WO1998042065A1

WO1998042065A1 - High power dc/dc converter

Info

Publication number: WO1998042065A1
Application number: PCT/EP1998/001578
Authority: WO
Inventors: Paolo Prestifilippo; Giovanni Baggione; Roberto Scibilia
Original assignee: Italtel S.P.A.
Priority date: 1997-03-14
Filing date: 1998-03-12
Publication date: 1998-09-24
Also published as: ITMI970583A1; IT1290082B1; EP0966789A1

Abstract

Power DC/DC converter adapted to supply telecommunication systems such as telephone exchanges. According to the invention this converter includes: a transformer (T1) with two primary windings (P1, P2) connected between two voltage sources connected in series (+ 400 V and - 400 V) and two secondary windings (S1, S2) with two common terminals and a load (R1) connected in parallel to the output filter capacitor (C); at each primary winding (P1, P2) of said transformer (T1) chopping means are connected which are implemented by a pair of controlled electronic switches (SW1 and SW2 or SW3 and SW4). Thanks to the technical solutions according to the invention it is possible to realize the converter with the minimum number (four) of switches necessary to manage, from the practical point of view, power levels of more than a few kW, and moreover it is possible to use switches presenting low breakdown voltage and reduced switching and conduction losses.

Description

"HIGH POWER DC/DC CONVERTER"

Field of the Invention The present invention relates to a DC/DC converter adapted to convert a

DC voltage in the range of 700-800 V in a DC voltage of a lower value in the range of 48-60 V with output current from 0 to 100 A or more.

The invention has a preferred application in the power supplies of telecommunication systems, for example telephone exchanges, and it will be described hereafter with specific reference to such use which however is not limited thereto since other uses may be made by those skilled in the art without departing from the scope thereof.

In the power supplies for telecommunication systems it is necessary to produce a continuous voltage (DC) of 48 V from a three-phase mains with or without neutral, and with an output power in the range 1 ÷10 kW with sinusoidal input currents from the mains or with unity power factor and limited distortion. For such direct conversion by the mains normally two cascade conversion stages are used, one preregulation stage and a final converter, respectively. From the practical point of view, considering the maximum value of the mains, the minimum voltage to be generated at the first stage and therefore at the AC/DC converter is 800 V, and this stage may be schematized by two 400

V generators connected in series. Starting from these generators the second conversion (from 800 to 48 V) has to be carried out. The second conversion stage is the subject of the present invention.

Background Art

According to a known solution illustrated in details in figure 1 , in order to realize the final conversion starting from the 800 V voltage available at the first conversion stage, a full bridge configuration is used which is operating on the total voltage of 800 V generated by the preregulation stage.

The disadvantage of this solution is the need of over 800V breakdown voltage switching elements which present high voltage drop and poor switching performances. Moreover using this topology on the entire voltage instead of the single voltages generated by the first conversion stage, we have no contribution to the balancing of the two voltages generated by the first conversion stage. As an alternative to the full bridge it is possible to use any other converter, but the breakdown voltage of the switching elements is always greater than 800V.

According to another known solution two series-connected DC/DC converters (i.e. full bridge type) are used, each one supplied by 400 V generated by the first conversion stage. One configuration of this type is shown in details in figure 2, comprising two DC/DC converters series connected at the input and in parallel at the output after the respective filters.

This solution allows the use of switching elements with a lower voltage, i.e. a nominal breakdown voltage of 400 V, but there is a considerable complexity of the circuit. In fact it is necessary to generate the modulation pulses (PIL1 , PIL2) for two DC/DC converters; there are also two output filters (L1 ,C1 and L2/C2) and moreover it is necessary to manage the control of each converter in order to balance the two voltages generated by the first conversion stage. Objects of the invention

The main object of the present invention is to realize a DC/DC converter adapted for the connection to a first AC/DC converter with a reduced number of switching components and having a low breakdown voltage, so that it has a low cost but a high switching speed. Another object is to realize a converter of the type described before adapted to perform an automatic balancing of the two voltages generated by the first conversion stage and being also adpted to recover the magnetic energy stored in the leakage inductance of the transformer. Disclosure of the invention The invention achieves the mentioned objects through a high power

