DE102011088426A1 - ELECTRONIC BALLAST AND METHOD FOR OPERATING AT LEAST ONE FIRST CASCADE AND ONE SECOND CASCADE OF LEDS - Google Patents

Abstract

The present invention relates to an electronic ballast for operating at least a first (K1) and a second cascade (K2) of LEDs, comprising an input having a first (E1) and a second input terminal (E2) for coupling to a supply AC voltage (UN); a rectifier (10) coupled to the first (E1) and second input terminals (E2), the rectifier (10) having an output having a first (A1) and a second output terminal (A2); at least one first unit (EH1) comprising at least the first cascade (K1) of LEDs; at least one second unit (EH2) comprising at least the second cascade (K2) of LEDs; and at least one linear current limiting device (SB1) serially coupled to at least the first unit (EH1); wherein the first unit (EH1) comprises a first storage capacitor (C1) connected in parallel with the first cascade (K1); and the second unit (EH2) comprises a second storage capacitor (C2) connected in parallel with the second cascade (K2). The invention also relates to a corresponding method for operating at least a first and a second cascade of LEDs.

Description

Technical area

The present invention relates to an electronic ballast for operating at least a first cascade and a second cascade of LEDs, comprising an input having a first and a second input terminal for coupling with an AC supply voltage, a rectifier coupled to the first and second input terminals, wherein the rectifier has an output having a first and a second output terminal, at least one first unit comprising at least the first cascade of LEDs, at least one second unit comprising at least the second cascade of LEDs, and at least one linear current limiting device connected in series is coupled to at least the first unit. It also relates to a corresponding method for operating at least a first and a second cascade of LEDs.

State of the art

1 shows in this connection a known from the prior art electronic ballast. There is a rectifier 10 On the input side via a first E1 and a second input terminal E2 with a supply AC voltage U _N , for example, a grid voltage coupled. The rectifier 10 provides at its output, which comprises a first A1 and a second output terminal A2, the rectified AC supply voltage. These are, in particular, rectified half sine waves. The series connection of an ohmic resistor R and several cascades K1, K2 of diodes is coupled to the output terminal A1, whereby different numbers of cascades of LEDs can be activated via assigned switches S1, S2. For example, by turning on the switch S2 (in the case of a non-switched switch S1), the cascades K1, K2 can be coupled between the rectifier output terminals A1, A2.

Although only two cascades and two switches are shown in the present case, this arrangement may comprise a plurality of cascades as well as a plurality of switches. By suitable control of the switch S _i as a function of the output A1, A2 of the rectifier 10 Currently provided rectified AC supply voltage, the forward voltage of the activated LED cascades can be adapted to the instantaneous AC supply voltage. The ohmic resistor R is used to limit the current.

This circuit does not require any inductors and therefore has advantages with regard to the requirements of the relevant radio interference regulations. In addition, the size is small and the power factor is good.

However, go with this known electronic ballast a variety of disadvantages: The LEDs are poorly utilized, since many LEDs are not used over long periods. In view of the high price of LEDs, this is undesirable. Furthermore, certain LEDs are operated much more frequently than other LEDs. For example, with reference to 1 the LEDs of the K1 cascade frequently operate as the LEDs of the K2 cascade. LEDs of the cascades to the right of the K2 cascade are operated even less frequently. This results in uneven service life expectancy and can lead to premature failure, although much of the LEDs have still not reached the intended operating life.

Furthermore, with regard to the majority of the LEDs, a luminous flux modulation with a degree of modulation of 100% results. This results in unwanted flicker phenomena. While the luminous flux is substantially semi-sinusoidal, the current consumption from the network is rectangular. This creates many harmonics. This results in a poor PF. Since there are regulations for luminaires with a power of more than 25 W, which require a largely sinusoidal current consumption, that is known from the prior art 1 illustrated electronic ballast for operating lighting devices with higher powers unsuitable.

Presentation of the invention

The present invention is therefore based on the object, a generic electronic ballast in such a way that it causes the lowest possible EMC interference, the existing LEDs as well as possible and uses the lowest possible Lichtstromodulation shows.

This object is achieved by an electronic ballast having the features of claim 1 and by a method having the features of claim 19.

The present invention is based on the finding that the abovementioned object can be achieved if in each case one storage capacitor is connected in parallel to the cascades. In this way, it is possible to supply the LEDs during the phases with electricity during which the rectified provided at the output of the rectifier Supply AC voltage has low voltage values. As a result, depending on the size of the storage capacitor used in each case, the degree of modulation of the light modulation can in principle be significantly reduced, in particular reduced to zero. According to the invention, the LED cascades can be operated permanently. This leads to a complete utilization of the existing LEDs, that is all LEDs can be permanently in operation with suitable dimensioning of the storage capacitor.

