Switching power-supply unit with improved start-up behavior

The voltage present at an input is chopped by a switch which is actuated by a pulse-width modulator controlled by the output voltage and is smoothed at least by a low-pass filter, consisting of an inductance and a capacitance, at the output. In order to reduce to a minimum the delay between the time an input voltage is applied and the time a rated value is at least approximately reached by the output voltage, the inductance is bypassed by a bypassing circuit which has a low inductance and which conducts if the output voltage is below a minimum voltage and in other cases is turned off. It contains, for example, a switch which is constructed as a switchable current source and which is actuated by a comparator comparing the output voltage with a reference voltage. In transformer-coupled embodiments, isolation between the output and the input is possible. The supply unit is suited, for example for supplying auxiliary devices of an instrument transformer from the current of the power-line to be monitored.

BACKGROUND OF THE INVENTION 
The present invention relates to a switching power-supply unit with a 
DC-voltage input, which unit contains a pulse-width modulator controlled 
by the output voltage and at least one switch, actuated by the pulse-width 
modulator, and, at its output side, a low-pass filter including of an 
inductance and a capacitance. In this arrangement, a DC-voltage applied to 
the input is chopped by the switch and the AC-voltage generated in this 
manner is smoothed out by the low-pass filter at the output. The 
pulse-width modulator is controlled by feedback in such a manner that the 
output voltage approximately maintains a pre-set rated value. 
Such switching power-supply units are used in order to efficiently generate 
a constant DC-voltage which is suitable, for example for powering of 
electronic units from a possibly fluctuating DC-voltage source. 
A switching power-supply unit (Buck-type regulator) of this general type is 
known (International Semiconductor Databook 1981, pp. 731-733 by Unitrode 
Corporation or U. Tietze, Ch. Schenk: Halbleiter-Schaltungstechnik, 5th 
edition, pp. 391-394, Springer-Verlag 1980), in which a switch actuated by 
the pulse-width modulator is located between the input and the low-pass 
filter, and the input of the low-pass filter is connected to zero, for 
example ground, through a free-wheeling diode. 
Furthermore, a switching power-supply unit of this type is known 
(International Semiconductor Databook 1981, pp. 733, 734) which contains a 
transformer having a symmetrical primary and secondary winding, the center 
tap of the primary winding being connected to a DC-voltage input and the 
outer connections each being connected by a switch, actuated by the 
pulse-width modulator, to ground and the secondary winding, in conjunction 
with two diodes, forming a full-wave rectifier the output of which is 
connected to the input of the low-pass filter. 
In such switching power-supply units, the inductance of the low-pass filter 
limits the rate of rise of the current to a maximum value permissible for 
the correct operation of the circuit. If an input voltage is applied to 
the de-energized supply unit, the current through the inductance rises 
only slowly and the capacitance of the low-pass filter is charged at a 
correspondingly slow rate; thus there is a considerably delay before the 
output voltage reaches its rated value. The lower the frequency of the 
chopper and the greater the dynamic range of the load current which has to 
be covered with the prescribed constant value of output voltage and the 
larger consequently the values of the components of the low-pass filter, 
the greater is this delay. 
Further symmetric chopper circuits are known which work with a constant 
duty-cycle and where only a capacitor is connected to the output of the 
full-wave rectifier for smoothing the output voltage. These choppers do 
not exhibit a voltage-regulator effect. The output voltage is proportional 
to the input voltage, the proportionality constant approximately equal to 
the ratio between the number of secondary turns to the number of primary 
turns on the transformer. When the input voltage is switched on, the 
capacitor at the output is rapidly charged; the rate of charging is 
determined only by ohmic resistances of the transformer windings and of 
the electronic switch in conjunction with the dimensioning of the 
capacitor. 
None of these circuits are suitable for applications, where, apart from the 
voltage-regulator effect, it is also required that the rated value of the 
output voltage should be reached as rapidly as possible after the input 
voltage has been applied. 
One case of application in which such requirements occur is, for example, 
that of supplying units for monitoring a power-line (for example 
instrument transformers) or auxiliary devices of such units powered from 
the current of the line to be monitored. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is intended to improve this condition by means of the invention. The 
present invention achieves the objective of creating a switching 
power-supply unit of the same class in which the output voltage at least 
approximately reaches its rated value with very little delay after an 
input voltage has been applied. 
