CA2168179A1

CA2168179A1 - Dc supply with power factor corrector

Info

Publication number: CA2168179A1
Application number: CA002168179A
Authority: CA
Inventors: Didier Cadinot; Daniel Petit
Original assignee: Thomcast
Current assignee: Thomcast
Priority date: 1995-01-27
Filing date: 1996-01-26
Publication date: 1996-07-28
Also published as: FR2730104A1; JPH08317644A; EP0724327A1; FR2730104B1

Abstract

The disclosure relates to the field of DC supplies, the energy of whichis given by a three-phase mains. There is proposed a supply wherein, to each of the three phases of the mains, there corresponds a rectifier assembly followed by a power factor correction cell with switching converter. The outputs of the three cells are parallel-connected. One of the converters is controlled to give a constant output voltage while the other two, owing to a master-slave assembly, are controlled to give a value of power identical to that delivered by the converter controlled to give a constant output voltage. Each converter has a protection against short-circuits and has galvanic insulation between its input and its output.

Description

DC SUPPLY WITH POWER FACTOR CORRECTOR
BACKGROUND OF THE INVENTION
The present invention relates to a DC supply whose power is provided by a three-phase mains and more particularly to a supply 5 where, to each of the three phases of the mains, there corresponds a rectifier assembly followed by a power factor correction cell based on a switching converter with the outputs of the three cells that are parallel-connected. One of the converters is controlled to give a constant output voltage while the other two, through a master-slave o assembly, are controlled to give power identical to that given by the converter that is controlled to give a constant output voltage.
A supply of this kind is described, for example, in the US patent US-A-~ 1~7 269. However, the supply proposed in this patent has drawbacks that may make its use impossible in certain cases. In 15 particular, it cannot be used to give voltage of any polarity desired with respect to the ground and it does not have any protection against short-circuits at output.
SUMMARY OF THE INVENTION
The present invention is aimed at circumventing these 20 drawbacks.
This is obtained especially by the use, in the supply, of converters with galvanic insulation, comprising protection against output short-circuits.
According to the present invention, there is provided a DC supply - 25 with a power factor corrector for three-phase mains, comprising three rectifier assemblies arranged in a triangle, each rectifier assembly having outputs, the corrector comprising a first, a second and a third power factor correction cell with a switching converter servo-controlled to impose a given value on the ratio of its instantaneous 30 input voltage to its instantaneous input current, these cells being respectively coupled to the three rectifier assemblies and having respectively three pairs of output conductors parallel-connected to form one and the same pair of output terminals, the first cell comprising means for the servo-control of its given value as a 35 function of the voltage at the output terminals, the second and third cells comprising means to servo-control their given value to the

2 21 68~ 79 difference between their delivered power and the power delivered by the first cell, the switching converter of each correction cell comprising two input terminals designed to be coupled to the outputs of the rectifier assembly to which the correction cell considered is 5 coupled, a first inductor and a second inductor magnetically coupled to each other, a power transistor with two first electrodes and a third electrode, a capacitor, a transformer with a primary winding and a secondary winding and a galvanic insulation between these two windings and a pair of output conductors with the flrst inductor and l0 the two first electrodes of the power transistor in series between the two input terminals, the third electrode constituting a switching control input and, in parallel on the two first electrodes, the capacitor followed by the primary winding of the transformer and with, in series between the conductors of the pair, the second inductor and rectifier 15 means comprising the secondary winding of the transformer associated with rectifier elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more clearly and other characteristics shall appear from the following description and from 20 the appended figures, of which:
- Figure 1 shows a general drawing of an exemplary embodiment of a supply according to the invention, - Figures 2 and 3 are more detailed drawings of parts of Figure 1.
In the different figures, the same elements are designated by the 25 same references.
MORE DETAILED DESCRIPTION
Hereinafter, there shall be proposed a DC supply made from a three-phase mains where the three phases are used to prepare three output signals directly connected in parallel to the output terminals of 30 the supply and where, in order to obtain a power factor very close to 1, means are implemented so that the energy values given by the three output signals are identical. Since these three output signals are parallel-connected, the equality of the energy values could be sought by causing the mean values of the currents pertaining to the three 35 output signals to be identical.

