Design example of LED drive circuit based on AC/DC power supply

1) Use AC offline power supply to power LED

There are many different applications where AC offline power is used to power LEDs, such as electronic ballasts, fluorescent lamp replacement, traffic lights, LED bulbs, street and parking lighting, building lighting, obstruction lights and signs. In these applications where high-power LEDs are driven from AC mains, there are two common power conversion technologies, namely the use of a flyback converter when galvanic isolation is required, or the use of a simpler buck topology when isolation is not required.

In terms of flyback converters, different flyback converters from ON Semiconductor can be used depending on the output power. For example, ON Semiconductor's NCP1013 is suitable for compact design applications with power up to 5 W (current of 350 mA, 700 mA or 1 A), NCP1014/1028 can provide up to 8 W of continuous output power, and NCP1351 is suitable for general-purpose applications with higher power greater than 15 W.

Take NCP1014/1028 as an example. This is an offline PWM switching regulator launched by ON Semiconductor. It has an integrated 700 V high-voltage MOSFET. It uses a 350 mA/22 Vdc transformer design and a 700 mA/17 Vdc configuration. The input voltage range is 90 to 265 Vac. It has output open-circuit voltage clamping, frequency jittering to reduce electromagnetic interference (EMI) signals, and built-in thermal shutdown protection. It is suitable for applications such as LED ballasts, building lighting, display backlighting, signs and channel lighting, and task lights. The application design diagram of NCP1014/1028 is shown in Figure 1 below. It is worth mentioning that this design has an open-circuit output protection function, which will clamp the output to 24 V voltage when the circuit is open. In this design, the current and open-circuit voltage can be adjusted by simply changing the resistor/Zener diode combination. It is worth mentioning that if an optional transformer is used for the 230 Vac line, the NCP1014 can provide up to 19 W and the NCP1028 can provide up to 25 W.

Figure 1: Application diagram of ON Semiconductor's offline second-generation LED driver NCP1014/1028.

In lighting applications, if the output power requirement is higher than 25 W, the LED driver faces the problem of power factor correction (PFC). For example, the European Union's International Electrotechnical Commission (IEC) has provisions for total harmonic distortion (THD) in its requirements for lighting (power greater than 25 W). In the United States, the Department of Energy's "Energy Star" project solid-state lighting standards have mandatory requirements for PFC (regardless of the power level), that is, the power factor is required to be higher than 0.7 for residential applications and higher than 0.9 for commercial applications. This standard is a voluntary standard and is not a mandatory requirement, but some applications may require a good power factor. For example, public utilities will promote the large-scale application of LEDs, and LEDs used at the utility level are expected to have a higher power factor; and when public utilities own or provide LED street light services, whether the LED has a higher power factor (usually greater than 0.95) depends on the willingness of the public utilities. If they are willing, the corresponding LED driver solution must meet this requirement.

Figure 2: Comparison of different architectures for LED driver applications requiring PFC.

In such applications where a PFC controller may be required, the traditional solution is a two-stage solution of PFC controller + PWM controller. This solution supports modularity and is simple to authenticate, but there will be a compromise in overall energy efficiency. For example, assuming that the energy efficiency of the AC-DC section is 87% to 90% and the energy efficiency of the DC-DC section is 85% to 90%, the total energy efficiency is only 74% to 81%. With the continuous improvement of LED technology, this architecture is expected to be transformed into a more optimized and more energy-efficient solution. Depending on the requirements, there are a variety of options available, such as: PFC + non-isolated buck, PFC + non-isolated flyback or half-bridge LLC, NCP1651/NCP1652 single-stage PFC solution.

On the other hand, as mentioned above, in applications where isolation is not required, a simpler buck topology can be used, which uses a much smaller inductor than a transformer and requires only a few components to implement this solution. This architecture uses peak current control (PCC) mode and operates in deep continuous conduction mode (CCM). This architecture has several advantages, such as eliminating the need for large electrolytic output capacitors, a simple control principle with "good" current regulation, and the ability to fully utilize ON Semiconductor's Dynamic Self-Power (DSS) technology capability to power the driver directly from the AC line. Figure 3 shows an application design diagram of ON Semiconductor's NCP1216 PWM current mode controller.

