Transistorized direct current voltage converters offer a number of advantages over other types because of their small size, low weight, high efficiency, high reliability, precision accuracy and wide range of control. There are three basic types of converter subsystems: series converter in which a voltage-controlled component is placed in series with the load; shunt converter in which a current-controlled component is placed in shunt with the load; and a switching converter in which a voltage-controlled component is turned on and off in series with the load. Converters employing any of these three types of subsystems can provide constant voltage, constant current, or constant impedance across the load. Design and application considerations, step-by-step design procedures, and the solution to sample design problems for the series, shunt, and switching types are presented

Lalli, V. R.

English

NASA Technical Reports Server

NASA TECHNICAL NASA TM X-71513
MEMORANDUM
UN
(L ASA-TM-X-71513) CONVERTER DESIGN N74-18870
IECHNIQUES AND APPLICATIGINS (NASA) 23 p
HC &4.25 CSCL 09E
Unclas
G3/09 33323
Z
CONVERTER DESIGN TECHNIQUES AND APPLICATIONS
by Vincent R. Lalli 6 4
Lewis Research Center 1; -
Cleveland, Ohio
TECHNICAL PAPER proposed for presentation at
Semiconductor Electronics Conference sponsored by the
Institute of Electrical and Electronic Engineers
Cleveland, Ohio, May 9, 1974
https://ntrs.nasa.gov/search.jsp?R=19740010757 2020-03-23T10:39:57+00:00Z
CONVERTER DESIGN TECHNIQUES AND APPLICATIONS
by Vincent R. Lalli
ABSTRACT
Transistorized direct current voltage converters offer a number of
advantages over other types because of their small size, low weight, high
efficiency, high reliability, precision accuracy and wide range of control.
There are three basic types of converter subsystems: series con-
verter in which a voltage-controlled component is placed in series with
the load; shunt converter in which a current-controlled component is
placed in shunt with the load; and a switching converter in which a
voltage-controlled component is turned on and off in series with the load.,
Converters employing any of these three types of subsystems can pro-
vide constant voltage, constant current, or constant impedance across
the load.
This paper discusses transistorized voltage converters of the series,
shunt, and switching types. Included are design and application consider-
ations, step-by-step design procedures, and the solution to sample design
problems.
2INTRODUCTION
The function of power converters in aerospace systems is to trans-
fer and transform electric power from the output terminals of a power
source (i. e., generator) to other forms as required by specific loads.
Efficiency, size, weight, reliability, ripple, regulation, electromag-
netic compatibility and stability are important parameters in aerospace
power processing systems. The design and application considerations
are discussed herein for transistorized voltage converters of the series,
shunt, and switching configurations. Power processing systems are
made up of power converter subsystems (ref. 1).
A typical series voltage converter is shown in figure 1. In this con-
verter, output voltage (Vo) is sensed by a resistor network (R2 and R3).
The analog voltage (V3 ) is compared to a reference voltage (VR) to gen-
erate an error signal (VE) to drive the series component. Bias for the
series component is obtained from the generator (VG) thru resistor (R1).
The load resistor (Ro) varies over the range from (Ro - AR ) to
(Ro + ARo). The voltage source (Vs) varies over the range from
(Vs - AVs) to (Vs + AVs) with a generator resistance (RG). The voltage
appearing at the generator terminals is (VG)'
A typical shunt voltage converter is shown in figure 2. The shunt
regulator includes a shunt component and a reference-voltage component.
The output-voltage (Vo) remains constant because the shunt-element cur-
rent (Ish) changes as the load current (Io) or input voltage (Vs) changes.
The shunt element current (Ish) change is reflected in a change of voltage
across the resistor (R1) in series with the load.
A. typical switching voltage converter is shown in figure 3. The
series component serves as a switch and is either shut off or full on
(saturated). When the current thru the series component is high, the
voltage drop across it is low, so the power dissipated in the series com-
ponent is minimized. The output voltage is sampled and compared to a
reference. The error signal is amplified in the voltage comparator.
3The amplified error signal drives the interface component to turn the
series component on or off depending on the source or load activity.
