This application claims priority from European Patent Application No. 14154986.5 filed Feb. 13, 2014, the entire disclosure of which is incorporated herein by reference.
The present invention concerns the field of electronic power devices for light emitting diodes with a battery as the electrical energy source. In particular, the invention concerns the case where the battery is of small dimensions and inexpensive, for example a small battery of small thickness which is generally arranged to deliver a low instantaneous current.
In a first variant, electronic system 2 continuously powers the LED. In a second variant, the LED is periodically powered. This latter case is often implemented since it enables electrical power consumption to be reduced and the periodic light signal provides sufficient or better visual information.
For electronic system 2 to be functional, the battery must be chosen to enable electronic device 8 and the LED to be powered directly. Thus, the battery must have limited internal resistance. Indeed, for electronic system 2 to operate, there must first be a voltage Vin, higher than or equal to the minimum LED voltage VLEDmin divided by the multiplier factor K (Vin>=VLEDmin/K), so that the battery power PBat(Vin) is higher than or equal to the minimum LED operating power PLEDmin multiplied by the yield η of electronic device 8, namely PBat(Vin)>=η·PLEDmin. Further, voltage Vin which satisfies this first condition must be higher than or equal to the minimum operating voltage Vinmin of electronic device 8 which includes the voltage converter and its electronic control unit.
A maximum value for internal resistance RB, beyond which it is not theoretically possible for the electronic system to be functional, can be deduced from the preceding considerations regarding the operating conditions of electronic system 2. Since the maximum power Pinmax that the battery can provide is given by the relation Pinmax=VBat2/4RB, the electronic system is not functional if the internal resistance RB>(η/4)·(VBat)2/PLEDmin. Further, if voltage Vinmin is higher than half of voltage VBat, this system is no longer functional once RB>η·(VBat−Vinmin)·Vinmin/PLEDmin. Let us consider an example for a specific application: To operate properly the LED requires a voltage VLED=Vsup=3.0 V and an electric current ILED=Isup=1.0 mA, the selected battery has a no-load voltageBat=1.5 V and the voltage converter is a voltage tripler with a minimum operating voltage Vinmin=0.9 V and a yield of 80%. For this application to be functional, internal resistance RB must therefore be lower than or equal to RBmax=144Ω. With regulation at Vin=1.0 V and if RB=125Ω, a supply current Isup of around 1.07 mA is obtained with no other losses; which is suitable for the continuous mode or periodic mode application. However, if there is a resistance RB=140Ω for this regulated voltage, the current Isup has a value of approximately 0.95 mA; which is lower than the 1.0 mA required.
The small inexpensive batteries are generally characterized by a relatively high internal resistance, for example between 200Ω and 400Ω. Indeed, in principle, the decrease in internal resistance results in an increase in the cost and/or size of the battery. Moreover, some commercially advantageous technologies are not possible at a lower cost if the internal resistance of the battery has to be limited, for example for flat and flexible batteries. There is therefore a need for an electronic power device for an LED which could have, as energy source, a battery with a relatively high internal resistance, in particular higher than the maximum theoretical value given above.
It is an object of the present invention to find a solution to the aforementioned problem, in order to be able to properly power a light emitting diode with a battery whose internal resistance is high.
To this end, the present invention concerns an electronic device for powering an LED as defined in claim 1.
As a result of the features of the present invention, it is possible to periodically power an LED, in particular with a higher power than the maximum power that the selected battery can deliver, while controlling the electrical current that flows through the LED so as to optimise the dimensions of the various electronic elements and electrical power consumption. The storage capacitor makes it possible to decouple the electrical power supplied by the battery and the electrical power supplied to the LED. It is therefore possible to optimise operating conditions, on the one hand, for the part of the electronic system formed by the selected battery, the voltage converter and the control unit, and, on the other hand, for the part of the electronic system formed by the LED and the current source.
The invention will be described below with reference to the annexed drawings, given by way of non-limiting example, and in which:
With reference to
Electronic device 18 further includes an electrical energy storage capacitor 24, arranged downstream of the voltage converter parallel to output pad 17, a current source 26 and a switch 28 controlled by a control unit 22 of the electronic device. Generally, the current source and the switch are arranged in series between storage capacitor 24 and a lower voltage terminal 30 on the electrical path provided for powering the LED. In the variant shown in
According to the invention, storage capacitor charging phases and LED actuation phases are provided alternately. To achieve this, control unit 22 is arranged to control switch 28 so as to make it alternately non-conductive during first periods T1 and conductive during second periods T2. In
During an LED actuation phase it is important that the supply voltage Vsup supplied by the electronic device to the diode remains higher than or equal to a minimum voltage Vsupmin corresponding to the sum of the LED operating voltage VLED and a minimum current source actuation voltage VCSmin. The supply voltage is equal to the voltage supplied by the storage capacitor Vcap in this embodiment, and therefore Vcap=Vsup>=VLED+VCSmin throughout the actuation phase and particularly for supply voltage V2 at the end of this actuation phase. Since voltage VLED is constant with a constant supply voltage Isup=ICS (curve 48 in
For the actuation phase, there are two variants depending on whether or not voltage converter 20 remains active during this actuation phase. In the first variant where the voltage converter is inactive, there is the following mathematical relation:
ΔV=V1−V2=ICS·T2/C (1)
where V1 is the supply voltage at the start of the actuation phase, T2 the duration of this actuation phase and C the storage capacitance value.
