A modular dc power supply power sharing technique was developed for the Apollo telescope mount electrical power sytem on Skylab. The advantages and disadvantages of various techniques used are reviewed and compared. The new technique design is discussed, and results of its implementation in the power system are reviewed

Kapustka, R.

Lanier, R., Jr.

English

NASA Technical Reports Server

NASA Technical Memorandum 78120
Power Supply Sharing in the Apollo
Telescope Mount Electrical Power System
Roy Lanier, Jr., and Robert Kapustka
SEPTEMBER 1977
NASA
https://ntrs.nasa.gov/search.jsp?R=19770025647 2020-03-22T08:15:59+00:00Z
NASA Technical Memorandum 78120
Power Supply Sharing in the Apollo
Telescope Mount Electrical Power System
Roy Lanier, Jr., and Robert Kapustka
George C. Marshall Space Flight Center
Marshall Space Flight Center, Alabama
NASA
National Aeronautics
and Space Administration
Scientific and Technical
Information Office
1977
TABLE OF CONTENTS
Page
INTRODUCTION 1
POWER SHARING DEVELOPMENT APPROACH 1
POWER SHARING IMPLEMENTATION 4
CONCLUSION 4
111
LI STOP ILLUSTRATIONS
Figure Title Page
1. ATM electrical power system 5
2. ATM EPS characteristics without power sharing 5
3. Typical power sharing methods 6
4. Simplified feedback diagram for remote sensing and pro-
grammed impedance 6
5. Reduction of feedback equation 7
6. ATM bus voltage regulation 7
7. ATM EPS characteristics with power sharing 8
IV
TECHNICAL MEMORANDUM 78120
POWER SUPPLY SHARING IN THE APOLLO TELESCOPE
MOUNT ELECTRICAL POWER SYSTEM
INTRODUCTION
This report addresses the technique used in solution of a problem that
arose during the design of the Apollo Telescope Mount (ATM) electrical power
system for the Skylab Mission. The problem was related to parallel operation
of the modular power supplies, called charger-battery-regulator modules
(CBRM's), used on the ATM (Fig. 1). The problem, a common one in
paralleling dc power modules, was unequal load or power sharing. The result
of the unequal load on the power supplies was unequal component stresses and,
with limited power modular sources as the ATM had, loss of system output
capability. Early in the program a study showed that a potential loss of 25 per-
cent of the ATM power system capability existed even though care had been
taken to provide precise output voltage regulation and programmed regulator
impedance. Figure 2 depicts the magnitude of this problem on the ATM.
Obviously, this was not acceptable and a solution was sought.
POWER SHARING DEVELOPMENT APPROACH
Existing methods widely used to obtain power sharing between power mod-
ules are shown in Figure 3. The advantages and disadvantages are relative to
an ATM type application. Note that none of these methods provided a satisfac-
tory solution for the ATM. Since none of the typical methods provided a satis-
factory solution, a different technique had to be developed.
The goal for the new system was to reduce the maximum bus power loss
from 25 percent to 3 percent or approximately 120 W. Achieving this goal
required satisfying two basic considerations: (1) utilize maximum possible
solar cell panel power and ( 2) discharge the batteries to equal depths of dis-
charge to assure they were not depleted during orbital "night" and to maximize
battery life. Both of these considerations were satisfied if all 18 CBRM regu-
lators always delivered the same amount of power. Therefore, a system which
provided the advantages of the master-slave system without its disadvantage of
a single system critical CBRM was sought.
An analysis of a modular power system shows that the reasons for the
variation in output power are twofold: (1) the output voltage levels of the mod-
ules are different and (2) the output impedances of the modules are different.
The mismatch problem may be solved by minimizing or eliminating these vari-
ables .
->
A simplified schematic of the technique chosen to provide the necessary
sharing is shown in Figure 4. In this closed-loop system, the bus voltage was
sensed and compared to a reference. The resulting error signal was then used
