Isolated dc-dc converters with high-output voltage for TWTA - Ivo Barbi

Isolated dc-dc Converters with High-Output Voltage
For TWTA Telecommunication Satellite Applications
Roger Gules and Ivo Barbi
Federal University of Santa Catarina -UFSC
Power Electronic Institute - INEP
P.O. Box 5 119 - 88010-970 - Florian6polis-SC-Brail
Phone: +55-48-33 1-9204 - Fax: +55-48-234-5422
E-mail: roger@,ineD.ufsc.br Intemet: http://www.ineD.ufsc.br
Abstract-Two alternatives for the implementation of an
isolated dc-dc converter with high output voltage are presented.
The proposed topologies are especially well adapted for the
implementation of Travelling Wave Tube Amplifiers (TWTA)
utilized in telecommunication satellite applications. The
converters studied follow different principles and the main
operation aspect of each topology is analyzed. The experimental
results obtained from two prototypes with 150W of output
power, total output voltage of 3.2kV and an input voltage
variable from 26V until 44V are presented. The efficiency
obtained operating with the nominal output power is higher
than 93.4 YO for both structures.
I. INTRODUCTION
There are different applications where is necessary the
use of high level DC voltages (thousands volt). Typical
examples are COz laser-based systems, X-ray equipment for
medical and industrial applications, cathode-ray-tubes and
telecommunication equipment with vacuum tubes, like
Travelling Wave Tube (TWT) utilized in communication
satellite.
The choice of most adequate solution depends of the
design requirements of each specific application as cost,
mass, volume, efficiency, reliability, etc.
Other point in the choice of the converter is the highvoltage
circuit non-idealities that present an important
influence in the converter operation and the topology adopted
must integrate these intrinsic parameters into the operation
principle. Besides the leakage inductance, the high-voltage
transformer also presents a winding capacitance referred to
the primary side that due to the high number of tums in the
secondary winding and the high transformer tums ratio,
becomes important in-the converter operation. Fig. 1 presents
the high-voltage transformer simplified model constituted by
the primary leakage inductance (L d), magnetizing inductance
(L,,,)and the equivalent capacitance referred to the primary
side (C,) [ 1,2].
-HK
nl: n2
Fig. 1. Simplified equivalent circuit of the high voltage
and high frequency transformer.
In the particular case of communication satellite
applications studied in this paper, severe requirements must
be attended due to the environmental conditions, reliability,
mechanical and electrical specifications.
The launch cost .is very high and there is a permanent
interest in the efficiency improvement and reduction of the
mass and volume in satellite equipment. High efficiency is
important to achieve since the primary power source of a
satellite is in general a solar array and batteries, presenting an
important impact on total mass, volume and cost.
The final stage amplifier in the transponder of the
communication satellite is the Travelling Wave Tube
Amplifier (TWTA). In this kind of satellite, the payload mass
and power consumption is mainly given by the presence of
the TWTA which can represent about 35% of the total mss
and from 70% to 90% of the overall DC power consumption
[3]. Therefore, the design of TWTA has been continuously
improved in order to realize more light and efficient
equipment.
The TWTA consist of a microwave amplifier tube TWT
mainly determining the radio frequency (RF) performance
and by an Electronic Power Conditioner (EPC) for power
matching of the DC interface. The high voltage dc-dc
converter is the most important part of the EPC because it is
the critical point in the design of a high efficiency and
competitive TWTA. The input voltage of the dc-dc converter
is an unregulated low voltage bus (from 26V until 44V) and
in order to produce the TWT supply voltages there are
multiple outputs in the dc-dc converter, resulting in a total
cathode-anode voltage of thousands volts.
11. HIGH VOLTAGE DC-DC CONVERTERS
There are different altematives for the implementation of
an isolated dc-dc converter with high output voltage,
however there are some operations characteristics that the
converter must present in order to obtain high efficiency and
reduced volume and mass. The main operation characteristics
are:
High switching frequency in order to reduce the reactive
elements likes inductors, filter capacitors and the power
transformer.