3 2 1 68 1 79 Figure 1 shows a DC supply which, using a three-phase mains S, gives a DC voltage V with a load E having one of its terminals grounded.
This supply has three rectifier assemblies A1, A2, A3 positioned in a triangle on the three phases of the mains S. Each rectifier assembly is formed by four diodes mounted as a bridge to perform a full-wave rectification.
The three rectifier assemblies A1, A2, A3 have their DC outputs respectively connected to th~e power inputs, referenced "+" and "-", of three DC/DC converters B1, B2, B3. These converters are switching converters controlled at a control input to impose a given value on the ratio of their instantaneous input voltage to their instantaneous input current. This given value is usually a function of their output voltage but it shall be seen hereinafter that this given value is determined slightly differently for the converters B1, B2 since it tends to keep the power delivered by each of these converters equal to that delivered by the converter B3. The converters B1, B2, B3 have their two output terminals referenced "+" and "-" connected respectively to the two terminals of the load E.
The controls of the converters B1, B2, B3 are given respectively by three integrated circuits UC3854 manufactured by the firm U NITRODE.
The control circuit E1 has its own internal reference voltage and receives the output voltage of the rectifier assembly A1, a signal le1 given by the sensor E1' representing the instantaneous current consumed by the converter B1 and a signal Vs1 delivered through a galvanic insulation circuit D1. The control circuit E1, through an input circuit F1, delivers a control signal for the converter B1. With one of the two output conductors of the converter B1, there is coupled a sensor C1' connected to a measuring circuit C1.
The description just given in the above paragraph may be repeated by replacing the number 1 by the number 2 or 1 by 3 whenever they appear. This amounts to stating that, to each phase of the mains there is connected a single-phase DC supply with, furthermore, a circuit to measure the current let through by this supply.

4 21 6~1 79 The measurement circuits C1, C2, C3 all have one output connected to a first terminal of a resistor R whose second terminal is grounded. Furthermore, the measuring circuits C1 and C2 have another output. This other output is connected, for the circuit C1, to s the base of an NPN transistor T1 whose emitter is connected to the ground by a resistor R12 and whose collector, which gives a signal Vs1, is connected by a resistor R11 to that one of the output terminals of the supply which is not connected to the ground. The description given in the above sentence for the other output of the circuit C1 can also be written for the other output of the circuit C2ifT1,R12,Vs1 and R11 respectively are replaced by T2,R22,Vs2 and R21.
The supply according to Figure 1 also has, between its output terminals, two resistors R31, R32 that are series-mounted with the resistor R32 connected to the ground. A signal Vs3is obtained at the 15 common point between the resistors R31 and R32.
The three-phase supply according to Figure 1, since it results from the parallel connection of the output voltages of three single-phase supplies, must necessarily comprise a galvanic insulation between its outputs and its inputs. This insulation is provided by the 20 converters B1, B2,B3 whose drawing is shown in Figure 2 and by the circuits D1, D2l D3 comprising a voltage/frequency converterfollowed by an optical coupling itself followed by a frequency/voltage converter. The voltage/frequency converters and frequency/voltage converters have reciprocal functions and, owing to the fact that the 25 useful information is contained not in the amplitude but in the frequency of the output signal of the voltage/frequency converter, this insulation is not dependent on the variation of the gain in the optical coupling.
Figure 2is a drawing of an embodiment of the converters B1, B2, 30 B3 of Figure 1.
The converter according to Figure 2 comprises, between its " I "
and "-" input terminals, an inductor La, the collector-emitter link of an NPN power transistor T and, in parallel on this link, a capacitor Ca followed by the primary winding of a transformer Tr. The control input 35 of the converter is connected to the base of the transistor T. The transformer Tr has a secondary winding at midpoint associated with a s 2 i 68 1 79 rectifier assembly with two diodes Da, Db whose cathodes are connected to one of the ends of an inductor Lb having its other end connected to the "~" output of the converter. A filtering capacitor Cb internal to the converter is connected between the "+" and "-" output terminals. A diode Dc has its anode connected to the "-" output of the converter and its cathode connected to the cathodes of the diodes Da, Db. This is a so-called free-wheel diode which, while it requires an operation of the converter in discontinuous conduction, prevents the application to the capacitor Ca of a destructive reverse voltage. The inductors La and Lb have a low coefficient of mutual induction so as to produce a smoothing of the output current of the converter.
The capacitor Ca gets charged when the transistor T is off and gets discharged when it is conductive. Thus, at its secondary winding, the transformer Tr gives an AC voltage that is rectified by the diodes Da, Db. As for the control of the conduction time of the transistor T, it determines the value of the input current and hence of the output current of the converter. The capacitor Ca is a protection against output short-circuits because, if the transistor T remains off, owing to the presence of the capacitor Ca, no energy is transmitted to the transformer Tr.
It must be noted that the power transistor T, in cases of the high-powered structures of the circuit according to Figure 2, may be made by MOSFET or IGBT technology. The gate of this transistor may then have an excessively high capacitance for the control circuit of the UC3854 type. This is why, in Figure 1, the input circuits F1, F2, F3 are interposed between the converters and their control circuits. These inputs circuits are, in the present exemplary embodiment, circuits such as those described in the European patent 0 230 810 filed under French priority on 13 December 1985.
It has been stated here above that the supply was designed so that the energy values given by the three output signals are identical.
These three output signals are those delivered by the converters B1, B2, B3 and the energy values that they deliver will be identical to one another if their output currents, referenced Is1, Is2, Is3 in Figure 1, are identical. This identity of currents is obtained, as can be seen in Figure 1, by a master-slave type assembly designed to prepare the 6 2 i 6 8 1 7q signals Vs1, Vs2, Vs3 delivered to the control circuits such as E1 through the galvanic insulation circuits such as D1, the signals being prepared as a function of the mean values of the currents such as Is1.
In this master-slave assembly, the converter B3 is the master for the signal Vs3 corresponds to its i, .stanlaneous output voltage multiplied by a constant coefficient constituted by the reduction ratio of the bridge of resistors R31-R32. The converters B1, B2 are the slaves since the signals Vs1 and Vs2 take account not only of the output voltage of the converter but also respectively of the divergence lO of the mean values of Is1, Is2 from the mean value of Is3.
Figure 3 is a drawing of a part of the supply according to Figure 1. This drawing is designed to show how the signals Vs1 and Vs2 are prepared. It has only the sensors C1', C2', C3', the measuring circuits C1, C2, C3, the transistors T1, T2 and the resistors R, R12, R22.
15 However, the measurement circuits are shown with details of their constituent elements.
The measurement circuit C1 has an integration circuit K1 that receives the signals given by the sensor C1' and gives an output signal representing the mean value of the current Is1 delivered by the 20 converter B1. To simplify its designation and make it easier to remember its meaning, this output signal is designated by the same reference Is1 as the current to which it corresponds.
The signal 1s1 is applied to the "+" inputs of two comparators M1 and N1. The output of the comparator M1 is connected to the anode 25 of a diode d1 whose cathode is connected to the "-" inputs of the comparators M1 and N1 and to the first terminal of the resistor R. The output of the comparator N1 is connected to the base of the transistor T1 .
The diagram of the measurement circuit C2 is the same as that of 30 the measurement circuit C1 apart from the fact that, in the references, the number 2 has been replaced by 1. As for the diagram of the measurement circuit C2, it corresponds to that of the measurement circuit C1 without the comparator N1 and its connections and with references where 1 is replaced by 3.
The unit represented in ~igure 3 by the elements M1 to M3, d1 to d3 and R as well as their connections is a circuit for the selection of 7 2 1 63 ~ 79 that output signal, among the signals Is1, Is2,1s3, which has the highest value. This signal which has the highest value is applied to the "-" input of the comparators N1 and N2. Two assumptions shall be considered here below to study the operation of the supply in s master-slave mode. These assumptions relate to the respective values of the output signals Is1, Is2,1s3 namely the mean values of the currents delivered by the converters B1, B2, B3. From these studies of operation, it will be easy to deduce the working of the supply for other assumptions on the possible respective values of these signals.
o Therefore, this study will not go beyond the above-mentioned two assumptions.
Under the first assumption, Is3>1s1 ~Is2, the comparators N1 and N2 make the transistors T1 and T2 highly conductive which, as can be seen in Figure 1, causes Vs1 and Vs2 to fall. The control circuits E1, E2 will, by feedback, increase the currents delivered by the converters B1, B2. By contrast, the current delivered by the master converter B3 will drop so as to keep Vs3 constant. The working is truly convergent.
Under the second assumption, 1s1 >1s3>1s2, the comparator N2 reacts as in the previous paragraph giving rise to an increase in the current 1s2 delivered by the converter B2 while the comparator N1 will turn off the transistors T1 and reduce the current 1s1 delivered by the converter B1. As here above, Vs3is kept constant by the master converter B3 controlled by the control circuit E3.
It must be noted that in this master-slave operation, it is necessary for the resistor R31 and, a fortiori, the resistor R32 to be not at values of zero so as to give Vs3 a value about which Vs1 and Vs2 may vary.
The present invention is not limited to the example described.
Thus, in particular, the controlled converters according to Figure 2 may be replaced by controlled converters that do not have their own galvanic insulation. It would then be necessary to provide for an input or output galvanic insulation circuit. Similarly, the converter according to Figure 2 may have its own filtering capacitor. As for the diode Dc, it is not indispensable for the converter to be made to work in discontinuous operation mode.

