1967, No. 8
251 .
Current-limiting circuits for transistorized power supplies
R. Gasser and R. Hug
One of the areas of electronics in which valves have nearly completely been superseded by
semi-conductors is that of stabilized d.c. power supply units. The very low internal resistance
of these units makes it necessary to safeguard them against excessive current drain. In this
paper the authors discuss the principles of the circuits used for this purpose.
Introduetion
In stabilized d.c. power supply equipment valves
have nowadays almost completely been replaced by
diodes and transistors. This has brought about a reduetion in size and in price of the apparatus involved.
For this reason the fields of application for stabilized
power supply units is continuously increasing. They are
now used in places where batteries or unstabilized reetifiers were formerly employed.
A well-stabilized power supply provides a d.c. voltage which is scarcely affected by variations of the mains
voltage or ofthe load current. Stabilization againstload
current variation implies that the units have very low
internal resistance.
Another requirement is that the output voltage
should be independent of temperature. As several semiconductor parameters depend on temperature, special
attention must be paid to this point in transistorized
equipment.
Circuit of a stabilized prwer supply
The operation of a stabilized power supply has been
previously discussed in Philips Technical Review by
Klein and Zaal berg van ZeIst [11. The principles, which
are those of a feed-back circuit, will be put in perspective with the aid of jig. 1. The unregulated voltage
supply (which is nearly always a two phase rectifier)
with e.m.f. U« and internal resistance Ri is connected
to the output terminals a and b via the control transistor TrI. This transistor is driven by the amplifier A,
which amplifies the difference between a fraction k of
the output voltage UI and a fixed reference voltage U».
The control transistor acts as a resistance in series with
the load RI, the value of this series resistance being affected by the difference amplifier A. The sign of the
amplification is such that an increase of UI causes an
increase of the equivalent resistance of TrI. With a
R. Gasser and R. Hug are with Philips A.G. Zürich.
A
Fig. I. Simplified diagram of a stabilized power supply unit. The
unstabilized voltage supply with e.m.f. U« and internal resistance
R; is connected to the output terminals a and b via the transistor
TrI. TrI is controlled by the amplifier A, which amplifies the
difference of a certain fraction k of the output voltage UI and a
fixed reference voltage Uv. RI load resistance. ft load current.
sufficiently large amplification of A the output voltage
can be effectively stabilized in this way. This includes
stabilization against variations caused by changes of
the uncontrolled voltage U« as well as those originating
from a changing load current Ir. The latter effect is
often called regulation [11.
As the internal resistance of a well-stabilized power
supply is very low, the connection of a smallioad resistance brings about a very large current, possibly damaging the power supply as well as the circuits connected
to it. As a means of proteetion a fuse can be used,
but in many cases a heavy overload or even a shortcircuit may occur so suddenly that a fuse is too slow
to prevent damage of the equipment. For this reason
electronic circuits are incorporated in the power supply
to prevent the current .from exceeding a certain limit.
[1)
See G. Klein and J. J. Zaalberg van Zeist, Combinations of
valves and transistors in a stabilized 2000 V power supply,
Philips tech. Rev. 25, ~81-190, 1963/64, G. Klein and J. J.
Zaal berg van Zeist, Instrumental electronics, Philips Techni- '
cal Library 1967.
PHILlPS
252
TECHNICAL
These circuits act so quickly that no harm can be
caused to any component. The design of these currentlimiting circuits forms the subject of this article.
VOLUME28
REVIEW
F
u'n
U,
t
Different protection methods
For controlling a current-limiting
circuit an indication is needed ofthe value ofthe load current. This can
be obtained by connecting a small "resistance Rp in
series with the load (jig. 2). The voltage across Rp controls the proteetion circuit P. When this voltage exceeds
a certain value, P supplies a signal to one of the stages
of the amplifier A. The control transistor is then affected
in such a way that the required proteetion is obtained.
The ways of operation
of such proteetion
circuits
may differ considerably; we shall now deal with some
of the usual methods.
/.