The objectives of the invention are essentially achieved by bypassing the 
inductance of the low-pass filter in a switching power-supply unit of this 
class with a bypassing circuit, the impedance of which has an inductive 
component which is low in comparison with the inductance of the low-pass 
filter and which conducts if the output voltage is below a minimum value 
which is not higher than the rated value of the output voltage, and in 
other cases does not conduct. This causes the capacitance of the low-pass 
filter to be quickly charged by a rapidly rising, high charging current 
immediately after an input voltage has been applied, until the selected 
minimum value of the output voltage is reached. As soon as sufficient 
current flows through the inductance of the low-pass filter for 
maintaining this minimum value, the bypassing circuit ceases to conduct.

In FIG. 1 a switching power-supply unit is shown which, in its basic 
construction, contains a pulse-width modulator 1 of the commercially 
available type (for example SG 1225A by Silicon General) which, controlled 
by the output voltage, actuates a switch 2 at the input 3 which is 
followed at the output by a low-pass filter consisting of an inductance 5 
located between the switch 2 and the output 4 and of a capacitance 6 
located between the output 4 and ground. The input of the low-pass filter 
is connected to ground via a free-wheeling diode 7. 
According to the invention, the inductance 5 is bypassed by a bypassing 
circuit 8. This circuit contains an impedence which, in its simplest 
embodiment, is constructed as a resistance 9 and a switch 10, which is 
actuated by a comparator 11 that compares the output voltage with a 
reference voltage applied to the reference input 12. 
In normal operation, that is when the output voltage is above a minimum 
value, the bypassing circuit 8 is not conducting. Under the control of the 
pulse-width modulator 1, the switch 2 periodically opens for the duration 
of an interval, the length of which in each case depends on the input and 
the output voltage. During the interval in which the input voltage is 
switched through to the low-pass filter, the current through the 
inductance 5 rises, since the input voltage is higher than the output 
voltage and the diode 7 is cut off. If the switch 2 is open, a decreasing 
current flows through the diode 7 and the inductance 5, since the output 
voltage is higher than the voltage present at the input of the low-pass 
filter. The output voltage is smoothed by the capacitance 6. 
If an input voltage is applied to the de-energized supply unit, the 
comparator 11 is activated and compares the output voltage with a 
reference voltage. Since the former is far below the minimum value, the 
comparator closes the switch 10, whereupon a high current flows through 
the resistance 9, which limits the current to the extent that an 
overloading of the switch 10 is prevented, which current charges the 
capacitance 5 until the output voltage has reached the minimum value, 
whereupon the comparator 11 opens the switch 10. As soon as sufficient 
current flows through the inductance 5 for maintaining the output voltage 
above the minimum value, the bypassing circuit 8 stays turned-off and the 
supply unit operates in normal mode. The minimum value of the output 
voltage, which is the threshold below which the bypassing circuit 8 will 
conduct, is determined in accordance with the application-related voltage 
stability requirements. In most cases 80% of the rated value should be 
adequate. 
FIG. 2 shows a transformer-coupled switching power-supply unit with a 
push-pull circuit and isolation of input and output. The center tap 15 of 
the symmetric primary winding of the transformer 13 is connected to the 
DC-voltage input 3, the outer connections 14a, 14b of the primary winding 
are alternately connected by electronic switch 2a and 2b to the zero point 
of the primary circuit. A pulse-width modulator 1 controls the closing 
times of these switches 2a, 2b as a function of the voltage at output 4, 
or as a function of a voltage which is proportional to the output voltage, 
respectively. On the secondary side, an arrangement consisting of the 
secondary winding having connections 16a, 16b and the diodes 17a, 17b, 17c 
and 17d forms a full-wave rectifier, the output of which is connected to 
the input of a low-pass filter consisting of the inductance 5 and the 
capacitor 6. This filter then forms the average of the pulse-width 
modulated voltage at the output of the rectifier. The feedback from the 
output 4 to the pulse-width modulator 1 is established by isolating 
transmission means 18, for example an isolation amplifier. 
According to the invention, the power-supply unit again has a bypassing 
circuit 8 consisting of an impedance, in the simplest case a resistance 9 
and a switch 10, actuated by a comparator 11, which circuit bypasses the 
inductance 5, the comparator 11 again comparing the output voltage with a 
reference voltage applied to its reference input 12. 
When an input voltage is applied to the de-energized supply unit, the 
capacitance 6 is again charged through the bypassing circuit 8 until the 
output voltage has reached the minimum value. In normal operation, the 
bypassing circuit 8 is not conducting. 
