Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-01-31
2002-07-16
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021040
Reexamination Certificate
active
06421255
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a forward-transfer DC/DC conversion circuit.
The present invention relates more particularly to a new forward-transfer converter circuit known as a “forward converter”.
BACKGROUND OF THE INVENTION
In the field of DC/DC converters, it is known practice to use converters to transform a unipolar voltage, which is preferably a direct voltage, into another unipolar voltage, which is preferably a direct voltage, while at the same time ensuring galvanic isolation. One type of converter is known as a “forward” converter.
Galvanic isolation consists in isolating the primary winding from the secondary winding of a transformer, also referred to hereinbelow as primary and secondary, respectively, and in making it possible to have a potential difference across the secondary which is different from that across the primary.
The transformer ratio is the voltage across the secondary relative to the voltage across the primary.
The “forward” converter has the drawback of accumulating magnetization energy on the core of the transformer.
It is known practice in the prior art to use an additional winding placed on the core of the transformer, referred to hereinbelow as a tertiary winding or more simply a tertiary, which returns this magnetization energy to the voltage source so as to extract this magnetization energy.
Several types of “forward” converter may be distinguished, in particular the one-switch forward converter, two-switch forward converter or parallel forward converter.
As regards the present invention, the one-switch forward converter forms the basis of the invention.
It has the property of being able to process very large currents, which, by actuating the switch from the open position to the closed position, make it possible to produce the non-conducting phase and the conducting phase.
The non-conducting phase serves to discharge the magnetization energy accumulated on the core of the transformer, while the conducting phase serves to convert the supply voltage.
However, in order to ensure complete demagnetization after the conducting phase, it is currently necessary for the non-conducting phase to be long enough and for it to last for at least 50% of the full cycle of the cumulative conducting and non-conducting phases.
In technical terms, this behavior is characterized by a duty cycle (time closed over total period) of less than or equal to 50%.
Two main problems arise in practice during the use of the one-switch forward converter.
First, the energy yield is reduced due to the fact that there is a conversion time, i.e. a conduction time of the switch which is shorter than the restoration or demagnetization time of the transformer, and, in addition, due to the fact that the energy contained in the leakage inductance is not recovered.
Secondly, it is necessary to oversize most of the components, and in particular the switch and the transformer, since they are subject to sudden changes in voltage or current that are very large in value.
For the one-switch “forward” converters, the prior art discloses the use of diodes of “clamping” type and/or non-dissipative switching circuits (“snubbers”).
These modifications of the standard forward converter make it possible, on the one hand, to return the energy stored in the magnetization and leakage inductances of the transformer to the voltage source, and, on the other hand, to limit, to a predetermined value, of the order of twice the input direct voltage, the overvoltages across the switch.
Thus, compared with the conventional assembly, document U.S. Pat. No. 4,268,898 proposes to insert a capacitor in series between the primary and tertiary windings of the transformer. The diode conducts in the direction of return of the energy to the source via the tertiary winding of the transformer. The capacitor between the primary and tertiary windings is similarly featured in the article by Machin and Dekter (Proceedings of: “19th International Telecommunications Energy Conference, INTELEC97—19/23 October, Melbourne, Australia”). By means of this capacitor, the diode begins to conduct once the voltage across the switch exceeds twice the input voltage.
In the assemblies disclosed in document DE-A36 34990 and in the article by Varga and Losic (Proc. 4th Annual Applied Power Electronics Conf. And Exp. (APEC), Baltimore, Mar. 13-17, 1989, pp. 40-45), it is proposed to place a second diode in series with the tertiary winding, in the same conducting direction as the primary winding. A capacitor is connected firstly between the primary winding of the transformer and the switch, and secondly between the two diodes.
As in the case of the conventional forward assembly, these variants still have the tertiary winding in the opposite direction to that of the primary winding and a cyclic ratio of less than or equal to 0.5. This is a considerable limitation with respect to the amount of energy which can be transferred to the load placed on the secondary side of the transformer.
In addition, the number of turns n
3
on the tertiary winding is generally equal to the number of turns n
1
on the primary winding (n
3
1
=1).
Document EP-A-0 474 471 discloses a converter for converting direct voltage by zero-voltage rectangular signal switching, including an input for a source of direct voltage, a transformer comprising a primary winding and, connected to the first input, a main switch connected in series with the primary winding and the input of the source of direct voltage, as well as a clock signal generator to control the main switch. This converter comprises an auxiliary switch and a magnetic reversal capacitor connected in series with the primary winding and with the input of the source of direct voltage, this auxiliary switch being controlled by the clock signal generator in phase opposition relative to the main switch, and serves to reset the transformer. It also includes switching delay means.
SUMMARY OF THE INVENTION
The aim of the present invention is to obtain an improvement in the conventional one-switch forward converter electronic circuit, while avoiding the drawbacks of the prior art.
Another aim of the present invention is to propose better exploitation of the electronic components which constitute the body of the electronic circuit and to distribute the energy better throughout all of the said components.
In particular, one aim of the present invention is to increase the power density both per unit volume and weight and per unit cost of the various components.
Another additional aim of the present invention is to recover the energy from the leakage and magnetization inductances of the transformer by a part of the circuit during the operating phases of the converter.
Another additional aim of the present invention is to improve the dynamic behaviour of the converter and to increase its handling range.
The present invention relates to a DC/DC converter controlled by a rectangular signal, comprising an input for a source of direct voltage, a transformer comprising a primary winding connected to the source of direct voltage and a secondary winding connected to the load, a single switch in series with the primary winding and the input of the source of direct voltage intended to produce the rectangular signal, as well as a tertiary winding located close to the primary and secondary windings on the core of the transformer.
The tertiary winding is preferably in series with a diode on a branch which is in parallel to the branch comprising the primary winding and the switch.
According to a first characteristic of the invention, the tertiary winding on the transformer has the same direction of winding as the primary winding.
The tertiary winding may be weakly coupled, optionally wound on a separate core.
According to a second characteristic of the invention, the converter comprises electronic components for making up a first current flow circuit to remove at least the energy contained in the leakage inductance and in the magnetization inductance of the transformer, and a second current flow circuit to r
Bleus Paul
Frebel Fabrice
Berhane Adolf Deneke
C.E. & T.
Knobbe Martens Olson & Bear LLP
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