,/
/'
/'
/'
"/
/'
/'
/'
/'
/'
/'
/'
/'
00
-1,
I'm
Fig. 3. Voltage-current characteristic of a stabilized power supply
with cut-out protection. UI output voltage with nominal value
Uin. fI output current with maximum value fIrn. When the fullload point F is exceeded the equipment is cut-out (dashed line).
such as the one occurring when a capacitive load is
connected, cuts offthe power supply. This disadvantage
may be partly overcome by introducing a certain time
delay in the cut-out process.
Another disadvantage, which is not eliminated by a
_--,
time delay, is that the power supply unit cannot be
I
used to find out the cause of an overload; the equipI
!
I
,il
ment simply "refuses" to deliver a voltage to a load
:UI! IR,
I
"
resistance which is too small.
I
Lr'
:tII
I
I
I
I
I
i
Current limiting
~----------------------~~--~~-J
L._._._j
b
&-
Fig. 2. To safeguard the equipment against overload, a proteetion
circuit P is included. This circuit is controlled by the voltage
across a small resistance Rp in series with the load. The circuit P
delivers the appropriate signal voltage to one of the stages of the
amplifier to give the required proteetion against overload.
The objections mentioned
do not occur when a
proteetion method is followed in which attainment of
the maximum allowable current Iu« does not cut off the
equipment. A decrease of the load resistance below the
lowest permissible value (Uln/ Irrn) may cause the voltage
tó be reduced in such a way that the current is kept at
the value Iu« (fig. 4). With this system, however, an-
Cut-out proteetion
With the first system the exceeding of a certain
value Iu« of the load current Ir causes the control transistor to be cut-off; the output voltage UI drops from
the nominal value Uln to zero and so does the current.
This is illustrated in jig. 3, which shows the voltagecurrent characteristic
of an ideally stabilized power
supply provided with the proteetion system mentioned.
Mostly the operation takes place in such a way that the
cut-out condition is only terminated after pushing a
special button or after switching the equipment off and
then on again. If the overloading is then still present,
the cut-out comes into action again.
In this way very effective proteetion may be obtained
with a simple circuit. However, some objections to
this method can be raised. It is often considered as inadmissible that a very shortlasting strong current pulse,
u,n~----------------------------~F
U,
t
°o~--------------------------~
_1,
I'm
Fig. 4. Proteetion by a pure current-limiting circuit. When the
full-load point F is reached the current does not exceed the value
IIn{. This entails however a heavy power loss in the control transistor.
.
1967, No. 8
CURRENT-LIMITING
other difficulty arises. The decrease of the output
voltage brings about an increase of the voltage on the
control transistor. The power dissipation in this transistor may reach a far greater"value than that occurring
when the power supply is fully loaded (point F); if
there is a short circuit of the terminals (Ul = 0), almost the whole nominal power is dissipated in the control transistor. This must be taken into account in the
design of the equipment: it may be necessary to put
two or more transistors in parallel and it may be
necessary to fit large cooling fins to obtain sufficient
cooling. The dimensions ofthe apparatus could be considerably increased in this way.
CIRCUITS
253
If we denote the constant power loss in the control transistor
TrI by P«, the equation for the curve FC in fig. 7 can be written in
the following form (see also fig. 2):
UI
=
Pir
UU-J;-lJ(Ri
+
Rp).
The slope of this curve at point Fis:
dUI)
(
dit
= (Ptr)F _ (Ri
F
fJm2
+ Rp),
Power limiting in the control transistor
Excessive power dissipation in the control transistor
is prevented if a proteetion system is used in which a
decrease of. the output voltage is accompanied by a
decrease of the current in such a way that the power
dissipation in the control transistor is kept constant,
The relation between voltage and current is now given
by a line similar to the one shown infig. 5. In this case
no additional power losses in the control transistor need
to be taken into account; when the power supply is
overloaded, or even short-circuited, the dissipation in
this transistor stays equal to the value at full load
(point F). As no extra measures for cooling need to be
taken, this may enable the designer to reduce the dimensions of the equipment. A more complicated circuit is however the price that must be paid. Another objection to this system is that jumps in the voltage and
the current occur at the start and finish of the overload
of the power supply. This is illustrated in fig. 6:
When the load current has its maximum value (point F)
even a small decrease of the load resistance makes the
voltage and current jump to the values corresponding
to point B. Further decrease of the load resistance is
accompanied by a continuous fall of voltage and
current till the short-circuit point C is reached. At a
subsequent increase of the load resistance the voltage
and current rise till point D is reached, and a jump to
point E then occurs.
