U.S. patent application number 09/761236 was filed with the patent office on 2001-07-26 for method for switching off a switched-mode power supply in the event of short-circuit, and a switched- mode power supply.
Invention is credited to Preller, Peter.
Application Number | 20010009517 09/761236 |
Document ID | / |
Family ID | 7628151 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009517 |
Kind Code |
A1 |
Preller, Peter |
July 26, 2001 |
Method for switching off a switched-mode power supply in the event
of short-circuit, and a switched- mode power supply
Abstract
In order to achieve complete short-circuit protection
irrespective of a location of a short-circuit in a switched-mode
power supply having a controllable switch in a primary circuit, the
controllable switch is opened when a voltage drop across an
electrolytic capacitor in a primary circuit falls below a threshold
value which can be predetermined. For this purpose, a capacitive
voltage device containing two capacitances is connected in parallel
with the electrolytic capacitor. A potential at a center tap of the
capacitive voltage divider is compared in a comparator with a
reference voltage, which opens the controllable switch when the
potential at the center tap falls below the reference voltage.
Inventors: |
Preller, Peter; (Munchen,
DE) |
Correspondence
Address: |
Lerner and Greenberg, P.A.
P.O. Box 2480
Hollywood
FL
33022-2480
US
|
Family ID: |
7628151 |
Appl. No.: |
09/761236 |
Filed: |
January 17, 2001 |
Current U.S.
Class: |
363/21.04 |
Current CPC
Class: |
H02M 1/32 20130101; H02M
3/335 20130101 |
Class at
Publication: |
363/21.04 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 2000 |
DE |
100 02 326.6 |
Claims
I claim:
1. A method for switching off a power supplying device in an event
of a short-circuit, which comprises the steps of: providing a
switched-mode power supply including: a bridge rectifier having AC
voltage inputs receiving an AC voltage and DC outputs; an
electrolytic capacitor; a transformer having a primary winding and
a secondary winding; a first series circuit containing the primary
winding of the transformer and a controllable switch having a
control input, the first series circuit connected in parallel with
the electrolytic capacitor and together defining a parallel circuit
connected to the DC outputs of the bridge rectifier; a control unit
having a control input and an output connected to the control input
of the controllable switch; and a second series circuit containing
a diode and a capacitance and connected in parallel with the
secondary winding of the transformer; and opening the controllable
switch if a voltage drop across the electrolytic capacitor falls
below a predetermined threshold value.
2. A switched-mode power supply, comprising: a bridge rectifier
having AC voltage inputs for receiving an AC voltage and DC
outputs; an electrolytic capacitor; a transformer having a primary
winding and a secondary winding; a first series circuit containing
said primary winding of said transformer and a controllable switch
having a control input, said first series circuit connected in
parallel with said electrolytic capacitor and together defining a
parallel circuit connected to said DC outputs of said bridge
rectifier; a control unit having a control input and an output
connected to said control input of said controllable switch; a
second series circuit containing a diode and a first capacitance
and connected in parallel with said secondary winding of said
transformer; a capacitive voltage divider formed of a second
capacitance, a center tap, and a third capacitance, said capacitive
voltage divider connected in parallel with said electrolytic
capacitor; and a comparator having a first input connected to said
center tap of said capacitive voltage divider, a second input to be
connected to a reference voltage, and an output connected said
control input of said control unit.
3. The switched-mode power supply according to claim 2, including a
resistor connected between said second capacitance and said third
capacitance of said capacitive voltage divider.
4. The switched-mode power supply according to claim 3, including:
a further resistor having a first terminal connected to said first
input of said comparator and a second terminal to be connected to a
voltage for applying the voltage to said first input of said
comparator; and a further diode connected in parallel with said
second capacitance of said capacitive voltage divider which is also
connected to said first input of said comparator.
5. The switched-mode power supply according to claim 2, including a
fourth capacitance connected in parallel with said controllable
switch.
6. The switched-mode power supply according to claim 2, including a
bistable multivibrator connected between said output of said
comparator and said control input of said control unit.
7. The switched-mode power supply according to claim 6, wherein the
bistable multivibrator is a flipflop.
8. The switched mode power supply according to claim 6, wherein
said comparator, said bistable multivibrator and said control unit
form an integrated circuit.
