse-Biased Gate

IDUT (A) tq (us) tq (us) tq (us) 29.6 5.8 7.8 6.00 36 5.2 8.6 6.00

41.6 6.4 7.8 6.80 50.4 6.2 7.6 7.40

Table 9 - Reverse-biased gate technique for fast SCRs (moderate and high power)

38

IX) Multi-Pulse Attempt

The multi-pulse technique consists of dividing the reverse pulse across the anode into

two. The first SCR begins the turn-off procedure and after 𝑡 a second SCR applies the

second pulse. Below are the waveforms obtained when using this method. 𝑡 minimum value

is around 4us, and it can be increased/decreased by using the switches on the circuit.

Figure 2 - Waveforms for the multi-pulse triggering method

First, a single pulse was used to define the default 𝑡 . Then, different settings of 𝑡

and capacitance combinations were used to try to achieve a scenario where a 𝑡 reduction could

be seen. This method did not show significant reduction on the turn-off time for small SCRs

running low currents.

Settings: DUT = S6020L, SCR2 = S6020L, SCR3 = TYN1225, Vs = 13V,

Vr = 37V, IDUT = 1.5A (steady), all circuit on protoboard.

Legend:

1) DUT Voltage

2) SCR3 Current

3) SCR2 Current

4) DUT Current

𝑡 : time between 3) and 2)

4

2 3

1

39

tdelay (us)

C1 (uF) C2 (uF) treverse (us)

C1+C2 Δ(C1+C2) (%) Turned off?

Single Pulse - 4.6 0 33.6 4.6 Yes Double Pulse 4 2.67 2.03 36.4 4.7 2.17 Yes Double Pulse 4 2.67 1.43 30.8 4.1 -10.87 No Double Pulse 4 2.67 1.64 32 4.31 -6.30 No Double Pulse 4 2.67 1.86 34 4.53 -1.52 Yes Double Pulse 4 1.86 2.67 34.4 4.53 -1.52 Yes Double Pulse 4 1.64 2.67 33.2 4.31 -6.30 No Double Pulse 10 1.64 2.67 34.8 4.31 -6.30 No Double Pulse 12.6 1.64 2.67 36 4.31 -6.30 No Double Pulse 4 1.44 2.89 33.2 4.33 -5.87 No Double Pulse 10 1.44 2.89 34.8 4.33 -5.87 No Double Pulse 4 1.64 2.89 34.4 4.53 -1.52 Yes Double Pulse 10 1.64 2.89 36.8 4.53 -1.52 Yes Double Pulse 12 1.64 2.89 38 4.53 -1.52 No

Table 10 - Multi-pulse results

40

X) Suggestion for Further Work

The results obtained were promising about impeding the increase of 𝑡 on fast SCRs due

to temperature effect with the reverse-biased gate technique. Also, temperature effect was not

studied for small SCRs. More data would give a better understanding on these two topics.

The effect of overdriving small SCRs was studied using forward current pulses up to 2.5x

the steady current rating. This test could be performed by applying forward current up to 𝐼 ,

which is usually more than 10x the steady current rating.

According to the literature, the rate can influence the turn-off. The rate of the

reapplied voltage could not be controlled by the employed circuit; it was defined by the

capacitor, the time constant, and initial/final voltage of the charge. Its typical value was below

2V/𝜇𝑠. An improved circuit version with control would be able to see how is the effect of

on 𝑡 .

41

Summary

The focus of this report was to study the turn-off conditions for power SCRs. The devices

employed in this experiment can be divided in two classes: fast and standard SCRs. There is a

significant 𝑡 difference between these two classes. In all tests with the various SCRs the

measured 𝑡 was always equal or less than the specified value on the datasheet.

In contrast to literature, which says that forward current is a major parameter on 𝑡 ,

applying steady state current in the ratio from 10 to 120% of the steady current rating resulted

only into a minor increase on 𝑡 , around 10 to 30%. The 𝑡 response for the current does not

comply with a linear or exponential trend. Instead, its response can be considered flat between 20

to 80% of average steady current rating. This behavior can be extended without much deviation

for SCRs of all power classes.

