Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of power converters and, more specifically, D.C./D.C. converters of switched-mode power supply type. The present invention applies to step-up or step-down converters intended to supply several loads independent from one another.  
         [0003]     The loads supplied by the power converter may be of different natures. An example of application relates to backlit screens of the type used in portable phones or personal digital assistants (PDA). Several series-associated light-emitting diodes (generally white diodes) form the different loads. According to the desired backlighting intensity, one or the others of the loads is supplied. Another example of application relates to a power dimming function performed by a halfwave between supplied branches.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  shows a first conventional example of a D.C./D.C. converter for supplying several loads independent from one another.  
         [0006]     It shows a voltage step-up converter intended to provide, between an output terminal  1  and ground  2 , a voltage Vout higher than a D.C. input voltage Vdc applied between an input terminal  3  and ground  2 . In a step-up converter, terminals  3  and  1  are connected to each other by an inductive element L in series with a diode D, the cathode of diode D being connected to terminal  1 . The output voltage is sampled across a capacitor C connecting terminal  1  to ground. A cut-off switch M is connected between junction point  4  of inductance L and of diode D and the ground. Switch M is controlled by a circuit  5  (for example, PWM CTRL) in charge of supplying pulses for turning on switch M according to a reference value (OR) and to a control signal FB. Block  5  also receives a clock signal f M  enabling it to generate the control pulses of switch M. The control performed by circuit  5  on the control pulses may be of pulse-width modulation type (PWM), of frequency-width modulation type (FWM), etc.  
         [0007]     The power converter is intended to supply several independent loads. In the example shown in  FIG. 1 , two loads  11  (Q 1 ) and  12  (Q 2 ) are connected to terminal  1 . Each of the loads is in series with a switch, respectively K 1 , K 2 , controlled by a signal A 1 , A 2  to select the load  11  or  12  which is to be supplied with voltage Vout. A resistor R 1  or R 2 , respectively, connects the switch of each of the loads to ground  2 .  
         [0008]     In a power converter such as illustrated in  FIG. 1 , the regulation of voltage Vout is only performed on one of the loads. In this example, signal FB is sampled at point  6  between load  11  and resistor R 1  which is used as a current-to-voltage converter to control voltage Vout according to reference voltage OR. For the regulation to be properly performed, resistors R 1  and R 2  must compensate for the impedance differences between the supplied loads  11  and  12 . Such ballast resistors increase system losses.  
         [0009]      FIG. 2  shows a second conventional example of a power converter intended to supply several loads. As compared with the assembly of  FIG. 1 , the only difference is the assembly of loads  11  and  12 . In this example, two loads  11  and  12  are in series with a resistor R between terminal  1  and ground  2 , each load being short-circuitable by a switch K 1 , K 2  controlled by one of signals A 1 , A 2 , respectively. The two switches K 1  and K 2  are in series between terminal  1  and junction point  6  of load  12  and resistor R.  
         [0010]     As compared with the assembly of  FIG. 1 , this assembly has the advantage of having a common regulation for the two loads. However, output voltage Vout must be higher to enable supply of the two loads at the same time, which imposes a larger switch M.  
         [0011]     Another disadvantage of this assembly is that it generates a permanent consumption in resistor R and thus forbids a true shutdown function of the system.  
         [0012]     Another disadvantage is that a power variation of the supplied loads is not possible independently from each other.  
         [0013]      FIG. 3  shows a third conventional example of a power regulation circuit intended to supply several loads. In this example, each load  11 ,  12  is supplied by a capacitor C 1 , C 2  which is specific thereto. Cathode  1  of diode D is connected to each of capacitors C 1 , C 2  via a switch K 1  or K 2 , respectively. Supply voltages Vout 1  and Vout 2  of loads Q 1  and Q 2  are respectively sampled across capacitors C 1  and C 2 . Circuit  5 ′ for providing the control pulse train to cut off switch M receives two control signals FB 1  and FB 2  respectively sampled across resistors R 1  and R 2 , each connecting loads  11  and  12  separately to ground.  
         [0014]     The solution of  FIG. 3  is close to a solution consisting of providing one full converter per load, which is not desirable for bulk reasons.  
         [0015]     This assembly enables independent regulation of each of the load supply voltages. However, it requires two output capacitors of the regulator as well as two full output voltage regulation loops. Further, the controls of loads Q 1  and Q 2  generate a complex management of the power stored in inductance L.  
         [0016]     Another disadvantage is that switches K 1  and K 2  must exhibit small on-state resistances to avoid generating any additional dissipation with respect to loads  11  and  12  with which they are in series.  
       BRIEF SUMMARY OF THE INVENTION  
       [0017]     One embodiment of the present invention provides a power converter of voltage step-up or step-down switched-mode power supply type which overcomes the disadvantages of known solutions.  
