Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-09-27
2003-08-12
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S330000, C327S532000
Reexamination Certificate
active
06605980
ABSTRACT:
The present invention relates to a rectifier circuit, in particular a rectifier circuit having a transformer, which has a primary side and a secondary side, the primary side having a first and a second input terminal for coupling in an AC voltage signal, and the secondary side comprising two secondary windings which are connected to one another and to a first output terminal, the first and the second secondary winding being connected by their other terminal to a first terminal of a first and a second diode disposed in the forward direction, and the second terminal of the first and of the second diode being connected to one another and to a second output terminal, it being possible for a load to be coupled between the first and second output terminals, the first and the second diode being realized by a first and a second MOSFET at least within a predetermined time range, by virtue of the fact that each MOSFET has a main current path with a main current direction HS and an auxiliary current path—disposed parallel to the main current path—with an auxiliary current direction NS and with a body diode disposed therein, the main current direction HS and the auxiliary current direction NS running opposite to one another, and it being possible for the first and the second MOSFET to be operated in such a way that the first and the second body diode realize the first and the second diode within the predetermined time range.
PRIOR ART
The literature discloses a rectifier circuit designated as single-phase center-tap connection. Rectifier circuits of this type are used, for example, to feed long rail and cable systems with electronic transformers. In this case, they serve for rectifying and smoothing the high-frequency output voltage. Without these two measures, the voltage drop along the line and the radio interference would be too high.
In a rectifier circuit of this type, the diodes required for rectification are often replaced by MOSFETs, in this case specifically by the body diode present in each MOSFET.
FIG. 1
illustrates a MOSFET by way of example, the three terminals being marked D for drain, G for gate and S for source. The body diode is designated by BD. The direction in which current normally flows through the MOSFET, the so-called main current direction HS, runs from drain to source. The auxiliary current direction NS is disposed parallel and runs oppositely to the main current direction HS, as indicated by the arrows. If, in this circuit, the gate is not driven by a signal, then the MOSFET, via its body diode BD, acts like a normal diode, i.e. it permits a current flow only in one direction, to be precise in the auxiliary current direction NS as defined above. The disadvantage of this rectifier circuit disclosed in the prior art consists in the high power loss. In order to be able to supply the lamps fitted to the rail and cable systems with sufficient power, very high currents flow in the rectifier circuit, for example in the range between 20 and 40 A. In the case of a current of 25 A, for example, a power loss of 17.5 W is therefore produced on the diode, given a voltage drop across the diode of 0.7 V.
The same problems arise in the case of electronic transformers for rail and cable systems, in which the rectification is realized using schottky diodes. As a consequence of the high power loss, either use only at low ambient temperatures is considered or a fan has to be used for cooling, the service life of said fan being highly limited and being between 10,000 and 20,000 hours.
SUMMARY OF THE INVENTION
Taking this prior art as a departure point, the object of the present invention therefore consists in developing a rectifier circuit of the type mentioned in the introduction in such a way that the power loss is considerably reduced.
This object is achieved according to the invention by virtue of the fact that a rectifier circuit of the generic type furthermore comprises a first and a second drive circuit, which are designed to drive the first and the second MOSFET with regard to the current flow in the main current path in such a way that the current flow in the main current path is effected in the auxiliary current direction NS.
The invention is based on the insight that, in order to realize a diode function, not only can the body diode of a MOSFET which lies in the auxiliary current direction NS be used, but also, by active driving in a suitable manner, the main current path of a MOSFET. It is furthermore based on the insight that MOSFETs can also be operated inversely with regard to their main current direction HS, which runs from the drain to the source, without incurring damage. In this case, the driving for inverse operation corresponds to the driving during forward operation, i.e. the crucial voltage is the voltage present across the MOSFET between gate and source. With the MOSFET being switched into the on state by suitable driving of the gate, the current flowing through it therefore only has to overcome a very low forward resistance, which is of the order of magnitude of 6 m&OHgr;. For the current of 25 A assumed by way of example above, a power loss of just 3.7 W is produced as a result. This low power loss makes it possible to operate a rectifier circuit according to the invention even at high ambient temperatures without using a fan.
In a preferred embodiment, the first and the second drive circuit can be operated in such a way that the current flow through the respective MOSFET is effected in the main current path in the auxiliary current direction NS at least in a first time range and in the auxiliary current path in the auxiliary current direction NS in a second time range. This measure makes it possible for the diode function to be realized by the main current path in a first time range—by suitable driving of the gate—and in a customary manner by the body diode of the MOSFET in a second time range. The realization of the diode function by the body diode should, of course, be considered with regard to the reduction of the power loss only when the total current flowing through the MOSFET has a very small value or the time range in which the current flows is very short and hence the power loss of this current on the body diode is very low. This is the case in particular in the event of commutation of the current from one transistor to the other.
Furthermore, it is preferably provided that the first and the second drive circuit can be operated in such a way that no current flow at all takes place through the respective MOSFET at least in a third time range.
In a particularly advantageous embodiment, the first and the second drive circuit comprise a first and a second auxiliary winding connected to the transformer. This makes it possible to generate the drive signals for the two MOSFETs in conjunction with a very low outlay. When a large current is transformed by the transformer, the auxiliary windings generate a large signal, which is used for driving the gate electrode of the respective MOSFET. With this large signal, the MOSFETs can be switched without any difficulty into the on state, which is why the current which is actually flowing through the MOSFETs exactly at this moment traverses the MOSFET in the main current path, but in the auxiliary current direction NS. If only a very small current is transformed by the transformer, the signal obtained via the auxiliary windings does not suffice, under certain circumstances, to switch the respective MOSFET into the on state by driving of the respective gate electrode. This is entirely unproblematic in the present case, however, since it is exactly then that the current is also small which now flows through the respective MOSFET in the auxiliary current path in the auxiliary current direction NS and thus only a low power loss is produced on the body diode. If the two auxiliary windings are, moreover, assigned to the secondary side in terms of potential, the DC isolation between primary side and secondary side is realized in a particularly cost-effective and simple manner in this way. As an alternative, however, pr
Bessone Carlo S.
Lam Tuan T.
Nguyen Hiep
Patent-Treuhand-Gesellschaft fuer elektrische Gluehlampen mbH
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