Effects of cobalt 60 gamma radiation on silicon control rectifiers used in switching mod

Smith, G. D.

English

NASA Technical Reports Server

- -  
, 
EFFECTS OF GAMMA RADIATION 
. ON SCR'S USED IN 
SWITCHING MODE AND 
IN AF!PlICATION 
TO ATTITUDE CONTROL 
MOTOR DRIVE BRIDGE 
_ -  GPO PRICE $ 
- CFSTl PRICE(S) $ 
MAY 18, 1965 /. da ..' . . ' Hard copy (HC) 
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F z  ff 653 July65 . L. - -  e 
I 
GODDARD SPACE FLIGHT CENTEk 
. GREENBELT, MARYLAND - 2, 
. 
. 
https://ntrs.nasa.gov/search.jsp?R=19660001912 2020-03-24T03:37:41+00:00Z
X-636-65-241 
EFFECTS OF GAMMA RADIATION ON SCR'S 
USED IN SWITCHING MODE AND IN APPLICATION 
TO ATTITUDE CONTROL MOTOR DRIVE BRIDGE 
Gail D. Smith 
EFFECTS OF GAMMA RADIATION ON SCR'S 
USED IN SWITCHING MODE AND IN APPLICATION 
TO ATTITUDE CONTROL MOTOR DRIVE BRIDGE - r- -~ .'\, 
I SUMMARY 
A study of the effect of Cobalt 60 gamma radiation upon silicon controlled 
rectifiers (SCR) during repetitive pulsing i s  presented. Six electrical character- ,' 
ist ics of each SCR were tested before, during, and after radiation exposure. All  ,' 
reveal appreciable changes in the characteristics with total accumulated dose an 
of these characteristics are shown graphically as a function of the total accum- 
ulated dose with the SCR's both in and out of the radiation field. These curves 
some evidence of dose rate effect. It is concluded that all observed changes are 
within design capability. That is ,  SCR circuits can be designed to reliably per- 
dose of 6.6 x 106 rads. I,, \ form the desired functions in a gamma field of 2 x 106 rads per 
iii 
CONTENTS 
Page -
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  INTRODUCTION 1 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  MOTOR DRIVE BRIDGE 1 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 TEST CIRCUIT.. 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  PARAMETER TESTING 3 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  TEST PROCEDURE 3 
RADIATION SOURCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4 
. . . . . . . . . . . . . . . . . . . . . . . . . .  DISCUSSION OF TEST RESULTS 4 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  CONCLUSIONS 6 
REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6 
LIST OF ILLUSTRATIONS 
Figure Page 
1 Motor Drive Bridge Circuit .......................... 7 
3 Oscillograms of Test Circuit During Irradiation Studies . . . . . .  9 
4 Gate Trigger Voltage v s  Total Gamma Dose. . . . . . . . . . . . . . .  10 
5 Gate Trigger Current vs  Total Gamma Dose . . . . . . . . . . . . . .  11 
6 Forward on Voltage v s  Total Gamma Dose . . . . . . . . . . . . . . .  12 
7 Holding Current v s  Total Gamma Dose . . . . . . . . . . . . . . . . . .  13 
8 Forward Blocking Current vs Total Gamma Dose . . . . . . . . . . .  14 
9 Reverse Blocking Current vs Total Gamma Dose . . . . . . . . . . .  15 
. . . . . . . . . . . . . .  2 Test Circuit to Simulate Motor Drive Bridge 8 
V 
EFFECTS OF GAMMA RADIATION ON SCR'S 
USED IN SWITCHING MODE AND IN APPLICATION 
TO ATTITUDE CONTROL MOTOR DRIVE BRIDGE 
INTRODUCTION 
This report is being written primarily to inform the advanced orbiting solar 
observatory (AOSO) project office of the findings to this date of the silicon con- 
trolled rectifier (SCR) radiation study. An extensive study i s  being conducted of 
high power SCR's during and after exposure to  various radiation fields. The 
SCR's in this study are  irradiated in three different modes: 
(1) Biased 
(2) Unbiased 
(3) Switched on and off several hundred times per second. 
This report covers only the portion concerned with the SCR's being switched on 
and off during radiation. Thus, the purpose of this report is to determine the 
effect of radiation upon the SCR's while being turned on and off as they are in 
the AOSO inertia wheel motor drive bridge circuit. 
While being turned on and off in the radiation field, various parameters of 
the SCR's were continuously monitored with a multichannel oscilloscope. Static 
tests were conducted by use of variable voltage regulated power supplies, an 
electrostatic microammeter, a multirange milliammeter, and a digital voltmeter. 