When the series component is on, the free wheeling diode (DF) is back-
biased by the generator voltage (VG). During saturation of the series
component the voltage across the filter choke (VF) is the difference be-
tween the generator voltage (VG) and the output voltage (Vo) which is
nearly a constant. Since the voltage across the choke during this time
is nearly constant, the current through the choke increases linearly with
time. When the output voltage reaches a predetermined level, the volt-
age comparator turns the series component off. When the series com-
ponent is off, the current in the choke starts to decrease, so the voltage
across the choke reverses polarity. This polarity change is in the direc-
tion to turn on the filter diode (DF), and the choke then becomes clamped
to the filter capacitor (CF), which has the output voltage (Vo) across it.
The choke current then decreases linearly during the off time interval.
When the choke current falls below the load current, the capacitor sup-
plies the difference to the load. This causes the capacitor voltage to fall.
When the output voltage falls below a level determined by the voltage ref-
erence, the series component is turned back on. The on/off ratio increases
with load and decreasing generator voltage.
SERIES CONVERTER DESIGN TECHNIQUE
The step-by-step technique which follows is recommended for the
design of transistorized series voltage converters. The design equations
used in this technique are derived in Ref. 2. Referring to the block
diagram in figure 1
1. Conditions and requirements typical for a spacecraft application:
V o = 28 Vdc AV = +0. 1 Vdc
V s = 56 Vdc AV s = ±6 Vdc; solar array battery buffered.
4RG = 7 ohms
R o = 40 ohms AR o = ±15 ohms
Spacecraft baseplate temperature -550 to +600 C
2. Select a silicon reference diode
Usually 0. 2 Vo 5 VR 5 0. 9 Vo; choose
VR c 0. 5 V - 14 Vdc use 15 Vdc
The values of VR, AV o and circuit gain are all interrelated. Higher
values of VR minimize circuit gain for a given AV o.
In selecting electronic components, a reasonable value of tempera-
ture rise (AT) for the components must be determined. Typical values
are 150 C for AT.
Motorola type (Ref. 3) IN965B IN4109
Power rating at 750 C 368 140 mW
Current rating at 750 C 24.5 9.35 mA
Cost 2.95 3.85 $
Reference supply (IRS) = 10 mA typical
Power dissipation (PDRC) = VR x IRS = 150 mW.
Select IN965B for reference diode
3. Determine V3
V3 is a function of VR, typically V3 = VR
.".. V3 = 15. OVdc
4. Select a series component
Io = 0.70 Adc AI = +0. 42
-0. 19
5VCE V + AV s - V + -AV o 2 56
max(steady state) Ro + ARo
+ 6 - 28 + 0.1 -- 30.3 Vdc
VCE Vs - AV - V 1 + - AV o s 56CE
min Ro- AR
- 6-28 (1 + i + 0, 1 1 4.2 Vde
ICmax = Ioma x
PDmax (V + AV- V o - AV)ICmax - Cm a  (RG)
- 34(1. 12) - (1. 12)2(7) = 29.2 Wdc at a maximum case
temperature of 750 C. Consider
Motorola type (NPN) 2N4231 2N4233
BVCEO 40 80 Vdc
max
VCEsat 2.0 2. OVdc
max
ICmax 1.5 1.5 Adc
PDTmax at 75 0 C 28.0 28. 0 W
max
hFE at 0. 70 Adc 50 50
VEB 5. 0 5. 0 Vdc
6hfe 30 30
gm at 750 C 4.0 4. 0 mhos
Cost 2.10 3.20 $
Choose the 2N4231
5. Select a Darlington driver
Choose IBn = 1.0 mAdc = input base current of nth transistor
C1 0.70hFE hFE. . 0 hFE l . -700
1 2 n IBn 1x10 - 3
> IC1 1 70014
hFE -2 IBn hFE 1 50
C1 0.70IC2 = IB1 - - 15 mAdc
h 50FE
PDT (max) CE 1 - VBE 1 C2 = (30.3 - 0.7)15x10 - 3
= 29. 6x15x10 - 3 = 445 mW
Consider Motorola type (NPN) 2N3946 2N3947
BVCEOmax 40 40 Vdc
ICmax 200 200 mAdc
PDTmax at75 C 0.96 0.96 W
max
hFE at 15 mAdc 100 150
VEB 6.0 6.0 Vdc
7gm 0.25 0. 35 mho
Cost 2.40 2.65 $
Choose 2N3946
6. Solve for VBE 21, where
VBE = VBE + VBE1 = 0. 77 + 0. 83 = 1. 60 Vdc
21 2 1
7. The design-center value for VCE 1 is given by
VoRGR o  (28)(7)(40)CE = Vs - Vo  56- 28- (28)(7)(40) 22.3 VdcVCE R 2 - AR 2 (40)2 - (15)2
0 0
8. Solve for R 1
V -VCE BEVCE1 2 1  22.3 -1.60R1 = = 10 = 20. 8 K ohms1X10
B2 1x10 - 3
Use 21 K ohms
AIC
9. Solve for gm3 = ; where gm3 is the mutual conductance of the
AVE
amplifier component (transistor).