The irregularities mentioned may be quite troublesome. It is possible to design the circuit in such a way
that the voltage and current jumps do not occur. Obviously the voltage-current characteristic should then
have a shape like that shown infig. 7: the straight line
connecting F with the origin should not interseet the
curved line Fe. However, a characteristic like this can
only be realized in a device having a ratherlow efficiency.
It can be shown that the efficiency (the ratio of the
power delivered to the load to the power delivered by
the uncontrolled supply (see figs. 1 and 2) is less' than
50 % if such a characteristic is used.
°o~------L-------------------~
-II
Fig. 5. Proteetion by a circuit which keeps the power loss in the
control transistor at a constant value.
UI
i
-II
flm
Fig. 6. With a constant power-loss system the operating point
will jump from F to B and from D to E. C is the-short-circuit
point.
Fig. 7. With a characteristic like the one shown (Fe), no voltage
and current jumps occur. This entails however a low efficiency
of the power supply unit.
PHILIPS TECHNICAL
254
(Ptr)F being the power loss in the transistor
in fully loaded condition. Putting this slope equal to Uln/hm or larger, the yalue of
this power loss is found as:
(Ptr)F ~ Ulnhm
+
hm2(RI
+
Rp).
The full-load efficiency of the power supply is:
17
=
.
Uln hrn
Uln
hm
.-
+ (Ptr)F + hrn2(RI + Rp) .;::"
- 2 (1 +
VOLUME
REVIEW
28
supply units act as constant current sources, no such
objections can be raised to connecting several of them
in parallel: the totalload current being properly distributed over the supply units in this case.
-=--:-~
Ri
+ Rp) ,
RIn
with RIn = Uln/hm. Obviously 1'/ is always less than t. This low
value of 1'/ also applies when the power supply is not fully loaded,
the efficiency being equal to Uud U«-
Utnl-----------------t-f
F
I
iLlUI
I
I
_t
Combined methods
The advantages and disadvantages of the proteetion
methods mentioned above have led to circuits in which
several systems have been combined. The characteristic
of a commonly used hybrid circuit, consisting of a
current limiting system and a cut-out system, is illustrated infig. 8. For a continuous decrease of the load resistance, the current is first limited to the maximum
permissible value Iu«. As has been explained above the
power loss in the control transistor now increases. This
increase is however limited in this case, the power supply being cut out when the output voltage has dropped
by a certain amount LIUI.
Another hybrid proteetion method is the combination of a current limiting system and a system with
constant power loss in the control transistor. The corresponding characteristic is shown in fig. 9. Although
with the proteetion methods of figs. 8 and 9 the power
dissipation in the control transistor may exceed the
value in fully loaded condition of the power supply,
the extra amount of power loss is limited and the cooling measures required are less severe than with a
pure current-limiting system.
Yet another system is the one whose characteristic is
shown infig. 10. Here after passing the fullload point
F, the voltage and current decrease, following a line
between the constant current and the constant power
loss systems. Here again the power loss in the control
transistor increases when the power supply is overloaded,
but the increase is less than the one occurring with a
pure current-limiting system.
Apart from proteetion of the power supply and the
equipment connected to it, circuits designed for the
constant current method of fig. 4 have another important feature. As supply units with voltage stabilization have a very low internal resistance it is not possible to obtain a large current output by connecting
several of these units in parallel: obviously small differences in the voltages of the units would cause the load
current to be very unequally distributed over the power
supplies. There could even be a reversal of the current
in one or more of the power units. When however the
-11
Fig. 8. Characteristic of a hybrid system. When the full-load point
F has been reached the voltage first decreases, the current staying
constant. At a certain value zl UI of the voltage drop the equipment is cut out.
Ulnl------------------=t-.
F
I
:LlUI
I
I
.!
-11
C
Fig. 9. Hybrid system combining a constant-current
a constant-power-Ioss circuit.
circuit and
Utn·I----------------rF
-11
c
Ilm
Fig. ID. Proteetion by a circuit with a characteristic which lies
between the constant-current and the constant-power-Ioss systems.
