U.S. patent application number 09/789344 was filed with the patent office on 2001-08-23 for switched-mode power supply.
Invention is credited to Preller, Peter.
Application Number | 20010015900 09/789344 |
Document ID | / |
Family ID | 7878226 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015900 |
Kind Code |
A1 |
Preller, Peter |
August 23, 2001 |
Switched-mode power supply
Abstract
The invention relates to a switched mode power supply, whereby
the number of components is reduced to a considerable extent in
comparison with conventional switched mode power supplies. The
switching transistor of the switched mode power supply is
controlled by a processor on the secondary side of said switched
mode power supply. Control thereof is determined by the software of
the processor and can be slightly modified and adapted. The
required processor functions can be taken over by a processor that
is already contained in the device which is powered by the switched
mode power supply.
Inventors: |
Preller, Peter; (Munchen,
DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
Post Office Box 2480
Hollywood
FL
33022-2480
US
|
Family ID: |
7878226 |
Appl. No.: |
09/789344 |
Filed: |
February 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09789344 |
Feb 20, 2001 |
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PCT/DE99/02538 |
Aug 13, 1999 |
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Current U.S.
Class: |
363/16 |
Current CPC
Class: |
H02M 3/33515
20130101 |
Class at
Publication: |
363/16 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 1998 |
DE |
198 37 919.6 |
Claims
I claim:
1. A switched-mode power supply, comprising: a rectifier
configuration having an output supplying a rectified voltage; a
transformer having a primary winding connected to said rectifier
configuration and a secondary winding; a switching transistor
connected to said primary winding for a clocked application of the
rectified voltage to said primary winding; a drive unit for driving
said switching transistor, said drive unit being coupled to said
secondary winding and being D.C.-isolated from said switching
transistor; and a voltage source connected on a secondary side of
said transformer for providing an operating voltage to said drive
unit during a startup of the switched-mode power supply.
2. The switched-mode power supply according to claim 1, which
comprises a further transformer connected between said drive unit
and said switching transistor.
3. The switched-mode power supply according to claim 1, wherein
said drive unit is D.C.-connected to said secondary winding.
4. The switched-mode power supply according to claim 1, wherein
said voltage source is a rechargeable battery.
5. The switched-mode power supply according to claim 1, wherein
said voltage source is an electrolytic capacitor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/DE99/02538, filed Aug. 13, 1999,
which designated the United States.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a switched-mode power supply having
a rectifier configuration which supplies a rectified voltage at its
output, a transformer with a primary winding and a secondary
winding for connecting a load, a switching transistor for the
clocked application of the rectified voltage to the primary
winding, and a drive unit which controls the switching
transistor.
[0004] In switched-mode power supplies the current flowing through
the primary winding of the switched-mode power supply transformer
is chopped by means of the switching transistor. The voltage pulses
induced on the secondary side of the transformer are rectified and
smoothed in order to supply a load with the D.C. voltage. The
turn-on and turn-off phases of the switching transistor are
controlled in dependence on the loading in such a way that the
secondary voltage is regulated to be constant and as far as
possible independent of load.
[0005] A power supply of the generic type is disclosed, for
example, in my earlier, commonly assigned U.S. Pat. No. 5,420,776.
The control unit there contains an integrated circuit designated
TDA 4605 which, for the purpose of supplying the operating voltage,
is connected via voltage dividers with a bridge rectifier. The
latter rectifies the input A.C. voltage. The output of the control
unit is connected to the control terminal of the switching
transistor which is coupled with the primary winding. The withstand
voltage of the control unit must thereby be chosen in dependence on
the strength input A.C. voltage.
[0006] FIG. 3 of published PCT application WO 97/50165 discloses a
switched-mode power supply which has a rectifier configuration at
the output of which a rectified voltage is available which, by
means of a switching transistor, is applied to a primary winding
belonging to a transformer and connected in series with the
switching transistor. A drive circuit of the switching transistor
is coupled to the secondary coil in that prior art switched-mode
power supply, and is connected to a control terminal of the
switching transistor by means of a transformer. Published PCT
application WO 98/03136 discloses a switched-mode power supply in
which a D.C. voltage which is available is applied by means of a
switching transistor to a primary winding belonging to a
transformer and connected in series with the switching transformer.
