Article source: ON Semiconductor
In recent years, as green environmental protection groups continue to expand the energy conservation and environmental protection boundaries, various standards and standards organizations continue to release new energy efficiency standards. At the same time, terminal products continue to develop toward higher integration and smaller size, reducing energy consumption and improving energy use. Efficiency has become a focus of attention among governments, industry organizations, semiconductor companies, electronics manufacturers and consumers in many countries around the world.
If we look at the power consumption of each application area, it is estimated that about 19% of the world's electricity is used for lighting. In view of this, the industry is constantly looking for more energy-efficient lighting solutions for the general lighting market.
Comparison of different light sources in the general lighting market
From a specific application point of view, the general lighting market covers a wide range of areas, including building lighting, signs, landscape lighting, retail, signal lights, street lighting and residential lighting. In the general lighting market, currently used light sources include incandescent lamps, compact fluorescent lamps (CFLs), linear fluorescent lamps, high-intensity discharge lamps (HIDs), and emerging high-brightness light-emitting diodes (HB LEDs ).
If we compare different light sources based on an energy efficiency benchmark, one important indicator for measuring illumination is the total output lumen to input power ratio, measured in lumens per watt (lm/W), called energy efficiency. Incandescent lamps are relatively less energy efficient in different lighting solutions. For standard 60 W incandescent lamps, the energy efficiency range is between 10 and 13 lm/W (total output is 600 to 800 lm). Relatively speaking, CFLs are much more energy efficient, with typical energy efficiency of 55 to 60 lm/W. However, the CFL is omnidirectional, and when installed in a luminaire, the light is directed, reversed, or obscured, resulting in optical loss, resulting in a net energy efficiency of only 55 to 50 lm/W for a 55 lm/W CFL luminaire. between. Other light sources, such as HID, also have higher energy efficiency than incandescent lamps. A 100 W metal halogen HID lamp can produce approximately 8,000 lumens of output, ie energy efficiency of 80 lm/W; however, like CFL, HID light The output is omnidirectional and there is a lot of loss in the light projection path.
Figure 1: Comparison of energy efficiency trends of different light sources
In comparison, LED is an emerging light source technology. The most common white LEDs are blue LEDs that are plated with phosphorus (which glows yellow when excited). LEDs are becoming more and more energy efficient. The industry's most recent white LED development capability has reached 132 to 136 lm/W and color temperature (4,500-6,000K). In fact, in recent years, the industry has become increasingly interested in using LEDs in the general lighting market. For general lighting, LEDs have many attractive features. For example, it is essentially a low voltage device that is small in size and produces directional light that produces multiple colors and white light. They produce infrared (IR) or ultraviolet (UV) radiation, and because they are solid state devices, they are mechanically strong and free of mercury, and can have a working life cycle of more than 50,000 hours when properly designed and used. Compared to standard incandescent lamps with a life of 1,000 hours, LEDs have a much longer life. These features of LEDs are particularly attractive for lighting applications that require long periods of continuous operation, limited usage, many extreme applications, or where geography is difficult to access or where downtime is costly. As a result, high-brightness LEDs have begun to replace incandescent lamps in a variety of applications; applications that are visible during the day include automotive central parking warning lights and traffic lights.
LED general lighting requirements and LED drive challenges
For the application of LEDs in general illumination, it is necessary to analyze the requirements from a system perspective. In general, LED solid-state lighting systems involve the following requirements:
* LED light source: The light source is compact and efficient, providing a wide range of colors and output power. * Power conversion: efficiently converts AC wall socket, battery, and solar cell power to a safe low-voltage DC power supply. * Control and drive: electronic circuit pairs LED regulation and control* Thermal management: In order to achieve a longer working life, junction temperature control is very important and requires analysis of heat dissipation* Optics: Focusing the light where it is needed requires the use of a lens or light guiding material
These requirements are important when developing energy efficient LED general lighting solutions. Among them, LED control and drive are the focus of this article. For LED drivers, the main challenge is the nonlinearity of the LEDs. This is mainly reflected in the fact that the forward voltage of the LED will vary with current and temperature. The forward voltage of different LED devices will be different. The "color point" of the LED will drift with current and temperature, and the LED must meet the requirements of the specification. Work within the scope to achieve reliable work. The main function of the LED driver is to limit the current within the operating conditions, regardless of the input conditions and the forward voltage. Figure 2 shows the basic working circuit diagram of the LED driver.
Figure 2: Basic operating circuit diagram of the LED driver
For the LED driver circuit, in addition to constant current regulation, there are other key requirements. For example, if LED dimming is required, pulse width modulation (PWM) techniques are required, while typical PWM frequencies for LED dimming are 1 to 3 kHz. In addition, the power handling capability of the LED driver circuit must be sufficient and robust, can withstand a variety of fault conditions, and is easy to implement. It is worth mentioning that since the LED is always "on" when it is optimal for current, its color does not drift.
Since there are often more than one LEDs in the system, this involves the problem of configuring the LEDs. In general, it is highly recommended to drive a single string of LEDs as this provides optimum current matching regardless of forward voltage variations or output voltage "drift". Of course, the user can also configure the LEDs in parallel or in series, in parallel, and so on. If a parallel configuration is used, the circuit will require a "matched" LED forward voltage; if one LED fails open, the other LEDs may be overdriven. Accordingly, multiple parallel or series, parallel cross-connect techniques can be used to try to mitigate the risk of failure.