1967, No. 8
CURRENT-LIMITING
Examples of proteetion 'circuits
The general remarks made above will now be illustrated by a brief discussion of two circuits for protection systems used in Philips power supply units.
Combined current-limiting
and cut-out system
In fig. 11 a simplified circuit diagram is shown of the
Philips power supply unit PE 4870. Fig. 12 shows a
photograph of this unit with its cover removed. The
output voltage is adjustable between 0.5 V and 60 V.
The control transistor is again denoted by TrI. Instead of one single transistor a cascade circuit of three
transistors Tri ... 3 is used. This arrangement is
used because such a circuit has a larger transconductance than TrI alone, resulting in a better regulation of
the output voltage against variations due to variations
in the load current. The supply voltage for this circuit
is delivered by the auxiliary (unstabilized) rectifier VI.
The control amplifier A is a two stage difference
amplifier (transistors Tr4... 7). lts power is supplied
by a second auxiliary (stabilized) rectifier, V2. The
A
:'_'_'_'_'-'-'-'-'-'_'_'_'_'!
CIRCUITS
255
reference voltage Us is obtained from,the voltage across
the series circuit formed by the Zener diode Zand
the diodes Dl and D2. The diodes Dl and D2 are included to compensate for variation of the Zener voltage with temperature.
The proteetion circuit P contains two transistors, Tr«
and Trs, a diode D3 and a few resistors and capacitors.
Normally Trs is non-conducting. Ifhowever the output
current exceeds a certain value this transistor starts to
conduct because of the increasing voltage on Rp. The
collector of Trs is connected to the base of the transistor Tr6 in the amplifier. In this way the transistor TrI
is controlled and a further increase in output current
is prevented. The value atwhich this current stabilization
takes place can be adjusted by means of potentiometer
RI to between 10% and 110% of the nominal output
current of the power pack; rather sensitive loads (e.g.
transistor circuits) can therefore be protected. When
the output voltage has decreased to a certain value, the
transistor Tr« starts to conduct. The additional voltage
drop now occurring on R2 causes a further decrease of
op
:'-'-'-'-'-'_'_'-'-',
i
I
I
I
I
+
i...._._.j
I.
I
Vt
r '-~o---'+.
---~___'(._.)L~-__+
I
I
i.
+04---------+--,
L._._j
a
Fig. 11. Simplified circuit diagram of the Philips power supply unit PE 4870. A is a difference
amplifier. P is the proteetion circuit which operates according to the hybrid system shown in
fig. 8. The equipment can be put into operation again by pushing the button B. VI and V2
are auxiliary voltage sources (separate rectifiers); V2 is itself stabilized.
PHILIPS
256
TECHNICAL
VOLUME
REVIEW
Fig. 12. The Philips power supply unit PE 4870 with cover removed.
control transistor can be seen at the rear.
---------
r---- - - -----------
---
The cooling
-----
fins of the
-
r11
r1,
I
I
I
I
I
I
I
I
I
R1
I
I
I
~---9------~---------+-----,
I
I
I
I
i----i
I
,~,")I\
T""-+1
\ ... _
I
I
I
Tr1t--Tr:;i---r---
...__:
,
''''~")(I
....-"
--~-t-}
r -t
I
' .......
"~""---r~
I
I
-l
I
I
I
I
I
I
I
rl~
I
ll
Tr7
1
il~
Rp2::
J
LT)
I
A
r---'
J
I
1
I
L-t--- ...----1
I
I
f:
+Ó--
I
J...:::-,
I
.11
I
',.--_...'
I
:Ul
-
o-_i
:
I
I
I
=:i:=
_:T:_
T
I
I
L
I
03
1
041
I
1
I
~.):------- ....
-1"------- - --:---
f-~
I
I
t----~---
,I
RpI::
i
I
I
A
iT\.
:
~
I
I
I
I
;._-,
I
,_...
I
I
I
I
I
;'&->1,
F \
I
I
I
I
I
I
LrJ
LTJ
1
---
--""1"--
:
I
I
I
~Ji
I ,
lrJ
-~7
i \
I
I
I
......
