Multi-output DC/DC power converter

Abstract: Converter device that transforms an input voltage (VE) into several outputs voltages (V1,..,VN) comprising a transformer (T), a first switching control device (PWM) that generates a first switching control signal (Q1) to control the switching of a first switching element (M1) and thereby regulate the main output of the converter, and a second switching control device (SDCC) that generates a second switching control signal (Q2) as a function of the first switching control signal (Q1) and of a second output voltage (V2). This second switching control signal (Q2) controls the switching of the second switching device (SDC) in order to regulate the second output voltage.

Description: OBJECT OF THE INVENTION This invention concerns a power converter device that transforms a direct current input voltage into several outputs. This multi-output converter provides, in a regulated condition, the main output and one of the auxiliary outputs and is of special application in equipments that require more than one regulated direct-current power supply voltage. BACKGROUND OF THE INVENTION The article by J. Sebastian, J. Blanco, J. Uceda, A. Lumbreras and F. Cañizales, "AN ALTERNATIVE METHOD FOR CONTROLLING TWO-OUTPUT DC-TO-DC CONVERTERS USING SATURABLE CORE INDUCTORS", PESC 1990, shows the regulation of two outputs of a converter using a single switching element and a saturable core inductor, as illustrated in figure 11b of the article mentioned. The regulation of the two converter outputs is achieved by modulation of the switching frequency and of the pulse width of the duty cycle of the MOSFET. The main output is regulated by means of the pulse width of the duty cycle of the MOSFET and the auxiliary output is regulated by the switching frequency of the MOSFET in question. Nevertheless, when oscillation occurs in the auxiliary output load, it is accompanied by a variation in the switching frequency. The switching frequency varies by a percentage of the oscillation in the load connected to the auxilliary output. The drawbacks resulting from variations in switching frequency reside in the greater size required of the magnetic components, transformer and inductors, for the lowest switching frequency. Other parts of the converter that are also penalised in the design process are the electromagnetic interference filters since the switching frequency moves over a broad range, meaning that the design of these filters is of greater complexity. Similarly the efficiency of the converter is also affected as switching losses in the MOSFET and in the magnetic circuit of the transformer are proportional to the switching frequency, making the losses increase and the efficiency fall. Thus, the problem to overcome lies in producing a multi-output converter that regulates a number of outputs and works at a fixed switching frequency with no increase in converter losses or in the number of components, thereby negatively affecting the converter both economically and in size. Figure 3 of the patent DE-A-39 12 849, shows a converter device, known as two switch forward, comprising a first switching element coupled to the end of a primary winding of a transformer, and the other end of the primary winding is coupled to a second switching element. The transformer comprises several secondary windings formed each of them a output of the converter. A control device controls the open and close of both switching elements to regulate two different output voltages by the forward respective flyback cycle. However, the two output of the converter device have the same ground, thereby there is not isolation between two output. Moreover, the duty cycle of the two switching elements are limited to a half of the switching period. CHARACTERISATION OF THE INVENTION The multi-output DC/DC converter of the invention has two of its outputs regulated and eliminates the problems caused by variation in the switching frequency. To achieve this, the multi-output DC/DC converter additionally includes a second switching element, a MOSFET, which is used to regulate the voltage of an auxiliary output instead of using the switching frequency. The converter transfers the energy in the primary winding into a secondary winding during the conductive period of the first MOSFET switching element. This secondary winding constitutes the converter main output, forward PWM topology, while the other secondary windings constitute the converter auxiliary outputs. The auxiliary output which is regulated, includes a secondary winding which during the conductive period of the second MOSFET releases the energy stored in the transformer during the conductive period of the first MOSFET, flyback topology, and works in discontinuous mode, that is, the current in the secondary winding attains zero value during part of the cut-off period of the first MOSFET. In this way, by controlling the time in which the energy stored in the transformer is released through the secondary winding, the voltage of an auxiliary output is regulated. This last feature is achieved by controlling the pulse width of the duty cycle of the second MOSFET. Consequently, the regulated auxiliary output voltage is given by: V2 = (VE/n2)*(δ/δ') where VE is the input voltage to the converter, n2 is the turns ratio between the primary winding and the secondary winding that forms the regulated auxiliary output, δ is the quotient obtained by dividing the conductive period of the first MOSFET by the total period, and δ' is the quotient obtained by dividing the conductive period of the second MOSFET by the total period, taking into account the current flowing in the parasitic diode of the second MOSFET. The multi-output converter of the invention regulates the main output and one of the auxiliary outputs by means of the pulse width of the duty cycles of the two MOSFETs, the switching frequency remaining fixed. The filters for electromagnetic interferences are simpler to design and, finally, the converter efficiency is high since the regulating technique used is non-dissipating. BRIEF FOOTNOTES TO THE FIGURES A fuller explanation of the invention is given in the following description based on the figures attached in which: figure 1 shows the block diagram of a multi-output DC/DC power converter according to the invention, figure 2 shows an implementation of the second switching device in which the switching is controlled by a second switching control device, all in accordance with the invention, figure 3 shows another possible arrangement of the second switching device in parallel with the first switching element, and figure 4 shows the waveforms of the switching signals of the two MOSFETs. DESCRIPTION OF THE INVENTION The multi-output converter of the invention, see figure 1, transforms an input voltage signal VE into several output voltages V1,...,VN. The number of outputs is exactly the same as the number of secondary windings DS1,...,DSN in the transformer T, and of which one winding is the main one and the others are auxiliary. Both the main output and one of the auxiliary outputs are regulated. The converter primary circuit comprises the primary winding DP of the transformer T connected in series with the drain of a first MOSFET M1 the switching of which is controlled by a first switching control device PWM, a pulse width modulator, by means of a first switching control signal Q1 that is applied to the gate of the MOSFET M1 mentioned. In addition, it includes a second switching device SDC, see figure 2, which includes a second MOSFET M2 whose source is connected to a capacitor C1, the other terminal of this capacitor C1 being connected to the drain of the MOSFET M1. Also, the drain of the second MOSFET M2 is connected to the input node A of the converter, to which is connected the primary winding DP of the transformer T. The switching of the second transistor MOSFET M2 is controlled by a second switching control device SDCC that generates a second switching control signal Q2 that is applied to the gate of said MOSFET M2. The first MOSFET M1 alternatively connects the primary winding DP of the transformer T to the input voltage signal VE, which is applied across the input nodes A, B of the converter. During the conductive period of the first MOSFET M1, a current flows through the primary winding DP producing the transfer of energy into a secondary winding DS1, this secondary winding DS1 constituting the main output of the converter. The transformer T stores energy during the conductive period TON1 of the first MOSFET M1 and releases this stored energy into a secondary winding DS2 during the cut-off period TOFF1 of MOSFET M1. This secondary winding DS2 works in a discontinuous mode, that is, the current in this secondary winding reaches zero value during part of the cut-off period TOFF1 of the first MOSFET M1 and, moreover, this cut-off period TOFF1 of the first MOSFET M1 is greater than the conductive period TON2 of the second MOSFET M2 (see figure 4). Consequently, by controlling the time of conduction of the second MOSFET M2 the voltage of one of the auxiliary outputs of the converter is regulated. To achieve this, the second switching control device SDCC generates the second switching control signal as a function of the voltage signal V2 that is regulated and of the first switching control signal Q1. Figure 3 shows another possible configuration of the multi-output converter, in which the second switching control device SDCC is connected in parallel with the first MOSFET M1, in such a way that the drain of the second MOSFET M2 is connected to the drain of the first MOSFET M1 and the terminal of the capacitor C1 is connected to the source of the first MOSFET M1. The converter works in a similar manner to the previous case.

