Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/40—Details of LED load circuits
  • H05B45/44—Details of LED load circuits with an active control inside an LED matrix
  • H05B45/48—Details of LED load circuits with an active control inside an LED matrix having LEDs organised in strings and incorporating parallel shunting devices
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/10—Controlling the intensity of the light

Definitions

• the present invention refers to circuit variants and to a method for reducing the flicker index of the LED light engines supplied directly from the public AC power grid at 230V, the so called ..constant current direct AC drivers", or shortly ..direct AC drivers".
• Direct AC drivers experience an accelerated advancement, in 2014 being commercially available more than 20 specialized integrated circuits.
• Specific to direct AC integrated circuits is that they optimize only the electrical efficiency and the power factor; the direct result of this optimization strategy is that all the commercially available solutions exhibit a time interval between 2.5 and 3.5 ms long during which the LEDs are not lighting, as shown in Figure 1 , i.e. they exhibit a flicker index greater than 0.34, the reference profile for calculating the flicker index being indicated in Figure 2.
• the driver mediates between two processes coupled in series and characterized by different dynamic parameters, as following: the power is sunk from the power grid at a rate as close as possible to the sine function, and is supplied to the LEDs at a rate as constant as possible.
• the Jean production" theory of industrial processes developed by global manufacturers like Toyota, Bosch, etc. indicates the solution for solving the conflicts generated by this type of series coupled processes, but characterized by different dynamic parameters, namely: the decoupling of the processes.
• the decoupling of the processes is done by using an accumulator, in which the first process supplies power at its own rate, and the second process sunk power, at a different rate than that of the first process.
• the two processes are shown coupled in series and having different rates.
• the only known solution is the one developed by Texas Instruments - http://www.ti.com/lit/ug/slvu965a/slvu965a.pdf.
• the solution implies the setting of electrolytic capacitors having the values of 33uF/100V, 68uF/50V and 120uF/25V respectively, in parallel of each of the 3 LED segments of a 10W module.
• the result of embedding these 3 electrolytic capacitors as energy accumulators is the reduction of the flicker index from 0.34 to approximately 0.12 and the reduction of the power factor from 0.98 to 0.96 over the whole range of grid voltage variation.
• the technical problem solved by the present invention is the significant reduction (10 times approximately) of the flicker index of the LED modules supplied directly from the public power grid by means of direct AC drivers, with no impact on the power factor.
• the method according to the invention comprises the step of flowing a current through the segments of a LED string, when the power grid voltage value is smaller than the voltage drop on the string made of segments, the flowing current equaling the discharge current of a capacitor set in parallel with the string made of segments, one lead of the capacitor being connected to the anode of the first LED of the first segment and the other lead of the capacitor being connected to the cathode of the last LED of the last segment.
• the direct AC driver does not cease its operation and continues to function with a progressively increasing flicker index, up to the initial value reached without the capacitor.
• Fig. 1 time intervals during which the classic direct AC solutions do not light
• Fig. 2 the reference profile defining the flicker index
• Fig. 3 the decoupling principle
• Fig. 4 the general electric diagram of a direct AC driver with 4 segments, compliant to the prior art
• Fig. 5 the electric diagram of a circuit as according to the invention, in a first embodiment, with 4 LED segments;
• Fig. 6 the profile of the light emitted by the circuit shown in Fig. 5;
• Fig. 7 the profile of the current passing through the last segment L N ;
• Fig. 8 the electric diagram of the circuit as according to the invention, in a second embodiment, with an arbitrary N number of LED segments;
• Fig. 9 the electric diagram of the circuit as according to the invention, in a third embodiment, with an arbitrary N number of LED segments;
• Fig. 1 0 - electric diagram of the circuit as according to the invention, in a fourth embodiment, with an arbitrary N number of LED segments;
• Fig. 1 1 - electric diagram of the circuit as according to the invention, in a fifth embodiment, with an arbitrary N number of LED segments.
• Figure 3 briefly presents the known principle of decoupling the power sunk from the grid, by the power supplied to the LEDs.
• the decoupling is done by an accumulator, wherein the first process is supplying energy with its own rate, different than the rate of the second process, which is sinking energy, where l SU nk is the current sunk from the grid, and I L ED is the current flowing through the LEDs.
• FIG 4 it is shown the known typical/classical direct AC LED driver schematic diagram, with 4 LED segments.
• the direct AC driver is sequentially coupling to the taps of the LED series a number of constant current sources so that the current sunk from the power grid by the circuit exhibits a stepped profile, as shown in Figure 1.
• the current flowing through LEDs is identical to the current sunk from the power grid, i.e. it also exhibits a stepped profile, the light emitted by the series of LEDs being therefore non- constant.
