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EDNE MARCH 2013

readerS SOLVE DESIGN PROBLEMS designideas An improved offline driver lights an LED string Yan-Niu Ren, Southwest Petroleum University, Chengdu, China A constant current is better than a constant voltage for driving LEDs. In this proposed circuit, a common constant voltage regulator is changed into a constant-current source for LEDs. In addition, a startup current limiter is used to suppress large current surges, and a voltage chopper is employed for a wide ac input of 96 to 260 VRMS. The concept presented here originates from two Design Ideas published in 2011 (references 1 and 2) and was developed to improve power efficiency at a low cost. The circuits shown in figures 1 and 2 both have the same brilliance of an inductorless chopper and the same controversial issue of power efficiency. To improve the power efficiency, you should observe two principles: The resistors of the chopper should dissipate as little power as possible, and the chopper should switch at the appropriate threshold voltage, VTH. In addition, VTH should be as close This aproach minimizes CCR power disipation while keping LED curent constant . as possible to the operating voltage across the LED string. This approach minimizes the power dissipation of the constant-current regulator (CCR) while maintaining a constant LED current. The circuit shown in Figure 3 is an DIs Inside 39 Low-duty-cycle LED flasher keeps power draw at 4 mW 40 Rotary encoder with absolute readout offers high resolution and low cost example that follows the principles described above, with a power efficiency of about 85%. Voltage regulator IC1 and R5 form a 20-mA CCR. The LED string has a sufficient number of LEDs to require 120V at 20 mA. The voltage across R6 provides a means for indirect measurement of the LED current. VTH is the diode bridge full-wave rectified output voltage above which, when divided by R1 to R3, the 68V bias of D5 is overcome, turning on Q1 and turning off Q2. C1 charges quickly to VTH while Q2 is on, then discharges slowly Q2 C1 22 F MMSZ15T1G NDD0350Z CCR NSI45020AT1G 22 LEDs R4 10 1% SENSE R3 R 56k 1 330k MMSZ5260BT1G R2 390k D2 1N4004 D1 1N4004 D4 1N4004 D3 1N4004 D6 15V D5 43V Q1 MPSA44 85 TO 265V AC 60 Hz Figure 1 This circuit drives a string of LEDs with a constant current over the entire worldwide range of ac-mains voltages. The resistor in series with the LED string provides a convenient point to measure LED current via its voltage drop. w ww.edn-europe.com MARCH 2013 | EDN Europe 37


EDNE MARCH 2013
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