Data processing: measuring – calibrating – or testing – Calibration or correction system – Circuit tuning
Reexamination Certificate
1999-08-24
2002-04-30
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Circuit tuning
C702S198000, C363S041000, C315S194000, C315S224000, C315S308000
Reexamination Certificate
active
06381551
ABSTRACT:
Method for providing an output signal having a desired value of a characteristic quantity at the output of a switched-mode power supply unit and circuit for carrying out the method
FIELD OF THE INVENTION
The present invention relates to a method for providing an output signal having a desired value of a characteristic quantity at the output of a switched-mode power supply unit, at least one low-voltage lamp being connected to the output of the switched-mode power supply unit, and also to a circuit for carrying out the method.
BACKGROUND OF THE INVENTION
The problems addressed by the present invention will be described using the example of the circuit arrangement known from the prior art and shown in FIG.
1
. In order to operate low-voltage lamps, for example incandescent halogen lamps
10
,
12
, they are connected individually or in a parallel arrangement to the output terminals of a switched-mode power supply unit
14
. The switched-mode power supply unit
14
is connected to an operating voltage U
B
, for example the mains voltage. The inputs
16
,
18
of the switched-mode power supply unit
14
are connected to a drive circuit
20
, which provides in-antiphase, pulse width-modulated squarewave signals U
e1
, U
e2
at its outputs, which signals have a high frequency in comparison with the mains frequency. The drive circuit could also provide just one of the two signals U
e1
or U
e2
to the switched-mode power supply unit
14
, in which case it would then be necessary to provide a component in the switched-mode power supply unit in order to generate from the signal U
e1
or U
e2
the respective other signal. A typical frequency for the square-wave signals is 50 kHz. The switched-mode power supply unit
14
illustrated by way of example comprises a half-bridge comprising the transistors T
1
and T
2
, two capacitors C
1
and C
2
and also a transformer Tr, the primary side of which is connected to a point between the two transistors T
1
, T
2
, on the one hand, and to a point between the two capacitors C
1
, C
2
, on the other hand. The secondary side of the transformer forms the output of the switched-mode power supply unit
14
.
The transformer is dimensioned for example for a maximum power of 300 watts. Depending on how many lamps are connected in parallel between the output terminals of the switched-mode power supply unit and/or the power for which the corresponding lamp or the corresponding lamps is or are rated, the input signals
16
,
18
must be varied in order to provide a desired root-mean-square value U
arms
of the output voltage U
a
, as required by the associated lamp specification, at the output of the switched-mode power supply unit
14
. Even slight deviations can lead to a considerable reduction of the service life of the lamp or lamps.
In accordance with the teaching of the prior art, in order to set the desired root-mean-square value U
arms
of the output voltage U
a
, the instantaneous root-mean-square value U
arms
of the output voltage U
a
is detected in a detection device
22
and compared with a predetermined reference value
26
in a comparator
23
. Depending on the comparison result, a control signal
28
is applied to the drive circuit
20
, which leads to the pulse width of the output signal of the drive circuit being modified in the drive circuit
20
in such a way that the root-mean-square value U
arms
of the output voltage U
a
of the switched-mode power supply unit
14
approximates to the reference value
26
. The output voltage U
a
is regulated via the feedback loop with the components
22
,
24
,
26
until the root-mean-square value U
arms
of the output voltage U
a
corresponds to the reference value
26
. In the event of changes in the operating voltage U
B
and associated changes in the root-mean-square value U
arms
of the output voltage U
a
, regulation to the reference value is effected in a corresponding manner by means of the aforementioned feedback loop.
The solution according to the prior art has two crucial disadvantages: firstly, the shape of the output voltage U
a
is dependent on the load connected between the output terminals of the switched-mode power supply unit, that is to say a change in the load leads not only to a change in the root-mean-square value U
arms
of the output voltage U
a
, but also to a change in the signal waveform. Therefore, merely detecting the amplitude of the output voltage U
a
does not suffice to determine the root-mean-square value U
arms
of the output voltage U
a
. Rather, it is necessary to carry out a genuine root-mean-square value measurement. In view of the high frequency components in the output voltage U
a
, this means a considerable outlay on circuitry.
Secondly, the safety requirements with regard to the accessibility of the output terminals of the switched-mode power supply unit (SELV) prescribe potential isolation between mains voltage and output voltage U
a
(4 kV test). The feedback loop between output voltage U
a
and drive circuit
20
would likewise have to satisfy these safety requirements and therefore have to be realized with a transformer or an optocoupler. This would likewise mean an additional outlay on circuitry.
SUMMARY OF THE INVENTION
The present invention is based on the object, therefore, of providing a method by means of which, irrespective of fluctuations in the operating voltage U
B
and irrespective of the number of connected incandescent halogen lamps or the power of the connected incandescent halogen lamp(s), the output voltage U
a
having the desired root-mean-square value U
arms
is provided at the output of the switched-mode power supply unit, in conjunction with the lowest possible outlay on circuitry.
Furthermore, the object of the present invention is to provide a circuit for carrying out a method of this type.
These objects are achieved by means of a method having the features in accordance with claim 1 and a circuit having the features in accordance with claim 10.
The particular advantage of the solution according to the invention is that the dependence of the root-mean-square value U
arms
of the output voltage U
a
on the operating voltage U
B
, the load current I
L
and the pulse width PW of the input signal or the input signals of the switched-mode power supply unit, that is to say the recording of the family of characteristic curves, can be performed at the factory using high-quality instruments and the dependencies that have been determined can be stored in an inexpensive memory module which is added to each switched-mode power supply unit. A further important advantage is that the output voltage U
a
of the switched-mode power supply unit is not used as the input quantity of the feedback path. The outlay on circuitry for complying with the abovementioned safety requirements for the feedback path is obviated as a result of this.
In a particularly advantageous embodiment of the present invention, in a switched-mode power supply unit having at least two transistors, which are in the on state alternately, for the purpose of determining the amplitude of the load current I
L
, the voltage dropped across a resistor in the output circuit of one of these at least two transistors while this transistor is in the on state is evaluated. In an even more favourable realization, for the purpose of determining the amplitude of the load current I
L
, the voltage dropped across the output terminals of one of these at least two transistors on account of the internal resistance of this transistor while this transistor is in the on state is evaluated.
Further advantageous developments of the invention are defined in the subclaims.
An exemplary embodiment is described in more detail below with reference to FIG.
2
.
REFERENCES:
patent: 5537006 (1996-07-01), Wilson et al.
patent: 5740021 (1998-04-01), Lecheler et al.
patent: 5961203 (1999-10-01), Schuda
Bessone Carlo S.
Hoff Marc S.
Patent-Treuhand-Gesellschaft fuer elektrische Gluehlampen mbH
Vo Hien
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