V0
AVs +- + -AR 0
AV0 V 3  RG 11 + gm3R 1  R+ogm -
V0 R0 Rgm1
8gm 3 2.61 mho, where
1 1 1 1
+ +
gml gm2hfel 4 (0.25)30
6+ 28 (7 + 0. 38)15
16000.1 =1600
(1500)(15) + 7 1
28 40 (40)(4)
7 94 c _7.82 0.097mho0.= " gm3 0. 097 mho
1. 181 + 804 gm3 (min) 80. 4
where gm3 is the calculated value (minimum value desired) of
mutual conductance.
10. A third Motorola 2N3946 can be used as the amplifier component;
at this current level (- 14 mAdc):
Dgm3 0. 15 hFEmin 40
where gDm3 is the mutual conductance of the selected transistor and
obtained using Ref. 3.
Since gD > g 3 only one amplifier stage is needed for this case
11. Select R2 and R3 as follows:
Choose 13 = 10 mAdc (typically 13 < 1% Io to minimize power
dissipation).
V 3  VR = 15. 0 Vdc
V3 15R 3  - -1500 ohms neglect IR13 10x10 - 3
9R2  Vo - V3 28-15 1300 ohms
13 10x10 - 3
Use 1300 ohms (the nearest standard value, 1200 ohms would re-
sult in a 4% error in Vo).
The complete regulator circuit is shown in figure 4.
12. Efficiency considerations;
Po0 2 19.6 2
rated x 102 19. x 102 50.3% where
Po + PL 38.9
Po = (0.70)(28) = 19. 6 W
PL = (0. 70)(4. 9 + 22.3) + (0. 011)(28) = 19 + 0.3 = 19.3 W
SHUNT CONVERTER DESIGN TECHNIQUE
A step-by-step technique for the design of shunt converters is ex-
plained in this section. Equations used in this example are derived in
Ref. 2. Referring to the block diagram in figure 2.
1. Conditions and requirements similar to those used for the series
example:
Vo =+28 Vdc AV o = 0. 0125 Vdc
Vs =+56 Vdc AV s = ±6 Vdc
Ro = 40 ohms AR o = ±15 ohms
RG = 7 ohms
Spacecraft baseplate temperature -550 to +600 C
10
2. Select a shunt component:
V o + AV o
I shm = 1  = 1. 12 Adc where I 0
shma x  max max R - AR
S28. 0125 1. 12 A
40 - 15
Ishma x is defined as the limiting case which exceeds the maximum
value of ICE obtained with a load equal to Ro - AR 0
Vshma x < V + AV o L 28 Vdc (forward biased steady state)
PDshmax = Ish maxVshmax = (28)(1. 12) = 31. 4 W at 750 C. Consider
'Motorola type (Ref. 3) MJE205 2N4921
BVCEO 50 40 Vdc
ICma x  5.0 3. OAdc
hFE at 1. 0, 0. 5 Adc 50 45
VCE at 1. 5, 0. 5 Adc 0.5 1. O Vdc
sat
PD at 75 0 C 38 21.6 W
Cost 2.10 1.33 $
Choose the MJE205
3. Select the base-to-emitter resistor for the shunt component:
Let 12 = 1. 5 mAdc, typically 12 -< 1% Io = 11. 2 ma
VBE 1 = 0. 8 VBE 2 = 0. 7 Vdc (on bias two stages)
2  1.5R2 - - = 1000 ohms
I2 1.5x10 - 3
neglecting 13 << 12
4. Select the number of stages required in the shunt component:
Total B = 1.12 A 1120
0. 001
Number of stages (hFE c 50 for:each stage)= 50 n - 1120
n=2
Choose a two stage series component.