A first preferred category of electronic ballasts according to the invention comprises at least a first and a second linear current limiting device, wherein the first linear current limiting device is associated with the first unit and coupled serially to the first unit, the second linear current limiting device being associated with the second unit and serial to the second unit is coupled, wherein the series circuit of the first linear current limiting device and the first unit on the one hand and the series circuit of the second linear current limiting device and second unit on the other hand, at least in phases parallel between the first output terminal of the rectifier and a reference potential, in particular the second output terminal of the rectifier coupled. A linear current limiting device may in its simplest embodiment constitute a variable resistor. Although this is cheap and produces no EMC interference, but this results in unwanted losses. It is therefore preferred to form the linear current limiting device as a linearly regulated current source. As a result, a specifiable current can be set. Preferably, such a linearly regulated current source is realized by an operational amplifier.

In a first embodiment, the first and the second linear current limiting device is coupled to the first output terminal of the rectifier without the interposition of an electronic switch. The respective storage capacitors are always recharged when the voltage at the first output terminal of the rectifier is above the forward voltage of the respective LED cascade. The difference between the voltage at the first output terminal of the rectifier and the voltage across the respective LED cascade drops at the respective linear current limiting device. This results in losses, which is why embodiments are preferred in which at least between the first output terminal of the rectifier and the second linear current limiting device, a first electronic switch is coupled. The electronic switch does not affect the good EMC, since the switch usually switches only twice per half cycle and thus slowly, that is, without generating EMC interference, can be switched.

If the respective LED cascades are optimized for different input voltage ranges, the respective LED cascade can be switched on and off depending on the level of the instantaneous rectifier output voltage by means of the first electronic switch, so that the respective units and the associated linear current limiting devices are activated only in the optimum voltage range are. In this case, the current sources are activated according to the different forward voltages of the LED cascades at suitable, different times, so that there is a harmonic reduced current consumption. Such an embodiment is therefore characterized by an excellent power factor.

The activation of the respective cascade takes place automatically when the rectifier output voltage becomes greater than the forward voltage of the respective LED cascade, the linear current limiting device having a diode action. This results in very low losses. Despite the switching on and off of the respective unit, the LEDs are operated permanently. For this purpose, the storage capacitor of the respective unit must be interpreted accordingly.

The combination of linear current limiting device and associated unit with the highest voltage may be permanently coupled to the rectifier output, the associated linear current limiting device should have a diode action, so that the storage capacitor when exceeding the threshold voltage resulting from the forward voltage of the parallel-connected LED cascade , is recharged and can only discharge via its associated LED cascade. The diode prevents the discharge of the capacitor when connecting a parallel path.

More preferably, a second electronic switch is coupled between the first output terminal of the rectifier and the first linear current limiting device, that is, associated with the highest flux voltage LED cascade. The use of a second electronic switch has the advantage that the time for switching on the associated LED cascade can be controlled and can be done later than in the case that the switching on the threshold voltage is defined. This results in an additional degree of freedom, which can be used for optimizing the harmonic correction or for optimizing the distribution of the power to the various LED cascades.

Preferably, the electronic ballast further comprises at least one further linear current limiting device, a further electronic switch and a further unit with a further cascade of LEDs and a further storage capacitor. The more such units are provided, the more sinusoidal the current consumption can be designed. The more sinusoidal the current consumption is, the better the power factor of such an electronic ballast. For systems over 25W, three to four LED cascades have been a good compromise between cost and power factor. The respective LED cascades and the associated linear current limiting device are each optimized for different input voltage ranges. The power of the respective LED cascades can be different, resulting in an additional degree of freedom, in order to achieve a high efficiency and an optimized harmonic correction of the current consumption.

A second preferred category of a ballast according to the invention is characterized in that the first and the second unit are coupled to each other in series. In this case, the electronic ballast comprises a linear current limiting device, which is associated with the first unit and the second unit, wherein the linear current limiting device is serially coupled to the series connection of the first and the second unit, wherein the series circuit of linear current limiting device, first unit and second unit at least in phases connected in series between the first output terminal of the rectifier and a reference potential, in particular the second output terminal of the rectifier. Preferably, a first diode is serially coupled between the first and second units. This ensures that the storage capacitor of the first unit with conductive second unit does not discharge via the LED cascade of the second unit.

