Abstract:
In various embodiments, an electrical power supply device may include a transformer with a primary winding having coupled thereto a main switch switchable on and off to connect and disconnect said primary winding from a supply source and a secondary winding having coupled therewith a main branch for feeding a load and at least one auxiliary branch to provide an auxiliary power supply; wherein said at least one auxiliary branch includes a switching regulator with hysteresis with a respective auxiliary switch sensitive to the voltage on said secondary winding and the voltage of said auxiliary power supply, said auxiliary switch configured for closing to draw current from said main branch toward said auxiliary power supply in the presence of a negative transition of the voltage on said secondary winding and for opening when the voltage on said auxiliary power supply reaches an upper reference level.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Italian Patent Application Serial No. TO2011A000041, which was filed Jan. 20, 2011, and is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to electrical power supply devices. 
         [0003]    In various embodiments, the description may refer to the field of isolated switching converters, e.g. in a flyback topology. 
         [0004]    In various embodiments, the description may refer to generating multiple synchronous outputs from the same transformer winding. 
         [0005]    In various embodiments, the description may refer to solutions adapted for example to supply light sources, such as light emitting diode (LED) light sources. 
       BACKGROUND 
       [0006]      FIGS. 1 to 3  show various solutions of electrical power supply devices, adapted to obtain multiple outputs resorting to a flyback topology. 
         [0007]    The three diagrams of  FIGS. 1 to 3  refer to solutions wherein a transformer T is provided having a primary winding P, supplied from a supply source V via an electronic switch Qp (typically a mosfet) which is switchable alternatively on and off by a regulator SW. 
         [0008]    Such basic arrangement, recalled by way of reference example, is kept for the primary side of the device in all embodiments presently considered by way of non limiting example only. 
         [0009]    In the diagram of  FIG. 1 , transformer T is provided with a plurality of secondary windings, in the presently considered example two windings S 1 , S 2 . Each secondary winding S 1 , S 2  generates, via a rectifying/filtering network comprised of a diode D 1 , D 2 , . . . and of a capacitor C 1 , C 2 , . . . , a respective output Out 1 , Out 2 , . . . . 
         [0010]    This way to obtain multiple outputs has several drawbacks. 
         [0011]    First of all transformer T, in itself a critical component, tends to become bulkier and to operate less and less ideally, being moreover costlier. In addition, the number of connection pins increases, which in turn worsens the insulation performance. 
         [0012]    Moreover, the fact that the ratio among various output voltages is ideally constant (as it depends on the number of turns of the various secondary windings S 1 , S 2 , etc.) means on the one hand that it is impossible to adjust one output voltage without varying the others, and on the other hand that the cross regulation is spoilt by the transformer&#39;s parasitics and the other circuit elements, therefore making the output voltage ratios dependent on load, frequency, temperature and other operating parameters. 
         [0013]    The solution according to  FIG. 2  has on the contrary one single secondary winding S and derives the auxiliary output(s) (output Out 2  in the presently considered example) from main output Out 1  through a regulator of the switching type, comprising an electronic switch (once again typically a mosfet) Qs driven by a related regulator SW&#39;. 
         [0014]    This solution is energy-efficient from the point of view of energy use, but requires a number of additional components, including regulator SW&#39;, a really efficient switch on the high side (i.e. Qs), an output inductor L 2  and a recirculating diode D 2 , to recirculate the current when switch Qs is open and, usually, a feedback network FB to drive regulator SW&#39; according to output voltage Out 2 . 
         [0015]    It is therefore an expensive solution which is quite space consuming, especially considering its use for generating a low power auxiliary supply. 
         [0016]    The solution in  FIG. 2  can however be implemented in a simplified design, as schematically depicted in  FIG. 3 : in this case as a switch Qs of the auxiliary circuit, a bipolar pass transistor or mosfet is used, which however no longer operates as a switch but as a linear regulator, driven by an error amplifier EA which drives transistor Qs according to the difference between a reference voltage Ref and the feedback voltage supplied by network FB. 
         [0017]    This solution is simple and easy to implement with few components, but has the drawback of a high dissipation of transistor Qs (which works in the linear operation region), consequently limiting the possible applications to cases wherein the voltage drop and the current supplied at the output have low values. 