5. For the second stage Q2:
hFE 1562
hFE - 39.1hFE
1 0.70
IC2 - 7 0 - 14 mAdc
hFE 50
Motorola type (Ref. 3) 2N3946 2N3947
BVCEmax 40 40 Vdc
max
Iomax 200 200 mAdc
PDTmax at 75 0 C 0.96 0.96 W
max
hFE at 14 mAdc 100 150
Cost 2.40 2.75 $
Choose the 2N3946
6. Select a reference component (RC)
Let Rf S 5 ohms
VR = Vo - V2 = 28 - 1. 5 = 26. 5 Vdc
I 1  =2 + 13 = 1.5 + 0.14 = 1. 64 mAdc
max
PDRC = 26. 5(1. 64) = 43.5 mW at 750 C. Consider
12
Motorola type (Ref. 3) IN4750 IN971
Selected zener voltage 26. 5 ±1. 0% 26. 5+1. 0% Vdc
PDz at 750 C 0.93 0.35 W
IRma x  34 15 mAdc
max
Zztma x at 9. 5 mAdc 35 41 ohms
Cost 1.26 2.35 $
Choose the IN4750 (selected)
7. Select a series resistor component:
Vs - AV s = Vo + (RG + R 1)IG
56 - 6 = 28 + (RG + R 1 )0.70
22 = (RG + R 1 )0. 70
R 1 = 31. 5 - 7 = 24.5 ohms
Choose 24 ohms (closest standard value).
The complete circuit for the transistorized shunt voltage converter
is shown in figure 5.
8. Efficiency considerations:
S= x 102 = 19.6 x102 =38.9
Po + PL 19.6 + 29. 5
Po = VoI 0 = (28)(0. 70) = 19. 6 W
PL IG(RG + R 1) + (Ish + IC2 + I1)Vo
= (0. 70)(31. 5) + (0.266)(28) = 29.5 W
SWITCHING CONVERTER DESIGN TECHNIQUE
A step-by-step technique for the design of switching converters is
explained in this section. Equations used in this example are derived
in Refs. 4 and 5. The block diagram of a switching converter is illus-
trated in figure 3.
1. Conditions and requirements similar to those used for the series
example:
Vo = +28 Vdc AVo = +0. 030 Vdc
Vs = +56 Vdc AVs = ±6 Vdc
Ro = 40 ohms AR o = *15 ohms
RG = 7 ohms
Spacecraft baseplate temperature -550 to +600 C
2. Select a choke for the output filter using energy considerations with
Kirchoff's laws:
(Vs - Vo)Vo (56 - 28)(28)LF - (56 - 8( 1. 12 mH, where f is
2fVs (AI o ) (2)(2. 5x10 4 )(56)(0. 25)
the desired frequency at maximum load
An air gaped 'C' core would be best suited for the choke component
3. Select a capacitor for the output filter:
(Vs - Vo)Vo (56- 28)(28)
8f2 (AV0 - AH)LFVs 8(2. 5x104)2(6x10- 2 )(1. 12x10- 3 )(56)
CF = 29. 5 IF where AH is the hysteresis of the voltage compar-
ator which in the case for a differential input amplifier is as-
sumed to be zero.
