Low power high efficiency dc to dc converter- regulator for electronic system

Roberge, J. K.

English

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1 tn - -  I Z 1 Electronic Systems Laboratory 
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Massachusetts Institute of Technology 
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Recent component and design advancer hove *enabled the operation P-I 
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of relatively large lectronic s y s t e m ~ . ~ u m r ~ l ~ V e l m  of f ract ionkof  \o 
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a Watt .  A To date, the majority of such low power syetems have been 5. 
derigned for  use in  the space environment. They often require a 
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number of well-regulated supply voltages to insure  ati is factory operation. 
Since these voltages a re  generally not available f rom the main apace - 
c ra f t  power supply, a DC to DC converter and regulator is  usually employed. 
Typical currently available converter -regulator de signs have an efficiency 
of 70 to 75  percent at the I wa t t  level and 8 0  to 90 percent a t  the 10 watt 
level. These designs usually deveiop maximum efficiency for  only a limited 
range of output p o w e r  ciose to the maximum levei.  Thus, particularly 
in applications :vt-.e re  Nide variations in required power level a r e  anticipated, 
many of the advantages of h w  power circuitry a re  negated by converter-  
regnlator inefficiencies. This paper presents  a design techriiq.de fo r  
t converter-regalators  which overcomes some of the limitations inherent 
- _  - 
in many previous types. 
The essential  elements of typical currently available high efficiency 
DC to DC converter-regulators a r e  shown in the block diagram of Figure 1. 
,Multiple output voltages a r e  obtained by rectifying the outputs f rom a 
conventional I3C to AC converter, consisting of a solid s t a t e  chopper and a 
t ransformer with multiple outputs. 
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Feedback from one o r  mdre  of the 
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output voltages i s  applied to a pre-regulator which controlr the input 
voltage to the DC to AC converter in orde r  to maintain the b r i r e d  output 
voltage 9 .  
A majority of the losses  in a converter of this type, particularly when 
operating at a fraction of maximum output power, arire in the DC to AC 
converter section, since the operating frequency of t h i s  element 18 errent i~ l ly  
indopondant of the power level. Thur, a8 output power is reduced, core 
l o r r e r  diraipate an increasingly larger fraction of the total input power. 
In m y  application where the input voltage is not extremely well 
regulated, pro-regulation is accomplirhed wi th  a switch driving a reronant 
charging network since the inefficienciar inherent in a disripative regulator 
operating over a wide dynamic r m g e  a r e  intolerable. 
effecting resonant regulation a r e  shown in Figure 2. 
requires  that Ein be more positive than Eout: 
cur ren t  increaser  in the inductor, charging the output capacitor and 
storing magnetic energy in the inductor. 
rhor t r  due to inductor voltage flyback, permitting t ransfer  of the energy 
rtored in the inductor to the capacitor. 
cu r ren t  d60 incrersss 1 2  the inductor when the rwitch i r  closed. 
rborod in the inductor i s  t ransferred to the output capacitor through tbo 
dio& whop the switch is opened. 
l o r r l 4 ~  converrion from one voltage level to another, since the cycle erdr 
when inductor current,  and thus the energy r tored in the inductor, i r  sero.  
In e i t b r  Came ,  control of E 
Two methodr fo r  
The f i r r t  method 
When the ewitch is closed 
When the switch opens the diode 
In the aecond configuration, 
The energy 
Both of there mothod. provide theoretically 
is achieved by varying switch duty cycle. 
out 
Examination of the recond method of accomplishing r e r o n m t  regulation 
hdicabr  that it should ba poarible to replace th hown wlth 
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a tapped inductor and include additional di0d.s and crpici tors .  
various output taps a re  tightly coupled, multiple output voltagos can ba 
If the 
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obtained which are related by the turns ratios of the various taps. It 
is also possible to store magnetic energy in the pr imary  magnetizing fpduc- 
Lulce of a t ransformer with multiple secondary windings, am shown in 
Figure 3. Areuming taro leakage inductances fo r  the transformor,  the 
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I various output voltages are simply related by tho relative turns ratio8 m o n g  
the multiple secondary windings. Since energy is placed on tho output 
capacitors during flyback, the output voltages can  be controlled fndepen&m.tly 
from the input voltage. 
convsraion to multiple output voltagea in 8 ringle stage. 
Thus the basic method parmitr both roguktion and 
If the switch 
drive is arranged so that a constant energy i s  t r l n e f s r r s d  from the input 
to the output on each cycle of operation, the output voltage can be regulated 