It will be noted that multiplier coefficient K is preferably provided so that voltage V1 is lower than or equal to K·Vin, with Vin higher than voltage Vinmin for the operation of electronic device 18 and corresponding to a given input current Iin regulated by the electronic device. In all cases, voltage V1 is lower than the non-load voltage VBat of the battery multiplied by K. In the second variant, converter 20 is active and the battery directly supplies part of the LED supply current. Therefore:
ΔV=V1−V2=(ICS−IBC)·T2/C (2)
Where IBC is the mean current supplied by the voltage converter during period T2.
The mathematical relation (1) or (2) above therefore connects various parameters of the electronic system. If period T2 and current ICS are given (and also IBC determined in the second variant), there is obtained a relation between ΔV and C, namely ΔV·C equal to a determined constant value Q. If V2 is equal to Vsupmin, to be able to supply current ICS during period T2, there is obtained an equality giving a higher minimum voltage V1min for a given value C, namely: (V1min−Vsupmin)=Q/C. There is therefore a direct relation between storage capacitance C and the higher minimum voltage V1min. On the other hand, for a determined higher voltage V1, there is obtained from the aforementioned relation a minimum capacitance value Cmin: Cmin=Q/(V1−Vsupmin). If value C is selected to be higher than Cmin in this latter case or if the value of V1 is selected to be higher than V1min in the other case, then the value of V2 at the end of the actuation period is higher than Vsupmin.
Taking, for example, an LED as described with reference to
In a storage capacitor charging phase during which the LED is deactivated (switch 28 open and thus non-conductive), the energy conservation law can be used to determine the charging duration T1 and vice versa. To simplify the calculation, the preferred variant will be considered where firstly, the current Iin at the input of electronic device 18 is regulated by control unit 22 to be substantially constant provided the voltage Wcap across the terminals of capacitor 24 is lower than or equal to the corresponding input voltage Vin multiplied by multiplier factor K of the voltage converter, and where secondly, the higher voltage V1 is provided to be lower than or equal to K·Vin. It will be noted that voltage Vin is provided to be higher than the minimum operating voltage Vinmin of electronic device 18.
The increase in energy in storage capacitor 24 is given by ΔE=C·(V12−V22)/2. This difference in energy is supplied by the battery via the voltage converter, the latter being in a transition phase since the initial voltage V2 is generally lower than K·Vin. During period T1, a mean yield η* of the electronic device can be defined, which is generally lower than yield η in steady state phase. Within the scope of the aforementioned preferred variant, the energy provided to the capacitor is defined by ΔE=η*·Vin·Iin·T1. The following mathematical relation is thus obtained:
C=2η*·Vin·Iin·T1/(V12−V22) (3)
Utilising the values of the example considered for the actuation phase and a yield of η*=0.6 (60%), a voltage Vin=1.0 V and the battery used in the preceding numerical examples (VBat=1.5 V and RB=300Ω), there is obtained a current Iin=1.66 mA. Next, with a period T1max=900 ms, there is obtained from mathematical relation (3) a maximum storage capacitance value Cmax=˜120 μF.
In the given numerical example, it is thus noted that, with T1=100 ms and T2=900 ms, namely T1+T2=1.0 s (one second), the storage capacitance value may be comprised between two end values defined, on the one hand, by the actuation phase and on the other hand, by the charging phase: 55 μF<C<120 μF. There is thus a certain freedom in the choice of storage capacitor according to the desired application. Thus, to decrease the duration of charging period T1, C will be set lower than but close to 120 μF. However, if it is desired to reduce energy consumption and thus to increase the longevity of the battery, C will be set higher than but close to 55 μF to reduce the energy dissipated by the battery. With, for example C=60 μF, the input power necessary is Pin=0.82 mW. The input voltage, corresponding to the voltage delivered by the battery, thus has a value Vin≈1.3 V and the corresponding current Iin=0.63 mA. The electronic device can therefore be arranged so that the current drawn from the battery is regulated at Iin=0.65 mA or, taking account of residual losses not included in the calculations, for example Iin=0.70 mA. It will be noted that with Vin=1.25 V, the voltage converter can have a multiplier factor K=4, instead of K=5 where Vin=1.0 V. On the other hand, it is also possible to select a higher capacitance C, for example C 150 μF, and to reduce the selected higher voltage V1, particularly to V1=3.9 V. In this latter case, it is then possible to further decrease the multiplier factor and to choose a voltage tripler.
Thus, according to the invention, storage capacitance value C, higher voltage V1 and the first and second periods T1 and T2, for a given LED and battery and for the elements of the electronic device other than the storage capacitor, are selected so that, on the one hand, the voltage across the storage capacitor terminals in each actuation period remains higher than or equal to a minimum supply voltage Vsupmin, equal to the sum of the LED operating voltage VLED and a minimum current source actuation voltage VCSmin, and so that, on the other hand, the voltage at the storage capacitor terminals at the end of each charging period is at least equal to higher voltage V1. The voltage converter is selected with a multiplier factor which allows at least this higher voltage to be achieved at the storage capacitor terminals for the battery provided.
Finally, as previously indicated, according to a preferred variant of the electronic device, control unit 22 is arranged to regulate the supply current Iin supplied by the battery to the electronic device so that the supply voltage Vin of the battery remains higher than a minimum supply voltage Vinmin of the electronic device.
Number | Date | Country | Kind |
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14154986.5 | Feb 2014 | EP | regional |