1
1
0
,~
I
I
- - - -
-l----6----.._-__...--._-_.._-__...-____________________________
Fig. 13_ Circuit diagram of the Philips proteetion
unit PE 4891, drawn together with a part of
the circuit of the Philips power supply unit PE 4868, to which it can be provided
as an additional unit. The circuit operates according to the system illustrated
in fig. 9 (constant-current
combined
with constant-power-loss).
Current-limiting
is brought about by the transistor
Tr3.
The constant-power-loss
system is driven by the currents in the diodes Dl and D2 which have
logarithmic
characteristics.
1
I
J
28
.--------------~----~~~~~~-~~-
1967, No. 8
---
CURR;ENT-LIMlTING
- ----------. --
CIRCUITS
257
the current in Tr«, thus decreasing the output current. about by the diodes Dl and D2. The current in Dl is
This brings about an avalanche effect and the equip- proportional to the collector-emi!ter voltage-of TrI and
ment is eventually cut out. The cut-out condition can Tr2, whereas the current in D2 (originating from the
be terminated by pushing the button B, which brings -collector current of the transistor Tr4) is proportional
Tr9 into the cut-off state for a moment. The same . to the' output current. As the characteristics of semieffect can be obtained by switching the equipment off conductor diodes are logarithmic over a certain range,
and on.
the voltage across the. two 4i.9desin series is proportional to the product of voltage and current ofthe control
Combined current-limiting and constant-power-loss systransistors, Le, to the power.dissipated in them.
I
tem
The temperature dependance of the diodes Dl and
The proteetion unit now to be described, type D2 is compensated by the use :of two other diodes D~
PE 4891, can be provided as an additional unit to the and D4 and a difference amplifier containing the tranPhilips power supply PE 4868 (voltage 0.5-30 V, cur- sistors Tr5 and Tr6. If large variations in temperature
rent 3 A). The circuit diagram has been drawn infig. 13 can occur, these elements should be housed in a contogether with a part of the circuit ofthe power supply stant temperature enclosure, In the- unloaded condimentioned.
tion of the power supply the current in D2 is set to zero
In this power supply two control transistors TrI and with the aid oî Tr» and RI, and'at the same time Tr6 is
Tr2 in parallel are used as well as two series resistors,
brought into the cut off condition by means of R2.
RpI and Rp2. The current-limiting process is obtàined
When the total voltage on Dl and D2 exceeds a certain
by the-transistor Tr3 in almost the same way as in the value, Tr5 is cut off and Tr6 starts to conduct, applying
unit previously described. The regulation with con- the voltage via the diode D5 to the output stage of the
stant power loss in the control transistors is brought control amplifier. As the diode D6 is now biased to
cut-off, further action of the;çurrent-limiting transistor
Tr3 is prevented and the load current is decreased to a
value giving a constant power loss in the control
25V
transistors. Fig. 14 shows the voltage-current characteristic obtained. This is ofthe type discussed with fig. 9,
./
20
providing a compromise between a current-limiting
system and a constant power loss system.
10
5
/
/
/
/
V
2
-It
3A
Fig. 14. Voltage-current characteristic of the Philips power
supply unit PE 4868, equipped with the proteetion unit PE 4891.
Summary. In transistorized d.c. power supply equipment electronie circuits are needed for safeguarding the supply units and
the circuits connected to them against damage due to overloadingorshort-circuiting.
Systems which cut offthe power supply
when overloading occurs can be made up from simple electronic .
circuits, but such systems have several disadvantages in practice.
If a pure current-limiting system is used, there is a large additional
power dissipation in the control transistor. Circuits have therefore
been developed which hold the power loss in the control transistor to a constant value. Hybrid circuits of these systems have
also been made. Two examples of these circuits, as used in Philips
equipment, are described.
;
,
PHILIPS
258
TECHNICAL
VOLUME 28
~VIEW
Recent scientific publications
These publications 'are contributed
by staff of laboratories and plants
part of or co-operate with enterprises of the Philips group of companies,
by staff of the following research laboratories:
which form
particularly
Philips Research Laboratories, Eindhoven, Netherlands
Mullard Research Laboratories, Redhill (Surrey), England
Laboratoires d'Electronique et de Physique Appliquée, Limeil-Brévannes
(S.O.), France
PhilipsZentrallaboratorium
GmbH, Aachenlaboratory,
Weisshausstrasse,
51 Aachen, Germany
Philips Zentrallaboratorium
GmbH, Hamburg laboratory, Vogt-Kölln. Strasse 30, 2 Hamburg-Stellingen,
Germany
MBLE Laboratoire de Recherche, 2 avenue Van Becelaere, Brussels 17
(Boitsfort), Belgium.
