Note: Descriptions are shown in the official language in which they were submitted.
12 016S21
PHT. 82.319
The invention relates to a current supply circuit
comprising a non-controlled switched-mode converter as a
d.c. voltage converter with a transformer and also at least
a controllable switch and a controller for pulse-width
control.
In the periodical Elektronik, 1978, Vol. 4,
pages 102-107 several current supply circuits are des-
cribed which comprise a d.c. voltage converter. There are
two basic types of d.c. converters: the blocking oscilla-
tor and the switched-mode type. The push-puIl converter
is a special type of switched-mode converter.
With all types of d.c. converters the input volt-
age is chopped by means of a switching transistor. The
square wave voltage thus obtained is transformed by means
of a transformer and subsequently rectified and filtered.
The filtered output voltage of the d.c. converter is
stabilized because of the fact that the duty cycle of the
square-wave voltage is controlled by means of a control
cirouit. Fluctuations of the voltage to be transformed or
of the load resistor to which the filtered output voltage
is applied consequently effect a change in the du*y cycle.
This control of the duty cycle is alternatively denoted
puIse-width control.
In the transmission technique, current supply
circuits which are remotely fed v a line from a d.c.
source are used in terminal equipment and repeaters. Such
a remote supply by means of a d.c. voltage as the remote
supply source is effective up to a line length of approx-
imately 2 km as the voltage losses alony the line then
still remain within acceptable limits. If in contrast
therewith voltage must be supplied over a longer distance
up to, for example, 130 km, as is often the case in the
transmission technique the d.c. voltage source is replaced
by a a.c. source which sends a constant current through
the remote-supply line to the individual load or loads, for
example, repeaters. When several loads are fed from the
same remote-supply source then all loads are arranged in
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PHT. 82.319 2
series with the current source when a direct current source
is used, while when a d.c. voltage source is used they are
arranged in parallel with the voltage source. To ensure
that the voltage losses along the line remain low, the
current must be small. Consequently, when a voltage source
is employed as the energy source, a high voltage is advan-
tageous whereas when a current source is used as the energy
source a small current is advantageousO When for the
supply from a current source different currents are required,
a current supply circuit which converts the remote supply
voltage into the operating current for the load is provided
for each load.
The current supply circuits described in the above-
mentioned periodical are however not so suitable for con-
verting a current produced by a direct current source intoa higher current, as when the circuits are used thus a
reduction of the load~at the output causes the input resis-
tance and consequently also the input power to be increased
instead or to be decreased. As a result thereo~ they have
a comparatively high dissipation.
The invention has therefore for its object to
provide a current supply cirauit which has a comparatively
low dissipation when supplied from a direct current source.
This object is accomplished, according to the
invention, by a current supply circuit comprising a non-
controlled switched-mode converter as a d.c. voltage
converter with a transformer and also at least a control-
lable switch and a controller for pulse width control,
characterized in that the input of the d.c. voltage con-
verter is connected to a direct current source, that afurther controllable switch, which is pulsed-width con-
trolled by the controller is~arranged in parallel with the
input of the d.c. voltage converter and that the control-
lable switches~are controlled such that they are never
simuItaneously conductive~
An advantageous embodiment of the invention con-
sists of a current supply circuit wherein the d.c. voltage
converter is a single-end switched-mode converter and the
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PHT. 82. 319 3
controllable switch of the single-ended switched-mode con-
verter is controlled in the push-pull mode compared with
the further controllable switch.
The invention will now be described in greater
detail by way of example with reference to drawings, in
which:
Flg. 1 shows a first embodiment of a current
supply circuit according to the present invention and
Fig. 2 shows a second embodiment of a current
supply circuit according to the present invention.
In the embodiment shown in Fig. 1, the d.c. con-
verter is in the form of a single-ended converter Gl. One
side of the current source Q is connected to the collector
of a transistor T2 via.a choke DR and the other side to
15 the emitter of the transistor T2. A series.arrangement
formed by the primary winding of a transformer U and the
collector-emitter path of:a transistor Tl is arranged in
parallel with the collector-emitter path of the transistor
T2. The terminals of the secondary winding of the trans-
20 former U:are interconnected.via a series-arrangement of a
diode Dl:and a capacitor C. Connected in parallel with
this capacitor C are the output A of the current supply
circuit and the input of:a con.troller R, whose output is
connected to both the base of the transistor T2 and, via an
25 inverter I, to the base of transistor Tl.