         [0018]     One embodiment of the present invention enables independent regulation of the respective supply voltages of the loads without requiring several output capacitors.  
         [0019]     One embodiment of the present invention also enables each of the loads to be able to stand a power variation function.  
         [0020]     One embodiment of the present invention also provides an integrable solution compatible with the use of a circuit for generating pulse trains for controlling a cut-off switch comprising a single regulation reference input.  
         [0021]     One embodiment of the present invention provides a power converter of switched-mode power supply type for providing a voltage to several loads independent from one another, comprising a circuit for generating strobe pulses of a D.C. supply voltage, and means for devoting to each load, within a period of relatively long duration as compared to the maximum duration of said strobe pulses and of relatively short duration as compared to the operating time of the load, at least one supply period during which said circuit regulates the voltage provided to the considered load.  
         [0022]     According to an embodiment of the present invention, each load is connected in series with a switch between a first terminal of provision of the output voltage and a terminal connected to ground by a common current-to-voltage conversion element.  
         [0023]     According to an embodiment of the present invention, the converter comprises a circuit for controlling said switches to assign to each of the loads its supply periods.  
         [0024]     According to an embodiment of the present invention, a control signal for said strobe pulse generation circuit is sampled across the current-to-voltage conversion element.  
         [0025]     According to an embodiment of the present invention, a current-limiting element is connected to the connection point of said switches and of the current-to-voltage conversion element, a control signal for said circuit for generating the strobe pulses being sampled at the respective junction points of each load with the corresponding switch, and synchronization switches being interposed between each of the sampling points and the corresponding input of said circuit for generating the strobe pulses.  
         [0026]     According to an embodiment of the present invention, the duty cycles of the supply periods of two loads are inverted with respect to each other.  
         [0027]     According to an embodiment of the present invention, the loads to be supplied are formed of light-emitting diodes in series.  
         [0028]     The present invention also provides a method for sharing a power converter between several independent loads, by devoting to each of the loads periodic supply time windows during which the power converter is respectively dedicated thereto, the periodicity of the time windows being selected according to the remanence time of said loads.  
         [0029]     The foregoing and other features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0030]     FIGS.  1  to  3 , previously described, are intended to show the state of the art and the problem to solve;  
         [0031]      FIG. 4  very schematically shows in the form of blocks an embodiment of a power converter according to the present invention;  
         [0032]      FIGS. 5A, 5B ,  5 C, and  5 D illustrate the operation of the converter of  FIG. 4 ;  
         [0033]      FIG. 6  shows a simplified embodiment of a selection element of the converter of  FIG. 4 ;  
         [0034]      FIGS. 7A, 7B , and  7 C illustrate, in the form of timing diagrams, the operation of the element of  FIG. 6 ;  
         [0035]      FIG. 8  shows a detailed embodiment of a control circuit of the power converter of  FIG. 4 ; and  
         [0036]      FIGS. 9A, 9B ,  9 C,  9 D, and  9 E illustrate, in the form of timing diagrams, the operation of the converter of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     Same elements have been designated with same reference numerals in the different drawings and the timing diagrams of  FIGS. 5, 7 , and  9  have been drawn out of scale. For clarity, only those elements which are useful to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the details constitutive of the control pulse train generation circuits of the cut-off switches of the shown converters have not been detailed and are no object of the present invention, the present invention being compatible with the use of any conventional pulse train generation circuit.  
         [0038]     The present invention will be described in relation with an example of application to step-up converters. However, it more generally applies to any converter, be it a voltage step-up or step-down converter, the assembly of the inductive element of the switch and of the diode, although different according to the converter type, having no influence upon the operation of the present invention.  
         [0039]     A feature of one embodiment of the present invention is to devote to each of the loads periodic supply time windows, different from one load to another.  
         [0040]      FIG. 4  very schematically shows in the form of blocks an embodiment of a power converter for supplying two loads  11  and  12  according to the present invention.  
         [0041]     In this example, loads  11  and  12  are series associations of light-emitting diodes forming, for example, the backlighting elements of a screen. For example, load  11  (Q 1 ) comprises four light-emitting diodes LED in series while load  12  (Q 2 ) only has two.  
         [0042]     The actual power conversion circuit uses many of the same components as the conventional circuit of FIGS.  1  or  2 . Thus, a cut-off switch M is connected to junction point  4  of an inductive element L with a diode D between a terminal  3  of application of a D.C. input voltage Vdc and a terminal  1  connected to ground  2  by a capacitor C for providing an output supply voltage Vout. A circuit  5  for providing control pulses of cut-off switch M is similar to the conventional circuit described in relation with  FIGS. 1 and 2 . Circuit  5  comprises an input for receiving a reference signal OR of the value of the desired output voltage, an input for receiving a regulation signal FB and an input for receiving a clock signal f M  of relatively high frequency (generally, several hundreds of kilohertz).  
