Offline High Power Factor LED Driver Design

    This article will introduce a 28 V, 3.3 A offline high power factor LED driver design for applications such as LED street lights. This design is based on the NCL30001 LED driver and the NCS1002 constant voltage and constant current controller. It uses a 90 to 265 V AC voltage supply, provides a maximum output power of 90 W, has a high power factor, meets relevant harmonic content standards, and can cooperate with pulse width modulation (PWM) dimming.

　　Single-stage topology LED driver + secondary-side CVCC controller combination

　　NCL30001 is a single-stage topology LED driver solution, and its flexible features are very suitable for new LED lighting products. This offline LED driver with single-stage integrated power factor correction (PFC) and isolated step-down AC-DC power conversion eliminates the dedicated PFC boost stage, which can reduce the number of components and solution costs, and improve the overall energy efficiency of the LED power system. It is used in applications such as LED street lights, low-bay lights, exterior wall lights and building lighting.

　　The NCL30001 operates in continuous conduction mode (CCM), which integrates PFC and isolated DC-DC conversion circuits and provides constant current to directly drive LEDs. It is equivalent to integrating the two circuits of AC-DC conversion and LED driving, both of which are located in the lighting fixture, eliminating the linear or DC-DC converter integrated in the LED light strip. This integrated solution has fewer power conversion stages, reduces the number of components used (such as optical components, LEDs, electronic components and printed circuit boards), reduces system costs, and supports higher overall energy efficiency of LED power supplies. Of course, this solution has a higher power density and may not be suitable for all area lighting applications. Its optical pattern may be more suitable for lower-power LEDs. Typical applications include LED street lights, exterior wall lights, wall washers and refrigerator cabinet lighting.

　　Other specifications of the NCL30001 controller include high-voltage startup circuit, voltage feed-forward to improve loop response, input undervoltage detection, internal overload timer, latch input, and high-precision multiplier to reduce input line harmonics. ON Semiconductor also provides an isolated single-stage power factor correction LED driver evaluation board for this device, which is used in applications with a power range of 40 W to 100 W that require direct constant current output. The current can be adjusted between 0.7 A and 1.5 A to match a wider range of high-power high-brightness LEDs.

　　The NCS1002 is a secondary-side constant voltage constant current (CVCC) controller suitable for area lighting applications such as LED street lights. When used with the NCL30001 controller, a high-efficiency LED driver demo board with dimming function can be designed.

　　90 W demo board based on NCL30001 and NCS1002

　　The 90 W constant voltage and constant current demonstration board can accept an extended universal input voltage of 90 to 265 Vac (supports 305 Vac under component replacement conditions), provides a constant current output range of 0.7 A to 1.5 A and a constant output voltage range of 30 V to 55 V (selectable through a resistor divider), with a maximum output power of 90 W, supports 50 to 1,000 Hz dimming control, and includes a 6-pin interface that can be connected to an optional dimming card for intelligent dimming applications such as analog current regulation/dual brightness level digital dimming. In addition, this demonstration board also provides a variety of protection features such as short circuit protection, open circuit protection, over-temperature protection, over-current protection and over-voltage protection. Tests show that the energy efficiency of this demonstration board is higher than 87% under the conditions of 50 W output power, 1,000 mA output voltage/48 V forward voltage drop, and the power factor is higher than 0.9 under 50% to 100% load conditions. At the same time, it meets the IEC61000-3-2 Class C equipment harmonic content standards.

　　Figure 1: Top view of the demo board

　　Since this design can have power levels comparable to the standard NCL30001 evaluation board (50V2A), the changes in the circuit principle lie in the design of the transformer and the constant current/constant voltage secondary side control circuit based on the NCS1002. Figure 2 shows the primary side circuit. This design's flyback transformer is derived from the secondary winding of the original design to meet the new maximum voltage and current requirements. The required inductance and overall structure of the primary winding are basically the same. The 277 VAC input can change the "X" and "Y" capacitors of the EMI input filter with the appropriate voltage rating.

　　Figure 2: Primary side schematic of the NCL30001 CVCC 90 W demo board

　　Using the NCL30001 continuous conduction mode power factor corrected single stage flyback controller, a constant current LED driver for area lighting applications such as overhead, tunnel, parking lot and roadway lighting can be implemented. This particular design is optimized for LEDs that require drive capability greater than 3A, such as the CSM360 and SST90 from Luminus Devices. This design can support up to 8 SST90s in series or 2 CSM360 devices. The CSM360 LED has a nominal forward voltage of 12.8 V at a drive current of 3.2A, which can produce 1600-3900 lumens depending on color temperature and luminous flux.

　　The secondary side control circuit is shown in Figure 3. In this design, the output voltage is determined by a resistor ratio, and the output current is also set based on a resistor ratio. Therefore, only two resistors need to be changed: one is the output voltage divider R34, and the other is R32 which is part of the current setting circuit. In addition, since the output voltage is lower and the output current is higher, the output capacitance is increased while reducing the voltage rating of the capacitor.

　　Figure 3: Secondary side schematic