Automatic digital print out or paper punch equipment was  not employed because 
of the short time period allotted to circuit design. This would have required 
the design of a system fo r  digitalizing current in the low nano-ampere range 
and the use of memory storage elements to remember the firing voltage and 
current after the SCR had fired and thereby lost the desired information. 
MOTOR DRIVE BRIDGE 
The optical axis of observation of the solar observatory is oriented by ro- 
tating inertia wheels on three separate mutually perpendicular axes. The de- 
s i red orientation is attained and retained by controlling the rate of rotation of 
these wheels. Each of the three motors used to  drive the inertia wheels is 
controlled in a circuit similar to that of Figure 1. A bridge circuit is employed 
to enable the dc motor to be driven in either the clockwise o r  counterclockwise 
1 
direction by a single power source. All transistors and both silicon controlled 
rectifiers a r e  operated in the off condition except when an input signal is present 
at one of the inputs. 
To obtain clockwise rotation, a se r i e s  of square pulses are injected at input 
C (clockwise). These pulses are negative with respect to the positive side of the 
28 volt power source. Each of these pulses turn transistor Q1 on which turns 
Q2, Q3 and SCR 1 on. Thus current flows from the positive side of the power 
source through SCR 1, M1, Q3, and R12. This current of approximately 10 
amperes causes the motor M 1  to  rotate its inertia wheel in the clockwise direc- 
tion. At the end of the pulse transistors,  Q1, Q2, and Q3 go off. As Q3 goes off 
the anode current through SCR 1 is reduced below the holding current and causes 
this SCR to go off. 
Counterclockwise rotation is obtained by introducing a similar train of 
pulses at input CC. Each of these pulses turn on Q6, Q5, Q4, and SCR 2. Thus 
current flows through SCR 2, M1, Q4, and R12. This current now flows through 
the motor in the opposite direction to that f o r  the clockwise case thus driving 
the inertia wheel in the counterclockwise direction. The transistors and SCR's 
turn off in the same manner as for  the clockwise portion of the circuit. A turn 
on pulse should not be introduced at  both inputs simultaneously because a very 
high current would surge through SCR 1 and Q4 and also through SCR 2 and Q3. 
Such a current would destroy the usefulness of several components in a very 
short period of time. This also reveals that it is imperative that the SCR's go 
off a t  the end of each pulse. Fo r  example, if SCR 1 does not go off when the 
input signal is removed, then when Q4 is turned on by a counterclockwise input 
a destructive current is allowed to surge through SCR 1 and Q4. 
When input pulses a r e  introduced at one of the inputs, they appear at a 
constant rate of 960 pulses per  second. The rate  of rotation of the motor or 
inertia wheel is controlled by pulse width modulation (PWM). That is, the motor 
speed is controlled by controlling the length of time current is allowed to flow 
through the motor each period. The width of the pulse can be modulated from 
approximately 2 to 98%. 
TEST CIRCUIT 
The circuit of Figure 2 was used to simulate the circuit of Figure 1 during 
irradiation of the SCR's. Resistor R5 is used for  the load in the bridge circuit 
instead of a motor and consists of a 5 ohm, 10 ampere potentiometer. The nega- 
tive Supply biases the transistors and SCR's off. A positive pulse is introduced 
at input C which turns Q1, Q2, Q3 and SCR 1 on. Thus current flows through 
2 
SCR 1, R5 and Q3. Resistor R5 is adjusted to give a current of 10 amperes. At 
the end of this square pulse Q1, Q2, and Q3 go off. SCR 1 also turns off because 
Q3 reduces the anode current below the holding current. After a short dwell 
time, a positive pulse i s  presented at input CC which causes Q6, Q5, Q4, and 
SCR 2 to  turn on. This allows current to  flow through resistor R5 in the opposite 
direction to that for a positive pulse at input C. At the end of the pulse the SCR 
and transistors go off as before. After another similar dwell time another turn 
on signal appears at input C. This cycle is repeated at a rate of approximately 
lKH,. 
This method of testing with the circuit of Figure 2 differs from operation of 
the actual circuit of Figure 1 in two respects. The square input pulse appears 
alternately at the two inputs whereas in the satellite a long train of pulses appear 
at one input a t  a time. Also the width of the input pulses are held constant. The 
reason for these deviations is to maintain approximately equal heating and aging 
of the two irradiated SCR's. 