Soft-commutation in the power switches, avoiding the
reduction of the efficiency with the increment of the
switching frequency.
0-7803-6618-2/011$10.000 2001 IEEE 296

Integration in the converter operation of the intinsic
elements of the high-voltage power transformer like
leakage inductance and distributed capacitance, avoiding
the dissipation of the energy stored in these elements.
Constant switching frequency, allowing the choice of an
optimum switching frequency with respect to the mass,
volume and efficiency.
0 Voltage source output characteristic, because the multiple
high-voltage outputs necessaries for to supply the TWT
become unfeasible the utilization of high-voltage output
filter inductors for a current source output characteristic.
Others operation characteristics are also desirable in TWTA
application:
Input current source behavior that minimizes the volume
of the input filter.
A step-up output characteristic, allowing the reduction of
the transformer turns ratio and minimizing the effects of
the transformer equivalent capacitance in the converter
operation.
Good load regulation and support a large variation of the
input voltage.
Considering as choice criterion of the converters all
operation characteristics presented above, two topologies
were selected and studied. Two dc-dc converters connected
in series (two-stage topology) compose most part of the
structures developed for this application and the first
topology adopted follows this principle. The second topology
studied is based on a new single-stage structure for TWTA
applications.
111. TWO-STAGE TOPOLOGY
A. Proposed Circuit
The first converter that composes the two-stage topology
is a regulator (normally a classical buck or boost dc-dc
converter). The second structure is an isolated dc-dc
converter operating in resonant way, generating the different
high voltage outputs for supply the TWT. In this case, the
isolated dc-dc converter operates in an optimized operation
point, with constant switching frequency and fix duty-ratio.
This configuration is normally utilized due to the difficulty in
to obtain high efficiency in the isolated dc-dc converter.
The two-stage topology studied in this work is presented
in Fig.2. The first converter is a classical boost dc-dc
converter composed by the input inductor L power switch
SI, diode D1 and a filter capacitor C The boost regulator
operates with hard switching but with the utilization of a
Schottky diode in its output, high-efficiency operating with
high switching frequency is reached.
The second converter is a resonant push-pull current fed
dc-dc converter composed by two power switches S and
$2, an input inductor L z, a resonant capacitor C and the
push-pull transformer. The outputs are composed by fullbridge
rectifier and filter capacitors. The intrinsic parameters
of the transformer are also presented in Fig.2.
The push-pull converter operates with zero current and
zero voltage switching techniques (ZCWZVS). The softcommutation
is not dependent of the load current because the
energy stored in the magnetizing inductance performs the
voltage transitions in the circuit capacitance.
The main theoretical waveforms of the isolated converter
are presented in Fig.3.
There are two resonant circuits in the operation of this
converter. During the conduction of one power switch S or
Sp2 (to-tl), the resonance between center tap capacitor (C T)
and the transformer leakage inductance (L d) occurs. When
both power switches are turned-off (tl-t2) there is the
resonance between the magnetizing inductance and the
intrinsic circuit capacitance (power switches and transformer
equivalent capacitance). The current (i) presented in Fig.3 is
the current in the center tap of the push-pull transformer.
Input voltage: Vin=26/44V
Total output voltage: Vo=32o0V
Nominal output power: Po=ISOW :
PI=SP2=APTZOM22LZVR
SI= IRFp2Y
D,= 8TQlOo
Reclifis diodes = MUR 1 lo0
.----_.
Ferrite colc ETD39-IPITIhomton
Fig.2. Power circuit implemented of the two-stage topology.
297

Reference [4] presents more details about the operation
of this converter.
VCI
I
The switch conduction period is represented by the
relative parameter (tr). The current and voltage effort
increase with the reduction of the switch conduction time (tr).
The relative frequency F is the relationship between the
resonance frequency of the leakage inductance and the center
tap capacitance, with the switching frequency. This
resonance occurs during the conduction of the power switch.