Claims: CONVERTER DEVICE that transforms an input voltage (VE) into several output voltages (V1, ..., VN) comprising a transformer (T) with a primary winding (DP) and a number of secondary windings (DS1, ..., DSN), a first switching element (M1) connected in series with said primary winding (DP) constituting a primary circuit and a first switching control device (PWM) which generates a first switching control signal (Q1) to control the switching of said first switching element (M1), characterised in that it also comprises a second switching device (SDC) is connected in parallel with the primary winding (DP) of the transformer (T), said second switching device (SDC) comprises a second switching element (M2) connected in series with a capacitor (C1); a second switching control device (SDCC) generating a second switching control signal (Q2) as a function of the first switching control signal (Q1) and of a second output voltage (V2), said second switching control signal (Q2) controlling the switching of the second switching device (SDC). CONVERTER DEVICE according to claim 1, characterised in that the second switching control device (SDCC) comprises a control logic process such that the second switching element (M2) is in a cut-off state (TOFF2) prior to a conducting state (TON1) of the first switching element (M1), remaining in this cut-off state (TOFF2) during the conductive period (TON1) of the first switching element (M1) and enters a conductive period (TON2) after the conductive period (TON1) of the first switching element (M1). CONVERTER DEVICE according to claim 2, characterised in that the conductive period (TON2) of the second switching element (M2) is less than the cut-off period (TOFF1) of the first switching element (M1). CONVERTER DEVICE according to claim 1, characterised in that the second switching device (SDC) is connected in parallel with the first switching element (M1).

Inventors: DE LA CRUZ MORENO ENRIQUE, OLLERO VELASCO SALVADOR, VAZQUEZ LOPEZ MANUEL, DE LA CRUZ MORENO, ENRIQUE, OLLERO VELASCO, SALVADOR, VAZQUEZ LOPEZ, MANUEL, Vázquez Lopez, Manuel
Applicants: ALCATEL NV, ALCATEL N.V.
Application year: 1995
Main classification: H02M3
IPC codes: H02M3