Switch Mode Power Supply Design Constraints for Space Applications
Mauro Santos1, Hugo Ribeiro1,2, M. Martins3, J. Guilherme1,2
1
Escola Superior de Tecnologia de Tomar, Estrada da Serra, Quinta do contador, 2300-313 Tomar, Portugal
2
Instituto de Telecomunicações, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
3
Rotacional, Aerospace Ldt, Parque Industrial Vale do Alecrim, lote 13, 2950-403 Palmela, Portugal
Phone: +351-249328196, Fax: +351-249328197, e-mail: jorge.guilherme@ipt.pt
Abstract – This paper describes the design trade-offs presented
in DC-DC power converters used in space applications. The
major concern, in the design of power supplies for space
applications, is to obtain a very high reliability level, which can
be very difficult to quantify, considering the relatively low
production quantities involved. Space converters have to deal
with radiation effects, vacuum environment, shock, vibration
and comply with EMI requirements. To cope with these
stringent requirements several considerations have to be taken.
Selection of the proper converter topology suitable to support
radiation effects is of major importance. Component selection to
resist radiation, mechanical dimensions together with thermal
analysis to support vacuum conditions are key parameters to
achieve a high reliability level.
I. INTRODUCTION
Trapped particles in the magnetic field of the Earth
(primarily protons and electrons) and cosmic rays (heavy ions
or protons of solar or galactic origin) can cause total
ionization dose (TID) damage, displacement damage, or
single event upsets (SEUs) in electronics. These very
different radiation effects in components can lead to degraded
performance, temporary loss of performance or even
catastrophic failures (e.g. burnout). Taking into account, the
radiation effects, the converter design requires specific
topologies and component selection with desirable
characteristics. The first step of the design is choosing the
appropriate power converter topology. Converters which have
a direct short circuit path from a low impedance source to
ground through switches (MOS or Bipolar transistors) will
always be sensitive to single event upsets due to possible
latchup. These converters include the half bridge, full-bridge,
voltage-fed push-pull and active clamp Forward. All singleended converters such as the Forward or the Flyback are
acceptable.
The requirement to obtain very high reliability levels, lead
to the use of “heritage” designs, i.e., the use of components
that have already been used onboard of spacecrafts,
considering that the new application will have the same
environment conditions. Usually the pieces parts, with history
of use in space applications, belong to the so-called preferred
parts lists. The disadvantage is that, because these designs are
typically several years old, the resulting power supplies are
larger, heavier and less competitive than current practice.
We wish to express our gratitude to Eng. Francisco Nunes for the tips he
gave us and Eng. Carlos Ferreira for the mechanical design drawings he
kindly provided.
Another drawback is that competitive space power supplies
will always need to use some non-standard parts, namely
custom ones, like magnetics or a full hybrid design, all chip
components.
Non-standard parts needs to be qualified according to ECSS
standards in terms of: physical properties, functional and
performance, humidity (optional), leak/pressure, acceleration
(optional), sinusoidal and random vibration, shock, corona
and arcing, thermal vacuum, thermal cycling, EMC/ESD,
microgravity and audible noise. The design will also have to
take into account harsher environment conditions, namely,
because of vacuum, heat can be dissipated only by conduction
and, because there is no protection like in earth, with
atmosphere, the equipment endures more severe radiations.
Space environment conditions, which are harsher than
terrestrial ones, are unique to each application. For instance,
radiation effects like total dose and SEU depend on the orbit
as well as time on orbit. Temperature conditions depend on
the spacecraft's thermal control system as well as the location
of the power supply.
The present study results from a project proposal to develop
a DC-DC converter for space applications under European
Space Agency (ESA) contract [1]. ESA specified the required
converter specifications which result in some of the presented
constraints.
This paper is organized as follows: Section II analyses the
design constraints imposed by the harsh space environment.
