The principle features and special testing of a high voltage high power transformer designed and developed for space application are described. The transformer is operated in a series resonant inverter supplying beam power to a 30 cm mercury ion thruster. Electrical requirements include operation of 2.3 kW continuous power output, primary currents to 35 amps rms, and frequencies up to 20 kHz. High efficiency was obtained through detailed considerations of the tradeoffs available in core materials, wire selection, coil configurations and thermal control. A number of novel heat removal techniques are discussed which control the winding temperature using only the available conductive cooling

Hansen, I.

English

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NASA Technical Memorandum 79138
(NASA-TM-79138) DESCRIPTION OF A 2.3 kW	 N79-23348
y	 POWER TRANSFUMEE FOE SPACE APPIICATICNS
(NASA) 12 p HC A02/MF A01
	 CSCL 09C
•.	 Unclas
G3/33 23984
DESCRIPTION OF A 2.3 kW POWER
TRANSFORMER FOR SPACE
APPLICATIONS
i
Irving Hansen
Lewis Research Center	 x i
Cleveland, Ohio
a
s
Prepared for the	 h^	 //v	 S
Aerospace High Voltage Workshop d
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cosponsored by the International Electronics 	 r -foF P"/^/`F
Packaging Society, Aerospace Electronic	
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Systems Society, and Institute of Electrical 	 6	 1+
and Electronics Engineers
Anaheim,' California, ,February 26-27, 1979
1=
tDESCRIPTION OF A 2.3 kW POWER TRANSFORMER
FOR SPACE APPLICATIONS
by Irving Hansen
NASA Lewis Research Center
Cleveland, Ohio 44135
INTRODUCTION
Solar Electric Propulsion (SEP) using mercury bombardment ion thrusters has
been in a research and development stage for well over a decade. The emphasis
has been on minimizing the propulsion system mass while maintaining or improving
the overall system efficiency thus enabling larger payloads. A major element of
the thrust system is the power conditioning which converts the solar array power to
match the load requirements of the 30 cm ion thrustor.
The emphasis in the development of the power: processor (refs. 1 to 3) has been
directed towards reliability and higher efficiency with reduced overall weight. The 	 j
W baseline power processor desigyi chosen for this application was a 2.3 kW series
resonant converter which requires a high voltage transformer. This paper deals
with the optimization of this major component to achieve high performance.
The emphasis on reliability is easily understood. An important part of compo-
nent reliability resides in the circuit design philosophy. SCR series resonant cir-
cuitry places major emphasis on minimizing electrical stress not only during normal
operation but during transient conditions such as startup, shutdown and load variations
from open circuit to arc faults.
The emphasis on reducing weight is obvious. Less obvious is the manner in
which improved efficiency will be reflected by corresponding weight reduction in
solar arrays and thermal control systems. Thermal control techniques used with
space hardware are substantially different than those encountered in terrestrial -
equipment. As convection does not occur in a vacuum, cooling is predominantly by
conduction. If the conduction path is interrupted by an interface separation, however
small, the thermal impedance is increased resulting; in higher component temper-
atures. However, the same item with the same separation may not display an
appreciable temperature rise when tested in air.
i
r 
2
DESIGN REQUIREMENTS
h	 Based on design tradeoffs, a series resonant inverter was selected as the main
!	 power circuit (screen supply) of the power processor (refs. 1 and 2). The circuit is
shown in figure 1. The resonant tank consisting of LA, CA, and CB, with control
by SCR1, energizes the load through the impedance matching transformer T during	 a
one half of the cycle and LB, CB, and CA with control by SCR2 operating T during
the alternate half cycle. The output is filterec' by C4 after full wave bridge rectifi-
cation by CR3.	 m
The screen supply power transformer requirements are summarized in figure 2.
A detailed description of the design follows, along with a rationale of component
selection.
RESULTS AND DISCUSSION
Configuration and Core Selection
The core selected was a shell type cut I, Cr t core or t I loop" design using two
Supermalloy cores and a single coil (see fig. 3). The use of cut cores permits the
coils to be made separately allowing the controlled placement of wire and insulation.
Additionally, the circuit requires an air gap to control the balance between half
cycles of operation. A controlled gap is easily implemented with a cut core.
The Supermalloy cores were fabricated of half mil tape to achieve lower losses.
Although the industry standard allows a 50% stacking factor for half mil material,
the vendor, Arnold Engineering, was able to provide cores with a stacking factor in
excess of 80% and losses of less than 6 W/lb measured at 5 kG and 10 kHz.
The use of sheet laminated cores was discarded for several reasons. Lamina-
tions are not suitable in this application because of the limited variations available,
the difficulty in removing the heat from each lamination, and the problems involved
