Patent WO-2014077718-A2 (WO)
Inventor: RUS ADRIAN IOAN
IPC: H05B44/00
Filed: 2013-04-19
Abstract: An EN61000-3-2 compliant lighting apparatus sized for minimum crest factor criteria or for maximum efficiency, that sinks from the grid rectangular current pulses AM, high frequency PWM, instrinsic PWM or random modulated, consisting of a rectifier bridge, a constant current source series-connected with a sequence of [N x M] high power [HP] and high brightness [HB] LEDs, series-connected and organized in N segments times M LEDs each, segments that can be introduced or removed from the circuit by short-circuiting them by an individual switch, except for the segment to the current source, a resistor voltage divider setting the voltage switching tresholds at which the LED segments are introduced or removed from the circuit and a voltage reference modulated or not by a signal un-correlated to the network voltage. Upon connecting the grid to the input terminals A and B of the circuit displayed in Figure 5, the grid alternate voltage is full wave rectified by the rectifier bridge and is applied to the string of N x M luminiscent diodes [LED] series-connected with the constant current source. Upon increasing the U AB  voltage between 0 and up to reaching the conduction value of the segment with the highest S N  parameter that is permanently present in circuit, in series with the constant current source, the current sink by the apparatus. At the hereinafter increase of voltage at AB terminals, the current through S N  segment and through the constant current source increases exponentially up to th value set by the constant current source, through the switch made by transistor Q N-1  by-passing segments S N-1 ... S 1 . Observing the serial presence of the constant current source, the voltage drop across segment SN is limited and consistent to the constant current set by the current source. For U AB  voltages lower than the maximum voltage drop on a series of LEDs supplied at that constant current, the full input voltage drops at the terminals of segment S N ; As input voltages exceed the above mentioned maximum drop voltage, the balance will drop on the constant current source. When the input voltage exceeds the threshold values set via the R 1 , R 2 +...+R N  resistive divider, the switch consisting the transistor Q N-1  and controlled by the corresponding comparator, gets switched off, thus introducing the segment S N-1  in the circuit, in series with S N . The sink current drops to a value set by U AB  voltage, quadruplicated to M N +M N-1  number of diodes from segments S N  si S N-1  present in series and their curent to voltage curve. Hereinafter, as the U AB  voltage successively exceeds the threshold values set by the R 1 , R 2 , R N  resistive dividers, the schematic introduces successively in the circuit, in series with the already existing segments, additional segments that undertake voltage drops from the constant current source. Upon the decrease U AB  voltage at AB terminals, the process is carried out in a reverse way, namely upon each drop in UAB voltage, below the threshold levels set by the resistive dividers made of R 1 , R 2 ,... R N , the schematic successively removes from the circuit and in a reverse order as above, LED segments, by-passing them with the appropriate switch. The power conversion efficiency and the crest factor are being set by simply fixing the N [the number of segments] and the switching threshold levels,. Following the above described successive switch, I have ensured a sequence of constant amplitude current pulses, approximately rectangular, starting at approximately 0.2mS and lasting up to approximately 9.8ms from the zero crossing of grid voltage is sinking through the LEDs. To get EN61000-3-2 compliance, the sink current is amplitude, HF-PWM, LF intrinsec PWM or random modulated so that the envelope of real time or average sink current follows the„ i trapezoid" trapezoidal profile, indicated in Figure 4. If setting the driver for maximum efficiency, the harmonic spectrum of the sink current does not comply with EN61000-3-2, its harmonic amplitudes between approximately 1kHz and 3kHz, being some 3% higher than the limit. For EN61000-3-2 compliance the spectrum of sink current is scattered by modulating the reference voltage by low frequency uncorrected to the grid. Spectrum scattering through such method reduces the amplitude of the harmonic components by some 8dB thus offering a safe margin to the limit. Comparing the maximum theoretical performances achievable through the various modulation methods, it is noticed that for a given structure of 10 x 10 LEDs, they are grouped. Under these circumstances, the election of the modulation method is technical and price based. In the real life the efficiency is affected by power losses in the current source, in switches and on rectifier bridge. A superficial calculation, sets these additional losses in the area of 1.5% which affects the theoretical efficiency to approximately 92%. Nevertheless the efficiency losses can be offset by simply gliding the switching thresholds to the point where the harmonics between 1kHz and 3kHz drop below the 3% limit.