• the LED direct AC flicker index reducing method comprises the step of flowing a current through the segments L-i , L 2 L N of a LED string, when the power grid voltage value is smaller than the voltage drop on the string made of the segments L-i , L 2 LN, the flowing current equaling the discharge current of a capacitor Ci set in parallel with the string made of the segments L-i , L 2 ,..., l_N, one lead of the capacitor C ' i being connected to the anode of the first LED of the first segment L-i , and the other lead of the capacitor Ci being connected to the cathode of the last LED of the last segment L N .
• the example is non-limiting, as according to the invention the series connected LED string may be split in any number of segments, for example 1 , 4, 9, etc.
• a circuit with a number of N segments will employ a number of N tap diodes and a number of N constant current sources.
• the capacitor C-i as shown in Figures 5, 8, 9, 10 and 11 , will store the energy sunk from the power grid at a rate set by the direct AC driver, but will supply it to the LED string at a quasi-constant pace.
• the capacitor will perform the decoupling of the two processes: the grid sunk power process set by the direct AC driver and characterized by an approximately sin 2 variation law, and respectively the LED series power supply process done at a quasi-constant rate.
• the circuit shown in Figure 5 functions as following : when connecting the AC voltage from the public power grid to the AC leads of the rectifier bridge PR, the AC voltage is rectified and fed to the parallel group formed by the LED string and the capacitor Ci respectively. Initially, the capacitor is discharged and the voltage drop at its leads is 0 Volts. This makes the instantaneous voltage potential on the anode of tap diode D 4 to equal the instantaneous value of the rectified voltage; depending on the connecting instance to the power grid, the magnitude of the instantaneous voltage can have any value between OVcc and the maximum of 325Vcc.
• the first result of this current increase through L1 segment is the increase of the voltage drop on the Li LED segment subsequent to the new value of the current, flowing through it, leading to a decrease of the voltage available to L 2 , L 3 and L 4 LED segments, which becomes equal to the difference between the voltage drop on the capacitor Ci leads and the voltage drop on the LED segment, which in turn lead to a lower discharge current of the capacitor C-i.
• the direct AC driver switches off the constant current source Si connected to the tap diode Di cathode and switches on the constant current source S2 connected to the tap diode D 2 cathode. Subsequently, the current flowing through Li and L 2 LED segments increases by the value set by the constant current source S 2 , which leads to a larger voltage drop on Li and L 2 LED segments, given to the new larger current. Subsequently, the voltage available for L 3 and L 4 LED segments decreases, which leads in turn to an even lower discharge current of the capacitor Ci.
• the direct AC driver switches off the constant current source S 2 connected to the tap diode D 2 cathode and switches on the constant current source S 3 connected to the tap diode D 3 cathode.
• the current flowing through Li, L 2 and L 3 LED segments increases with respect to the discharge current of the capacitor by the value set by the S 3 current source, which leads to a larger voltage drop on L-i, L 2 and L 3 LED segments given the new, larger current. Subsequently, the voltage available for L 4 LED segment decreases even more, further decreasing the discharge current of the capacitor Ci .
• the direct AC driver switches off the constant current source S 3 connected to the tap diode D 3 cathode and switches on the constant current source S 4 connected to the tap diode D 4 cathode.
• the load of the constant current source S 4 becomes the parallel group formed by the series connected L-i, l_2, L3, L LED segments and the capacitor C-i, respectively.
• the current flowing through the LEDs will correspond to the voltage drop on the leads of the capacitor C-i.
• the capacitor Ci will be charged with a constant current equal to the difference between the constant current set by the constant current source S 4 and the current flowing through the LEDs.
• the capacitor continues to charge until the instantaneous value of the grid voltage decreases again under the voltage drop on its leads, when the direct AC driver switches off the constant current source S 4 connected to the tap diode D 4 cathode and switches on the constant current source S3 connected to the tap diode D 3 cathode.
• the charge of the capacitor Ci i.e. the storage of electric energy in it ceases and the discharge begins.
• the current flowing through L-i, L2 and L 3 LED segments equals the discharge current of the capacitor Ci to which is added the current set by the constant current source S3; the current flowing through L 4 LED segment stay equal to the discharge current of the capacitor.
• the direct AC driver switches off the constant current source S 3 connected to the tap diode D 3 cathode and switches on the constant current source S 2 connected to the tap diode D 2 cathode.
• the current flowing through Li and L 2 LED segments equals the discharge current of the capacitor Ci plus the current set by the constant current source S 2 , and respectively the current flowing through L 3 and L LED segments equals the discharge current of the capacitor C-i.
• the direct AC driver switches off the constant current source S 2 connected to the tap diode D 2 cathode and switches on the constant current source Si connected to the tap diode Di cathode.
• the current flowing through Li LED segment equals the discharge current of the capacitor Ci plus the current set by the constant current source S-i, and respectively the current flowing through l_2, L 3 and L 4 LED segments equals the discharge current of the capacitor d.