In order to drive the switching transistor, a pulse generator is
provided, which is coupled to the control connection of the
switching transistor by means of a transformer. In that case, the
pulse generator is connected to a secondary coil of the
transformer. In order to permit the switched-mode power supply to
be started up, a start circuit configuration is provided on the
primary side, which picks up energy from the D.C. voltage that is
present on the primary side and which is connected to the control
terminal of the switching transistor.
[0007] 2. Summary of the Invention
[0008] The object of the present invention is to provide a switched
mode power supply which overcomes the above-noted deficiencies and
disadvantages of the prior art devices and methods of this general
kind, and which is simplified in terms of its circuitry on the
primary side.
[0009] With the above and other objects in view there is provided,
in accordance with the invention, a switched-mode power supply,
comprising:
[0010] a rectifier configuration having an output supplying a
rectified voltage;
[0011] a transformer having a primary winding connected to the
rectifier configuration and a secondary winding;
[0012] a switching transistor connected to the primary winding for
a clocked application of the rectified voltage to the primary
winding;
[0013] a drive unit for driving the switching transistor, the drive
unit being coupled to the secondary winding and being D.C.-isolated
from the switching transistor; and
[0014] a voltage source connected on a secondary side of the
transformer for providing an operating voltage to the drive unit
during a startup of the switched-mode power supply.
[0015] In accordance with an added feature of the invention, the
drive unit is connected to the switching transistor via a further
transformer.
[0016] In accordance with an additional feature of the invention,
the drive unit is D.C.-connected to the secondary winding.
[0017] In accordance with a concomitant feature of the invention,
the voltage source is a rechargeable battery or an electrolytic
capacitor.
[0018] The invention has the advantage that commercial
microprocessors can be used for driving the switching transistor.
The drive unit does not have to be designed for the mains voltage;
instead it can be designed for the lower voltage on the secondary
side.
[0019] The driving function of the microprocessor can be undertaken
by a processor which is already present in the device to be
supplied with power by the switched-mode power supply, for example
the deflection processor in television sets.
[0020] Driving the switching transistor by means of the
microprocessor is very flexible and, by changing the software, can
be adapted with a relatively low outlay and even for relatively
small numbers. Driving is preferably carried out digitally, but can
also be carried out analog.
[0021] Since the entire control outlay on the primary side is
dispensed with, the latter substantially only contains the power
transistor, the bridge rectifier, the mains filter and, possibly, a
current pump for the sinusoidal current uptake.
[0022] Mains-voltage monitoring, short-circuit disconnection and
further special functions, such as burst operation in standby, can
likewise be implemented as software.
[0023] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0024] Although the invention is illustrated and described herein
as embodied in 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.
[0025] 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
[0026] FIG. 1 is a schematic circuit diagram of a
processor-controlled switched-mode power supply; and
[0027] FIG. 2 is a graph showing the variation of the drive voltage
VG under different loads.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The invention is based on the principle of driving a
switched-mode power supply on the secondary side, via a processor,
rather than on the primary side. Referring now to the figures of
the drawing in detail and first, particularly, to FIG. 1 thereof,
there is seen a processor-controlled switched-mode power supply. A
mains filter F, to which an input A.C. voltage U.sub.E can be fed,
is connected to the input of a rectifier configuration BR. The
rectifier configuration BR is normally a bridge rectifier. The
output of the rectifier BR is connected to a primary winding L1 of
a transformer Tr1. A primary capacitor C1 is connected between the
output of the rectifier BR and a primary reference potential VSS1.