The effect on 𝑡 when overdriving small SCRs was studied. Even with forward current at

2.5x of steady state current rating, the 𝑡 had consistent response with operation under regular

settings.

Also, there was a suspicion that the available charge at the capacitor might be critical to

turn off the SCR. The tests results showed that the charge is not a critical factor, but the C1 time

constant. The 𝑡 /𝜏 ratio is kept almost constant for small SCRs up to 50% of steady current

rating.

The turn-off time is directly related to the junction temperature. As the temperature

increases, the SCR will need to stay reverse-biased for a longer time during the turn-off, and in

some cases, the 𝑡 increase can be more than 30%. An appropriate heat sink or dissipation

system to cool down the SCR can impede the increase of 𝑡 .

42

The reverse-biased gate technique was found to be an effective way to reduce the 𝑡 for

some small SCRs, especially the standard ones. No significant gain has been seen with the

moderate and high power SCRs at room temperature. However, in the real world situation where

the SCR is hot, the reverse-biased gate technique reduced the 𝑡 increase of fast SCRs due to

temperature effect.

The multi-pulse technique did not show significant 𝑡 reduction for small SCRs at low

currents. As this method was tested with one small SCR, the influence of it on 𝑡 for moderate

and high power SCRs is unknown. Despite the small influence on 𝑡 , the multi-pulse trigger

allows to split the single-pulse capacitance into two smaller ones.

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Appendices

Appendix A: Commutation Classes Circuits and Waveforms

a) Class A – Self commutated by resonating the load

b) Class B – Self commutated by an L-C circuit

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c) Class C – C or LC switched by another load-carrying SCR

d) Class D – LC or C switched by an auxiliary SCR

e) Class E – External pulse source for commutation

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f) Class F – AC line commutated

Appendix B: Design Considerations

a) Bill of Materials

Part Name/ Value Quantity Resistor 0.2Ω / 3W 8 Resistor 0.1Ω / 5W 8 Resistor 8Ω / 5W 10 Resistor 100Ω / 0.5W 5 Resistor 1kΩ / 0.5W 1 Resistor 2.7kΩ /0.5W 1 Resistor 4.7kΩ /0.5W 2 Resistor 10kΩ /0.5W 6 Capacitor 3.3mF/100V 3 Capacitor 22uF/100V (unpolarized) 10 Capacitor 10uF/250V 1 Capacitor 1uF/250V (unpolarized) 6 Capacitor 0.5uF/250V (unpolarized) 4 Capacitor 0.2uF/250V (unpolarized) 2 Capacitor 0.047uF/250V (unpolarized) 1

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Part Name/ Value Quantity TYN1225 (as SCR2 and SCR3) 2 ZVP2106A 3 7414N 1 MSP430G2553 1 Push-Button 3 LM311 1

b) Main Board Layout

The software ARES Professional was used to design this layout.

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c) Main Board Schematic

The software ISIS Professional was used to create the layout netlist.

48

d) Circuit Pictures

Figure 3 - Trigger and main circuit, with the capacitor array

Figure 4 - Main board

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Figure 5 - Load resistance

e) Suggestions for Board Improvement

The trigger circuit and the capacitor array didn’t have a proper board. The best practice

would be to have only one board, with the capacitors, main and trigger circuit, so the impedance

and inductance due to long wires are minimized.

The capacitive coupling seemed to be enough to isolate the trigger circuit from the high

power system. However, for higher voltages and currents an opto-isolator with a high-side driver

should be considered to completely isolate both circuits.

The load was also off-board. At first, the circuit was designed for steady state

measurements, which would require high power resistors (50W each 8 ohm resistor, as showed

in Figure 5). The 1ms measurement procedure proved to be valid, and later on 3W resistors were

used without problems.

50

This is a prototype for the trigger-circuit board. It was intended to be connected

underneath the main board.

52

Appendix C: Trigger Software Guidelines

The software was developed using the MSP430 LaunchPad and CodeComposer Studio

5.3.0. The LaunchPad can be bought directly from Texas Instruments website, and to download

the CodeComposer it is first necessary to register on their website.