in handling large thin laminations of strain sensitive material. Ferrite materials
were also discarded for this application due to their thermal constraints. Ferrite
materials have a much lower thermal conductivity. The cross section needed would
have to be larger than the Supermalloy to operate it within its lower limit of flux
density. When operating thick sections in vacuum, its thermal conductivity is so
low that the interior reaches Curie saturation. Additionally, the coefficient of
thermal expansion of ferrite is different enough from potting materials that local
forces experienced during thermal cycling are sufficient to cause core fracture of
the material.
3Wire
Litz (Litzuindraht) wire was selected for the primary and power secondary.
This wire consists of many strands of individually insulated fine wire with the entire
bundle twisted to insure equal strand length and uniformly distributing the position
of each strand throughout the bundle.
One advantage of this wire is that at high frequencies there is reduced loss of
utilization due to skin effect. Another advantage is reduction of circulating currents
within the wire as the twisted strands are equally affected in a nonuniform field. The
disadvantage of this wire lies in the poor volumetric utilization and problems with
stripping. The latter can be overcome by the use of Nylese © insulation which per-
mits solder pot tinning when done with care. There are several types of solderable
insulation available, of these Nylese© has the highest temperature rating, 130 0 C.
Cooling
The cooling design which includes several novel techniques, begins with mini-
mizing losses. The low loss cores and low loss windings, together with all other
losses, dissipate only 28 W when handling the full load of more than 3000 VA.
Frame
The coil assembly is placed hi a one piece aluminum frame shown in figure 4.
This frame was computer modeled to provide a performance versus weight tradeoff.
The one piece construction minimizes interfaces to improve heat flow, The volume
between the square core and the round coil form is filled with four berylium oxide
sections which provide high thermal conductivity heat paths to the frame.
Coil
The coil was wound on a tubular form to preserve physical and electrical sym-
metry of all windings. Symmetry reduces mechanical stress concentrations and
helps prevent tiny hairline fissures from forming during thermal cycling, thereby
reducing the corona inception voltage.
Phelps Dodge Copper Products Corporation Registered Trademark.
4The winding order used was .
 primary, electrostatic shield, power secondary
and auxiliary secondary. This sequence provides the lowest 1211 losses and best
heat removal.
An electrostatic shield is necessary to protect the primary circuit semiconductors
from voltage transients reflected when the secondary operates into plasma are faults.
Without a shield, energy is stored in the dielectric between the primary and the high
voltage secondary. In the event of a secondary short to ground the stored energy
would then appear between the primary and ground elevating the potential of the pri-
mary circuit. The purpose of the shield is to provide a ground path for the stored
energy thereby removing this source of primary voltage stress.
Leadsinks
The primary leads handle over 33 arms and are sized accordingly. The physical
size of the Litz bundle is about 0. 2 in. diameter. The primary ends are used as
heat sinks insulated with berylium oxide insulators as shown in figure 4. These
r t leadsinks' I provide a shunt heat path for the primary generated losses directly
into the baseplate, additionally reducing the temperature of the secondary. This
heat sinking method was found to be so effective that the unused center tap of a two
layer primary was specifically brought out to take further advantage of the technique
(see fig. 5).
Core Cooling
The core is built of many layers of halt mil thick strip having a magnesium oxide
coating on both sides to provide electrical insulation. This oxide layer acts as a
thermal barrier so that lieat sinking the broad strip face is not effective. Instead it
is necessary to sink the edge of each tape layer. ' A review of the assembly shows
the effective cooling path provided for the core. To'maIntaui this interface between
tape layers and the base, there are G clamps distributed around tlie core which over-
come any inclination for separation resulting from thermal exercising.
ELECTROSTATIC SHIELD (ESS) COOLING
.Another cooling technique involved thermal sinking of the electrostatic shield..
Since the shield was necessary for electrical purposes and since it is in the region
of the highest temperature rise, it was exploited as a thermal heat path to the frame
5by extending it through the coil for thermal connection to the frame via brass hard-
ware ► risers' I as shown in figures 3 to 5.
ANALYTICAL MODELING
The above features and the basic thermal design were computer modeled along
with other proposed cooling techniques. This analysis allowed their effectiveness
to be assessed and optimized without costly iterative hardware builds.
HIGH VOLTAGE DESIGN
The high voltage design was based on experience gained from the Communica-
tions Technology Satellite Transmitter Experiment Program (ref. 4) which oper-