Further preferably, a first electronic switch is coupled in parallel to the series connection of the first diode and the second unit. In this way, the second LED cascade can be short-circuited, so that a small voltage, which results from the forward voltage of the first LED cascade, can be coupled to the rectifier output. When the electronic switch is nonconductive, the series combination of first and second LED cascades is coupled between the rectifier output terminals. Thus, a higher voltage at the rectifier output can be used largely lossless to operate the LED cascades.

This variant may also comprise at least one further diode, a further electronic switch, a further unit with a further cascade of LEDs and a further storage capacitor for optimizing the harmonic correction and minimizing the losses.

Preferably, the respective storage capacitor is generally, regardless of the selected Verschaltungsvariante the LED cascades, dimensioned so that sufficient energy to supply all the LEDs of the respective cascade is present during a half cycle of the AC supply voltage. As a result, the light modulation can be reduced to values close to zero as desired. Preferably, the two or more cascades have a different forward voltage. As a result, an optimal adaptation to the semi-sinusoidal profile of the rectifier output voltage can be achieved, which results in very low losses on the one hand and a largely sinusoidal current consumption on the other hand.

As already mentioned, the respective linear current-limiting device can preferably represent a linearly controllable current source or a current-limiting resistor.

Further preferably, the electronic ballast comprises a control device for controlling at least the first and / or the second and / or the further electronic switch. In this context, it is particularly preferred for a parallel connection of the LED units if the control device is designed to switch the first electronic switch nonconducting as soon as current flows through the first cascade. This state can be easily detected, for example, by a small shunt resistor.

Furthermore, in this context, the control device can be designed to switch the first electronic switch to conducting as soon as no current flows through the first cascade.

In general, the control device can be designed to switch the respective electronic switch in such a conductive or non-conductive manner that the temporal integral of the power loss of all linear current-limiting devices is below a predefinable threshold value over a half-wave of the alternating supply voltage, in particular is minimal. This results in particularly low-loss implementations.

It can also be provided that a number of all to be operated by means of the electronic ballast LEDs is specified, the division of the LEDs in the individual cascades in such a way that the temporal integral of the power loss of all linear current limiting devices is less than a predefinable threshold value over a half-wave of the AC supply voltage, in particular is minimal. This procedure takes into account the fact that the losses can be minimized by a clever division of the number of LEDs specified, for example, on the basis of the power to be achieved.

In this case, the at least one linear current-limiting device is preferably controlled in such a way that the mains current consumption corresponds to predefinable requirements for the mains harmonics or a power factor or predefinable requirements in the form of a predefinable waveform.

Further preferred embodiments emerge from the subclaims.

The preferred embodiments presented with respect to the electronic ballast according to the invention and their advantages apply correspondingly, as far as applicable, to the method according to the invention for operating at least one first and one second cascade of LEDs.

Short description of the drawing (s)

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Show it:

1 a schematic representation of a known from the prior art electronic ballast;

2 a schematic representation of a first embodiment of an electronic ballast according to the invention;

3 the time course of various sizes of the electronic ballast according to 2 ;

4 the time course of various sizes of an electronic ballast according to the invention, in which compared to the embodiment of 2 the electronic switch S1 is replaced by a short circuit;

5 a schematic representation of a second embodiment of an electronic ballast according to the invention; and

6 the time course of various sizes of the electronic ballast according to 5 ,

Preferred embodiment of the invention

The referring to 1 introduced reference numerals are used in the following for the same and same-acting components.

2 shows a schematic representation of a first embodiment of an electronic ballast according to the invention. Accordingly, the rectifier output terminal A1 is coupled via a first linear current limiting device SB1 to a unit EH1, which comprises the parallel connection of a storage capacitor C1 and an LED cascade K1. The voltage dropping between the output terminals A1, A2 of the rectifier is denoted by U _Gl . The output terminal A1 can also be coupled via an electronic switch S1 to a second linear current limiting device SB2, which in turn is coupled to a unit EH2.

The latter comprises the parallel connection of a storage capacitor C2 and a second LED cascade K2.

The LED cascades K1 and K2 have a different number of LEDs and therefore different forward voltages. The current flowing in the first linear current limiting device SB1 is denoted by I1, while the current flowing into the second linear current limiting device SB2 is indicated by I2.

3 shows the time course of various sizes of the electronic ballast of 2 , The between the output terminals A1 and A2 of the rectifier 10 decreasing voltage U _Gl is semi-sinusoidal. The switch S1 is already turned on at time t1. As soon as the voltage U _Gl exceeds the forward voltage U2, the capacitor C2 is charged and a current I2 begins to flow into the unit EH2. Until time t3, the linear current limiting device SB2 is active, wherein at time t3, the switch S1 is switched non-conductive. Due to the fact that the voltage U _Gl exceeds the forward voltage U1, the first linear current limiting device SB1 becomes active and the current I1 starts to flow.