DC/DC converter having the characteristics illustrated in the main claim. Object of the present invention is also a rectifier of the type comprising two cascade connected conversion stages realized in conformity with what disclosed in claim 12.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. Brief description of the drawings

The invention, together with further objects and advantages thereof, may be understood with reference to the following description, taken in conjuctionwith the accompanying drawings, and in wich: figures 1 and 2 show the same number of converters of the known type;

Fig. 3 shows the electric scheme of the DC/DC converter realized according to the present invention;

Fig. 4 illustrates the drive signals of the four switches SW of Fig. 3; Fig. 5 illustrates schematically an off-line rectifier which uses (like the second conversion stage) the DC/DC converter realized according to the present invention. Detailed description of a preferred embodiment of the invention

Referring to the electric scheme of Fig. 3 the converter according to the invention is made by two sections, one of which is supplied with a +400 V voltage (between +400 V and 0 V) and the other with a -400 V voltage (between -400V and 0 V).

The terminal labelled +400 V will hereafter also be indicated as the terminal with the highest potential, the one labelled -400 V as the terminal with the lowest potential and the terminal labelled 0 V as the common terminal or with intermediate potential.

In other words the converter according to the invention may be considered as supplied by two voltage sources in series.

The converter comprises a single transformer T1 with two independent primary windings P1, P2, and two secondary windings S1, S2 with a common point. The two terminals of secondary windings S1 and S2 are connected respectively to the anode of the diodes D5 and D6. The load R1 is connected to the common point of windings S1 and S2 and downstream of the inductor L to the common cathode of diodes D5 and D6. Finally a capacitor C is connected in parallel to the load R1.

The primary winding P1 is connected to the terminals with +400 V and 0 V, respectively, through two diodes in series D1 e D2 which are inversely polarized. More exactly a first terminal - indicated by PP1 in Fig. 3 - is connected to the anode of a diode D1 which cathode is connected to the terminal with + 400 V, while the other terminal or second terminal SP1 of the same primary winding P1 is connected to the cathode of a second diode D2 which anode is connected to the common terminal with 0 V. Moreover the first terminal PP1 is connected to the terminal with 0 V through a controlled electronic switch indicated by SW2, and the second terminal SP1 is connected to the terminal with +400 V trough a controlled electronic switch indicated by SW1.

A similar configuration is foreseen for the second primary winding P2 where the first terminal PP2 is connected through a diode D3 to the major potential of the second source (0 Volt in this case) and the second terminal SP2 is connected through a diode D4 to the minor potential (-400 V in this case). The diodes D3 and D4, analogously to what happens for the diodes D1 and D2, turn therefore out to be inversely polarized. Even in this case a pair of switches called SW3 and SW4 connects the first terminal PP2 to the terminal 0 V and the second terminal SP2 to the terminal with -400 V.

The switching means generally indicated by SW1 , SW2, SW3 and SW4 may be transistors or MOSFETs or IGBTs (Insulated Gate Bipolar Transistors) and they are controlled by the signals DS1 , DS2, DS3, DS4 shown in Fig. 4 and characterized by rectangular waveforms with a switching period indicated by "T". The conduction time DT of the individual switches can not be more than 0,5T (duty cycle smaller than 50%).

The circuit works as a full bridge converter, that means that the waveforms of the voltage and power on the four switching elements are absolutely similar to those of the switching elements of a full bridge.