       SUMMARY 
       [0018]    In various embodiments, an electrical power supply device may include a transformer with a primary winding having coupled thereto a main switch switchable on and off to connect and disconnect said primary winding from a supply source and a secondary winding having coupled therewith a main branch for feeding a load and at least one auxiliary branch to provide an auxiliary power supply; wherein said at least one auxiliary branch includes a switching regulator with hysteresis with a respective auxiliary switch sensitive to the voltage on said secondary winding and the voltage of said auxiliary power supply, said auxiliary switch configured for closing to draw current from said main branch toward said auxiliary power supply in the presence of a negative transition of the voltage on said secondary winding and for opening when the voltage on said auxiliary power supply reaches an upper reference level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will now be described, by way of non-limiting example only, with reference to the enclosed views, wherein: 
           [0020]      FIGS. 1 to 3  have already been described in the foregoing; 
           [0021]      FIG. 4  is a block diagram of an embodiment; 
           [0022]      FIG. 5  is a block diagram of an embodiment; and 
           [0023]      FIG. 6  is an implementation diagram of an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or several specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments. 
         [0025]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0026]    The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
         [0027]    Various embodiments propose a solution to the drawbacks of the previously outlined arrangements. 
         [0028]    In various embodiments, the same secondary winding may supply multiple voltage-regulated outputs. 
         [0029]    In various embodiments, the derived outputs can be regulated independently one from the other. 
         [0030]    In various embodiments, it is possible to avoid resorting to a linear passive regulator. 
         [0031]    In various embodiments, the number of components in each auxiliary regulator may be remarkably lower as compared to a complete buck regulator. 
         [0032]    In various embodiments, an improved efficiency can be achieved by operating in ZVS (Zero Voltage Switching) conditions. 
         [0033]    In various embodiments, as regards the pulse generation, the or each auxiliary regulator is intrinsically synchronous with the main switch, unlike the case of a buck converter (see for example  FIG. 2 ). 
         [0034]    In various embodiments, wherein the generation of multiple auxiliary outputs is provided, it is possible to share a few parts of the circuit among multiple regulators, therefore decreasing the overall number of components; this is not possible in a traditional buck implementation. 
         [0035]    In  FIG. 4 ,  FIG. 5  and  FIG. 6  reference  10  denotes in general a supply device, adapted to supply, from an input voltage V, a load L, for example comprised of a light source, such as a light emitting diode (LED) light source. In various embodiments, load L can be comprised of a so called LED string. 
         [0036]    It will be noted however that, although being shown in some of the figures, load L does not belong in itself to device  10 . 
         [0037]    Device  10  in  FIG. 4 ,  FIG. 5  and  FIG. 6  has the same general design as in the previously described diagrams of  FIG. 1  to  FIG. 3 . Specifically, in all three embodiments referred to, by way of example and not limitation, in  FIG. 4  to  FIG. 6 , the primary side of device  10  may be the same as described with reference to  FIG. 1  to  FIG. 3 . For this reason parts, elements and components which are identical or equivalent to parts, elements and components already described with reference to  FIG. 1  to  FIG. 3  are denoted in  FIG. 4  to  FIG. 6  with the same reference numbers, and the description of such parts, elements and components will not be repeated in the following for the sake of brevity. However, the description of the elements of  FIG. 1  to  FIG. 3  are incorporated in its entirety into the description of the elements and embodiments of  FIG. 4  to  FIG. 6 . 
         [0038]    In the three diagrams of  FIG. 4 ,  FIG. 5  and  FIG. 6  the secondary side of device  10  may include a main branch adapted to supply a load L with an output voltage Out 1 . 
         [0039]    This takes place according to criteria known in themselves, which do not therefore require a detailed explanation in the following. For this reason, in all the diagrams in  FIG. 4 ,  FIG. 5  and  FIG. 6  said main branch is shown simply as a rectifying (diode D MAIN ) and filtering (capacitor C MAIN ) network, supplied directly from secondary winding S of transformer T, operating in a flyback mode. 
         [0040]    As will be better understood in the following, the diagram in  FIG. 5  highlights the possibility, in various embodiments, of having a plurality of outputs implemented substantially in the same way, although they may have mutually different features (for example as regards voltage). 
         [0041]    In this case, the components of the main branch have not been identified with suffix MAIN, but simply with reference number  1 , providing therefore the presence of a rectifying (diode D 1 )/filtering (capacitor C 1 A) network. 
         [0042]    The diagram in  FIG. 4  refers to embodiments wherein the output consists of the main output Out 1  and in addition of a single auxiliary supply output (Aux supply). 
         [0043]    Various embodiments may have, on the secondary side of circuit  10 , a secondary or auxiliary rectifier D AUX , the anode whereof is connected to the anode of main rectifier D MAIN , with the consequent ability to shunt (according to criteria better explained in the following) current from the input line which goes towards main output Out 1 . 
         [0044]    Downstream secondary rectifier D AUX  there is provided an electronic switch Q AUX  (typically a metal oxide semiconductor field effect transistor (mosfet), such as mosfet n) driven by a driver D. 