to determine the regulator output current. The system worked as follows:
The heart of the system was a master reference containing a reference voltage,
an amplifier, and an isolation network to prevent external faults from causing
a circuit failure. The circuit sensed both ATM main bus voltages and selected
the lower of the two voltages. The sensed bus voltage, V , was compared withB
a master reference, V , , and the difference or error between the two was
HI
operated on and distributed to each CBRM as a signal that was proportional to
the desired CBRM regulator current required for the bus voltage. Inside the
CBRM a signal, I , equivalent to the actual regulator output current was com-
pared with the desired current signal, I , from the master reference. The
error between these two current signals was operated on to provide a correction
voltage signal to be added to the internal CBRM regulator reference voltage,
VT,., to give the regulator output voltage, V , necessary to supply the requiredR2 o
current. When all CBRM's had the same output current, the output powers were
different by only the difference in distribution line losses which were small on
the ATM. Proper selection of system gains minimized this difference. The
equation describing the loop is:
(1)
where
V,., = lowest ATM bus voltage
V = master reference voltageRl
V = CBRM reference voltagei\Z
A = master reference gain that determines the programmed
impedance
A = CBRM gain with which the regulator responds to the pro-
grammed current error
I_ = signal equivalent to CBRM output currentK
I = signal equivalent to the desired output current
V = CBRM regulator output voltage
Z = impedance of distribution line.
_LJ
If the circuit gains, A and A , are selected properly, the equation may be
1 £i
approximated as shown in Figure 5. The original equation may be simplified
as follows:
(2)
'
Note that in equation ( 2) the reference voltage, V_,. , is in the master reference
Kl
and is common to all regulators. Therefore, variation of this parameter only
results in similar variations in bus voltage and do not affect the power sharing
among the CBRM's. The dependence of regulator output current on bus voltage
generates a resistance characteristic described by the term (A + Z /! + A A )
Z -L I Z
If the gains are chosen properly, variations of the line impedances, Z , haveLJ
negligible effect on the net impedance. On the ATM,maximum variations of line
impedance resulted in less than 1 percent change in the net impedance. The
effects of regulator voltage reference variations have been reduced by the fac-
tor (I/A A ) . Again, if the gains were chosen properly, variations of the1 ^
regulator voltage references, V , had negligible effect on the output voltage.
K2
On the ATM,,maximum variations of CBRM reference voltage resulted in less
than 0.1 percent change in output voltages. The result was negligible effect of
individual CBRM parameters on the power sharing among the CBRM's.
POWER SHARING IMPLEMENTATION
A circuit was designed to implement the technique discussed. The
sensing circuit, reference supply, comparator, and amplifier were designed
with redundancy so that no single component failure would cause a loss of out-
put. This redundant circuitry was packaged on one 7. 8 by 11.4 cm printed cir-
cuit board. The isolation network for the 18 outputs were packaged on a second
board. Since the redundancy technique used precluded the elimination of failures
that would cause the circuit output to demand a current higher than required,
(I signal high) a second redundant power sharing unit was also used on the ATM.
The relays for switching the desired unit were located on the board with the
isolation network.
The resulting circuit, except for the relays, lends itself well to micro-
circuit techniques. The entire circuit, including the isolation network compo-
nents, could be packaged in one microcircuit, should the need arise.
CONCLUSION
The goal of introducing a power sharing scheme into the ATM electrical
power system was to increase the system power capability as stated. An addi-