Essential for the correct functioning are the directions of the windings in the single transformer indicated in a conventional way through dots on the scheme which assure together with the associated driving pulses the balancing of the Volts X second area applied to the transformer (this guarantee the core will not saturate). Looking at the driving signal patterns of Fig. 4, during the first half period of the switching frequency the core is magnetized in a direction from the simultaneous closing of the switches SW1 and SW2 (while SW3 and SW4 remain opened). That means to apply the voltage + 400 V at P1 primary winding of the transformer T1. In the remaining part of the switching period the core is magnetized in the oppositre direction by the simultaneous closing of the switches SW3 and SW4 (while SW1 and SW2 remain open). That means to apply the voltage - 400 V at other primary P2 of the transformer T1. If both voltages +400 V and -400 V are equal, the closing times of the switches must be equal in order to assure the balancing of the Volt X second areas applied to the transformer (to avoid the saturation of the transformer ). Studies carried out by the applicant have shown that slight differences in the magnitude of the two supply voltages +400 V and -400 V can be tollerated if a current mode modulator is used for the control of the converter. In particular if the driving signals are applied to the switches SW1 , SW2, SW3, SW4, of fig. 3 by means of a current mode modulator (not shown in fig. 3) the ON time of the switches SW1 , SW2 respect to SW3, SW4, are automatically generated in order to assure the non-saturation of the transformer.

According to the invention in any case, the presence of the diodes D1 , D2 and D3, D4, permits the balancing of the two voltages +400 V and - 400 V by means of a magnetic coupling betweens the two primaries windings. In fact when the switches SW3 and SW4 are closed the voltage existing across the capacitor C2 can be reflected through the transformer T1 across the capacitor C1 trough diodes D1 , D2.

Similarly the voltage across C1 is reflected across C2 by means of diodes D3, D4. In this way it is possible to balance pulse by pulse the voltages across the two capacitors C1 and C2 and in the last analysis the two voltages +400 V and - 400 V are forced to be equal. Thanks to this tight balancing it is possible to use four switches with a breakdown voltage equal to 500 V. Preferably the invention propends for the use of IGBTs which present a low input capacitance and are able to handle high currents and they can be driven by a small transformer. The transformer T1 is preferably realized by means of two smaller transformers having the primary windings connected in series and the secondary windings connected in parallel downstream of the rectification elements implemented by the diodes D5 and D6.

Moreover the diodes D1 - D4 carry out the function of the recovering of the energy stored on the leakage inductance of the transformer T1. In particular this energy is transferred back through the diodes D1 and D2 to the capacitor C1 and trough the diodes D3 and D4 to the capacitor C2.

The proposed topology according to the invention shows a minimum number of switches (four) wich are able to manage a level of power from 1 to 10 kW.

This tolology allows the use of switches with a breakdown voltage wich is a half of the one necessary for the full bridge configuration. These kind of switches, available from the market, show better characteristics in terms of on- resistance and switching losses compared to the high breakdown voltage switches. As a result the efficiency obtained by using this topology is much higher compared to the efficiency of standard topologies.

The converter according to the invention implements the self-balancing of the two voltages generated by the first stage without any additional circuits acting on the first stage or forcing the driven pulses to do a particular job in the second stage. In other words other toplogies need more complexity to assure a stable functioning of the two stages.

Thanks to the above-mentioned solutions the DC/DC converter shows an efficiency more than 95% even working in hard-switching mode.

Figure 5 shows a rectifier for telecommunication systems with two conversion stages connected in cascade.

The first stage is a preregulation, stage and it is composed by an input filter IN_F connected to a switching circuit SW_C suitable to convert the three- phase mains in two DC voltages equal to +400 V and -400 V, respectively. The input filter IN_F is connected to the switching circuit SW_C through three inductors L1 , l_2, and L3.

The second stage comprises the DC/DC converter, according to the invention, which is supplied by the preregulating stage. It generates a DC 48 V supply for the telecommunications equipments.

It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departing from the scope and spirit thereof. For example in order to increase the level of the power of the converter it is possible to use more transformers connected in series at the primary side and in parallel to the secondary side after rectification diodes. In principle it is possible to use transformers connected in parallel, however in this case there is no guaranty of the balancing of the volt x second areas applied to the single transformers.