E
M
L
A
H
B
. Reprints of most of these publications will be available in the near future. Requests
for reprints should be addressed to the respective laboratories (see the code letter) or
to Philips Research Laboratories, Eindhoven, Netherlands.
P. Billard, J. Donjon & G. Marie: Application
modulation de Iumière aux télécommunications.
Acta electronica 9, 305-313, 1965 (No. 4).
de la
L
G. Blasse: On the Eu3+ fluorescence of mixed metal
oxides, IV. The photoluminescent
efficiency of Eu3+activated oxides.
J. chem. Phys. 45, 2356-2360, 1966 (No. 7).
E
G. Blasse & A. Bril: On the Eu3+ fluorescence in mixed
metal oxides, IlL Energy transfer in Euê+activated
tungstates and mol yb dates of the type Ln2 W06 and
Ln2Mo06.
J. chem. Phys. 45, 2350-2355, 1966 (No. 7).
E
G. Blasse & A. Bril: Broad band u.v. excitation
Sm3+-activated phosphors.
Physics Letters 23, 440-441, 1966 (No. 7).
of
E
B. J. Mulder: Photo-injection
of electrons into anthracene from electrolytic electrodes.
Solid State Comm. 4,615-617, 1966 (No. 11).
E
R. F. Pearson & R. Cooper: Magnetic crystals that
manipulate light.
New Scientist 32, 92-94, 1966 (No. 517).
M
G. W. Rathenau: Einige Gesichtspunkte zur Wechselwirkung in nichtleitenden magnetischen Kristallen.
Z. angew. Physik 21, 277-282, 1966 (No. 4).
E
H.-D. Rüpke: Über die Ankopplung von Mikrowellenresonatoren zur Messung von Materialeigenschaften.
Archiv elektr. Übertr. 20, 617-620, 1966 (No. 11). H
P. C. Scholten: Indium contacts on CdS.
Solid-State Electronics 9, 1142-1143, 1966 (No. 11/12).
E
E. Schwartz: Empirische Synthese verlustloser, symmeJ. Donjon & G. Marie: Modulateurs
de lumière à large
bande utilisant l'effet Pockels.
Acta electronica 9, 315-385, 1965 (No. 4).
L
J. A. Geurst: Theory ofinsulated-gate
field-effect transistors near and beyond pinch-off.
Solid-State Electronics 9, 129-142, 1966 (No. 2).
E
W. Kischio: Halbleitendes Cadmium- und Zinkdiphosphid.
Z. Naturf. 21a, 1733-1734, 1966 (No. 10).
A
trischer Zirkulatoren.
Archiv elektro Übertr.
20, 621-625, 1966 (No. 11). A
P. J. W. Severin & A. G. van Nie: A simple and rugged
wide-band gas discharge detector for millimeter waves.
IEEE Trans. on microwave theory and techniques
MTT-14, 431-436, 1966 (No. 9).
E
J. G. Siekman: Lassen met een elektronenbundel.
Ingenieur 78, 099-108, 1966 (No. 47).
T. L. Tansley: Heterojunction
boundary
J. appl. Phys. 37, 3908, 1966 (No. 10).
conditions.
T. Klein: Contour deposition - a new epitaxial deposition technique for semiconductor devices and integrated circuits.
Solid-State Electronics 9, 959-966, 1966 (No. 10). M
D. J. Vinney: Possible travelling-wave parametrie
plifier using the Gunn effect.
Electronics Letters 2, 357-358, 1966 (No. 10).
J. R. Mansell: Travelling-wave
J. Volger: Progress in superconductivity.
IEEE Trans. on magnetics MAG-2, 159-164,
(No.3).
Int. J. Electronics
phototube theory.
20, 467-488, 1966 (No. 5).
Volume 28, 1967, No. 8
M
pages 231-258
E
M
amM
1966
E
Published 15th July 1967