Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-09-03
2004-03-23
Berhane, Adolf D. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000
Reexamination Certificate
active
06711034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a DC—DC converter, a regulation method for a DC—DC converter, and a switched-mode power supply.
2. Description of the Related Art
DC—DC converters, also denoted converters, are known for converting an input-side DC voltage into an output-side DC voltage. A DC voltage present on the input side is first converted (chopped) into a switched AC voltage, and this AC voltage feeds a circuit comprising the primary side of a transformer. In this way, at least one voltage, which, after rectification, is available as an output DC voltage, is generated on the secondary side of the transformer.
Such DC—DC converters are also known as a decisive assembly of switched-mode power supplies. They comprise a switched-mode power supply input circuit for connection to the electricity power grid and for generating an intermediate circuit DC voltage. The intermediate circuit DC voltage feeds the incorporated DC—DC converter.
Numerous circuits for DC—DC converters are known. They comprise, on the one hand, very simple circuits in which the inverter only consists of a single controlled switch. In these simple circuits, the switched AC voltage is directly fed to the primary side of the transformer, so that the primary-side circuit thus consists of only the primary side of the transformer. On the other hand, also resonant converters are known in which a primary-side circuit is supplied with a switched-mode AC voltage generated by a half-wave bridge or a full-wave bridge, the primary-side circuit being built up from at least one capacitance and at least one inductance, the inductance often not being present as a discrete component, but as the stray inductance of the transformer.
European Patent Application No. EP 0 898 360 discloses a method and a device for controlling a DC rectifier with an AC intermediate circuit. An inverter with a controlled half-wave bridge or full-wave bridge generates an AC voltage which is transformed via a transformer and rectified on the output side. The secondary side of the transformer then comprises a winding with a middle tap, while the output voltage is rectified by two-way rectification. In this publication, the problem is discussed of the saturation of the magnetic flux in the transformer core. To achieve a waveform here which shows the least possible saturation it is proposed that the switched-mode AC voltage generated by the inverter is symmetrical, i.e., that within a predefined time interval, a voltage pulse is generated that is as long positive as it is negative. This publication, however, exclusively discusses a control method and not a regulation of the output voltage. In this case, a converter topology is provided which does not utilize a resonant arrangement.
In resonant converters, it is a known fact that the output voltage is regulated, the drive of the inverter forming the setting variable then. If, for example, a half-wave bridge generates a pulse-width modulated voltage for supplying power to a resonant arrangement mentioned above, the output voltage can be regulated by varying the frequency of the pulse-width modulated voltage, i.e., the closer the switching frequency is to the resonant frequency, the more noticeable is a resonant voltage increase and the higher the (rectified) output voltage is.
U.S. Pat. No. 5,986,895 describes a DC current converter comprising an inverter driven by a pulse-width modulation for supplying power to a resonant circuit in the form of a resonant capacitance and the primary side of a transformer. The secondary-side voltage of the transformer is rectified and filtered at a secondary winding having a middle tap thereby forming two branches each including one diode and each generating a DC output voltage. The output voltage is regulated via the drive of the inverter with variable pulse width. To reduce losses during switching, the current through the resonant circuit is measured and the switching times are selected such that, when a minimum and a maximum frequency are adhered to, switching only takes place when the current reaches a lower threshold value. U.S. Pat. No. 5,986,895 additionally describes the problem of losses during the rectification of asymmetrical output voltages. This problem is solved by the use of secondary-side synchronous rectifiers instead of diodes.
As a matter of fact, secondary-side asymmetries in the paths containing rectifier elements (diodes) form a problem for DC—DC converters. secondary-side asymmetries arise in that, with multipath rectification (for example, by a four-diode bridge or by two single diodes with a winding with a middle tap), the individual branches, and thus the individual rectifier elements, are loaded differently, i.e., different powers, voltages or currents are to be managed. This leads to the fact that when the circuit is designed, the asymmetrically loaded components are to be designed according to their respective maximum load. For example, components are to be dimensioned with a larger, for example, double power dissipation, so that, partially, additional cooling measures are necessary. Also, any output filter for filtering the rectified output voltage needs to be able to process the lower frequency portions (sub-harmonics) resulting from asymmetry and to have a larger structure for this and also for a higher current standing wave ratio.
A reason for such asymmetries may be, for example, tolerances in the stray inductance of the output-side windings or tolerances in the forward voltages of the diodes. Building the circuits for avoiding such asymmetries with the aid of components that have smaller tolerances leads to highly increased cost. Furthermore, the tolerances cannot always be avoided because, for example, the forward voltage of a diode depends on temperature.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a DC—DC converter and a regulation method for a DC—DC converter in which asymmetrical loads of the secondary-side rectifier are avoided to a maximum extent.
This object is achieved by a DC—DC converter comprising an inverter for generating a switched AC voltage, a primary-side circuit supplied with power from the inverter, the primary-side circuit comprising the primary side of a transformer, the transformer having a secondary side including at least one rectifier for generating at least an output DC voltage (Vo), a measuring unit for measuring an electrical magnitude of DC—DC converter, a symmetry calculation unit for calculating, from the measurement, a parameter for the symmetry deviation of the electrical magnitude, and a symmetry regulation unit for changing the drive of the inverter in dependence on the symmetry deviation so that the symmetry deviation is minimized.
According to the invention, asymmetrical loads are avoided in that an electrical magnitude is measured and a symmetry criterion is applied thereto. Accordingly, driving the inverter then provides that it is excited so that the output load is symmetrical.
In the DC—DC converter according to the invention, a measuring arrangement is provided for measuring an electrical magnitude where various primary and secondary-side magnitudes are involved. For example, the primary-side current can be measured by the transformer. Likewise, it is possible to measure the voltage at a primary-side capacitance. A further possibility comprises the measuring of a secondary-side magnitude, preferably the rectified secondary-side voltage. Accordingly, suitable measuring arrangements are sufficiently known to the person of ordinary skill in the art.
A symmetry calculator unit uses the measuring result to calculate therefrom a parameter for the symmetry deviation from the electrical magnitude. Preferably, the waveform of the electrical magnitude is considered here, and for an observation interval, the symmetry thereof is determined, i.e., the deviation from a symmetrical waveform. Various criterions may be used for this, according to which the symmetry is judged and a parameter for the deviation of symmetry is form
Duerbaum Thomas
Elferich Reinhold
Sauerlaender Georg
Berhane Adolf D.
Koninklijke Phillips Electronics N.V.
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