Use 2, 15 gF ±10%, 50 WVDC aluminum electrolytes in parallel
to minimize equivalent series resistance
14
4. Select a transistor for the series switching component:
ICmax 1. 0 Adc
max
Assuming a converter efficiency of 80%, the following relationship
for transistor power dissipation is:
PDT - 0. 2 Po = (0.2)(28)(0. 75) = 4.2 W at 750 C
VCEO 62 Vdc VCEsa t  0. 3 Vdc
sat
tT(on) = tT(off) 0. 3 psec (switching times). Consider
Motorola type (Ref. 3) 2N4912 2N4923
IC  at IB = 0. 1 Adc 1. 0 1. OAdc
max
PDT at 7 50 C 20 21W
VCEO 80 80 Vdc
VCES 0.3 0. 3 Vdc
fT 3.0 3. OMHz
tT(on/off switching time) 0.3 0. 3 psec
Cost 2.10 1.80 $
Use the 2N4923.
5. Select a diode for the output filter (Ref. 7):
IDmax = 1. 0 Adc PIVD  28 Vdc
max
tfr 5 0.2 psec trr 5 0.3 psec. Consider
Unitrode type (Ref. 6) UTR 02 UTX 205
IDmax at 750 C 1.54 1. 54 Adc
max
PIVD 50.0 50.O V
tfr 0.1 0.01 jsec
trr 0.25 0.075 gsec
Cost 1.55 3, 55 $
Use UTR 02.
15
6. Efficiency considerations.
7= o x 102 19. 6 x102 =87.2%
Po + PL 19.6+ 2.9
P o = (28)(0. 70) = 19. 6
Losses:
a. Choke dc loss = I2RCH =(1.12)2(1) = 1.26 W
2 2 - I
b. Choke ac loss = f2 L2  s max -
6Q Vo(Vs - Vo)
2. 5x10 4 ) 1. 12x10-3) 56 1)3 -(0. 70)3
(6)(20) 28(56 - 28)
= 0. 216 W
c. Transistor on loss - VCEsatVoIo _ (0. 3)(28)(0. 70)
v s  (56)
= 0. 104 W
(Vs - Vo) _ (0. 6)(0. 70)(28)d. Diode on loss ' Vfr o V s  56
= 0.21 W
16
e. Transistor switching losses:
fVsI maxT(off) + t2ff
fIpVstrr(off) + max rr = (3)(56)(0. 25x10-6)(2. 5x10 4 )
6 t T(off)
+ (2. 5x104)(56)(1)(0. 09x10 - 12 + 0. 06x1012 )
(6)(0. 3x10 - 6 )
= 1.05 + 0. 0128 = 1. 063 W
VB IoVo (5)(1. 12)(28)f. Auxiliary circuit losses VI (5)(1.12)(28)
hFEVs (10)(56)
= 0.28 W
f
Total losses =D Pi - 3. 13 W
i=a
The complete schematic at the switching converter is shown in fig-
ure 6. The design techniques for the auxiliary circuit components are
explained in existing literature (see Ref. 8). The important requirements
for these circuits are that hysteresis be held to a minimum and rise and
fall times are less than the series component.
A study of optimum methods to transfer and transform electric power
in aerospace applications showed that no single method was best for all
cases. The optimum method was very much dependent upon available
devices, generating sources, and load parameters. These are constraints
difficult to identify for future aerospace missions.
Table I shows some of the important typical converter parameters
to consider when faced with the task of choosing the type of converter for
use in a given mission.
Low efficiency increases power source area and raises costs; i.e.,
solar arrays cost about $8000 per square foot at about 4 watts per square
foot. It is not unusual to spend $106 for a 500 watt, 125 square foot array.
17
CONCLUDING REMARKS
The design of transistorized voltage converters of the series, shunt,
and switching types are developed and explained in this paper.
Past aerospace missions have used either the series, shunt and
switching converters or combinations thereof to transfer and transform
electric power. The shunt converter has the smallest size, lowest
weight and parts count. Regulation and stability are very good but effi-
ciency is poor. The series converter is somewhat larger in size, heavi-
er, uses more parts and has an order of magnitude (10 to 1) decrease in
regulation performance than the shunt converter. Stability of both types
are good at light or full loads. A ten percent increase in efficiency
justifies its use in some cases.