         [0045]    The current shunted by auxiliary rectifier D AUX  and auxiliary switch Q AUX  is sent towards a capacitor C SMOOTH , across which the voltage Aux supply is present. 
         [0046]    In various embodiments, voltage Aux supply may be the output of a low pass filter LC, optionally arranged in cascade connection with capacitor C SMOOTH , with the function of an antinoise filter, such filter being comprised of an inductor L FILTER  and a capacitor C FILTER . 
         [0047]    In various embodiments, downstream diode D AUX  and upstream switch Q AUX  there may be provided a capacitor C X  which, although eliminating the ZVS condition, is adapted to improve the circuit efficiency by decreasing the opening current of switch Q AUX , with the further result of reducing EMI problems. 
         [0048]    In the present embodiment, illustrated by way of example only, reference  12  denotes a comparator, the non-inverting input whereof being connected to ground and the inverting input being connected to secondary winding S of transformer T via a network N 1  of a proportional or derivative type. 
         [0049]    The structure comprised of comparator  12  and network N 1  is adapted to sense that the voltage on secondary winding S of transformer T has experienced a down-going transition (for example that it has become negative). 
         [0050]    Reference  14  on the other hand denotes another comparator, which is adapted to compare the auxiliary output voltage Aux supply (for example the voltage across capacitor C SMOOTH ), downscaled via a resistive voltage divider R A  and R B , to a reference voltage V REF . 
         [0051]    Reference R C  denotes a resistor, connected between the non-inverting input and the output of comparator  14 , in such a way as to induce into comparator  14  itself a hysteretic behavior. 
         [0052]    Finally, reference  16  denotes a latch circuit, adapted to drive (through driver D) auxiliary switch Q AUX , on the basis of the output signals of both comparators  12  and  14 . 
         [0053]    The described circuit implements a hysteretic switching regulator, wherein auxiliary switch Q AUX  is closed (i.e. made conductive) when the voltage on secondary winding S is negative or experiences a sudden down-going transition. Thereafter switch Q AUX  is opened again when the output voltage becomes higher than the preset value represented by value V REF  (and/or by the division ratio of divider R A , R B ). 
         [0054]    As has already been stated,  FIG. 5  shows the possibility to extend the solution shown in  FIG. 4  to the generation of a general number n of output signals Out 1 , Out 2 , . . . , whereof the first (or any other chosen output) may constitute the main output, adapted to supply load L. 
         [0055]    In the diagram of  FIG. 5 , in each of the auxiliary branches there is provided a respective latch circuit  16 , with driver D of the auxiliary switch associated therewith, in this case denoted by Q 2 , Q 3 , . . . , Qn, while the corresponding auxiliary rectifier is denoted by reference D 2 , D 3 , . . . , Dn. 
         [0056]      FIG. 5  shows that, in implementing such a “multiple” solution, it is possible to have all auxiliary branches, adapted to generate output voltages Out 2 , Out 3 , . . . , Outn, share the same comparator  12  and the same network N 1 . 
         [0057]    The smoothing capacitor (C SMOOTH  in  FIG. 4 ) is denoted in the diagram of  FIG. 5  as C 2 A, C 3 A, . . . , CnA. 
         [0058]    In a similar way, components LC of the output filter (if present) are denoted as L 2 , L 3 , . . . , Ln and C 2 B, C 3 B, . . . , CnB. 
         [0059]    From the same diagram of  FIG. 5  it is clear that auxiliary branches can also share the same reference voltage V REF , as a function of the comparing operation performed in comparator  14 , with the voltage drawn from the output (divider R A , R B  of  FIG. 4 , denoted in general as FN 2 , FN 3 , . . . , FNn in  FIG. 5 ). 
         [0060]    In order to highlight the overall symmetry with auxiliary branches, in  FIG. 5  rectifier D MAIN  of the main branch is denoted by D 1 , while capacitor C MAIN  and components LC of the output filter are denoted respectively by C 1 A, L 1  and C 1 B. For the same reason, the network sensing the output voltage is denoted in  FIG. 5 , referring to the main branch, by FN 1 . 
         [0061]    It will be noted moreover that in the diagram of  FIG. 5  the result of the comparison between voltage V REF  and the output voltage of network FN 1  is used in the main branch by a comparator  20  performing, for example through an opto-isolator  22 , a feedback function towards regulator SW, provided on the primary side of the device. The performance criteria of such a feedback regulating operation are well known in the state of the art and therefore do not require a detailed description in the following. 
         [0062]    The operation of the embodiments of  FIG. 4  or  FIG. 5  may be described as follows, by referring to a continuous operation with steady-state (i.e. non transient) voltage levels. Moreover, it is to be noted that in any case the voltages drawn on the auxiliary outputs Aux Supply or Out 2 , Out 3 , . . . , Outn are lower than (or at most virtually equal to) the voltage of main output Out 1 . 