PARAMETER TESTING 
Periodically during the radiation study the forward blocking current (IFB), 
reverse blocking current (IRB), dc gate trigger voltage (VGT), dc gate trigger 
current (IGT), forward on voltage (VF), and holding current (IH) were  measured 
f o r  each SCR. The forward blocking current is the anode current with the posi- 
tive side of a 28 volt supply connected to the anode, and the negative to the 
cathode with the gate open. Similarily the reverse blocking current is the anode 
current with the negative side of the 28 volt supply connected to the anode and 
the positive to the cathode with open gate. The dc gate trigger voltage and dc 
gate trigger current are the dc voltage and current required at the gate to turn 
the SCR on with a 28 volt supply and a load (approximately 2.7 ohms) to give an 
anode current of 10 amperes. The forward on-voltage is the anode to cathode 
voltage while conducting a current of 10 amperes with open gate. The holding 
current is the minimum forward anode current required to hold the SCR in the 
"ont' state with an open gate. 
TEST PROCEDURE 
The SCR's and temperature sensors were mounted on a small aluminum 
heat sink. Lead wires ,  fifteen feet long, were connected to the assembly for 
remote operation, testing and temperature monitoring. The pre-radiation 
measurements were made of the six aforementioned SCR parameters. The 
assembly was then placed in the Cobalt-60 radiation field while being operated 
3 
as described for  the circuit of Figure 2 and monitored with meters and an oscil- 
loscope. After some period of time in the radiation chamber the six parameters 
were measured again. The devices were removed from the radiation field and 
tested once again. Then the SCR assembly was placed in the Cobalt 60 field for 
another period of operation and testing. This cycle was repeated until a total 
dose of approximately 6.6 x 10 rads were received. 6 
RADIATION SOURCE 
Al l  radiation tests were conducted by use of a Cobalt 60 gamma cell. This 
source emits two gamma rays (1.1733 and 1.3 328 MEV) at approximately equal 
rates. The dose rate in the test chamber is 2 x 106 rads per hour. Reference 
(a) indicates that one rad of these gamma rays is approximately equivalent to 
3 x lo7  electrons. Thus the dose rate in the test chamber is approximately 
equivalent to 6 x 1013 electrons per hour. 
DISCUSSION OF TEST RESULTS 
The test results a r e  shown graphically in Figures 3 through 9. More un- 
published test findings are available from the extensive study of SCR's in radia- 
tion environments mentioned in the introduction. Frequent reference is made 
to this more extensive study to reveal the maximum and minimum parameter 
variation for a large number of SCR's. 
I 
The pictures of the oscilloscope traces shown in Figure 3 were taken during 
operation of the SCR's in the radiation field, The dose rate is 2 x l o 6  rads  per 
hour and the total dose received is recorded for  each picture. The curves of 
this figure reveal details of the circuits (Figure 2) operation. When the clock- 
wise input signal (trace VIN at Q1) is received Q3 and SCR 1 are turned on. The 
collector voltage of Q3 (trace VCE Q3) rapidly reaches its saturation voltage. 
At the same time the voltage across Q4 (trace VCE Q4) goes up to the supply 
voltage minus the saturation voltage of SCR 1. The load current (trace ILOAD) 
gradually climbs to its preset peak of 10amperes. A t  the end of the input pulse 
(trace Vm at Q1) Q3 goes off. Note the rapid fall of load current. This sudden 
change in current through the 15 foot leadwires and load resistor gives an  in- 
ductive kick (trace VCE Q3 and trace VCE Q4). Immediately after this kick (at 
100 ps)  the two collector voltages begin to settle at a value equal to the supply 
voltage minus the anode to cathode drop of SCR 1. At this instant SCR 1 turns 
off and both collector voltages rapidly attaina voltage determined by the leakage 
resistance of Q3, Q4, SCR 1, and SCR 2. During the time when these components 
a r e  off, the leakage resistance of the SCR's is approximately a thousand times 
that of the reverse biased transistors Q3 and Q4. 
4 
The cathode to gate voltage required to turn the SCR on with a 28 volt anode 
supply and a 2.7 ohm load is shown in Figure 4. These curves reveal a pronounced 
increase with total dose of radiation in the gate voltage required to f i re  the SCR. 
Tests on a large number of SCR's reveal no change for some and an increase of 
as much as 3.5 percent per  megarad for others. These curves reveal a marked 
decrease in the gate firing voltage upon entering the radiation field and a corre- 
sponding increase upon being removed from the field. Similar tests with several 
SCR's reveal that some do not change whereas others change as much as 5 per- 
cent f o r  a field of 2 x 106 rads per hour. 
