designideas L1 2.2 mH C1 100 nF R1 10 1W 1N4004 1N4004 1N4004 1N4004 LINE NEUTRAL Figure 2 The chopper operation is similar to the circuit of Figure 1; the larger LED series resistor, instead of a constant-current source, provides the current-limit function. Table 1 Power eficiency of improved circuit VRMS ac at 50 Hz 96 140 180 220 260 Power efficiency (%) 90 87 86 85 82 into the LED string until the next halfcycle R2 39k R3 10k of the incoming ac. VTH must be no less than required to maintain the LED operation voltage of 120V at the end of C1’s discharge and no more than 1.414 times the VRMS of the lowest ac level. With 120V required for the LEDs, plus the 3V input-to-output D3 1N4004 C2 220 F 63V + I=20 mA Q2 IRF830 R5 100 R4 470k 1W D2 15V D1 39V Q1 BC547 differential required by IC1, plus 1.25V developed across R5, the minimum C1 voltage will be 124.25V. For simplicity, this figure can be rounded up to 125V. As shown in Figure 4, the C1 discharge time is much longer than the charge time during a 50-Hz half-cycle of 10 msec. During this period, the peakto peak voltage across C1 is almost 20 mA×10 msec/22 μf=9.09V. Thus, UC1_MAX=125V+9.09V=134.09V. For simplicity, this result can be rounded up to 135V. This is VTH; any voltage above this turns Q1 on and gets chopped off by Q2. When Q1 switches on, the power consumption of R4 in Figure 3 is less than 20 mW at 260VRMS input, and the R1-R2-R3-D5 divider dissipates less than 100 mW. This result is almost negligible compared with the 2.4W consumed by the LEDs. These resistors are large value so as to consume as little power as possible. R3 allows fine adjustment of VTH to match the actual drop across the LED string. A startup current limiter has been included to limit the large inrush current surge through C1 and Q2 that would occur if the ac were switched on at a time in its cycle just before VTH was reached. A current-limiting resistor would reduce efficiency on every cycle, but R9 limits only the surge to 1.35A at power-up until C2 charges sufficiently to turn on Q3. As the ac input increases, the power consumption of the chopper rises a little and power efficiency decreases somewhat, as shown in Table 1. This improved circuit can run at 96V to 260V ac (at 50 Hz). For a larger LED current, increasing the capacity of C1 and decreasing the resistance of R5 VOLTAGE CHOPPER EDNDI5305 Fig 2.eps DIANE Q2 IRF830 R4 R 3.3M 1 649k D6 BZX55- C7V5 D5 BZX55C68 D3 1N4004 D4 R2 649k R3 20k LM317AH VIN VOUT ADJ R10 6k C1 22 F 200V + IC1 Q1 MPSA44 D8 STARTUP CURRENT LIMITER Q3 IRF830 D7 BZX55C7V5 R7 2.67M C2 0.1 μF R8 178k 1N4004 R6 5 1% R5 63.4 20 mA AT 120V D1 1N4004 4A 50 Hz D2 + _ 1N4004 96 TO 260VRMS AC R9 100 Figure 3 This circuit achieves an efficiency improvement by using tight control of the switching threshold to provide just barely enough LED voltage. 38 EDN Europe | MARCH 2013 www.edn-europe.com

EDNE MARCH 2013

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