Creating a new project through New> CCS Project in CodeComposer Studio v5.3, the

Device settings should be as the following:

Family: MSP430; Variant: MSP430G2553;

a) Standard Trigger Code

#include <msp430.h> #define DUT BIT0 #define SCR2 BIT4 #define SCR3 BIT2 //BIT 7 NOT WORKING!!!! #define BUTTON1 BIT5 #define BUTTON2 BIT7 #define TURN_ON_TIME 2000 #define TIME_BEFORE_TURN_OFF 20000 #define TRIGGER_SCR3 50000 #define TURN_OFF_TIME 35000 #define STEP 1 #define DELAY_SCR3 1; #define DELAY 100; //#define BUTTON1_1 BIT3; /* * main.c */ unsigned int timerCount = 0; unsigned int check = 0; unsigned int timeBeforeTurnOff = TIME_BEFORE_TURN_OFF; unsigned int triggerSCR3 = TRIGGER_SCR3; unsigned int delaySCR3 = DELAY_SCR3; unsigned int i =0; int main(void) { WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer BCSCTL1 |= (RSEL3 + RSEL2 + RSEL1 +RSEL0); DCOCTL |= (DCO2 + DCO1 + DCO0); //BCSCTL1 &= ~RSEL0; // set the DCO to around 10MHz P1DIR |= (DUT + SCR2 + SCR3); //P1OUT |= (DUT + SCR2 + SCR3); P1OUT &= ~(DUT + SCR2 + SCR3);

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//P1OUT |= SCR2; //P1OUT &= ~BUTTON2; // This is needed to make the onboard BUTTON1 work P1IES |= BUTTON1 + BUTTON2; // Edge Selector -> High to low P1IFG &= ~(BUTTON1 + BUTTON2); //Clear the flag interrupt P1IE |= BUTTON1 + BUTTON2; P1REN |= BUTTON1 + BUTTON2; __enable_interrupt(); while (1) { if(check){ if (timerCount == 1) { P1OUT |= DUT; P1OUT &= ~SCR2; TACCR0 = TURN_ON_TIME; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } else if (timerCount == 2) { P1OUT |= DUT; P1OUT &= ~SCR2; TACCR0 = timeBeforeTurnOff; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } else if (timerCount == 3) { P1OUT &= ~DUT; P1OUT |= SCR2; i = delaySCR3; while(i--){ __delay_cycles(5); } P1OUT |= SCR3; TACCR0 = triggerSCR3; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } check = 0; } } } #pragma vector=PORT1_VECTOR __interrupt void Port_1(void) { if(P1IFG & BUTTON1){ P1IFG &= ~BUTTON1; //timeBeforeTurnOff += STEP; //triggerSCR3 += STEP; if(delaySCR3 <65000) delaySCR3 += STEP; timerCount = 1; check = 1; } else if(P1IFG & BUTTON2){

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P1IFG &= ~BUTTON2; //timeBeforeTurnOff -= STEP; //triggerSCR3 -= STEP; if(delaySCR3 > STEP) delaySCR3 -= STEP; timerCount = 1; check = 1; } } // Timer A0 interrupt service routine #pragma vector=TIMER0_A0_VECTOR __interrupt void Timer_A(void) { TACCTL0 &= ~CCIE; if (timerCount == 1) { P1OUT |= DUT; P1OUT &= ~SCR2; TACCTL0 &= ~CCIE; } else if(timerCount == 2){ TACCTL0 &= ~CCIE; } else if(timerCount == 3){ P1OUT &= ~DUT; P1OUT &= ~SCR2; P1OUT &= ~SCR3; TACCTL0 &= ~CCIE; } timerCount++; check = 1; }

b) Multi Pulse Code

#include <msp430.h> #define DUT BIT0 #define SCR2 BIT4 #define SCR3 BIT2 #define BUTTON1 BIT7 #define BUTTON2 BIT5 #define TURN_ON_TIME 250 #define TIME_BEFORE_TURN_OFF 20000 #define TRIGGER_SCR3 50000 #define TURN_OFF_TIME 35000 #define STEP 1 #define DELAY_SCR3 1; #define DELAY 100; //#define BUTTON1_1 BIT3; /* * main.c */ unsigned int timerCount = 0;