Fr2 is the relationship between the resonant frequency of
the magnetizing inductance and equivalent circuit
capacitance, with the switching frequency. This resonance
occurs when both switches are tumed off.
Substituting the specification and parameters in (1) yields:
VCI
VCI
11 U 0 %
Fig.3. Main theoretical waveforms.
B. Simplified Design Procedure
1) Specifications andparameters
The following specifications are considered in the design:
Input voltage:
Vi,,=26/44V
Total output voltage
V0=3200V
Output power:
P0=150W
Push-pull switching frequency:
Fs=801

,=atan.( z.Fr .(I - t ) )=0.549
voltage is presented in Fig.7. The lowest efficiency obtained
with nominal output power was 93.4% for the minimum
input voltage.
TO* *coo 25 OMS,*
715 ACqc.
is:
The switch RMS current is:
is, = ./%- = 3.09A (13)
The maximum blocking voltage across the switch (VS pk)
7
Zn = .,/$
= 1.34
a
Fig.5.Power switch voltage and current (25V/2A/2p/div).
vs, =-
cos(,)
+2.V2 =IOW
With the increment of the leakage inductance L d due to
the constructive characteristic of the transformer, the
capacitor CT must be lower for the same resonant frequency
F, as shown in (9). Thus the parameter Zn calculated by (14)
and the voltage efforts increase with the increment of L d.
5) Output Filter Voltage Ripple
The parameterized output voltage ripple is calculated by
(16) and (17).
-
AV=
AVc,,*C;F, -
_-.
1 2
I O z F;cos(~)
a=otang( w.F,.(l- t,) )
Where:
I, - Average output current.
CO - Filter capacitor
AV,, - Output voltage ripple
C. Experimental Results
A laboratory prototype was implemented following the
optimized design procedure developed and some waveforms
obtained from this prototype operating with the minimal
input voltage and the nominal output power are presented.
The details about the power circuit implemented are shown in
Fig.2. A typical configuration of load resistance representing
the TWT for the test of the power supply is used. The
different current i d voltage level in each output and the
nominal output power are defined by the kind of TWT
considered in the design.
Fig.5 presents the soft-commutation obtained in the
power switch of the push-pull converter. Fig.6 shows the
voltage and current in a rectifier diode of the push-pull. The
efficiency curve of the two-stage topology (boost and Pushpull)
operating with nominal output power and variable input
S.
Fig.6. Rectifier diode voltage and current (lOOV/50mA/2p Jdiv).
9s
94.5
t1W) 94
93.5
94-s-
93
25 30 3s
Vin (V)
40 45
Fig.7. Efficiency operating with nominal output power
and variable input voltage.
IV-SINGLE STAGE TOPOLOGY
The main drawback of the two-stage topologies is the
converter series connection that causes a reduction of the
overall efficiency. Both converters must present a very high
efficiency (close to 96%) in order to maintain an adequate
efficiency for a satellite communication application (higher
than 92%).
Therefore, a single stage topology can result in a higher
efficiency and a lower mass and volume than the two-stage
topology. However, the isolated dc-dc converter operates
with a variable operation point (non-optimum) due to the
converter's regulation action. Thus, the single stage topology
candidate must present all suitable operation characteristics
for high-voltage applications described before and maintain a
high efficiency in all range of the input voltage.
A. Proposed Circuit
For the implementation of a competitive high-voltage
isolated dc-dc converter, a single-stage high-efficiency
topology is proposed. The power circuit adopted, presented in
299

-____.
Fig.8, is. based -on the current-fed push-pull dc-dc
converter operating with PWM modulation, active clamping
and ZVS commutation [SI.
Two main switches (S p1 and SP2), two auxiliary clamping
switches (Sal and Sa), a clamping capacitor C G and a pushpull
transformer compose the converter. The current feeding
is provided by the voltage source V in in series with the input
inductor Li,. The outputs are formed by full-bridge rectifiers
and by filter capacitors. The anti-parallel diode and intrinsic
capacitance of the MOSFET are used in the circuit operation.