As soon as the voltage U _Gl again falls below a predefinable threshold value, that is to say in this case at the time t4, the linear current-limiting device SB1 is deactivated and the linear current-limiting device SB2 is activated. It starts again to flow a current I2. At time t5, the current limiting device SB2 becomes inactive. Accordingly, the mains current consumption takes place between times t2 and t5 in accordance with the indicated currents I1 and I2 flowing at these times.

As can be seen from the thick double arrows, a current I1 flows in the voltage range UB1 of the voltage U _Gl , whereas the voltage range UB2 is covered by the then current I2 flowing in phases. As would be obvious to one skilled in the art, by providing further voltage limiting devices SBI and units EI, an improved adaptation of the current consumption to the course of the voltage U _{Gl could be} effected, which would result in a further optimization of the power factor as well as in a reduction of the power loss. The respective power loss is determined in particular as the product of the current flowing through the respective current-limiting device and the voltage difference across the respective voltage-limiting device. This is proportional to the gap between the curve U _Gl and the respectively indicated current curves.

The respective current limiting devices SB1, SB2 and the forward voltages U1, U2 are selected such that the second linear current limiting device (current I2) operates at a lower voltage than the first current limiting device. The activation takes place in each case when the voltage U _{Gl is} greater than the forward voltage of the respective LED cascade. The respective linear voltage limiting devices SB1, SB2 are preferably designed as linear regulators with diode action.

4 shows the time course of various sizes of an embodiment of an electronic ballast according to the invention, in which compared to the in 1 illustrated embodiment, the switch S1 is replaced by a short circuit. In this variant, the unit EH2 is permanently active, resulting in higher losses, as the voltage range is greater, to which it must be designed. The mains power consumption is, compare with this 3 , with the in 2 illustrated embodiment comparable.

5 shows an embodiment of an electronic ballast according to the invention, in which the units EH1 and EH2 are connected in series with the interposition of a diode D1. The switch S1 is connected so that it bridges the series connection of diode D1 and unit EH2 in its conducting state. As can be seen, in this embodiment, a linear current limiting device SB1, which is coupled in series with the units EH1 and EH2, is sufficient. The diode D1 serves to prevent the switch C1 is turned on, above the LED cascade K2, when the switch S1 is turned on. When switch S1 is not switched, the two cascades K1, K2 are connected in series.

Regarding 6 the switch S1 is turned on at time t1. As soon as the voltage U _Gl exceeds the forward voltage U1 of the first LED cascade K1, a current begins to flow through the cascade K1. As soon as the voltage U _{Gl is} greater than the sum U2 of the forward voltages of the cascades K1 and K2 as well as the diode D1, the switch S1 is switched non-conducting. This is the case at time t3. Then a current begins to flow through both cascades K1 and K2. As soon as U _Gl again falls below the voltage U ₂ , the switch S 1 is again turned on, whereby the current flow is limited again to the cascade K 1.

Claims (19)