ated a 500 W high voltage transformer at voltages up to 11.3 kV nominal.. Solid
dielectric stresses were held to less than 60 V/mil, and surface dielectric stress
held to less than 5 V/mil.. The impregnation technique for the potting material,
an OSHA approved system of a Urethane polymer, included material degassing,
hot pour into a heated degassed assembly and alternate schedules of pressure and
vacuum, The corona requirement of less than 5 picocoulombs corona initiation for
1. 3 times working voltage assures that corona does not exist in operation. This
eliminates one of the most important life related wearout mechanisms enabling
long mission life.
THERMAL VACUUM TESTING
One of the designer s greatest concerns is to objectively search for problems
and identify design flaws or potential wearout mechanisms. It was for this reason
that a vacuum thermal profile test was devised, Its intent is to check the analytical.
thermal analysis, and determine the maximum winding temperature rise under
actual thermal vacuum conditions, thus demonstrating the thermal and corona sta-
bility of the design as a function of thermal exercising. This test provided both a
powerful engineering proof test and a manufacturing screening test. The test con-
sisted of the following sequential events:
Corona test to inception
Tull load operation in vacuum by actual circuit on thermal sink elevated to
maximum operating temperature.
k
GContinuous recording of 12 thermocouples providing a thermal profile. Please
refer to figures G and 7.
After thermal stabilization, the winding resistances are measured using a
technique which determines resistance at time of power shut-off. This estab-
lishes a baseline for later comparison.
Magnetic assembly removed from vacuum chamber and subjected to two days of
thermal cycling from -50 0
 C to +100 0
 C in 10 cycles of about 4. 5 hr each.
Assembly replaced in vacuum chamber with initial test conditions and thermo-
couple measurements.
Repeat corona test to inception.
Compare I  before'' and I  after'' readings of
Thermocouple measurements
Winding temperature rise
Coron test values to inception
This test procedure served to detect deficiencies and check corrective modifications.
It also served to detect manufacturing imperfections such as poor soldering on the
ESS or imperfect interfaces between core and base or between BeO and core. The
sensitivity of this test was both surprising and gratifying because it.'also provided
confidence in units exhibiting no changes.
SPECIAL TESTING
There are other design proof and screening tests worth mentioning. Each core
was tested for losses at both the factoryand at TRW. There was a special calorim-
eter test to check calculated losses for the whole assembly which corroborated the
28 W loss figure. Finally a number of design material screening tests were per-
formed on a coil basis to help in the selection of the final insulation system. These
tests were effective enough to detect the difference in Litz twisting that occurred
A
' fbetween two if erent procurements of supposedly identical wire.
CONCLUSIONS
A high frequency high power low specific weight (0. 57 kg/kW) transformer was
developed for space use. The magnetic is capable of handling '3 kVA continuously
4F
7
with primary currents to 35 A at frequencies up to 20 kHz. High efficiency greater
than 99% was obtained through careful detailed design. A number of unique heat
removal techniques were developed which served to limit the maximum winding
temperature rise to 350
 C in a simulated space environment. An analytical thermal
model enabled frame weight and thermal tradeoffs to be made without the usual
iterations and fabrications.
A special thermal vacuum test was developed to establish a thermal profile and
maximum internal temperature. After thermal cycling of the transformer any
deviation from the established profile flags a potential problem even though the
winding temperature has not changed. This highly sensitive test has merit both as
a design proof test and as a screening test due to its ability to detect minor fabri-
cation defects.
ACKNOWLEDGEMENTS
The author wishes to thank Mr. Morris Chester of TRW Corp. not only for his
efforts in producing the transfdrmer described, but also for his assistance in the
n' nt of t1his repo -t L.
The author also wishes to thank Mr. Malcom Sheppard of Arnold Engineering
for his efforts in producing the cores used in this program.
REFERENCES
1. J. J. Biess, A. D. Schoenfeld, and E. Cohen, Power Processor for a 20 CM
Ion, Thruster, " TRW-20384-6002-RU-00, TRW Defense and Space Systems
Group, Redundo Beach, Calif. , October 1973. (NASA CR-121160)
2. J. J. Biess, L. Y. Inouye, A. D. Schoenfeld, and J. H. Shank, 1tThryistor
Power Processor For The 30 cm Mercury Electric Propulsion Engine,'
AIAA Paper 75-433, Mara 1975.
3. J. J. Biess and L. Y. Inouye, t t Electric Prototype Power Processor for a 30 cm
Ion Thruster, tf TRW- 28014-6001-TU--00, TRW Defense and Space Systems
Group, Redundo Beach, Calif., March 1977. (NASA CR-135287)
4. Edward F. Miller, J. L. Fiala, and I. G. Hansen, Performance Character-
istics of the 12 GHz, 200 Watt Transmitter Experiment Package for CTS,
NASA Lewis Research Center, Cleveland, Ohio. NASA TM X-71773.
I=	 ,
rTOP SECTOR BeOi
\ .-COIL FORM
1ARY LEADS
Ll	 L2
i
SCR1
	