The transformer U, the diode Dl, the capacitor C
and also.the transistor Tl provide a non-controlled,
sin.gle-ended converter without storage choke in the output
circuit. Its mode of operation will not be described in
detail here, as it is already sufficiently known from the
:above-mentioned periodical.
The duty cycle of the square-wave.voltage for the
transistors Tl and T2 is chan.ged by means of the controller
R. The controller R directly drives the transistor T2 and
35 the transistor Tl via:the in.verter I, so that the two tran-
sistors are operated in the opposite phase. As transistor
Tl conducts - the transistor T2 is then in the non-
con.ductive state - the current source Q applies energy to
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PHT. 82.319 4
the d.c. voltage converter Gl. Since the transistor T2
conducts during the non-conducting state of the transistor
Tl, the current source Q is not operated in the no-load
mode but, when the line resistance is disregarded, is short-
circuited. The two transistors must not both be in thenon-conductive state simultaneously, but they may conduct
simultaneously. Consequently no high voltage can build-up
at the input of the current supply circuit, as would occur
in no-load operation, so that the dissipation of the
current supply circuit and consequently also the load on
the current source remain low. For a better understanding
of the way in which the invention functions it should here
be borne in mind that a voltage source~and a current source,
both when short-circuited~and operated in the no-load mode
are loaded differently: a voltage source is absolutely not
loaded when operated in the no-load mode, but is highly
loaded when short-circ,uited. For a current source this is
just the other way around. A current source is loaded very
highly at~a high-~mpedance load, whereas it is absolutely
not loaded when short-circuited.
The choke DR has for its object to prevent cur-
rent peaks in the transistors Tl and T2 and sudden voltage
transients on the line, which may occur during switching
of the two transistors due to parasitic line capacitances.
Without the choke DR the line to the current source Q were
consequently not interference-free, the current source Q
would in that case be connected to a not nonreactive
current supply circuit.
Fig. 2 shows a second embodiment of the invention,
in which the d.c. voltage converter is a push-pull switched-
mode converter.
Arranged in parallel with the poles of the direct
current source Q is~a series arrangement formed by a choke
DR and the collector-emitter path of a transistor T2, whose
emitter is'connected to the emitters of two further tran-
sistors Tl and T3. The collectors of the two transistors
Tl and T3 are interconnected via the primary winding of the
transformer U, whose cen,tre tap is connected to the collec-
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PHT. 82. 319 5
tor of the transistor T2. The terminals of the secondarywinding are interconnected via two oppositely poled diodes
Dl and D2. A capacitor C is arranged between the junction
of the two diodes Dl and D2 and the centre tap of the
secondary winding, the output A of the current supply
circuit and also the input of the controller R are arranged
in parallel with this capacitor. The base of the transis-
tor T2 is connected to the output of the controller R. The
bases of the other transistors Tl and T3 are connected to
a pulse generator TG, which controls them in t}le push-pull
mode with a square-wa~e Yoltage whose duty cycle is 1/2.
The frequencies of the controller R and the pulse generator
TG are in synchronism... The push-pull converter is pro-
vided by the transformer U, the transistors Tl and T3, the
diodes Dl and D2 and the capacitor C. Its mode of opera-
tion is described in the:above-mentioned periodical.
The transistor T2 i5 pulse-width controllçd by
the controller R. As long.as this transistor is in the
non-conductive state, the.remote-supply voltage flows from
the current source Q either.via the transistor Tl or T3 in
the primary circuit of the.transformer U,:as the two tran-
sistors Tl and T3:are controlled in the push-pull mode, so
that:always one of them is conductive. The longer the
transistor T2 conducts, the lower is the voltage at input E
of the current supply circuit:and consequently the load of
the current source~
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