Claims

WHAT IS CLAIMED IS:

1. A DC supply with a power factor corrector for three-phase mains, comprising three rectifier assemblies arranged in a triangle, each rectifier assembly having outputs, the corrector comprising a first, a second and a third power factor correction cell with a switching converter servo-controlled to impose a given value on the ratio of its instantaneous input voltage to its instantaneous input current, these cells being respectively coupled to the three rectifier assemblies and having respectively three pairs of output conductors parallel-connected to form one and the same pair of output terminals, the first cell comprising means for the servo-control of its given value as a function of the voltage at the output terminals, the second and third cells comprising means to servo-control their given value to the difference between their delivered power and the power delivered by the first cell, the switching converter of each correction cell comprising two input terminals designed to be coupled to the outputs of the rectifier assembly to which the correction cell considered is coupled, a first inductor and a second inductor magnetically coupled to each other, a power transistor with two first electrodes and a third electrode, a capacitor, a transformer with a primary winding and a secondary winding and a galvanic insulation between these two windings and a pair of output conductors with the first inductor and the two first electrodes of the power transistor in series between the two input terminals, the third electrode constituting a switching control input and, in parallel on the two first electrodes, the capacitor followed by the primary winding of the transformer and with, in series between the conductors of the pair, the second inductor and rectifier means comprising the secondary winding of the transformer associated with rectifier elements.