Section III and IV discuss the trade-offs of converter
topologies suitable for space environment. Section V presents
some of the problems raised by the EMI requirements
presented to the converter. Section VI presents a proposed
method of construction of the converters and their attachment
to the spacecraft’s board. Experimental results are presented
in Section VII followed by the concluding remarks, Section
VIII.
II. SPACE DESIGN CONSTRAINTS
A. Component Selection
Aging of components in space does not happen at the same
rate or speed as it does on earth, the speed at which the
components age depends on the orbit at which the equipment
will operate, it also depends on the type of component. For
bipolar transistors the gain will decrease with exposure to
radiation, for MOS devices the threshold voltage will change,
making N-type devices turn on at higher gate-source voltages
and P-type devices turn on at lower source-gate voltages. In
each case this makes circuits behave differently or even cease
to work. That change must be predicted and accounted for at
the time of the design of the circuit.
D. Cost
B. Space Constraints
The objective of the project is to build a converter with
commercial of the shelf (COTS) parts preferably
manufactured in Europe. Space qualified parts are usually
much more expensive than the COTS counterpart, the
manufacturers are usually not European (most are American)
and the parts are usually considered military grade. This
imposes another problem, the need to get an approval to
import the parts, which makes the design more expensive and
the design cycle longer.
As an example the main supplier of space qualified MOS is
International Rectifier, IR, and because this manufacturer is
not European, using qualified parts from this company has the
above mentioned problems. Another possibility is the
qualification of new parts, this approach is very expensive
due to the specific nature of the tests required to obtain the
flight models. The qualification is time consuming due to the
amount of required tests and the outcome is unpredictable.
Selection of components for best performance can be
difficult when the available space is reduced. The design
requires low losses so the circuit has a high efficiency. The
design may also need certain component values that are
difficult to attain due to board size requirements. One of the
first problems is the output filter capacitor value constraints,
no electrolytic capacitors can be used due to the risk of
explosion. MKT and MKP capacitors have better
performance than other ones, but they have a lower capacity
density.
Output voltage sense is also a problem to take into account
in the design of the converter. There are two possibilities to
sense the output voltage: the use of an auxiliary winding or
the use of an optocoupler. The use of an auxiliary winding
may be difficult in a multi output converter where the
available space for power windings is already small; on the
other hand the use of an optocoupler can introduce problems
in the output voltage regulation, due to their loss of linearity
with aging.
The leakage capacitance between primary and secondary
circuit must be lower than 5nF at the switching frequency (by
ESA specification). Due to this fact the number of winding
turns can’t be too high. To reduce the number of winding
turns, a larger and heavier ferrite core is needed. This
objective is difficult to achieve when the dimensions and
weight are an important constrain.
C. ESR Effect on Output Filter Response
The Equivalent Series Resistance (ESR) of the output
capacitors must be small to provide low losses and good
attenuation of the output filter. Figure 1(a) shows the typical
LC output filter topology and Figure 1(b) shows the effect of
higher ESR in the attenuation and the phase of the output
filter. A higher ESR leads to lower attenuation and higher
dissipation on the capacitor, leading to an efficiency
reduction. The problem with high ESR is usually solved by
using a larger capacitor or several capacitors in parallel using
more space on the Printed Circuit Board (PCB).
III. STUDIED CONVERTER TOPOLOGIES
A. Flyback Converter
The Flyback converter is one of the possible topologies
which has the lower component count. The output voltage
isolation is guaranteed with two coupled inductances, a
primary inductance, LP, and a secondary inductance, LS. On
the input side there is only one winding, LP, which means that
only one power transistor is required. On the output side there
is only the output capacitor, CO, and one switch, which can be
a power diode or a power transistor in the case of
synchronous rectification. Therefore there is no need to use
an external inductor to filter the output voltage, which means
less space required. Figure 2 shows a simplified schematic of
the Flyback converter with a diode as an output switch.
iLP
iLS
D1
NS
vP
vS
CO
VO
NP
VI
Or with multiple outputs
vT1
T1
VO1
Bode Diagram
Magnitude (dB)
0
ESR=0.001
ESR=0.05
VO2
-50
Fig. 2. Flyback converter.