An electronic ballast for operating at least a first (K1) and a second cascade (K2) of LEDs, comprising: - an input having a first (E1) and a second input terminal (E2) for coupling to a supply AC voltage (U _N ); A rectifier ( 10 ), which is coupled to the first (E1) and the second input terminal (E2), wherein the rectifier ( 10 ) has an output with a first (A1) and a second output terminal (A2); At least one first unit (EH1) comprising at least the first cascade (K1) of LEDs; At least one second unit (EH2) comprising at least the second cascade (K2) of LEDs; and at least one linear current limiting device (SB1) serially coupled to at least the first unit (EH1); characterized in that the first unit (EH1) comprises a first storage capacitor (C1) connected in parallel with the first cascade (K1); and the second unit (EH2) comprises a second storage capacitor (C2) connected in parallel with the second cascade (K2). Electronic ballast according to claim 1, characterized in that the electronic ballast comprises at least a first (SB1) and a second linear current limiting device (SB2), wherein the first linear current limiting device (SB1) of the first unit (EH1) is assigned and serial to the first unit (EH1) is coupled, wherein the second linear current limiting device (SB2) of the second unit (EH2) is associated and coupled in series to the second unit (EH2), wherein the series circuit of the first linear current limiting device (SB1) and first unit (EH1) on the one hand and the series connection of the second linear current-limiting device (SB2) and the second unit (EH2), on the other hand, at least in phases parallel between the first output terminal of the rectifier ( 10 ) and a reference potential, in particular the second Output connection (A2) of the rectifier ( 10 ) is coupled. Electronic ballast according to claim 2, characterized in that at least between the first output terminal (A1) of the rectifier ( 10 ) and the second linear current limiting device (SB2) is coupled to a first electronic switch (S1). Electronic ballast according to claim 3, characterized in that between the first output terminal (A1) of the rectifier ( 10 ) and the first linear current limiting device (SB1) a second electronic switch (S2) is coupled. Electronic ballast according to one of claims 2 to 4, characterized in that the electronic ballast further at least one further linear current limiting device (SB _i ), another electronic switch (S _i ) and a further unit (EH _i ) with a further cascade (K _i ) of LEDs and a further storage capacitor (C _i ). Electronic ballast according to claim 1, characterized in that the first (EH1) and the second unit (EH2) are coupled to each other in series, wherein the electronic ballast comprises a linear current limiting device (SB1), the first unit (EH1) and the second Unit (EH2) is associated, wherein the linear current limiting device (SB1) is serially coupled to the series connection of the first (EH1) and the second unit (EH2), wherein the series circuit of linear current limiting device (SB1), first unit (EH1) and second unit (EH2) at least in phases serially between the first output terminal (A1) of the rectifier ( 10 ) and a reference potential, in particular the second output terminal (A2) of the rectifier ( 10 ) is coupled. Electronic ballast according to claim 6, characterized in that a first diode (D1) is coupled in series between the first (EH1) and the second unit (EH2). Electronic ballast according to Claim 7, characterized in that a first electronic switch (S1) is coupled in parallel to the series connection of first diode (D1) and second unit (EH2). Electronic ballast according to one of claims 6 to 8, characterized in that the electronic ballast further at least one further diode (D _i ), another electronic switch (S _i ) and a further unit (EH _i ) with a further cascade (K _i ) of LEDs and a further storage capacitor (C _i ). Electronic ballast according to one of the preceding claims, characterized in that the respective storage capacitor (C1; C2) is dimensioned such that sufficient energy is available for supplying all the LEDs of the respective cascade (K1; K2) during a half cycle of the alternating supply voltage (U _N ) , Electronic ballast according to one of the preceding claims, characterized in that the two (K1, K2) or more cascades have a different forward voltage. Electronic ballast according to one of the preceding claims, characterized in that the respective linear current limiting device (SB1; SB2) represents a linearly controllable or controllable current source or a current limiting resistor. Electronic ballast according to one of claims 3 to 12, characterized in that the electronic ballast further comprises a control device for controlling at least the first and / or the second and / or the further electronic switch. Electronic ballast according to claim 13 in combination with one of claims 2 to 5, characterized in that the control device is designed to switch the first electronic switch (S1) nonconducting as soon as current flows through the first cascade (K1). Electronic ballast according to one of Claims 13 or 14, characterized in that the control device is designed to switch on the first electronic switch (S1) as soon as no current flows through the first cascade (K1). Electronic ballast according to one of claims 13 to 15, characterized in that the control device is designed to switch the / the respective electronic switch (S1, S2, S _i ) so conductive or non-conductive that the time integral of the power loss of all linear Current limiting devices over a half-wave of the AC supply voltage (U _N ) is below a predetermined threshold, in particular is minimal. Electronic ballast according to one of claims 13 to 16, characterized in that a number of all to be operated by means of the electronic ballast LEDs is specified, wherein the division of the LEDs in the individual cascades (K1, K2, K _i ) is such that the temporal Integral the power loss of all linear current limiting devices over a half-wave of the AC supply voltage (U _N ) is below a predetermined threshold, in particular is minimal. Electronic ballast according to one of Claims 16 or 17, characterized in that the at least one linear current-limiting device (SB1; SB2; SB _i ) is controlled in such a way that the mains current consumption can prescribe requirements for the network harmonics or a power factor or predefinable requirements in the form of a predefinable waveform equivalent. A method of operating at least a first (K1) and a second cascade (K2) of LEDs by means of an electronic ballast comprising an input having a first (E1) and a second input terminal (E2) for coupling to a supply AC voltage (U _N ); a rectifier ( 10 ), which is coupled to the first (E1) and the second input terminal (E2), wherein the rectifier ( 10 ) has an output with a first (A1) and a second output terminal (A2); at least one first unit (EH1) comprising at least the first cascade (K1) of LEDs; at least one second unit (EH2) comprising at least the second cascade (K2) of LEDs; and at least one linear current limiting device (SB1) serially coupled to at least the first unit (EH1); characterized by the following steps: a) providing a first and a second storage capacitor; and b) parallel connection of the first storage capacitor of the first cascade (K1) and parallel connection of the second storage capacitor of the second cascade (K2).

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