         [0063]    When the voltage on secondary winding S becomes negative (or in general undergoes a rapid transition in the negative direction) because switch Q p  on the side of the primary winding has closed, or because the primary circuit in a free self-oscillation period, comparator  12  undergoes a transition, which controls latch  16  to close auxiliary switch Q AUX  (or Q 2 , Q 3 , . . . , Qn in the case of  FIG. 5 ). 
         [0064]    At this point there is no current flowing towards capacitor C SMOOTH  (C 2 A, C 3 A, . . . , CnA in the diagram of  FIG. 5 ). This is due to the fact that diode D AUX  (D 2 , D 3 , . . . , Dn in the diagram of  FIG. 5 ) stays open. Therefore, closing the auxiliary switch takes place in a condition of zero voltage and current. 
         [0065]    However, when main switch Q p  opens, the voltage on the secondary winding changes polarity and diode D AUX  (D 2 , D 3 , . . . , Dn in the case of  FIG. 5  embodiment) starts conducting, so that the current coming from secondary winding S starts to charge capacitor C SMOOTH  (C 2 A, C 3 A, . . . , CnA in the diagram of  FIG. 5 ) through transistor Q AUX  (Q 2 , Q 3 , . . . , Qn in the diagram of  FIG. 5 ). 
         [0066]    The capacitance value is chosen on the basis of the voltage ripple allowed for that auxiliary output, whence a corresponding dv/dt value is derived according to the current pumped into the capacitor. 
         [0067]    Main rectifier D MAIN  (or D 1  in the diagram of  FIG. 5 ) stays open because the voltage on the main output is higher than the one on C SMOOTH  (C 2 A, C 3 A, . . . , CnA). Secondary winding S works therefore as a current generator the voltage whereof is actually clamped by C SMOOTH  (C 2 A, C 3 A, . . . , CnA in the diagram of  FIG. 5 ). 
         [0068]    When the output voltage (Aux Supply or Out 2 , Out 3 , . . . , Outn—i.e. in practice the voltage across capacitor C SMOOTH , C 2 A, C 3 A, . . . , CnA) exceeds the set threshold on comparator  14 , the same comparator acts on latch circuit  16 , which turns off, i.e. opens, switch Q AUX , Q 2 , Q 3 , . . . , Qn. 
         [0069]    Now the voltage on secondary winding S rises until main diode D MAIN  or D 1  starts conducting, so that the current of the winding flows towards the output capacitor (C or C 1 A) and towards load L. At the same time, capacitor C SMOOTH  or C 2 A, C 3 A, . . . , CnA starts discharging, supplying the current towards the respective auxiliary output. 
         [0070]    In case the voltage on capacitor C SMOOTH  (C 2 A, C 3 A, . . . , CnA) does not reach the desired threshold in a single switching cycle, switch Q AUX , Q 2 , Q 3 , . . . , Qn stays closed for several cycles, until the threshold is reached. 
         [0071]    In case a fixed hysteresis is employed for comparator  14  and if latch circuit  16  is of the reset predominant type, internal auxiliary switch Q AUX , Q 2 , Q 3 , . . . , Qn can stay open for several cycles, until the lower threshold level is reached. 
         [0072]    On the contrary, by sizing the parts so as to ensure in any case that also auxiliary switch Q AUX  closes at every closing cycle of main switch Q p , the effect is obtained of reducing, when Q p  opens, the voltage at secondary winding S to value Out AUX : in this way the voltage reflected in the primary P is decreased as well, reducing the problem of an extra voltage at the opening due to the dispersed inductance of transformer T. 
         [0073]    The diagram of  FIG. 6  (wherein parts, elements and components which are equal or equivalent to those already described before are denoted by the same references, and will not be described again) shows the structure of opto-isolator  22 , including a “transmitting” part  22   a , coupled to load L, and a “receiving” part  22   b , which acts on regulator SW. 
         [0074]    To this respect we highlight the possibility to implement the circuit functions previously described with specific reference to capacitors  12  and  14  and to latch  16  by resorting to one single comparator  124  implementing the various logic functions shown in  FIG. 4 . 
         [0075]    The function of logical latch is implemented by a derivative network N 1 , connected to secondary winding S of transformer T, and by a hysteresis network N c , around comparator  124 . The function of driving auxiliary switch Q AUX  is achieved thanks to a level-shifted BJT voltage follower D, with a start-up resistor R su  from the main output. It will be appreciated that the whole circuit of  FIG. 6  may be supplied with the drawn auxiliary voltage Aux supply itself. 
         [0076]    While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.