-100
Phase (deg)
-150
0
(a)
-45
-90
-135
-180
2
10
10
4
10
6
10
8
Frequency (Hz)
(b)
Fig.1. Typical LC output filter with load: (a) configuration;
(b) frequency response versus capacitor ESR variation.
Figure 3 shows some waveforms present in the Flyback
converter operation. During ton, energy is stored in the LP
inductance, during toff the stored energy is transferred to the
output, resulting on a current in LS inductance which
forward-biases the output diode. The next equations give the
relation between VI and VO and the maximum voltage on T1
as a function of duty cycle δ = ton / TS [2].
δ 1
VO =VI ⋅
⋅
1−δ n
N
n= P
NS
(1)
V
= V ⋅ 1 / (1−δ ) → δ
⇒ VT 1
T 1 peak
I
MAX
MAX
(2)
The high current ripple in CO, has the value of 2⋅IO, and
requires the use of a low ESR capacitor or a parallel of
capacitors to provide the required output voltage ripple.
When the ESR is neglected the output voltage ripple, ∆vO , is
given by :
I
∆vO = O ⋅δ ⋅TS
CO
and D1 off. The next equations give the relation between VI
and VO and the maximum voltage on T1 as a function of the
duty cycle δ =ton / TS [3].
V ⋅δ
V = I
O
n
with n =
NP
NS
and δ ≤
V
= V ⋅ (1+ N P / N D )
T 1MAX
I
1
ND
1+
NP
(4)
(5)
(3)
The LC filter is designed to offer high attenuation at the
switching frequency, fS. Typically the cut-off frequency, fC, is
30 to 100 times lower than the switching frequency, fS. When
the ESR is neglected the output voltage ripple, ∆vO , is given
by :
∆vO =VO ⋅
π2

⋅(1−δ )⋅
2

fC 

fS 
2
(6)
and fC is given by:
fC =
Fig. 3. Flyback converter waveforms.
1
2⋅π LO ⋅CO
The current ripple in the capacitor CO is equal to the current
ripple in LO which is given by:
V
∆iLO = O ⋅(1−δ )⋅TS
LO
B. Forward Converter
(7)
(8)
The Forward converter uses a transformer to guarantee the
isolation between input and output voltage. The isolation
transformer has three windings: a primary winding, NP, a
secondary winding NS and a demagnetizing winding, ND. In
the primary circuit a voltage inverter is made with a transistor
and a diode. On the secondary side there is a capacitor, an
inductor and two switches, these switches can be two power
diodes or two power transistors in the case of synchronous
rectification. Figure 4 shows a simplified schematic of the
forward converter with diodes used as output switches.
LO
D1
vP
vX
vND
vS
Fig. 5. Forward converter waveforms.
CO
D2
VI
NP
ND
VO
IV. CONVERTER TOPOLOGIES COMPARISON
NS
Or with cross regulation
vT1
vD1
A. Operation Limitations
D
T1
VO1
VO2
Fig. 4. Forward converter.
Figure 5 shows some waveforms present in the Forward
converter operation. In this Figure iLM is the magnetizing
current. During ton the voltage applied to the primary, vP, is VI
and the energy is transferred to the output circuit. The diode
D1 is forward-biased and the current in LO has a positive
slope. During toff1, the diode D is forward-biased and the core
is demagnetized, the current iLO flows through D2 and has a
negative slope. During toff2, iLM is zero and D is reversedbiased. In the secondary circuit we have the same state, D2 on
The duty cycle range which can be used is limited on both
converters. On the Forward converter this limitation is due to
the need to demagnetize the core of the transformer, therefore
the maximum duty cycle achievable can be controlled by
changing the relation between primary winding turns and
demagnetizing winding turns. The voltage on the switch also
limits the duty cycle. A high duty cycle results in a high
voltage on the transistor, this degrades the efficiency because
high voltage transistors have poor specifications.