• the direct AC driver switches off the constant current source Si connected to the tap diode Di cathode.
• the current flowing through L-i, L 2 , L3 and L 4 LED segments equals the discharge current of the capacitor C ⁇
• FIG. 8 shows a circuit according to the invention, which comprises:
• a rectifier bridge PR whose positive output is series-connected to the anode of a string made of a plurality of series-connected LED segments Li, L 2 L N ;
• a control circuit which controls a plurality of constant current sources S-i, S 2
• Each constant current source S-i , S 2 ,..., S N having the cathode connected to the tap between the SENS lead of the control circuit and the lead connected to the SENS lead of a resistor RSET, the resistor RSET having its other lead connected to the ground;
• a capacitor Ci set in parallel with the string made of the LED segments L 1 f L 2 ,..., LN, a lead of the capacitor C-i being connected to the anode of the first LED of the first segment L-i, and the other lead of the capacitor Ci being connected to the cathode of the last LED of the last segment L N ;
• a varistor MOV having a lead connected to the cathode of the tap diode D N and the other lead connected to the ground.
• Figure 9 shows a circuit according to the invention which, compared to the circuit from figure 8, further comprises a capacitor C 2 set between the anodes of the tap diodes D N- i and DN, as well as a resistor R-i set between the positive output of the rectifier bridge PR and the anode of the first LED of the segment Li.
• the resistor Ri limits the in rush current of the capacitor C-i, which increases the operation life of the capacitor C ⁇
• the current flowing through the last segment of LEDs L N may generate harmonics in excess of the harmonic standard EN61000-3-2 limits.
• the capacitor C2 filters these harmonics below the limits set by the harmonic standard EN61000-3-2.
• R1 may have a value of some 200 ohm
• Ci may have a value of 10-20 F (in which case the in rush current of Ci may reach 1 ,5 A) and C 2 may have some 100 nF.
• the varistor MOV may be, for example, a 5mm diameter disc varistor.
• the current ratios ii / i 2 /... / of the most popular direct drive integrated circuits are fixed, their effective values being set with only one resistor RSET-
• the voltage drop on RSET given the instantaneous current flowing through it and through a number of series connected LED segments is compared to a reference voltage Vref at the SENS lead. Based on this comparison, the control circuit enables or disables incrementally/decrementally a current source S-i , S 2 ,..., S N switching on or off, successively, the LED segments, and providing the desired profile of the current sunk by the direct AC driver from the grid.
• Dimming of the light source can be achieved by varying RSET- But the variation of RSET is not a wise strategy, given that through RSET are flowing large currents, namely the currents through LEDs.
• R S ET ranges around tens of ohms and therefore Vref ranges in hundreds of mV.
• Vref is set to the lowest voltage potential in the schematic, which is GND, and also noticing that through the GND lead do not flow large currents (the currents through LEDs), but only the currents biasing the control circuit, it was identified a new light engine dimming method by varying the voltage potential of the GND reference lead.
• the voltage potential of the GND lead it is possible to modify the currents , i 2 ,..., I , set by the sources S-i , S2,..., S N , and flowing through the LEDs, and implicitly dim the power sunk by the light engine from the grid.
• the dimming control voltage U D IM is supplied to the schematic through the resistive voltage divider RA/RB translating the standard variation of U D I between 0 and -10V relative to the ground, to a reference potential variation of the GND lead between 0 and -Vref.
• the direct AC driver sinks the nominal current for which it was set.
• the power sunk by the direct AC driver is decreasing, to the point in which when the voltage potential of the GND reference lead becomes equal (and of contrary sign) to the internal reference voltage Vref, the direct AC driver sinks no current and thus the LEDs are no more lighting.
• Figure 10 shows a circuit according to the invention, in which the light source is dimmed. Mainly, it is the same circuit as shown in figure 9 to which a resistor RB is further added, having one lead connected to the ground and the second lead connected to the GND lead of the control circuit as well as a resistor R A having one lead to which a dimming voltage UDI is applied and the second lead connected to the GND lead of the control circuit.
• the light engines shown in figures 5 and 8 can be dimmed as described in the previous paragraph, by adding the resistors R A and RB to the schematic and by applying the voltage U D
• Figure 11 shows a thermally stabilized schematic according to the invention. It is mainly the schematic shown in figure 10 that was thermally stabilized by adding a diode D B having its anode connected to the GND lead of the control circuit and the cathode connected to the ground.
• the thermal stabilization can be done as described in the previous paragraph (by adding a properly connected diode DB) to the schematic shown in figure 10 where the resistor Ri and the capacitor C2 are missing.
• the capacitor Ci may preferably be an electrolytic capacitor.

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• Control Of Indicators Other Than Cathode Ray Tubes (AREA)
• Circuit Arrangement For Electric Light Sources In General (AREA)

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• 2015
  • 2015-05-12 WO PCT/RO2015/000012 patent/WO2015174877A2/en active Application Filing

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