NASA/TM--2002-211804 IECEC-2002-20032

Efficient Design in a DC to DC Converter Unit

Joel E. Bruemmer and Fitch R. Williams

The Boeing Company, Canoga Park, California

Gregory V. Schmitz

Glenn Research Center, Cleveland, Ohio

August 2002
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NASA/TM--2002-211804 IECEC-2002-20032

Efficient Design in a DC to DC Converter Unit

Joel E. Bruemmer and Fitch R. Williams

The Boeing Company, Canoga Park, California

Gregory V. Schmitz

Glenn Research Center, Cleveland, Ohio

Prepared for the

37th Intersociety Energy Conversion Engineering Conference

sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society

Washington, DC, July 28-August 2, 2002

National Aeronautics and

Space Administration

Glenn Research Center

August 2002
NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at _sa. _ov
IECEC 2002 20032

EFFICIENT DESIGN IN A DC TO DC CONVERTER UNIT

Joel E. Bruemmer and Fitch R. Williams

The Boeing Company

6633 Canoga Avenue

Canoga Park, CA 91303

Gregory V. Schmitz

National Aeronautics and Space Administration

Glenn Research Center

21000 Brookpark Road

Cleveland, OH 44135

ABSTRACT

Space Flight hardware requires high

power conversion efficiencies due to limited

power availability and weight penalties of cooling

systems. The International Space Station (ISS)

Electric Power System (EPS) DC- DC Converter

Unit (DDCU) power converter is no exception.

This paper explores the design methods and

tradeoffs that were utilized to accomplish high

efficiency in the DDCU. An isolating DC to DC

converter was selected for the ISS power

system because of requirements for separate

primary and secondary grounds and for a wellregulated secondary output voltage derived from

a widely varying input voltage. A flybackcurrent-fed push-pull topology, or improved

Weinberg circuit, was chosen for this converter

because of its potential for high efficiency and

reliability. To enhance efficiency, a nondissipative snubber circuit for the very-low-Rdson Field Effect Transistors (FET)s was utilized,

redistributing the energy that could be wasted

during the switching cycle of the power FETs. A

unique, low-impedance connection system was

utilized to improve contact resistance over a

bolted connection. For improved consistency in

performance and to lower internal wiring

inductance and losses, a planar bus system is

employed. All of these choices contributed to

the design of a 6.25 KW regulated dc to dc

converter that is 95% efficient.

The methodology used in the design of

this DC to DC Converter Unit may be directly

applicable to other systems that require a

conservative approach to efficient power

conversion and distribution.

INTRODUCTION

Power converters used for spacecraft

have more unique requirements than their

terrestrial counterparts. Due to physical

constraints, solar arrays can produce only a

limited quantity of power. Since this energy is

restricted, efficiency plays a very important part

in the choice of topology for the design of a

spacecraft power converter. In addition to limited

power availability, the heat generated by the

conversion process must be radiated into space.

The weight burden for these radiators may be

substantial. These were two of the many unique

problems facing the design team on the

International Space Station when developing the

DC to DC Converter Unit.

WHY A DDCU?

The International Space Station is

modularized. Each section of the station is

assembled and tested on earth and then

attached to the growing structure on orbit. It is

analogous to building a house a room at a time

with all the wiring, mechanics, and support

structures in place before adding it to the main

house. The DC distribution system for each of

these modules requires isolation so that

individual grounds will not interact with each

other. The solution to this electrical problem

was a power distribution system that would

isolate each of these grounds and also

NASA/TM_002-211804 l
distribute power from the arrays to each

interconnected module.

The Sequential Shunt Unit coarsely

regulates the voltage supplied by the arrays to a

preset value between 115 to 173 volts. The

many uses of this DC power supplied by the

arrays vary from simple lighting to sophisticated

S-band transmitters and receivers. Each of

these loads requires an input voltage that is free

from noise spikes that may be generated by the

primary switching and a tightly regulated input

which will simplify its internal power conversion

needs. Therefore, what was needed was a unit

that provided DC isolation and regulation.

WHAT IS A DDCU?