The intrinsic parameters of the transformer are also presented
in Fig.8.
The main operation waveforms are shown in Fig.9.
Fig.8. Power circuit implemented of the single-stage topology.
- - - __
The- active clamping allows the operation with softcommutation
in all switches until a minimum load and the
voltage across the blocking switch is equal the clamping
voltage.
An important characteristic of this structure is to operate
with a main switch duty-ratio (D) higher than and lower than
0.5 (with overlapping and without overlapping). The
auxiliary switches operate in a complementary way in
relation the respective main switch.
For a main switch duty-ratio lower than 0.5, the converter
operates like a sepic dc-dc converter and with a step-down
output characteristic. For the operation with a main switch
duty-cycle higher than 0.5, the converter operates like an
isolated boost converter with a step-up output characteristic.
Therefore, this converter does not present inrush current for a
progressive variation of the duty-cycle and support a large
variation of the input voltage.
There is the resonance between the leakage inductance
and the parasitic transformer capacitance when both main
switches are conducting. During this period the input
inductance (Lin) stores energy and the oscillations does not
present influence in the converter's operation.
I .
"spl " Sal ' Spl
Gat ,
, ' Sp2 Sa2 I'SP2 '* I
' -4.T " M.T,, 4.T , , .PI 4" '
T >
D.T
>'
Fig.9. Main theoretidl waveforms.
t
B. Simplified Design Procedure
1) Specifications and parameters
The following specifications are considered in the design:
Input voltage:
Vin=26144V
Total output voltage
V,=3200V
Output power:
P,=150W
Push-pull switching frequency: FS=80k&
The parameters of the implemented circuit are:
Magnetizing inductance:
Lm=85W
Leakage inductance:
Ld26@
Winding capacitance:
Cp=8.75nF
2) Operation point of the Push-pull converter
The output voltage referred to the primary side (V op)
adopted in the design is equal to SOV, considering the main
300

switch duty-ratio higher than 0.5 and a step-up output
characteristic.
The static gain, operating with the minimum and
maximum input voltage are determined respectively by:
gm = -%- = -
50
= 1.9231
Vin,, 26
vop 50
4, =-- - - = I. I364
Vin,, 44
(19)
The nominal input current operating with the minimum
and maximum input voltage, considering the operation
without losses is:
Po
Iin, =-=5.77A
Vin (20)
Po
Iin, = - = 3.41A
Vin m,
The parameter y represents a reduction of the effective
converter duty-ratio due to the presence of the active
clamping circuit. This characteristic is common in the most
part of the ZVS-PWM converters. This parametet is
proportional to the leakage inductance and input current.
For the minimum and maximum input voltage results:
Iin, * L, . F,
Yvm = = 0.08308
VOP
lin, . L, . F, =
Y, =
0.04909
VOP
The nominal converter duty-ratio for the minimum and
maximum input voltage is:
Auxiliary switch:
iSa, = lin, .&e,/-12 = 0.945A (29)
4) Current efforts operating with the maximum input
voltage
Main switch:
iSp, = lin, . -. ,/15.y,+13-7.d, ~2.4324 (30)
12
Auxiliary switch:
5) Soff-commutation range
The minimal input current that maintains the softcommutation
is defined by:
Where:
Z, =E (33)
The soft-commutation range increase with Z , and Ld. The
soft-commutation was obtained in all input voltage variation
range and in an adequate load variation range.
6) Output Filter Voltage Ripple
The parameterized output voltage ripple is variable with
the static gain (4) and calculated by (34).
2) Voltage efforts
The clamping voltage operating with nominal output
power and minimum and maximum input voltage is:
2
V,, = Vin,, - = 147V
l-dvm
(26)
L
V,, = fin,, = 112.5V
1 - d”X
Thus the maximum blocking voltage across the active
switches is equal to 147V.
3) Current efforts operating with the minimum input
voltage
Main switch:
Where:
I, - Average output current.