9. The switched-mode power supply according to claim 2, wherein
said controllable switch is a field-effect transistor.
Description
BACKGROUND OF THE INVENTION
[0001] Field of the Invention
[0002] The invention relates to a method for switching off a
switched-mode power supply in the event of a short-circuit, and to
a switched-mode power supply. In the switched-mode power supply, an
AC voltage is applied to AC voltage inputs of a bridge rectifier to
whose DC outputs a parallel circuit containing an electrolytic
capacitor and a series circuit formed of a primary winding of a
transformer and of a controllable switch is connected. A control
input of the controllable switch is connected to the output of a
control unit, and in which a series circuit containing a first
diode and a first capacitance is connected in parallel with the
secondary winding of the transformer.
[0003] Switched-mode power supplies that produce a regulated output
voltage are equipped with a transformer to whose primary winding
the DC voltage outputs of a rectifier, for example a bridge
rectifier, can be connected by a controllable switch which is
switched by a control unit or by a regulator. A series circuit
containing a diode and a capacitance across which the regulated
output voltage can be tapped off is connected in parallel with the
secondary winding of the transformer. The controllable switch is
opened and closed by switching pulses produced by the control unit
or by the regulator, depending on the load on the output of the
switched-mode power supply.
[0004] By virtue of their construction, switched-mode power
supplies are resistant to short-circuits on their output. It is
known for the primary current through the controllable switch and
through the primary winding to be measured by a shunt resistor in
order to ensure short-circuit protection on the primary side, as
well.
[0005] A disadvantage of such short-circuit protection is that
losses occur in the shunt resistor.
SUMMARY OF THE INVENTION
[0006] It is accordingly an object of the invention to provide a
method for switching off a switched-mode power supply in the event
of a short-circuit, and a switched-mode power supply which overcome
the above-mentioned disadvantages of the prior art devices and
methods of this general type, in which only a negligibly small
power loss occurs.
[0007] With the foregoing and other objects in view there is
provided, in accordance with the invention, a method for switching
off a power supplying device in an event of a short-circuit. The
method contains the steps of providing a switched-mode power supply
including:
[0008] a bridge rectifier having AC voltage inputs receiving an AC
voltage and DC outputs;
[0009] an electrolytic capacitor;
[0010] a transformer having a primary winding and a secondary
winding;
[0011] a first series circuit containing the primary winding of the
transformer and a controllable switch having a control input, the
first series circuit is connected in parallel with the electrolytic
capacitor and together define a parallel circuit connected to the
DC outputs of the bridge rectifier;
[0012] a control unit having a control input and an output
connected to the control input of the controllable switch; and
[0013] a second series circuit containing a diode and a capacitance
and connected in parallel with the secondary winding of the
transformer; and
[0014] opening the controllable switch if a voltage drop across the
electrolytic capacitor falls below a predetermined threshold
value.
[0015] With regard to the method, the controllable switch is opened
when the voltage drop across the electrolytic capacitor falls below
a threshold value that can be predetermined.
[0016] With the foregoing and other objects in view there is
further provided, in accordance with the invention, a switched-mode
power supply. The switched-mode power supply contains a bridge
rectifier having AC voltage inputs for receiving an AC voltage and
has DC outputs. An electrolytic capacitor and a transformer having
a primary winding and a secondary winding are provided. A first
series circuit containing the primary winding of the transformer
and a controllable switch having a control input is provided. The
first series circuit is connected in parallel with the electrolytic
capacitor and together define a parallel circuit connected to the
DC outputs of the bridge rectifier. A control unit having a control
input and an output is connected to the control input of the
controllable switch. A second series circuit containing a diode and
a first capacitance and is connected in parallel with the secondary
winding of the transformer. A capacitive voltage divider formed of
a second capacitance, a center tap, and a third capacitance, and
the capacitive voltage divider is connected in parallel with the
electrolytic capacitor. A comparator having a first input is
connected to the center tap of the capacitive voltage divider, a
second input is to be connected to a reference voltage, and an
output is connected the control input of the control unit.
[0017] With regard to the circuit, the object is achieved in that a
capacitive voltage divider containing the second and the third
capacitance is connected in parallel with the electrolytic
capacitor, and that the center tap of the voltage divider is
connected to the first input of the comparator. The second input of
the comparator receives a reference voltage and the output of the
comparator is connected to the control input of the control
unit.