tional advantage of improved bus regulation also resulted. The improved bus
regulation is shown in Figure 6. Theoretically, the regulation may be further im-
proved by increasing the gain A as noted in Figure 5. However, the practical
aspects of line impedance, regulator output voltage, and system stability led to
the particular voltage characteristic shown for the ATM.
The effect of incorporating the power sharing technique discussed on the
ATM power system output is readily apparent by comparing Figure 7 and Fig-
ure 2. Note that the dispersion between the highest output CBRM regulator and
an average output regulator is now less than 5 W. Before adding the power
sharing circuitry this dispersion could be almost 60 W. The net system power
loss with sharing was less than iOO W compared to approximately 1000 W before
this circuit was added. The operation of the system with the power sharing cir-
cuit was demonstrated on the Skylab power breadboard and on the ATM. No
problems resulted from implementing the sharing technique, and the predicted
advantages were realized. Thus, all goals were reached, the development
successfully completed, and a new approach to power sharing between dc mod-
ules was demonstrated.
CHARGER/BATTER?/
REGULATOR MODULE
CBRM II OF IB)
Figure 1. ATM electrical power system.
320
280
240
200
160
120
80
40
0
1 HIGH CBRM
0.16 0 20 0 24 0 28 0.32 0.36
LOAD RESISTANCE (OHMS)
Figure 2. ATM EPS characteristics without power sharing.
METHOD ADVANTAGES DISADVANTAGES
EQUALIZE LINES > SIMPLE ADDED WEIGHT
ADDED POWER LOSS
INADEQUATE SHARING
POOR BUS REGULATION
PROGRAM REG
IMPEDANCE • EFFICIENT
INADEQUATE SHARING
POOR BUS REGULATION
MASTER-SLAVE
SYSTEM
• EFFICIENT
• GOOD BUS REGULATION
• GOOD POWER SHARING
MASTER MODULE IS
SINGLE POINT
FAILURE
CURRENT LIMITED
REGS
• EFFICIENT
• GOOD BUS REGULATION
• INADEQUATE SHARING
Figure 3. Typical power sharing methods.
REGULATOR (1 OF 18)
HA, A2) v" + (i + A,AJ) V«' " ( I !A,AJ) '«
Figure 4. Simplified feedback diagram for
remote sensing and programmed impedance.
(1)
MINIMIZE EFFECTS OF VARIATIONS IN UNE RESISTANCE (Rt) => A2 »Z v
MINIMIZE EFFECTS OF VARIATIONS IN A2 ITSEIF =>A2 > 1
PROGRAM IMPEDANCE FOR"HARD" BUS =>fc|»l
(i)THEREFORE: V, a -5-^  VM * V,, - ^ I,
Figure 5. Reduction of feedback equation
1«17UCBRM'S
160 16I7I8CBRM'S
40 BO 120 160 200 240 MO
BUS OUTPUT CURRENT (AMPS)
Figure 6. ATM bus voltage regulation.
HIGHEST CBKM
0 0.1S 0.22 0.26 0.30 0.34
LOAD tESISTANCE (OHMS)
Figure 7. ATM EPS characteristics with power sharing.
TECHNICAL. REPORT STANDARD TITLE PAGE.
1 REPORT NO.
NASA TM 78120
2. GOVERNMENT ACCESSION NO.
4 TITLE AND SUBTITUE
Power Supply Sharing in the Apollo Telescope
Mount Electrical Power System
7. AUTHOR(S)
Roy Lanier, Jr. , and Robert Kapustka
9. PERFORMING ORGANIZATION NAME AND AD
George C. Marshall Space Flighl
Marshall Space Flight Center, A
DRESS
t Center
labama 35812
12 SPONSORING AGENCY NAME AND ADDRESS
National Aeronautics and Space Administration
Washington, D.C. 20546
3 RECIPIENT'S CATALOG NO.
5. REPORT DATE
September 1977
6 PERFORMING O R G A N I Z A T I O N CODE
8 PERFORMING ORGANIZATION REPORT *
M-232
10. WORK U N I T NO.
1 1. CONTRACT OR GRANT NO.
13. TYPE OF REPORT ft PERIOD COVERED
Technical Memorandum
14. SPONSORING AGENCY CODE
IS. SUPPLEMENTARY NOTES
Prepared by Electronics and Control Laboratory, Science and Engineering
16. ABSTRACT
This report presents a new type of modular dc power supply power sharing technique
that was developed for the Apollo Telescope Mount (ATM) electrical power system on the
Skylab. The advantages and disadvantages of various techniques used in the past are reviewed
and compared to the new method. The new technique design is discussed, and results of its
implementation in the ATM power system are reviewed .
17. KEY WORDS
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18. D I S T R I B U T I O N STATEMENT
STAR Category 44
20. SECURITY CLASSIF. (of thi. page)
Unclassified
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Power supply sharing in the Apollo telescope mount electrical power system

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