From a black box standpoint, the DDCU

must regulate the widely varying voltage from

the arrays (115 to 173 volts) to a constant 124.5

+ 1.5 volts. Loading may be very light (< 1 Amp),

very heavy (>50 Amps), and reach overload

conditions (= 80 Amps). The output must be

short circuit proof. A fault in one area of the

station must not render the whole power system

inoperative. The DDCU is required to

communicate via a MiI-Std-1553-type bus where

all the modules in the power system share their

status over telemetry channels. The input

current and voltage, and output current and

voltage are required to be internally monitored

and reported over this bus. In addition, the

output of the DDCU needs to turn off if the input

voltage exceeds the 173-volt maximum, if the

output current exceeds the 52 Amp maximum by

125% and 150%, or if the input current exceeds

a predefined safe limit. This Input Current

Shutdown is a safety measure should something

go wrong with the DDCU internally.

The DDCU is manufactured in three

versions: an external type (DDCU-E) used on

the Integrated Equipment Assemblies (lEA) for

distribution of power to each of the four main

structures; an internal type (DDCU-I), used

inside pressurized elements for distribution of

power to experiments and other internal loads; a

passively cooled type (DDCU-HP), used only on

the Z1 truss segment and is a heat-pipe-cooled

variation of the external type. The internal type

only, is capable of being paralleled with another

DDCU-I so that the power delivered can be

doubled while the input power is distributed

between the arrays (the input to each of the

paralleled DDCU-Is is from a different source).

Arrays

I m

DC I I DC to DC

Switching Unit _l Converter 17nit

I

Battery

Charge

Discharge

TTrip

Figure 1. Power System Block Diagram of

the Integrated Equipment Assembly (IEA)

CHOICE OF TOPOLOGY

Efficiency was the major driver for the

topology chosen. The primary function of

generating power is to deliver it to the loads. On

orbit, there is a limited amount of power

available from the arrays and batteries. Waste

heat is also a major consideration. Heat is

rejected through an ammonia-cooled baseplate

and radiator system facing deep space. An

estimated 5% efficiency decrease in the DDCU

would increase the cooling system weight by

100 lb. on one Integrated Equipment Assembly

(lEA).

For these reasons, a flyback-current-fed

push-pull (Weinberg) topology was chosen. The

advantages are high power conversion ratio

(efficiency), continuous conduction to the output

node, lack of flux imbalance in the power

transformer, and no possibility of damaging the

switching elements due to switching overlap.

One shortcoming of this topology is that

the power- switching elements must be rated at

a higher voltage than those of other circuit types.

The minimum voltage impressed on the

switching elements is twice the input voltage due

to the push-pull configuration of the power

transformer. Added to this is the voltage spike

generated each switching cycle by the flyback

inductor. Finally the combined leakage

inductance of both magnetic units provides

another voltage overshoot that further increases

the instantaneous voltage seen by the switching

elements.

NASA/TM_002-211804 2
The choice of power switching element

was very important to efficient design. Highcurrent bipolar devices require substantial basedrive power, which hurts efficiency unless

regenerative drives are used. High gain bipolar

devices, such as Darlingtons, have a high

saturation voltage drop. Insulated-gate-bipolartransistors (IGBTs) have a similar problem with

conduction losses. Thyristors are not noted for

radiation hardness and are subject to noise/rate

triggering as well as junction voltage drop

losses. Power Field Effect Transistors (FET)

were the optimum choice for this application due

to their potential for radiation hardness and very

low 'on-resistance.' However, FETs that were

capable of the required power (low Rds_on)

were not available. This void necessitated the

development a new breed of power FET capable

of reliably switching 100 Amps and rated at

500 volts, minimum.

÷

DC input bus

JkA

Figure 2. Improved Weinberg Topology

_- DC

Output

- bus

After a substantial development time, a

FET that could switch the required power was

produced and successfully demonstrated in a

prototype DDCU. During this time, the basic

design of the DDCU was formulated and the

necessary building blocks were assembled to

implement a reliable product.