The g a k  current required to turn the SCR on with a 28 volt anode supply and 
a 2.7 ohm load is shown in Figure 5. These curves reveal the gate triggering cur- 
rent to increase with total dose of radiation received. Similar tests on several 
SCR's reveal that some do not change in this respect whereas others increase as 
much as 3.5 percent per megarad. A change in this firing current is sometimes 
observed upon entering or  leaving the radiation field however the change is usually 
very small. 
The anode to cathode voltage with an anode current of 10 amperes is shown 
in Figure 6 .  The "on" voltage increases approximately linearly with total dose. 
The increase in this voltage upon removing it from the radiation field is small 
and is frequently not detected. 
The minimum value of forward anode current required to maintain the de- 
vice in the r'onl' state is shown in Figure 7. This holding current increases with 
total dose and approximates a straight line. Some SCR's reveal no change in 
this parameter whereas others increase as much as 5.5 percent per megarad. 
The change upon removing the devices from the radiation field is small and 
frequently indistinguishable. 
The forward blocking current is shown in Figure 8. This indicates a large 
increase in current upon being placed in the radiation field and a corresponding 
decrease upon being removed from the field. Auxiliary studies reveal this change 
t o  be related to the dose-rate as well  as the total dose. The current change while 
in the radiation field is indictive of the findings of auxiliary studies. That is, for  
some SCR's, this current does not change, for some it changes irregularly and 
rapidly, and for some it depends upon the manner driven electrically while in 
.the radiation field. Further studies are planned to determine the effect of bias, 
repetition rate and anode current upon this and other SCR parameters. The 
SCR's with the high rate of change in this current are of concern because it 
would become impossible to hold them off once they are turned off. The magni- 
tude of blocking current at which this failure occurs depends upon the circuit 
design, magnitude of the supply voltage, and gate trigger current. 
5 
The reverse blocking current is shown in Figure 9.  These curves reveal a 
large increase in the current upon being placed in the radiation field and a de- 
crease upon being removed from the field. Auxiliary studies point toward a 
relationship between this change and the dose rate as well as the total accumu- 
lated dose. 
CONCLUSIONS 
The observed changes in SCR parameters due to gamma radiation of the 
total accumulated dose and dose rate employed in these tests are considered 
to be within design capability. That is, SCR circuits can be designed to reliably 
perform the desired functions before and after receiving 6.6 x l o 6  rads of 1 to 2 
MEV gamma rays while in a 2 x lo6  rad per hour field. The maximum and 
minimum change in parameters are given to enable the designer to predict 
parameter values after a perscribed radiation history. Total accumulated 
doses greater than 6.6 x l o 6  rads may reveal turn off o r  more properly hold 
off problems. That is, the forward leakage of some units may become so high 
that the SCR will not remain off with the full supply voltage across  it. Further 
studies are  being conducted of this problem and the details should be available 
soon. 
REFE RENCE 
(a) Correlation of Proton to Neutron and Electron to Photon Radiation 
Damage in Transistors and Diodes by G. L. Keister, Document No. 
D2-90407, The Boeing Co., Seattle, Washington. 
6 
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Figure 3 - Oscillograms of Test Circuit During Irradiation Studies 
9 
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0 
16 HOURS 
AFTER EXPOSURE 
i lATlON FIELD 
7 
16 HOURS 
AFTER EXPOSURE 
1 2 3 4 5 6 6.66 
TOTAL GAMMA DOSE IN MEGARADS 
Figure 4 - Gate Tr igger Voltage v s  Tota l  Gamma Dose 
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TOTAL GAMMA DOSE IN MEGARADS 
Figure 5 - Gate Trigger Current vs Total Gamma Dose 
11 
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5 6 
0 
16 HOURS 
AFTER EXPOSURE 
16 HOURS 
AFTER EXPOSURI 
16 
Figure  6 - Forward On Voltage v s  Tota l  Gamma Dose 
1 2  
20 
18 
16 
OUT OF RADIATION 
0 
16 HOURS 
AFTER EXPOSURE 
Ei 
16 HOURS 
AFTER EXPOSURE 
2 3 4 5 6 6.66 c 1 
TOTAL GAMMA DOSE IN MEGARADS 
Figure 7 - Holding Current vs Tota l  Gamma Dose 
13 
80 
70 
60 
50 
40 
30 
20 
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Figure 8 - Forward Blocking Current v s  Total  Gamma Dose 
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TOTAL GAMMA DOSE IN MEGARADS 
16 HOURS 
AFTER EXPOSURE 
0 
El 
16 HOURS 
AFTER EXPOSURE 
6 
Figure 9 - Reverse Blocking Current vs Total Gamma Dose 
15 


Effects of gamma radiation on SCR'S used in switching mode and in application to attitude control motor drive bridge

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