[0018] The invention is based on the knowledge that an electrolytic
capacitor represents a series circuit containing an equivalent
series resistance, an inductance and the actual capacitance. Since
the primary current rises in the event of a shortcircuit, the
voltage across the electrolytic capacitor falls owing to the
voltage drop across its equivalent series resistance. The voltage
across the electrolytic capacitor is compared in a comparator with
a reference voltage, which is used as a threshold value. If the
voltage across the electrolytic capacitor falls as a result of a
short-circuit, irrespective of the point at which the short-circuit
is located, to the threshold value, then the controllable switch is
opened, and the switched-mode power supply is thus switched
off.
[0019] The invention therefore represents perfect short-circuit
protection for a switched-mode power supply since it switches the
switched-mode power supply off when a short-circuit occurs at any
point. A major advantage of the invention is that there is no
longer any need for a shunt resistor, which produces losses. The
short-circuit protection according to the invention operates with
virtually no losses.
[0020] In accordance with an added feature of the invention, a
resistor is connected between the second capacitance and the third
capacitance of the capacitive voltage divider.
[0021] In accordance with an additional feature of the invention, a
further resistor having a first terminal is connected to the first
input of the comparator and a second terminal is to be connected to
a voltage for applying the voltage to the first input of the
comparator. A further diode is connected in parallel with the
second capacitance of the capacitive voltage divider that is also
connected to the first input of the comparator.
[0022] In accordance with another feature of the invention, a
fourth capacitance is connected in parallel with the controllable
switch.
[0023] In accordance with a further feature of the invention, a
bistable multivibrator is connected between the output of the
comparator and the control input of the control unit.
[0024] In accordance with another added feature of the invention,
the bistable multivibrator is a flipflop.
[0025] In accordance with a further additional feature of the
invention, the comparator, the bistable multivibrator and the
control unit form an integrated circuit.
[0026] In accordance with a concomitant feature of the invention,
the controllable switch is a field-effect transistor.
[0027] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0028] Although the invention is illustrated and described herein
as embodied in a method for switching off a switched-mode power
supply in the event of a short-circuit, and a switched-mode power
supply, it is nevertheless not intended to be limited to the
details shown, since various modifications and structural changes
may be made therein without departing from the spirit of the
invention and within the scope and range of equivalents of the
claims.
[0029] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a circuit diagram of a first exemplary embodiment
of a switched-mode power supply according to the invention;
[0031] FIG. 2 is a circuit diagram of a second exemplary embodiment
of the switched-mode power supply;
[0032] FIG. 3 is a circuit diagram of an equivalent circuit of an
electrolytic capacitor; and
[0033] FIG. 4 is a graph of pulsed diagrams for the switched-mode
power supply.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] In all the figures of the drawing, sub-features and integral
parts that correspond to one another bear the same reference symbol
in each case. Referring now to the figures of the drawing in detail
and first, particularly, to FIG. 1 thereof, there is shown a first
exemplary embodiment of a switched-mode power supply according to
the invention. The switched-mode power supply has a voltage UW
applied to AC voltage inputs of a bridge rectifier BR. DC voltage
outputs of the bridge rectifier BR are connected to a parallel
circuit formed of an electrolytic capacitor Cp and a series circuit
formed of a primary winding Lp of a transformer and of a
controllable switch T1. A series circuit containing a diode D2 and
a capacitance C3 is connected in parallel with a secondary winding
Ls of the transformer . A capacitive voltage divider containing a
capacitance C1 and C2 is connected in parallel with the
electrolytic capacitor Cp, and a center tap of the voltage divider
is connected to a first input of a comparator K. A diode D1 is
connected in parallel with the capacitance C2. A voltage Vp is
applied via a resistor R2 to the first input of the comparator K,
in order to clamp it at a rest potential. A reference voltage Vs is
applied to a second input of the comparator K. The output of the
comparator K is connected to a control input of a control unit SE,
whose control output is connected to a control input of the
controllable switch T1.
[0035] Since a short-circuit results in the primary current Ip
through the electrolytic capacitor Cp rising severely, the voltage
across the electrolytic capacitor Cp falls, and thus the potential
at the center tap of the capacitive voltage divider containing the
capacitances C1 and C2 also falls. If the potential at the center
tap of the capacitive voltage divider falls below the reference
voltage Vs, the comparator K emits a control signal to the control
unit SE, which then immediately opens the controllable switch T1.