by varying the frequency of operation of the switch. In this case,  conversion 
losses  are directly related to the power being t ransferred,  permitting a 
design with efficiency which i e  e Beentially independent of operating power 
level. 
The f i r s t  prototype of a power supply utilixing this principle employed 
This design had the 2 a blocking oscillator as the energy transfer element. 
advantage of extreme simplicity (only two transistor. were used) but &a 
maximum efficiency obtained war only 70 percent at the 200 m w  level. 
The inefficiency resulted primarily f rom rather poor control of the rwitch 
trans iebr  in the blocking o scillator configuration. 
A detailed block diogrun for  a more recently t e a t e d  version is ahown 
in  Mgure 4. One of the seven output voltage6 i o  cornparad with a saner 
reference,  and the e r r o r  signal applied to a voltage to frequency converter. 
This v to f converter has extremely sharp c h a r a c t e r i a ~ c s ;  with the 12 volt 
output approximately 50 m v  below the referonce, the frequency output i a  
a mucirnum (30 KC for the experimental model) while a 50 r n t  high voltage 
k 
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(4) 
cuts off the v to f converter. Since the reference network and tha r tat ic  
power requirements of the v to f converter m u r t  be rupplied continuourly 
and  thus represent  a power overhead, there circui t .  a r e  darignad for  
extremely low power operation, with a combined dimripation of approximately 
v 
250 pw. 
vibrator which drives the switch transistor.  
Output pulres from the v to f converter t r igger  a one-rhot mdti- 
Since the static power 
requirement8 of the one-shot murt  bo rupplied continuourly, 8 monortable 
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version of the bai ic  four t ransis tor  complementary rwitching circuit is 
ured  which rerultr in a static dioripation of rpprorimrtmly 50 pw. Output 
pulre width of the one-shot i r  inversely related to the input supply voltage 
in orde r  to keep the energy t ranr fer red  from input to output pe r  cycle 
constmt.  
being delivered to the lord. 
Thus the frequency of operation i r  linearly related to the power 
The experimental converter described a b v e  delivers a maximum 
power of one watt. 
i s  approximately 0. 5 mw. When the converter is loaded with equal value 
r e r i s t o r s  connected between the tl2, t6, and -9  volt output8 and ground, 
an incremental  efficiency in exce i r  of 84 percent i o  obtained. The efficiency 
f ~ r  m y  nparating - _  power level can be obtained f rom the empir ical  relationrhip 
Total dissipation with no load connected to the converter 
7 7 =  L- Power Out - Power Out 
Power In 
- ~~ 
(1. 19) Power Out t 5 x 
where all powerr a r e  exprerred in wattr. 
exceeding 80 percent is obtained at a 10 mw output power level. 
Thur, for  example, 8 x 1  efficiency 
The converter function8 adequately with no measurab18 change in 
efficiency for  input voltage variationr f rom 10 to 30 voltr. The t12 
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vult uutput i s  the best regulated since feedback is obthined f rom thio 
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output. Regulation of the t 1 2  volt output is  within f 2 percent for all 
combinations u f  output powe r and input voltage teated over the temperature 
r ange  of -60°C to f 110 C. 
poorer due primariiy to t ransformer leakage inductance. However, if 
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Regulation of the other outputs is  somewhat 
the ratios of currents  supplied from the various output voltage8 are 
maintained constant within a factor of 2: 1, regulation to within f 10 percent  
is obtained from the 3 volt outputs, with regulation fo r  all other outputr 
to within i 5 percent. 
Approximately half of the converter incremental  loseer are attributable 
to the forward drops of the output diodes. 
increase in  efficiency can be obtained by using synchronous rectification. 
Additionally, carefui redesign of the reference network, v to f converter 
and one-shot should  reduce the power overhead to 0. 2 m w  o r  less .  
Thug it seems that a further 
Acknowledgment - 
This urork was supported under NASA Kesearch G r a n t ,  NSG-234-6 1. 
The author i s  grateful tu  J. A. Bosco for h i s  helpful comments concerning 
the basic  de sign. 
BIBLIOGRAPHY 
1 Schmidt, W .  G . ,  e t  al, "Electronic Instrumentation of Gamma-Ray 
Telescope Aboard Orbiting Solar Observatory S-57, ' I  Record of tho  
1964 International Space Electronics Symposium, October, 1964. 
2 Wardcomeky, R. E., "A Regulated Power Supply fo r  Spacecraft 
Use ,  " Massachusetts Institute of Technology S .  B. Thesis ,  June, 1964. 
3 Baker, R. H.,  "LvZaximum Efficiency Transis tor  Switching Circuits, I '  
Lincoln Laboratory Report No. 110, March, 1956. 
CAPTIONS 
Figure 1 - Conventional DC to DC converter and regulator. 
Figure 2 - Two methods for resonant regulation. 
Figure 3 - Method for achieving regulation and conversion to multiple 
voltages in a single stage. 
Figure 4 - Block diagram of experimental 1 watt converter-regulator.  
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A Low-power High-efficiency Dc to Dc Converter

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