A further connection of the primary winding L1 is connected to the
primary reference potential VSS1 via the drain/source path of a
switching transistor T1. On the secondary side there are a first
secondary winding L2 and a second secondary winding L3 of the
transformer Tr1. The second secondary winding L3 forms a series
circuit with an output diode D2 and an output capacitor C4. An
output voltage V2 can be tapped off via the output capacitor C4.
The first secondary winding L2 likewise forms a series circuit with
a supply diode D1 and a supply capacitor C3. A supply voltage V1 is
dropped across the supply capacitor C3 and is fed to a processor
.mu.P. In the exemplary embodiment, a rechargeable battery ES is
connected in parallel with the supply capacitor C3. The capacitors
C3 and C4 in each case are used to smooth the voltages induced in
the windings L2 and L3 on the secondary side.
[0029] The anode connection of the supply diode D1 forms a common
node with a connection of the first secondary winding L2 and a
first resistor R1. Via this node, the first resistor R1 is
connected to an input connection IN of the processor .mu.P. An
output connection OUT of the processor .mu.P is connected to a
secondary reference potential VSS2 via a series circuit comprising
a secondary resistor R2, a series capacitor C2 and a first winding
L5 of a transformer Tr2. The primary reference potential VSS1 is
connected to the control electrode of the switching transistor T1
via a second winding L4 of the transformer Tr2. Across the second
winding L4 of the transformer Tr2 there is a drive voltage VG.
[0030] The input filter F, the bridge rectifier BR, the primary
capacitor C1 and the switching transistor T1 form the primary side
of the switched-mode power supply. The secondary side is formed by
the processor .mu.P, the first and second resistor R1, R2, the
series capacitor C2 and the two output circuits, each having a
secondary winding of the transformer Tr1. The transformer Tr1 and
the transformer Tr2 are located between the primary and secondary
sides.
[0031] The supply voltage V1 across the supply capacitor C3 is
tapped by the processor .mu.P via supply terminals VA. One terminal
of the supply connections VA is connected to the secondary
reference potential VSS2. The processor evaluates the supply
voltage V1 and, at its output OUT, outputs a pulse-like output
signal AS on the basis of this voltage V1. That signal is fed via
the second resistor R2 and the series capacitor C2 to the first
winding L5 of the transformer Tr2. In the second winding L4 of the
transformer Tr2, pulses are accordingly induced and are passed on
to the control electrode of the switching transistor T1. In
accordance with these pulses, the switching transistor T1 is turned
on and off. On the basis of the changes in the magnetic flux, in
each case a voltage is induced in the secondary windings L2, L3.
The voltage induced in the first secondary winding L2, reduced by
the voltage drop across the supply diode D1, is present as a supply
voltage V1 on the supply connections VA of the processor .mu.P. The
latter therefore regulates its supply voltage V1 via the path
comprising transformer Tr2, switching transistor T1 and transformer
Tr1. It is likewise possible to regulate a different output voltage
than V1, if this voltage is fed to the processor .mu.P via a
voltage divider. Because of the magnetic coupling of the secondary
windings L2 and L3, a voltage across a load, which is connected in
parallel with the capacitor C4, is also regulated.
[0032] The transformer Tr2 replaces the optocoupler, which is
otherwise usual in the feedback path in switched-mode power
supplies, for the D.C. isolation of primary and secondary side. In
addition, it ensures transformation of the pulse-like output
signals AS from the processor .mu.P, which have a peak value of 3.3
V or 5 V, for example, to the higher voltage of the pulses VG,
which are 12 V, for example. The transformer Tr2 ensures that the
switching transistor T1 is driven with a higher voltage than that
supplied by the processor .mu.P.
[0033] In order that the current cannot exceed the saturation limit
in the second winding L5 of the transformer Tr2, the series circuit
comprising the second resistor R2 and the series capacitor C2 is
connected between the input of the winding L5 and the output OUT of
the processor .mu.P. Depending on the duty factor of the pulse-like
output signals AS, a corresponding D.C. voltage level of the drive
signal VG is then established.
