The present invention relates to a power rectifier of the type comprising
an output for connection to a supply line leading to a load, in which line there
is a cut-off device for breaking the connection between the rectifier and the
supply line in case of a malfunction of the rectifier.
There are known devices of this type. These are used when a plurality
of power rectifiers is connected in parallel to a single supply line leading to
a load, which has a considerable current requirement. In this situation, if a fault
occurs in one of these rectifiers, it is necessary to prevent the current flowing
in the supply line from entering the faulty rectifier, as this would cause a severe
disturbance in the common supply voltage (bus output).
At the present time, an arrangement of diodes is provided at the output
of the rectifier (on either the positive or the negative terminal) to prevent such
an occurrence, these diodes entering the cut-off state when the current at the
output of the rectifier tends to be inverted as a result of a fault in the rectifier.
These known devices have the advantage of a rapid response speed,
but have high dissipation when high currents are present. This not only reduces
efficiency, but also makes it necessary to provide efficient heat sinks, in thermal
contact with the diodes, to remove the heat generated by the Joule effect in the
diodes during the normal operation of the rectifier.
The object of the present invention is to provide a power rectifier
of the type described above which does not have the drawbacks of the conventional
Essentially, according to the invention the rectifier is characterized
in that the cut-off device comprises, instead of a set of diodes, a plurality of
MOSFETs in parallel and a control circuit which makes the MOSFETs non-conducting
when the output current from the rectifier tends to become inverted.
In a particularly advantageous possible embodiment of the invention,
the control circuit comprises an operational amplifier to whose inputs are applied
voltages proportional to the source and drain voltages of the MOSFET. A reactive
circuit with a capacitor is advantageously provided between the inverting terminal
and the output of the operational amplifier, to prevent oscillation of the output
of the operational amplifier in the proximity of the trigger threshold at which
the MOSFET becomes non-conducting.
Further advantageous characteristics and embodiments of the device
according to the invention are indicated in the attached dependent claims.
The invention will be more clearly understood from the description
and the attached drawing, which shows in the single figure an electrical circuit
diagram of the device according to the invention.
With reference to the drawing, the number 1 indicates in a general
and schematic way a power rectifier, whose internal structure is known to those
skilled in the art and will not be described in detail in this document. The rectifier
typically operates over a power range from hundreds to thousands of watts, for
example at levels above 100 watts, and in particular above 400 watts, and with
an output voltage which is typically of the order of several tens to several hundreds
The positive and negative terminals, indicated by 3 and 5 respectively,
of the rectifier 1 are connected to a power supply line 7A, 7B to which are connected
in parallel other rectifiers which are similar to that illustrated in the figure
and which are not shown. The supply line 7A, 7B is connected to a load indicated
in a general and schematic way by Z.
The cut-off device associated with the power rectifier 1 can be connected
to the positive output terminal 3 or to the negative output terminal 5 of the rectifier.
In the example illustrated in the figure, it is connected to the negative terminal,
and is indicated in a general way by 9. The cut-off device 9 comprises a plurality
of MOSFETs, indicated by 11A, 11B, 11C ... 11N. The MOSFETs 11A-11N are connected
in parallel with each other and are connected to the negative output terminal 5
of the rectifier in such a way that the current supplied by the latter is distributed
among the various MOSFETs and flows between the source and drain of each of them.
The gates of all the MOSFETs are connected, via corresponding resistors
13A, 13B, 13C, 13N to the emitter of a transistor 15. The collector of the transistor
15 is connected to the negative output terminal of the rectifier 1 before the group
of MOSFETs 11A-11 N, while the base of the transistor 15 is connected to the output
of an operational amplifier 17 forming the main element of the control circuit,
indicated as a whole by 19, associated with the MOSFETs 11A-11N.
The non-inverting input of the operational amplifier 17 is connected,
via a resistor 21, to the negative output terminal of the rectifier 1, before the
MOSFETs 11A-11N according to the conventional direction of flow of the current
supplied by the rectifier, while the inverting terminal of the operational amplifier
17 is connected, via a resistor 23, to the negative output terminal 5 of the rectifier
1 after the MOSFETs 11A-11N.
A reactive filter containing a capacitor 25 is provided between the
inverting terminal and the output of the operational amplifier 17, to prevent
oscillations of the output of the operational amplifier 17 in the proximity of the
The operation of the circuit described above is as follows. In normal
operating conditions, the rectifier 1 provides the supply line 7A, 7B with a current
I, which is added to the current supplied to the other rectifiers connected to
the supply line 7A, 7B, to supply the load Z. In these conditions, a limited voltage
drop is established between the source and drain of each MOSFET 11A-11N, and is
kept approximately constant until the value of the current tends toward zero, for
example if the rectifier 1 suffers an internal fault, which could lead to an inversion
of the current at the output of said rectifier, a situation which must be avoided.
When this type of fault situation arises, the voltage drop between
the sources and drains of the various MOSFETs 11A-11N causes a reduction of the
output voltage of the operational amplifier 17. The reduction of the output voltage
of the operational amplifier causes the transistor 15 to conduct. Since the transistor
15 is connected via the resistors 13A-13N to the gates of the MOSFETs 11A-11N,
its switching to the conducting state rapidly makes all the MOSFETs non-conducting.
Thus the connection between the supply line 7A-7B and the power rectifier 1 is
rapidly broken. The reactive filter formed by the capacitor 25 prevents any possible
oscillations of the output of the operational amplifier 17 in the proximity of
the intervention threshold.
It is to be understood that the drawing shows only an example provided
solely as a practical demonstration of the invention, which can be varied in respect
of its forms and arrangements without departure from the scope of the guiding principle
of the invention.