This results in reliable short-circuit protection, irrespective of
the point at which the short-circuit occurs in the switched-mode
power supply.
[0036] FIG. 2 shows a second exemplary embodiment of the
switched-mode power supply according to the invention.
[0037] The AC voltage UW is applied to the AC voltage inputs of the
bridge rectifier BR. The parallel circuit containing the
electrolytic capacitor Cp and the series circuit containing the
primary winding Lp of the transformer and the path through a
field-effect transistor T1 is connected to the DC voltage outputs
of the bridge rectifier BR. A capacitance Cs is connected in
parallel with the path through the controllable switch T1 in the
form of a field-effect transistor T1. The capacitive voltage
divider, which is in the form of the series circuit containing the
capacitance C1, a resistor R1 and the capacitance C2, is connected
in parallel with the electrolytic capacitor Cp. The common junction
point of the resistor R1 and the capacitance C2 is connected to the
first input of a comparator K, to whose second input the reference
voltage Vs is applied. A diode D1 is connected in parallel with the
capacitance C2. A voltage Vp is applied via the resistor R2 to the
first input of the comparator K, whose output is connected to the
set input of a bistable multivibrator EFF, for example a flipflop.
An output of the bistable multivibrator is connected to the control
input of the control unit SE, whose output is connected to a gate
electrode of the field-effect transistor T1. On the secondary side
of the switched-mode power supply there is the series circuit
containing the diode D2 and the capacitance C3, across which the
regulated output voltage of the switched-mode power supply can be
tapped off, in parallel with the secondary winding Ls of the
transformer .
[0038] FIG. 3 shows the equivalent circuit of an electrolytic
capacitor which represents a series circuit containing an
inductance L, an equivalent series resistance R and the actual
capacitance C. When the current through the electrolytic capacitor
Cp rises, as is the situation in the event of a short-circuit, the
current through the equivalent series resistance R also rises, and
causes a voltage drop across the resistance. Owing to the voltage
drop across the equivalent series resistance R, the potential
across the electrolytic capacitor Cp falls and is drawn to ground.
When the voltage across the electrolytic capacitor Cp falls, the
potential at the first input of the comparator K also falls. If the
potential at the input of the comparator K falls below the
reference voltage Vs, the comparator K emits a signal to the
bistable multivibrator EFF, which emits a control signal to the
control unit SE, which immediately switches off the field-effect
transistor T1. When the field-effect transistor T1 is switched off,
no current flows either in the primary circuit or in the secondary
circuit of the switched-mode power supply, so that perfect
short-circuit protection is achieved.
[0039] The quiescent potential at the first input of the comparator
K is governed by the voltage Vp, the resistor R2 and the clamping
diode D1. The capacitors C1 and C2 form the capacitive voltage
divider. Together with the capacitance C2, the resistor R1 forms a
low-pass filter for attenuating any RF components that are present.
The capacitance Cs is used as a so-called snubber capacitor. Since
a capacitive voltage divider is provided instead of a shunt
resistor, the power loss in the short-circuit protection is
negligibly small. The comparator K, the bistable multivibrator EFF
and the control unit SE can be provided in a suitable manner in an
integrated circuit IC1.
[0040] FIG. 4 shows pulse diagrams of the switched-mode power
supply illustrated in FIG. 2.
[0041] In normal operation, the primary current Ip rises within the
permissible limits between times t0 and t1.
[0042] Owing to a short-circuit, the primary current Ip rises
severely between times t2 and t3. The voltage Vcp1 which is tapped
off at the center tap of the capacitive voltage divider and is
applied to the first input of the comparator K thus falls further.
As soon as it falls below the reference voltage Vs at the time t3,
the comparator K emits a control signal to the control unit SE,
which switches off the field-effect transistor T1. The voltage VD
across the primary winding Lp of the transformer thus rises while,
at the same time, the primary current Ip falls suddenly to the
value 0, and the potential Vcpl at the first input of the
comparator K rises once again.
[0043] The switched-mode power supply according to the invention
has the advantage that it is switched off immediately irrespective
of the location of the short-circuit.
* * * * *