[0034] Referring now to FIG. 2, there is shown the voltage waveform
for a very low and a very high output power from the switched-mode
power supply. The coherent, hatched areas are voltage-time
products. The area of the left-hand hatched areas is approximately
equal to the area of the right-hand hatched areas. The drive
signals VG lie in the right-hand voltage range when the ratio
between the lowest possible (left-hand pulse waveform in FIG. 2)
and the highest possible (right-hand pulse waveform) voltage of the
drive signals is about 2. In the exemplary embodiment, the lowest
and the highest voltage of the drive signal VG are approximately
between 10 V and 20 V. The duty factors of the drive signals VG in
switched-mode power supplies normally are between 0% and 50%.
[0035] The drive power which the processor .mu.P must apply in
order to drive the switching transistor T1 via the transformer Tr2
is not substantially higher than the power which is needed to drive
the switching transistor T1 directly without any transformer, since
the transformer has a high efficiency. The drive power of the
switching transistor, for example a power MOSFET, is given by the
periodic charge reversal of its gate capacitance. If, for example,
the gate capacitance is 1 nF and the voltage of the drive signal is
12 V, and if the switching frequency of the switching transistor T1
is about 50 kHz, then the drive power is about 0.5*1 nF*(12
V).sup.2*50 kHz=3.6 mW. This low drive power is provided, for
example, by an integrated driver stage of the processor .mu.P.
Recently introduced, so-called cool-MOS power transistors exhibit
only about 1/5 of the gate capacitance with the same turn-on
resistance as conventional SIPMOS transistors.
[0036] Even the startup of the switched-mode power supply must take
place from the secondary side, since there is no longer any
circuitry on the primary side. There are a number of possibilities
of providing the operating voltage for the processor .mu.P on the
secondary side. The energy for the first startup of the
switched-mode power supply, that is to say if no voltage has yet
been induced on the secondary side by the transformer Tr1, can be
made available by the voltage source ES, which may be a
rechargeable battery, an appropriately large electrolytic
capacitor, which is protected from being discharged in standby
operation by regular switching pulses, or by a standby power supply
unit which may possibly already be present. Since, before a device
is switched on for the first time, an electrolytic capacitor is
completely empty and the power supply unit could not start up, a
coil coupled to the mains switch of the device would be expedient,
in which coil, when the switch is operated, a sufficiently high
voltage for the startup is induced. The advantage would be that,
when the mains plus of the device is plugged in with the main
switch already switched on, the device cannot switch itself on.
[0037] Even mains-voltage monitoring can be carried out on the
secondary side. When the switching transistor T1 is turned on, a
negative voltage is induced in the first secondary winding L2 of
the transformer Tr1. This voltage is proportional to the voltage
across the primary capacitor C1. Mains voltages, under voltages or
over voltages are registered, and appropriate control measures are
initiated by the processor .mu.P.
[0038] The processor .mu.P also performs monitoring for a secondary
short circuit. It is supplied with the comparison voltage V1 via
the supply connections VA. At the input connection IN, the
processor .mu.P is supplied with the zero crossings of the
transformer Tr1 via the first resistor R1. These zero crossings are
also evaluated in the event of a short circuit, and in this way
oversaturation of the transformer Tr1 and destruction of components
is prevented. The processor .mu.P then outputs appropriate signals
to turn off the switching transistor T1.
[0039] The switched-mode power supply according to the invention
can be constructed more cost-effectively as compared with
conventional switched-mode power supplies and, at the same time,
can be employed more flexibly with regard to its special functions.
Since the special functions are implemented in software, different
control specifications can be achieved with a single circuit
construction.
[0040] In the event of damage occurring, the switched-mode power
supply can be checked very simply, by a pulse generator being
connected to the control electrode of the switching transistor T1
and the connection to the transformer Tr2 being broken. It is then
possible to establish whether the fault is located in the
switched-mode power supply itself, on the secondary side or in the
processor.
* * * * *