         [0043]     Each load  11  or  12  is connected in series with a switch K 1  or K 2 , respectively, between terminal  1  and a first terminal  6  of a current-to-voltage conversion resistor R having its other terminal connected to ground  2 . Feedback signal FB is sampled from terminal  6 .  
         [0044]     Each switch is controlled by a signal CT 1  or CT 2 , respectively, provided by a circuit  7  (μC), for example, a microcontroller. Circuit  7  receives, for example, one or several reference signals CT setting the control needs of loads  11  and  12 , and defines the time periods assigned to each load with a relatively low frequency as compared to the relatively high cut-off frequency of supply voltage Vdc.  
         [0045]      FIGS. 5A, 5B ,  5 C, and  5 D illustrate, in the form of timing diagrams, the operation of a power converter such as shown in  FIG. 4 .  FIG. 5A  illustrates the on periods (ON) of switch K 1 .  FIG. 5B  illustrates the on periods (ON) of switch K 2 .  FIG. 5C  illustrates the periods during which circuit  5  is active (ACT), that is, provides a control pulse train to switch M to regulate output voltage Vout.  FIG. 5D  illustrates an example of a turn-on pulse train (ON) of switch M.  
         [0046]     The present invention takes advantage from the fact that the loads that the converter must supply (especially light-emitting diodes) have a proper operation, even if they do not permanently receive a voltage. In particular, for diodes, their lighting has a sufficient remanence to enable periods when their supply is stopped. To achieve this, account is taken of this remanence of the diodes (or more generally of the periods during which the load, for example, a motor, can temporarily receive no supply) to set the repetition frequency (period T,  FIG. 5C ) of periods T 1 , respectively, T 2 , of supply of each of the loads. For the system to properly operate, another condition is that the frequency (1/T) of the respective supply periods of the different loads is smaller than control frequency f M  of the cut-off switch. This condition is illustrated by  FIG. 5D  which shows that period T M  of the pulses provided by circuit  5  is very low as compared to repetition period T of the control sequences of loads  11  and  12 .  
         [0047]     Repetition period T of periods T 1  and T 2  assigned to loads  11  and  12  is short as compared to the average on time of the loads (at least a few seconds in the case of backlighting diodes) and long as compared to the duration (the longest in the case of an FWM frequency modulation) of the strobe pulses. For example, period T is at least 100 times greater than the duration of the strobe pulses and at least 10 times smaller than the average on time of the loads.  
         [0048]     An advantage of the converter of  FIG. 4  is that it enables an independent regulation on each of the branches supplied by the converter.  
         [0049]     Another advantage resulting therefrom is that the loads can thus be regulated in power variation independently from each other. It is enough to synchronize reference OR with periods T 1  and T 2 . This power variation is, for example, directly conditioned by signal OR provided to circuit  5  and made variable by microcontroller  7  according to a power reference value that it receives for the considered load.  
         [0050]     Another advantage, more specifically as compared to the diagram of  FIG. 2 , is that it avoids a permanent consumption in the circuit and thus enables true shutdown of the converter and of the supplied loads.  
         [0051]     Another advantage is that it preserves the use of a single power converter whatever the number of loads to be supplied. In particular, as illustrated in  FIG. 4 , microcontroller  7  may provide one or several additional control signals CTi to other loads. The only condition is that all loads be likely to be periodically supplied with a frequency which is compatible with their “remanence” and which is smaller than the switched-mode power supply frequency.  
         [0052]     Preferably, the respective load supply periods (periods T 1  and T 2 ) do not overlap. Accordingly, at most, the duty cycle of the two control signals CT 1  and CT 2  of switches K 1  and K 2  is inverted.  
         [0053]     A resulting advantage is that the converter of  FIG. 4  enables optimizing the size of cut-off switch M since the maximum output voltage corresponds to the voltage required by the greatest load.  
         [0054]      FIG. 6  illustrates a simplified embodiment of a circuit  7 ′ for providing signals CT 1  and CT 2  in the case where the periods assigned to the two loads  11  and  12  are complementary (for example, 60% and 40%, 20% and 80%, etc.). In this case, circuit  7 ′ comprises a simple inverter INV receiving a control signal CT as an input, and provides two outputs respectively with the reproduced input signal CT (signal CT 1 ) and this signal CT inverted (signal CT 2 ).  
         [0055]      FIGS. 7A, 7B , and  7 C illustrate in timing diagrams the operation of control circuit  7 ′. They show an example of control signal CT ( FIG. 7A ), signal CT 1  ( FIG. 7B ), and signal CT 2  ( FIG. 7C ).  
