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Enhanced offline driver lights an LED string

07 Feb 2013  | Yan-Niu Ren

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As shown in figure 4, the C1 discharge time is much longer than the charge time during a 50Hz half-cycle of 10 msec. During this period, the peak-to-peak voltage across C1 is almost 20 mA×10 msec/22µf=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.

Figure 4: The yellow and blue traces, respectively, present the voltage across C1 and R6 in the circuit at 220VRMS (at 50Hz ac). The two traces remain at the same position when the ac input changes from 96VRMS to 260VRMS.

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 start-up 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 50Hz). For a larger LED current, increasing the capacity of C1 and decreasing the resistance of R5 are suggested. For a different LED operation voltage, some parameters should be recomputed in the same way as in the foregoing analysis. The lower the LED operation voltage is, the lower the ac input voltage can be. This Design Idea can also apply to ac at 60Hz.

Author's notes
1.Use high-voltage through-hole resistors or series surface-mount resistors to achieve at least 400V withstand. A fuse is suggested for safety against shorts.
2.Safety warning for novice experimenters: Lethal voltages are present in this circuit; use caution when testing and operating it. If scoping, use an isolation transformer to float the circuit's ac input from earth ground; do not float the oscilloscope chassis. The scope ground cannot be connected to the circuit without isolation.
3. Do not push the button with ac voltage applied. For safe maintenance, keep pressing the button to discharge C1 through R10 until D8 goes out.

References
1. Sheard, Steve, "Driver circuit lights architectural and interior LEDs," EDN, Aug 11, 2011, pg 41.
2. Babu, TA, "Offline supply drives LEDs," EDN, April 21 2011, pg 58.

About the author
Yan-Niu Ren is from Southwest Petroleum University in Chengdu, China.

To download the PDF version of this article, click here.


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