High-Brightness LED Driving Challenges in the General Lighting Market and ON Semiconductor Solutions

In recent years, as green environmental protection groups continue to expand the boundaries of energy conservation and environmental protection, various standard organizations continue to release new energy efficiency standards. At the same time, terminal products continue to develop in the direction of higher integration and smaller size. Reducing energy consumption and improving energy efficiency have become a common focus of governments, industry organizations, semiconductor companies, electronic product manufacturers and consumers in many countries around the world.

Comparison of different light sources in the general lighting market

From the perspective of specific applications, the general lighting market covers a wide range of fields, including building lighting, signs, landscape lighting, retail, signal lights, street lighting and residential lighting. In the general lighting market, commonly used light sources include incandescent lamps, compact fluorescent lamps (CFL), linear fluorescent lamps, high-intensity gas discharge lamps (HID) and emerging high-brightness light-emitting diodes (HB LED).

If we compare different light sources based on an efficiency benchmark, an important metric for measuring lighting is the total output lumens to input power ratio, measured in lumens per watt (lm/W), known as efficiency. Among different lighting solutions, incandescent lamps have a relatively low efficiency. For a standard 60 W incandescent lamp, the efficiency range is between 10 and 13 lm/W (total output is 600 to 800 lm). In contrast, CFLs are much more efficient, with a typical efficiency of 55 to 60 lm/W. However, CFLs are omnidirectional, and when installed in a lamp, the light is directed, reversed, or blocked, resulting in light loss, making the net efficiency of a 55 lm/W CFL lamp only between 28 and 50 lm/W. Other light sources, such as HID, also have higher energy efficiency than incandescent lamps. A 100 W metal halide HID lamp can produce an output of about 8,000 lumens, which is an energy efficiency of 80 lm/W; however, like CFL, the light output of HID is omnidirectional, and there will be a lot of losses in the light projection path.

LEDs are an emerging light source technology. The most common white LEDs are blue LEDs coated with phosphors (which emit yellow light when excited). LEDs have increasingly higher energy efficiencies, with the industry recently announcing that the strongest white LEDs have developed capabilities of 132 to 136 lm/W and a color temperature of (4,500-6,000K). In fact, in recent years, the industry has been increasingly interested in using LEDs for the general lighting market. For general lighting, LEDs have many attractive properties.

LED general lighting requirements and LED driver challenges

For the application of LED in general lighting, it is necessary to analyze its requirements from the perspective of the system. In general, the LED solid-state lighting system involves the following requirements:

LED light source: compact and efficient, providing a wide range of colors and output power

Power conversion: Efficiently convert power from AC wall sockets, batteries, and solar cells to safe low-voltage DC power

Control and drive: Use electronic circuits to stabilize and control the LED

Thermal management: To achieve longer operating life, junction temperature control is very important and heat dissipation needs to be analyzed

Optics: Focusing light where it is needed requires the use of lenses or light-guiding materials

These requirements are important when developing energy-efficient LED general lighting solutions. Among them, LED control and driving are the focus of this article. For LED drivers, the main challenge is the nonlinearity of LEDs. This is mainly reflected in the fact that the forward voltage of the LED will change with current and temperature, the forward voltage of different LED devices will vary, the LED "color point" will drift with current and temperature, and the LED must operate within the range required by the specification to achieve reliable operation. The main function of the LED driver is to limit the current within the operating conditions, regardless of how the input conditions and forward voltage change.

For LED driver circuits, in addition to constant current and stabilization, there are other key requirements. For example, if LED dimming is required, pulse width modulation (PWM) technology is required, and the typical PWM frequency for LED dimming is 1 to 3 kHz. In addition, the power handling capacity of the LED driver circuit must be sufficient, and the function must be strong, can withstand a variety of fault conditions, and be easy to implement. It is worth mentioning that since the LED is always in the "on" state at the most suitable current, its color will not drift.

Since the number of LEDs that need to be used in the system is often more than one, this involves the issue of configuring the LEDs. Generally speaking, it is strongly recommended to drive a single string of LEDs because this provides the best current matching, regardless of forward voltage changes or output voltage "drift". Of course, users can also configure LEDs in parallel or series, parallel cross connection, etc. If a parallel configuration is used, the circuit requires "matched" LED forward voltages; if an LED fails and opens, other LEDs may be overdriven. Accordingly, the use of multiple parallel or series, parallel cross connection techniques can be used to try to reduce the risk of failure.

LED driver application examples

Depending on the application, LEDs may be powered by different power sources such as the AC line, solar panels, 12V car batteries, DC power supplies or low-voltage AC systems, or even alkaline and nickel-based batteries or lithium-ion batteries.

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 replacements, 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 of more 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. Both use 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. 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 another optional transformer is used for the 230 Vac AC line, the NCP1014 can provide up to 19 W of power and the NCP1028 can provide up to 25 W of power.

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.

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, you can choose the appropriate linear or switching power supply topology, such as linear, buck, boost, or buck-boost. The NCP3065/3066 from ON Semiconductor is a multi-mode LED controller that integrates a 1.5A 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 40V, 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.

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 premature 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).

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

Summarize

Compared with traditional light sources such as incandescent lamps, LEDs have many advantages such as high energy efficiency, long life, and good directionality, 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. This article focuses on LED drivers, analyzes the challenges faced by LED drivers in the general lighting market, and combines ON Semiconductor's high-performance LED driver solutions to explore different LED driver application examples, such as powering LEDs with AC isolated power supplies and powering LEDs with wide input range DC-DC power supplies; finally, it also introduces ON Semiconductor's LED shunt protection solutions that can be used in LED applications that require high reliability and continuity.