NON-DISSIPATIVE SNUBBER

In this Weinberg topology, the power

switching transistors are in series with the power

transformer primary. During the turn-off of the

switching element, the leakage inductance of the

power transformer generates a voltage in

excess of the expected "twice input" voltage

+

DC input bus

overshoot. If unmanaged, this excursion may

exceed the voltage rating of the power FET.

Normally, an RC snubber is used to limit this

overshoot. However, this is wasted energy. A

"non-dissipative snubber" provides a solution

and increases the overall converter efficiency.

Just as in a conventional RC snubber, the nondissipative snubber uses a capacitor to slow up

the switching element rise time. However, it

does not get rid of the resulting charge on the

capacitor by discharging it through a resistor.

Instead, the energy stored in the capacitor is

transferred to an inductor that is later forced to

return its stored energy back into the DC input

bus. The use of steering diodes helps to

accomplish this recovery.

The details can be seen in Figure 3. As

the FET turns off, its drain voltage starts rising;

the upper diode between the DC input bus and

capacitor turns on, and the capacitor slows the

voltage rise time. One end of the capacitor is

driven up to approximately twice the input

voltage and the other end is clamped to the DC

input bus through the diode. When the FET

turns on, the charged capacitor is switched

across the inductor with the lower diode in

series. This results in an oscillatory current ring

with a frequency Fr = 1/2_(LC) v2. At the end of

this half period, the stored energy in the

capacitor has been changed to stored energy in

the inductor. During the next half period, the

voltage at the top of the inductor rings positive

enough for the upper diode to conduct and the

energy stored in the inductor flows back to the

DC input bus.

Weinberg

Inductor

Main

Transformer

Figure 3. Non-dissipative Snubber

Unfortunately, this circuit has one

drawback: the time required to fully discharge

the inductor may be interrupted by a short 'ontime' of the FET. This results in a shortened

discharge pulse and the consequential leakage

NASA/TM 2002-211804 3
inductance overshoot may exceed the rating of

the FET. Since the DDCU has a large variation

in output load, this scenario may occur at light

loads and high line input voltage. A compromise

was struck between the non-dissipative and the

standard RC snubbers. A light RC snubber was

added to just keep the leakage inductance

voltage overshoot under the rating of the FET at

no load. This prevented the FET from reaching

the avalanche level and also returned as much

energy as possible to the input from the nondissipative snubber.

HIGH-CURRENT CONTACTS

Electromagnetic Compatibility (EMC)

requirements for the DDCU necessitated a

design in which the switching frequency and

harmonic noise generated by the FET could not

escape through the input or output buses. To

this end, the power section and input and output

filters were compartmentalized. Power

connections from one compartment to another

were accomplished via feed-thru filters, an

isolated connection point with a capacitor

connected to chassis.

In the early design of the DDCU, these

feed-thru filters were off-the-shelf devices in

which the connection point was a threaded stud,

secured via a crimp-on ring lug terminal. The

power loss of this type of terminal proved to be

unacceptable. For example, if only one contact

had a 1 milli-Ohm resistance, at 90 Amps the

loss would be 8.1 Watts. These connections

were at best a point contact that created an

unacceptable power loss. Although a reliable

connection could be made with gold contacts

and guaranteed flatness, the threaded stud

method did not lend itself to flatness or any

acceptable design alternative. Another way had

to be found.

Consulting industry, a Iouvered, highcurrent contact was found which was used on

diesel locomotives and for submarine ship to

shore power. This system was adapted to and

then checked out in the DDCU. These contacts

proved to be very successful and were

employed for all high current connections within

the DDCU. This system provides an additional

advantage of mechanical design flexibility with

an easy, push-on circuit connection. These

contacts also helped to solve a rather tricky

stability problem that will be discussed next.

Figure 3. High Current Louvered Contact

PLANAR BUS

In the DDCU, the output voltage is

sensed via a resistor divider at the output filter

and summed within the pulse width modulator

(PWM) circuit located on the Feedback

Assembly. The PWM output dictates the 'ontime' for each FET. Before being applied to the

PWM, the voltage sense signal is conditioned by

the compensation amplifier. The values of the

components comprising this amplifier circuit are

critical to the stability of the DDCU. If the

stability were compromised, the control circuit

might oscillate, rendering the DDCU inoperative

and possibly damaging its loads.