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unsigned int check = 0; unsigned int timeBeforeTurnOff = TIME_BEFORE_TURN_OFF; unsigned int triggerSCR3 = TRIGGER_SCR3; unsigned int delaySCR3 = DELAY_SCR3; unsigned int i =0; int main(void) { WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer BCSCTL1 |= (RSEL3 + RSEL2 + RSEL1 +RSEL0); DCOCTL |= (DCO2 + DCO1 + DCO0); //BCSCTL1 &= ~RSEL0; // set the DCO to around 10MHz P1DIR |= (DUT + SCR2 + SCR3); P1OUT &= ~(DUT + SCR2 + SCR3); //P1OUT |= BUTTON1; //P1OUT &= ~BUTTON2; // This is needed to make the onboard BUTTON1 work P1IES |= BUTTON1 + BUTTON2; // Edge Selector -> High to low P1IFG &= ~(BUTTON1 + BUTTON2); //Clear the flag interrupt P1IE |= BUTTON1 + BUTTON2; P1REN |= BUTTON1 + BUTTON2; __enable_interrupt(); while (1) { if(check){ if (timerCount == 1) { P1OUT |= DUT; P1OUT &= ~SCR2; TACCR0 = TURN_ON_TIME; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } else if (timerCount == 2) { P1OUT &= ~DUT; P1OUT &= ~SCR2; TACCR0 = timeBeforeTurnOff; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } else if (timerCount == 3) { P1OUT &= ~DUT; P1OUT |= SCR2; i = delaySCR3; while(i--){ __delay_cycles(5); } P1OUT |= SCR3; TACCR0 = triggerSCR3; TACTL = TASSEL_2 + ID_0 + MC_1 + TACLR; TACCTL0 = CCIE; } check = 0; } } }

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#pragma vector=PORT1_VECTOR __interrupt void Port_1(void) { if(P1IFG & BUTTON1){ P1IFG &= ~BUTTON1; //timeBeforeTurnOff += STEP; //triggerSCR3 += STEP; if(delaySCR3 <65000) delaySCR3 += STEP; timerCount = 1; check = 1; } else if(P1IFG & BUTTON2){ P1IFG &= ~BUTTON2; //timeBeforeTurnOff -= STEP; //triggerSCR3 -= STEP; if(delaySCR3 > STEP) delaySCR3 -= STEP; timerCount = 1; check = 1; } } // Timer A0 interrupt service routine #pragma vector=TIMER0_A0_VECTOR __interrupt void Timer_A(void) { TACCTL0 &= ~CCIE; if (timerCount == 1) { P1OUT &= ~DUT; P1OUT &= ~SCR2; TACCTL0 &= ~CCIE; } else if(timerCount == 2){ TACCTL0 &= ~CCIE; } else if(timerCount == 3){ P1OUT &= ~DUT; P1OUT &= ~SCR2; P1OUT &= ~SCR3; TACCTL0 &= ~CCIE; } timerCount++; check = 1; }

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References

“Thyristor Theory and Design Considerations.” ON Semiconductor. n.p, Nov. 2006. Web. 01

Aug. 2013. <http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF>

“Parameter list for SCRs, TRIACs, AC switches, and DIACS.” ST Electronics. n.p, March 2008.

Web. 15 Jun. 2013.<http://www.st.com/st-web-

ui/static/active/en/resource/technical/document/application_note/CD00183570.pdf>

“Commutation of Thyristor-Based Circuits Part-I.” Indian Institute of Technology. n.p, n.d. Web.

09 Jun. 2013. <http://www.nptel.iitm.ac.in/courses/Webcourse-

contents/IIT%20Kharagpur/Power%20Electronics/PDF/L-

19(SSG)(PE)%20((EE)NPTEL)%20.pdf>

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/CD00183570.pdf

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/CD00183570.pdf

http://www.nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Power%20Electronics/PDF/L-19(SSG)(PE)%20((EE)NPTEL)%20.pdf



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