CO - Filter capacitor
AVco - Output voltage ripple
C. Experimental Results
A laboratory prototype was implemented following an
optimized design procedure developed and some waveforms
obtained operating with the minimal input voltage and the
nominal output power are presented. The details about the
power circuit implemented are presented in Fig.8.
Fig.10 shows the main switch current and voltage
waveforms. The soft-commutation is obtained and the
maximum switch voltage is equal the clamping voltage.
The auxiliary switch voltage and current waveforms are
presented in Fig. 1 1. The auxiliary switch also presents soft-
301

commutation and the RMS current and the conduction losses
are very low.
The efficiency curve operating with nominal output
power and variable input voltage is present in Fig.12. The
lowest efficiency obtained with nominal output power was
94.1 Yo.
TeK StOL 25 OMS/5
2
503 ncqs
FiglO. Main switch voltage and current (5OV/2A/2pddiv).
TeK 5tOP 25 OMW5
114 ncqs
I
The proposed converters are suitable for low input voltage
applications, because the blocking voltage switch is higher
than two times the input voltage for both structures.
The operation characteristics were verified by the
implementation of the laboratory prototypes operating with a
variable input voltage (26V/ 44V) and with 3.2kV of total
output voltage. The lowest efficiency obtained operating with
the nominal output power is equal to 94.1% for the singlestage
topology and equal to 93.4% for the two-stage
topology. The single-stage topology proposed presents a
lower mass and volume and higher efficiency than the two
stages-topology studied.
The output voltage ripple of the single-stage topology
operating in the worst operation condition (higher static gain)
was 30% higher than the two-stage topology with the same
output filter.
REFERENCES
a
I I I II/F I I I I
Fig.11. Auxiliary switch voltage and current (50V/2A/2pddiv).
94.9
94.7
94.5
943
94.1
*
93.7 93*9
25 30 35 40 45
Vin (V)
Fig. 12. Efficiency operating with nominal output power
and variable input voltage.
V. CONCLUSIONS
[ ‘1 B. Tala-Ighil, J-M. Nyobe-Yome and C. Glaize, “High-Voltage Variable-
Frequency Double-Resonant DC-DC Converters Utilizing the
l
Tmsfomer Parasitic Elements,” Proceedings of the European Space
Power Conference, Austria, p. 245-250, Aug. 1993.
[ 1 J. Uceda, C. Blanco, M. A. Ptrez and M. RICO, “Design of the Delay
Line Power Supply of a TWT,” Proceedings of the European Space
Power Conference, Spain, p. 2123-2128, Aug. 1995.
[3] LCeruti, M. Gambarara and D.Vigano, ‘Yew Generation EPC for
Medium Power TWTs,” Proceedings of the European Space Power
Conference, Spain, p. 299-316, Sep. 1998.
[4] A. H Weinberg and L.Ghislanzoni, “A New Zero-Voltage Zero-Current
Power Switching Technique,” IEEE Transactions on Power Electronics,
vol. 7, no. 8, p. 655-665, Oct. 1992.
[5] F. J. Nome and 1. Barbi, “A ZVS Clamping Mode Current-Fed Push Pull
DC-DC Converter,” ISIE’98, Pretoria, South Afric, pp. 617-621, Ju1.1998,
[6] I. Arens and F. Tonicello, “ Conductance Control with a Boost Regulator
for a High-Voltage Power Conditioner for a TWTA,” Proceedings ofthe
European Space Power Conference, Ita&. p. 343-350, Sep. 1991.
r71 P. Delporte, P. Fa9 and E. Pequet, “EPC and TWTA for
Telecommunication Satellites,” Proceedings of the European Space
Power Conference, Spain, p. 305-310, Sep. 1998.
Two alternatives for the implementation of a highefficiency
isolated dc-dc converter with high-output voltage
for TWTA application were proposed and studied. Both
structures present several operation characteristics suitable
for high output voltage applications supplied by an
unregulated input voltage.
302