On the Flyback converter the maximum switch voltage is
directly related to the maximum duty cycle used, therefore
higher duty cycles can only be achieved by using switches
capable of blocking higher voltages. The Flyback converter
would require the use of an output filter to keep the ripple
voltage under the limits, due to capacitor ESR. The use of this
filter increases the number of system poles, degrading the
stability and increases the space needed on the PCB.
B. Efficiency
For any of the two converters the use of synchronous
rectification allows a higher efficiency at the expense of a
higher cost and an added degree of complexity. In the
Flyback converter a voltage snubber in parallel with the
primary transistor is required, which can reduce efficiency.
The advantages and drawbacks of each converter are
summarized in the following table.
Table I: Converter topologies comparison
Topology
Flyback
Forward
Advantages
Drawbacks
Hard to control;
Simple coupled inductor
δMAX limited;
design;
Poor cross regulation;
Low component count
High ∆iC;
Snubber needed
Simple control;
High component count;
Low ∆iC;
δMAX limited;
Output cross regulation Complex transformer
(a)
(b)
Fig. 6. Support scheme for the converter PCB: (a) single
board converter; (b) double board converter.
VII. EXPERIMENTAL RESULTS
A prototype of the forward converter was built to assess the
performance using COTS parts and design difficulties due to
space availability. Figure 7 shows the results obtained with
the prototype converter. The top waveform shows the output
voltage ripple and the lower waveform shows the voltage at
the drain of the MOSFET. The switching frequency was set to
1 MHz and the measured efficiency at full load was 88%.
This prototype was not tested against typical space
conditions, required to comply with ESA requirements as
stated in the Statement of Work, (SoW [1]).
V. EMI COMPLIANCE
The converters must comply with Electro Magnetic
Interference (EMI) regulations. To comply with conducted
interference, an input filter must be added to the converter
which can degrade the stability of the converter. To comply
with radiated interference special care must be taken in the
design of the converter PCB where short traces and small
loops are preferable. The choice of magnetic components will
not only affect the design of the converter but also the amount
of interference emitted by the converter, adding another
degree of complexity to the project.
VI. MECHANICAL DESIGN
Usually mechanical design is not a big issue. For space
applications, the parts assembly requires special attention,
because the circuit must resist launch without damage. The
converter design objective is to use only one PCB to
implement the full converter or at most two PCB’s, one for
the power components and another for the converter control.
Surface Mount Components (SMD) are preferable, not only
for their reduced space usage but also due to their better
mechanical behaviour. Fixation of the PCB is very important,
because the mechanical stress is distributed by several
support points. For a single board converter we propose a
scheme of five support points as can be seen on Figure 6(a),
for a double board converter we propose the scheme of
Figure 6(b). Despite the mechanical robustness of SMD
components, special techniques need to be used in the
soldering of components for space applications.
LO=25µH, CO=2µF
VI=30V, VO=5V
RL=2.5Ω, Fs=1MHz
η=88%, ∆VO/VO=1%
Po=10W
Fig. 7. Prototype converter waveforms (Time base
500ns/Div, CH1 20V/Div, CH2 50mV/Div).
VIII. CONCLUSIONS
The most promising topology is the forward in terms of
number of components and cross regulation. The Flyback
converter despite having an initial lower component count,
would require the use of an output filter to keep the ripple
voltage under the imposed limits due to the capacitor ESR.
The use of this filter not only increases the number of
components as well as the number of system poles, with the
consequence of stability degradation. Therefore the Forward
topology presents the best approach to implement these space
DCDC converters.
REFERENCES
[1] ESA SoW, Efficient Low Cost Power Conversion for Standard
and Advanced Fast Digital Electronics, September 2006.
[2] N. Mohan, T. M. Undeland, “W.P. Robbins, Power
Electronics: Converters Applications and Design, John Wiley
& Sons, 1989.
[3] Robert W. Erickson, Dragan Maksimovic Fundamentals of
Power Electronics Second Edition, KAP, 2001.