         [0056]      FIG. 8  shows an embodiment of a circuit  10  for synchronizing switches K 1  and K 2  according to an optional embodiment of the present invention. It shows all the elements described in relation with  FIG. 4 , except the number of light-emitting diodes LED of the loads (load  11  here comprises three light-emitting diodes LED while load  12  comprises  4 ).  
         [0057]     In this example, switches K 1  and K 2  are formed of MOS transistors.  
         [0058]     The function of circuit  10  is to operate transistors K 1  and K 2  in linear mode during the supply transition from one load to the other. To achieve this, a current-limiting element  13  receives a reference REF on a first terminal while its second terminal is connected to node  6  of connection of switches K 1  and K 2  to resistor R. The output of current-limiting element  13  is connected to the respective gates of switches K 1  and K 2  via switches  14  and  15  respectively controlled by signals CT 1  and CT 2 .  
         [0059]     According to this embodiment, signal FB is sampled upstream of switches K 1  and K 2 . Accordingly, two switches  16  and  17  respectively connect the interconnection nodes of loads  11  and  12  with their switches K 1  and K 2  to the input terminal of signal FB of circuit  5 . Their switches  16  and  17  are respectively controlled by signals CT 1  and CT 2 . Finally, two switches  18  and  19  connect the respective gates of MOS transistors K 1  and K 2  to ground  2 . Switch  18  associated with transistor K 2  is controlled by signal CT 1  while switch  19  associated with transistor K 1  is controlled by signal CT 2 .  
         [0060]      FIGS. 9A, 9B ,  9 C,  9 D, and  9 E illustrate in timing diagrams the operation of the circuit of  FIG. 8 .  FIG. 9A  illustrates the on periods (ON) of switches  14 ,  16 , and  18  controlled by signal CT 1 .  FIG. 9B  illustrates the on periods (ON) of switches  15 ,  17 , and  19  controlled by signal CT 2 .  FIG. 9C  illustrates the shape of current IL 1  in load  11 .  FIG. 9D  illustrates the shape of current IL 2  in load  12 .  FIG. 9E  illustrates the shape of voltage Vout.  
         [0061]     It is assumed that at a time t 0 , the power converter is activated and microcontroller  7  sets a first period T 1  of conduction of the first load  11 . Switches  14 ,  16 , and  18  are on while switches  15 ,  17 , and  19  are off. Starting from a discharge state, voltage Vout increases from zero to reach a voltage level V 1  corresponding to the reference value provided by microcontroller  7 . Current IL 1  in the load increases at the same time, to reach a nominal current Inom adapted to light-emitting diodes LED. At the end of period T 1 , switches  14 ,  16 , and  18  are turned off (time t 1 ). It is assumed in the left-hand portion of the timing diagrams of  FIG. 9  that the duty cycles are not inverted. Accordingly, time t 2  of beginning of the supply of load  12  and of turning-on of switches  15 ,  17 , and  19  is delayed with respect to time t 1 . Load  12  comprises more light-emitting diodes than the first one, voltage Vout must, for a same current Inom, be higher (level V 2 ) than on supply of load  11 . In the case of a power variation conditioned by reference value OR on circuit  5 , levels V 1  and V 2  are accordingly adapted. On the side of current IL 2 , the presence of current-limiting element  13  avoids a peak linked to the turning-on of the different switches.  
         [0062]     It is assumed that at a time t 3 , period T 2  of supply of the second load stops. Level Vout remains at level V 2  until the next time to of starting of the first load. At this time, level Vout falls to level V 1  while current IL 1  increases in the first load. In the vicinity of level V 1 , a slight drop in level Vout (point p) due to the regulation can be observed.  
         [0063]     In the right-hand portion of the timing diagrams of  FIGS. 9 , a duty cycle of 50% is assumed for each of loads  11  and  12 . Times t 1 ′ (end of periods T 1 ) and t 2 ′ (start of periods T 2 ) are confounded, and times t 0 ′ (start of periods T 1 ) and t 3 ′ (end of periods T 2 ) are confounded due to the 50% duty cycle. As in the previous case, current limiter  13  avoids current peaks at times t 2 ′.  
         [0064]     Of course, the present invention is likely to have various, alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, although the present invention has been described in relation with a voltage step-up converter, it also applies with no modification of the controls to a voltage step-down converter. The only difference lies in the actual conversion stage, which remains conventional.  
         [0065]     Further, the generation of the control signals adapted to the operation of the power converter and of the controlled loads is within the abilities of those skilled in the art based on the functional indications given hereabove and by using conventional tools.  
         [0066]     Moreover, more than two loads can be controlled independently from one another.  
         [0067]     Finally, within a same period T, a different number of periods from one load to another may be provided instead of one period, respectively, T 1  or T 2  for each load. For example, a unity duration of supply of all the loads is set as a quotient of period T and a unity number of durations is assigned to each load according to the power desired for this load.  
         [0068]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Technology Category: h