Introduction to the Evolutionary LED String Offline Driver

For LED driving, the constant current method is better than the constant voltage method. In the circuit proposed in this article, the constant voltage regulator commonly used for LEDs is replaced with a constant current source. In addition, a starting current limiter is used to suppress large inrush currents, and the voltage chopper can be used for an AC input voltage range as wide as 96VRMS~260VRMS.

The concept of this article is derived from two design examples published in 2011 (Reference 1 and Reference 2), which have been improved to improve power efficiency at a low cost. The circuits in Figure 1 and Figure 2 have the same inductorless chopper and controversial power efficiency issues. To improve power efficiency, the following two principles should be followed: the heat loss of the chopper resistor should be minimized; the chopper should switch at the appropriate threshold voltage VTH. In addition, VTH should be as close as possible to the operating voltage of the LED string. This method minimizes the power dissipation of the constant current regulator (CCR) while maintaining a constant LED current.

The circuit in Figure 3 is an example of following the above principles, with a power efficiency of about 85%. Regulator IC1 and R5 form a 20mA CCR. The LED string contains enough LEDs to require 120V at 20mA. The voltage across R6 provides an indirect way to measure the LED current.

VTH is the output voltage of the full-wave rectifier bridge. After the voltage divider from R1 to R3, it exceeds the 68V bias voltage of D5, turning Q1 on and Q2 off. When Q2 turns on, C1 quickly charges to VTH and then slowly discharges through the LED string until the next half cycle of the AC line. At the end of C1 discharge, VTH must not fall below the 120V voltage required to maintain LED operation, nor exceed 1.414 times the minimum AC level VRMS. When the LED requires 120V voltage, add the 3V input-output voltage difference required by IC1, plus the 1.25V voltage of R5, so the minimum C1 voltage will be 124.25V. For simplicity, this figure is rounded to 125V.

As shown in Figure 4, in a 10 ms 50 Hz half cycle, C1 discharges much longer than it charges. During this period, the peak-to-peak value across C1 is approximately 20 mA × 10 ms / 22 μF = 9.09 V. Therefore, UC1-MAX = 125 V + 9.09 V = 134.09 V. For simplicity, this value is rounded to 135 V. This value is VTH, and any voltage greater than this value will turn Q1 on and be cut off by Q2.

When Q1 is on, R4 in Figure 3 dissipates less than 20mW at 260VRMS input, while the R1-R2-R3-D5 divider dissipates less than 100mW. This is negligible compared to the 2.4W dissipation of the LED. These resistors are large values, so the power dissipation is low. R3 is used to precisely adjust VTH to match the actual voltage drop across the LED string.

There is a startup current limiter in the circuit to limit the inrush current through C1 and Q2, which occurs in the cycle just before VTH is reached, just when the AC line is applied. The current limiting resistor reduces the efficiency of each cycle, and R9 only limits the power-up surge to 1.35A until C2 is charged enough to turn on Q3.

When the AC input increases, the power consumption of the chopper increases slightly, while the power efficiency decreases slightly, as shown in Table 1.

This improved circuit can work at 96 V ~ 260 Vac (50 Hz). For larger LED currents, it is recommended to increase the capacity of C1 and reduce the resistance of R5. If the LED operating voltage is different, some parameters should be recalculated in the same way as above. The lower the LED operating voltage, the lower the AC input voltage can be. This design example is also applicable to 60Hz AC.

Author's Note:

1. Use a high-current through-hole resistor or several surface-mount resistors in series that can withstand a voltage of at least 400V. Considering the safety issue during a short circuit, it is recommended to use a fuse.

2. Safety warning for beginners: This circuit contains lethal voltages. Be especially careful during testing and use. If possible, use an isolation transformer to suspend the circuit's AC input to the ground, not to the oscilloscope case. The oscilloscope ground is not isolated, so it cannot be connected to the circuit.

3. Do not press the button when AC voltage is applied. To ensure safety during maintenance, press the button to fully discharge C1 through R10 to D8.