Figure 3: NCP1216 non-isolated offline LED driver application using peak current control.

It takes full advantage of high voltage process technology to power the controller directly from the AC mains, further simplifying the circuit. This design is suitable for 120 Vac conditions, and a few components such as power FETs and capacitors need to be changed for 230 Vac conditions. Since this is a non-isolated AC-DC design, high voltage is present. Also, this is a floating design, and the IC and LED are not referenced to ground. The LED must be connected to the board before powering the device.

One limitation of this type of buck control is when the number of controlled LEDs decreases, because the duty cycle becomes very narrow. Also, the switch controller has a leading edge blanking circuit of 200 to 400 ns before the current is sensed. In this case, the switching frequency must be reduced to accommodate normal operation and the voltage is kept to a minimum by a half-wave rectified input circuit. In this approach, the basic architecture can be easily expanded by component modification to drive longer LED strings.

2) Use a DC-DC power supply with a wide input range to power the LED

There are a range of high brightness LED applications that operate from power sources ranging from 8 to 40 VDC, including lead acid batteries, 12-36 VDC adapters, solar cells, and low voltage 12 and 24 VAC AC systems. This type of lighting applications are numerous, such as event lighting, landscape and road lighting, automotive and traffic lighting, solar powered lighting, and display case lighting.

Table 1: DC-DC LED applications with wide input range.

Even if the goal is to drive LEDs with constant current, the first thing to understand is the input and output voltage variations of the application. The forward voltage of the LED is determined by material properties, junction temperature range, drive current, and manufacturing tolerances. With this information, the appropriate linear or switching power supply topology can be selected, such as linear, buck, boost, or buck-boost. The NCP3065/3066 from ON Semiconductor is a multi-mode LED controller that integrates a 1.5 A switch and can be set to buck, boost, inversion (buck-boost)/single-ended primary inductor converter (SEPIC) and other topologies. The NCP3065/3066 has an input voltage range of 3.0 to 40 V, a low feedback voltage of 235 mV, and an adjustable operating frequency of up to 250 kHz. Other features include: cycle-by-cycle current limiting, no need for control loop compensation, operation with all ceramic output capacitors, analog and digital PWM dimming capabilities, and internal thermal shutdown when hysteresis occurs.

Figure 4: Schematic diagram of ON Semiconductor NCP3065 in LED constant current buck control application.

Provide protection for LED

As mentioned earlier, LEDs are extremely long-life light sources (up to 50,000 hours). In addition to choosing the right LED driver solution for your specific LED application, you also need to provide proper protection for your LEDs, because occasionally they fail. The reasons vary, from early LED failure to local assembly defects or transient phenomena. Preventive measures must be taken against these possible failures, especially because some applications are critical (high downtime costs), safety-critical (headlights, lighthouses, bridges, aircraft, runways, etc.), or geographically inaccessible (difficult to maintain).

In this regard, ON Semiconductor's NUD4700 LED shunt protection solution can be used. Figure 5 is a schematic diagram of the application and principle of this shunt protection solution.

Figure 5: Application diagram of ON Semiconductor NUD4700 open-circuit LED shunt protector.

When the LED is working normally, the leakage current is only about 100 μA; when encountering transient or surge conditions, the LED will be open-circuited, and the shunt channel where the NUD4700 shunt protector is located will be activated, and the voltage drop caused is only 1.0 V, which will minimize the impact on the circuit. This device uses a small space-saving package and is designed for 1 W LEDs (rated current is 350 mA @ 3 V). If the heat dissipation is properly handled, it also supports operation with a current greater than 1 A.

Summarize

Compared with traditional light sources such as incandescent lamps, LEDs have many advantages such as high energy efficiency, long life, and good directivity, and are increasingly favored by the industry for use in the general lighting market. The application of LEDs in the general lighting market involves many requirements that need to be considered from a system perspective, such as light source, power conversion, LED control and drive, heat dissipation, and optics.