During the early development of the

DDCU, the compensation amplifier component

values were constantly changing. The values

necessary for stability varied from unit to unit.

Small changes in wiring within the DDCU power

section would require additional changes to

these component values to eliminate

oscillations. The cause of the instability was

traced to inductive losses due to the type of

circular wiring being used to connect the high

power components (FET, transformer, snubber,

Weinberg inductor, etc.), and the variation of

these losses from unit to unit. A solution was

realized by the combination of the high-current,

push-on contacts and insulated, flat copper

conductors, i.e., the Planar Bus.

Planar buses are preformed, flat copper

sheets, coated and separated by an insulator

and fashioned so that the effects of equal and

opposing currents in the power flow will be

canceled out.

NASA/TM_002-211804 4
Figure 4. DDCU Primary Planar Bus

This power component interconnect

scheme provided close coupling of opposing

currents and thereby reduced magnetic field and

'noise' generation. The net effect was to reduce

the length of high current paths and their related

inductances. This, in turn, provided the

necessary wiring consistency and allowed the

compensation amplifier component values to be

optimized for stability.

An additional benefit was to enable

more efficient power component location and

simplified assembly due to these preformed

power buses.

CONCLUSION

Power converters for spacecraft

generally have very unique requirements. Due to

cooling system limitations and power availability,

efficiency plays a significant role in converter

selection and design. These configuration

choices utilized in the ISS DDCU can be directly

applied to commercial and industrial designs

where efficiency may not be quite as important,

but conservation of energy as a whole would

benefit.

The power system for the International

Space Station required a converter that provided

electrical ground isolation and load

simplification. The DDCU was designed to fulfill

the requirements of high efficiency and isolation.

Design criteria indicated that the DDCU

efficiency should be at least 90% from medium

to full load. Meeting these requirements was

aided through the use of Weinberg topology, a

non-dissipative snubber, high-current contacts

and planar bus technology. This allowed for

efficiencies greater than 95% and a design that

reduced the overall cooling requirements.

Innovations and technology

developments from the U.S. space program are

regularly applied to industrial and commercial

products. The efficiency enhancements

discussed herein can similarly be adapted by the

private sector for improvements in its power

conversion products.

NASA/TM_002-211804 5
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Efficient Design in a DC to DC Converter Unit

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Joel E. Bruemmer, Fitch R. Williams, and Gregory V. Schmitz

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11. SUPPLEMENTARY NOTES

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical and

Electronics Engineers, Electron Devices Society, Washington, DC, July 28-August 2, 2002. Joel E. Bruemmer and

Fitch R. Williams, The Boeing Company, 6633 CanogaAvenue, Canoga Park, California 91303; Gregory V. Schmitz,

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13. ABSTRACT (Maximum 200 words)

Space Flight hardware requires high power conversion efficiencies due to limited power availability and weight penalties

of cooling systems. The International Space Station (ISS) Electric Power System (EPS) DC- DC Converter Unit (DDCU)

power converter is no exception. This paper explores the design methods and tradeoffs that were utilized to accomplish

high efficiency in the DDCU. An isolating DC to DC converter was selected for the ISS power system because of requirements for separate primary and secondary grounds and for a well-regulated secondary output voltage derived from a widely

varying input voltage. A flyback-current-fed push-pull topology or improved Weinberg circuit was chosen for this convertm

because of its potential for high efficiency and reliability. To enhance efficiency, a non-dissipative snubber circuit for the

very-low-Rds-on Field Effect Transistors (FETs) was utilized, redistributing the energy that could be wasted during the

switching cycle of the power FETs. A unique, low-impedance connection system was utilized to improve contact resistance

over a bolted connection. For improved consistency in performance and to lower internal wiring inductance and losses a

planar bus system is employed. All of these choices contributed to the design of a 6.25 KW regulated dc to dc converter

that is 95 percent efficient. The methodology used in the design of this DC to DC Converter Unit may be directly applicable to other systems that require a conservative approach to efficient power conversion and distribution.

14. SUBJECT TERMS

Spacecraft power; Power converter

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