In both embodiments, it is of advantage to construct the switch 10 in the 
bypassing circuit 8 as a switchable constant-current source, since in this 
manner the current can be given an optimum value largely independent of 
the voltage drop across the bypassing circuit. 
FIG. 3 shows an illustrative embodiment of the bypassing circuit 8 for a 
positive voltage polarity, having a switchable constant current source and 
the low-pass filter feeding the output 4 of a DC-to-DC converter according 
to the invention with the bypassed inductance 5 and the capacitance 6. The 
constant-current source of known type is formed by a transistor 19, an 
emitter resistance 20, a Zener diode 21 and a base-collector resistance 
23. The value of the current is determined by the Zener diode voltage and 
the emitter resistance 20. The emitter output of the current source is 
connected to the output of the filter through a diode 22. The diode 22 
prevents the current source from being reversed in polarity during the 
time interval in which the output voltage of the rectifier is zero because 
of pulse-width modulation and the voltage at the output 4 has a positive 
value. The comparator 11 contains a second transistor 24 and a voltage 
divider 25, which is connected between the output voltage and ground, and 
which drives the transistor 24, the collector of which is connected to the 
base of the transistor 19 and its emitter is grounded via a Zener diode 
26. 
If the output voltage is low, the voltage at the base of the transistor 24 
is lower than the sum of the base-emitter voltage of the transistor 24 and 
of the breakthrough voltage of the Zener diode 26. The transistor 24 is 
cut off and the transistor 19 conducts. If the output voltage reaches the 
minimum value, the voltage at the base of the transistor 24 increases to 
such a value that this transistor and the Zener diode 26 used as reference 
voltage source begin to conduct. The base voltage at the transistor 19 
drops and it is cut off. This embodiment of the bypassing circuit 8 is 
particularly simple and reliable. 
In FIG. 4, a switching power-supply unit is shown the basic construction of 
which corresponds to that of the power-supply unit explained in connection 
with FIG. 2 except that the transformer 13 also has a center tap 27 which 
permits the full-wave rectifier to be constructed with only two diodes 
17a, 17b. 
According to the invention, the secondary winding of the transformer 13 has 
two further taps 28a, 28b which in each case branch off, preferably 
symmetrically with respect to each other, between the center tap 18 and 
one of the outer connections 16a or 16b, respectively, the bypassing 
circuit 8 consisting of two diodes 29a, 29b via each one of which one of 
the taps 28a, 28b is connected to the output 4. Under normal conditions of 
operation, the voltage between the taps 28a, 28b is a fraction of the 
voltage between the connections 16a, 16b as given by the relative number 
of turns between the respective connections. In normal operation, 
therefore, the voltage supplied by the latter, rectified by the diodes 
17a, 17b and smoothed by the inductance 5 is higher than that supplied by 
the taps 28a, 28b and rectified by the diodes 29a, 29b and the latter are 
cut off. However, if an input voltage is applied to the de-energized 
supply unit, first a high voltage drop is created across the inductance 5 
because of its retarding action on the current increase and the 
capacitance 6 is rapidly charged through the diodes 29a, 29b. In order to 
achieve DC-isolation between the input 3 and the output 4, the output 
voltage is simulated at least approximately by the following circuit: a 
secondary auxiliary winding 30 has a center tap 31, which is connected to 
the primary circuit zero potential and two outer connections 32a, 32b 
which are connected by diodes 33a, 33b, which form a full-wave rectifier, 
to the input of a low-pass filter. The low-pass filter consists of an 
inductance 34 and a capacitance 35 in parallel with which a resistance 36 
is connected. At its output, a voltage divider formed by resistances 37, 
38 is located from which the voltage controlling the pulse-width modulator 
1 is taken. This avoids the pulse-width modulator 1 being connected to the 
output 4. 
This embodiment can be recommended as being especially simple, reliable and 
cost-effective particularly in those cases where the value of the input 
voltage at the time of switch-on is accurately known and it is not 
expected that this value will be significantly exceeded in operation. 
The devices, shown in connection with FIG. 2 and FIG. 4 for feedback with 
isolation, naturally can also be combined with the respective other 
embodiment of the invention or, if necessary, can also be totally omitted 
and the output 4 can be connected directly or through an impedance to the 
pulse-width modulator 1 as in the embodiment of FIG. 1. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as being limited to the particular forms disclosed, since these 
are to be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.