Claims

1. DC/DC converter of the type including: a transformer which primary windings are connected to a power supply source and to which secondary winding the load is connected; ^" interruption means connected to the primary winding of said transformer to chop the voltage supplied by said power supply source; a plurality of diodes connected to the primary winding of said transformer to recuperate the energy stored in the leakage inductance of said transformer; ^" at least a pair of diodes connected to the secondary winding of said transformer to rectify the voltage corresponding on the secondary winding of said transformer; converter characterized in that: said transformer (T1) is structured in such a way to include two primary windings (P1 , P2) connected between the two voltage sources in series and two secondary windings (S1 , S2) with two common terminals and a load

(R1) connected between the other two terminals by means of output diodes

(D5, D6). to each of the primary windings (P1 , P2) of said transformer (T1 ) are connected a pair of controlled electronic switches (SW).

2. Converter according to claim 1 , characterized in that said two voltage sources in series are connected in such a way to comprise a common terminal (0 V), a high potential terminal (+400 V) and a terminal with lower potential (-400 V).

3. Converter according to claims 1 and 2, characterized in that the first terminal (PP1) of one (P1) of said primary windings is connected to the anode of a first diode (D1) which cathode is connected to the terminal with higher potential (+400 V) of said voltage sources, and the second terminal (SP1) of the same winding is connected to the cathode of a second diode (D2) which anode is connected to the common terminal (0 V) between said sources, and moreover the terminal with higher potential (+400 V) is connected to the second terminal (SP1) of one (P1) of said primary windings through a first controlled electronic switch (SW1) of said first pair of switches, and that said first terminal (PP1) is connected to the common terminal (0 V) through a second controlled electronic switch (SW2) of said first pair of switches; 4.

4. Converter according to claims 1 and 2, characterized in that the

5 first terminal (PP2) of the other (P2) of said primary windings is connected to the anode of a third diode (D3) which cathode is connected to the common terminal (0 V) of said voltage sources, and the second terminal (SP2) of the same winding (P2) is connected to the cathode of a fourth diode (D4) which anode is connected to the terminal with lower potential (-400 V) of said

10 sources, and moreover the common terminal (0 V) is connected to the second terminal (SP2) of the other (P2) of said primary windings through a first controlled electronic switch (SW3) of said second pair of switches, and that said first terminal (PP2) of the other (P2) of said windings is connected to the terminal with lower potential (-400 V) through a second controlled electronic

15 switch (SW4) of said second pair of switches.

5. Converter according to claims 1 and 2, characterized in that the anode of a first of said output diodes (D5) is connected to the dotted terminal of the secondary winding (S1) of the transformer (T1 ), and the anode of a second of said output diode (D6) is connected to the undotted terminal of the secondary

20 winding (S2) of the transformer (T1), and moreover that the cathodes of said first and second output diodes (D5) and (D6) are together connected to one terminal of the inductor (L), the second terminal of the inductor (L) being connected to a filter capacitor (C) and to the load resistor (R1); the other two terminal of said capacitor (C) and load resistor (R1) being together connected

25 to common terminal of secondary windings (S1 and S2) of the transformer (T1 ).

6. Converter according to claim 2, characterized in that between the common terminal (0 V) and the terminal with higher potential (+ 400 V) a first capacitor (C1) is connected, and between the common terminal (0 V) and the terminal with lower potential (- 400 V) a second capacitor (C2) is connected.

30 7. Converter according to claim 1 , characterized in that said two sources are generated by a three-phase AC/DC converter.

8. Converter according to claims 2 and 3, characterised in that said controlled electronic switches (SW1 , SW2, SW3, SW4) are set up by devices adapted to work at high switching frequency.

9. Converter according to claim 8, characterized in that said devices able to work at high switching frequency are MOSFET transistors or IGBT devices.

10. Converter according to claim 1 , characterized in that said transformer (T1) is implemented by means of two transformers having the primary windings connected in series and the secondary windings connected in parallel downstream of said rectification means.

11. Converter according to the previous claims, characterised in that said controlled electronic switches (SW1 , SW2, SW3, SW4) are piloted by pulse signals through a current mode modulator.

12. Rectifier of the type with two stages connected in cascade, characterised in that it comprises: a pre-regulation stage adapted to render available at the autput three levels of continuous voltage (+ 400 V, 0 V, - 400 V); a final stage comprising a DC/DC converter realized in conformity with what disclosed in the claims from 1 to 11.