Many current aerospace missions are going to some form of the
switching converter. It tends to be a compromise with increased size,
weight, and circuit complexity to gain in efficiency and regulation over
a series converter. A switching converter will usually exhibit ringing
in the output filter for some types of loads so it has only fair stability
performance.
The best designed product is only as good as the people and mater-
ials finally used to make it. The task of determining that people with
the required skills and materials of the proper specified quality are used
to build a product must be included in power processor design by other
means. This task requires participation of many disciplines including
rigorous reliability and quality assurance engineering (Ref. 9).
18
RE FERENCES
1. Schwarz, F. C.: Power Processing. NASA SP-244, 52, 1971.
2. Anon.: Transistorized Voltage Regulators. AG ICE-254, RCA,
Somerville, New Jersey, 12, 1961.
3. Anon.: The Semiconductor Data Book. RRD 97084, Motorola,
Phoenix, Arizona, 450, 1970.
4. Hauser, J. A.: Get With Switching Voltage Regulators. Electronic
Design, 62-66, April, 1968.
5. Schoenfeld, A. A. : The Application of the Analog Signal to Discrete
Time Interval Converter to the Signal Conditioner Power Supplies.
TRW Inc., TRW Systems Group; NASA CR-120906, 89 (1972).
6. Anon.: Design Guide and Short Form Catalog. Catalog SF-1A,
Unitrode Corp., Watertown, Mass., 32, (1971).
7. Eby, Ed: Choose the Right Commutating Diode for Switching Regu-
lators. EDN/EEE, 22-24 (1971).
8. Millman, J. and Taub, H.: Pulse, Digital, and Switching Waveforms.
McGraw-Hill, 1965.
9. Lalli, V. R.; and Vargo, D. J.: Aerospace Reliability Applied to
Biomedicine. NASA TM X-67942 (1971).
19
TABLE I. - TYPICAL CONVERTER COMPARISON
Type Efficiency, Size Weight Parts Regulation Stability
percent count
Series 50 1. 1 1.2 7 ±0. 10 Good
Shunt 40 1.0 1.0 5 ±0. 0125 Good
Switching 85 1.3 1.6 24 ±0.03 Fair
'Cl SERIES I°
RG COMPONENT I
R1 R2
VOLTAGE VG oAMPLIFIER Ro Vo
SOURCE v1 COMPONENT
+IR
VE REFERENCE VF R 3
Figure 1. - Typical series voltage converter.
Ish 1o
REFERENCE
COMPONENT
VOLTAGE V SHUNT 3 Vo Ro
SOURCE G COMPONENT I2
V2 R2
Figure 2. -Typical shunt voltage converter.
COMPONENT COMPARATOR ICOMPONENT
SEII
R SERIES LFCOMPONENT 
Io
VG D CF Vo Ro
Figure 3. - Typical switching voltage converter.
Q1
2N4231
Q2
R1 2N3946 R2
21KQ 1200M
V
VG Q3 +28 Vdc
56 ±6 Vdc 2N3946
R3
IN965B + 1500
15 V
Figure 4. - Schematic diagram for 28 V series converter.
R1
24 OHMS
CR1
IN4750
VG Q2
+56 ±6Vdc 2N 46 0
+28 Vdc
QI R2
MJE205 1000 OHMS
Figure 5. - Schematic diagram for 28 V shunt converter.
R2 R6
4. 7k 4. 7k
Q2 Q3
2N4314 R3 R5 2N4314
3 k 300 5 V
R1 Q4
27 3 k R4 C3 2N222
R8 0. 001 pF D2
+ 1 470 5V - IN4002
2N4923
R 7 1 F 20 k2. 7 k Q5 Q6
VG ODF R11 2N2221 Ro
56 6 Vdc UTRO2 15 k IN965
LF 15 V R10 F
1. 12 mh 150 k 30 +28
Figure 6. - Schematic diagram for 28 V switching converter.
NASA-Lewis


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