CRl
CA	 CR3
C1	 C2 LA
	 T
^ Il
' L-4 i6 +-o
C4
	
uL6	 '
R1
CR2	 CR2	
CB G
Figure 1. - Series resonant inverter with auxiliary diode configuration.
1
	
j	 POWER SECONDARY
	
I	 1100 V AT 2.7 ARMS
PRIMARY	 I 1060 V WKG
90 VAT 33.7 ARMS
650 V WKG	 I
550 V AT 0.026 ARMS
1520 V WKG
ELECTROSTATIC SHIELD IESS ► G_j
PRIMARY SOURCE:
SQUARE WAVE VOLTAGE, APPROXIMATE SINE
WAVE CURRENT, 20 kHz MAX
REPETITION RATE VARIABLE WITH INPUT VOLTAGE & COMB.
TEMP:
0 TO 500 C BASEPLATE OPERATE, -55 TO 100 0 C NONOPERATE
MAX AVG WINDING RISE 350
 C WHEN FULLY LOADED MOUNTED ON 50P C BASEPLATE
EFF
APPROXIMATELY 99 percent TRADED OFF AGAINST WEIGHT
Figure 2. - Screen supply power transformer requirements.
GAP 4 PLACES
"C" CORE-J'
	
v"C" CORE
LBeO 2 PLACES
SECONDARY	 BOTTOM BrO SECTOR-J
POWER LEADS-J
Figure 3. — Shell type construction.
Figure 4. - Screen transformer,
Figure 5. - Screen trai sformer, rear view.
" te r_	
-^C..e^^	 1	 ^i a^	 a>^'w
Figure 6. - Thermocouple instrumentation for thermo profile'est.
— — — — —
	 BLFORE TEMP. CYCLE
---	 AFTER 18 CYCLES
— — —	 AFTER REPAIR_
— -- —	 REPAIR +5 CYCLES
-----	 ESS OPEN THERMAL)
1ST PRODUCTION
SECONDARY #2
SECONDARY #1
PRIMARY /
BOBBIN
SHIELD
B. C. 0. TOP i 
CORE (REAR)
CORE (FRONT)
BRACKET
is
BRACKET
LEAD CU II I	 I
LEAD B. E. 0.
BRACKET (LOW (^i
PLATE
BASE
100 120	 140	 160	 180	 200	 220	 240	 260	 ?80
' TEMPERATURE, of
I
Figure 7. - Transformer thermal profile.
1


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