LED driver application example
Depending on the application, LEDs may be powered by different power sources, such as AC lines, solar panels, 12 V car batteries, DC power supplies or low voltage AC systems, or even alkaline and nickel based batteries or lithium ion batteries.
1) Powering the LEDs with an AC offline power supply
In applications that use AC offline power to power LEDs, there are many different applications, such as electronic ballasts, fluorescent replacements, traffic lights, LED bulbs, street and parking lighting, building lighting, obstacle lights and signs. In these applications that drive high-power LEDs from AC mains, there are two common power-conversion techniques, using a flyback converter when galvanic isolation is required, or a simpler drop when isolation is not required. Pressure topology.
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 up to 5 W (350 mA, 700 mA or 1 A), NCP1014/1028 can deliver up to 8 W of continuous output power, while NCP1351 is suitable for larger than 15 W for larger power general purpose applications.
Take NCP1014/1028 as an example. This is an off-line PWM switching regulator from ON Semiconductor with integrated 700 V high voltage MOSFETs, all with 350 mA/22 Vdc transformer design and 700 mA/17 Vdc configuration, input voltage range 90 to 265 Vac, with output open circuit voltage clamping, frequency jitter reduction electromagnetic interference (EMI) signal, and built-in thermal shutdown protection for LED ballasts, building lighting, display backlighting, signage and channel lighting and Applications such as work lights. The application design diagram of NCP1014/1028 is shown in Figure 3 below. It is worth mentioning that this design has an open output protection function that clamps the output to 24 V when the circuit is open. In this design, the current and open circuit voltage can be adjusted by simply changing the resistor/sina diode combination. It is worth mentioning that if another optional transformer is used for the 230 Vac AC line, the NCP1014 can deliver up to 19 W and the NCP1028 can deliver up to 25 W.
Figure 3: Schematic diagram of the application of ON Semiconductor's offline second-generation LED driver NCP1014/1028
In lighting applications, LED drivers face power factor correction (PFC) problems if the output power requirement is above 25 W. For example, the International Electrotechnical Commission (IEC) of the European Union has provisions for total harmonic distortion (THD) for lighting (power greater than 25 W). In the United States, the ENERGY STAR project's solid-state lighting standards have mandatory requirements for PFC (regardless of the power level), that is, for residential applications, the power factor is higher than 0.7, and for commercial applications. The required power factor is higher than 0.9. This standard is a voluntary compliance standard and is not mandatory, but some applications may require a good power factor. For example, public utilities will promote large-scale application of LEDs, and LEDs used at utility level are expected to have higher power factor; and whether LEDs have higher power factor (usually greater than when LEDs have or provide LED streetlight services) 0.95) Depending on the wishes of the public sector, if they wish, the corresponding LED driver solution must meet this requirement.
Figure 4: Comparison of different architectures in LED driver applications requiring PFC
In such applications where PFC controllers may be required, the traditional solution is a two-stage solution for the PFC controller + PWM controller. This solution supports modularity and simple authentication, but there is a tradeoff in overall energy efficiency, such as assuming an energy efficiency of 87% to 90% in the AC-DC segment, DC-DC energy efficiency. For 85% to 90%, the total energy efficiency is only 74% to 81%. As LED technology continues to improve, this architecture is expected to translate into a more optimized, more energy efficient solution. Depending on the requirements, there are several 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 that do not require isolation, a simpler buck topology can be used, which uses much less inductance than a transformer and requires only a few components to implement this. solution. This architecture uses a peak current control (PCC) mode that operates in deep continuous conduction mode (CCM). This architecture has several advantages, such as the ability to eliminate the use of large electrolytic output capacitors, a simple control principle with "good" steady current, and the ability to take advantage of ON Semiconductor's Dynamic Self-Powered (DSS) technology capabilities directly from the AC line. The drive is powered.
Figure 5 shows the application design of the ON Semiconductor NCP1216 PWM current mode controller.
Figure 5: NCP1216 Non-Isolated Offline LED Driver Application with Peak Current Control
It takes full advantage of the high-pressure process technology and directly powers the controller from the AC mains, further simplifying the circuit. This design is suitable for 120 Vac conditions and requires a few components, such as power FETs and capacitors, to be used for the 230 Vac condition. Since this is a non-isolated AC-DC design, there is a high voltage. And this is a floating design, IC and LED are not ground reference. The LED must be connected to the board before powering the device.
For this type of buck control, when the number of LEDs being controlled is reduced, one of its limitations arises because the duty cycle becomes extremely narrow. Moreover, 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 through a half-wave rectified input circuit. In this approach, the basic architecture can be easily extended by component modifications, which can also drive longer LED strings.
2) Powering the LED with a wide input range DC-DC power supply
Table 1: Wide Input Range DC-DC LED Applications
There are a range of high-brightness LED applications operating in the 8 to 40 VDC range, including lead-acid batteries, 12-36 VDC adapters, solar cells, and low-voltage 12 and 24 VAC AC systems. There are many lighting applications of this type, such as mobile lighting, landscape and road lighting, automotive and traffic lighting, solar powered lighting, and showcase lighting.
Even if the goal is to drive the LED with a constant current, the first thing to understand is the application's input and output voltage variations. The forward voltage of the LED is determined by material properties, junction temperature range, drive current, and manufacturing tolerance. With this information, you can choose the right linear or switching power supply topology, such as linear, buck, boost, or buck-boost. ON Semiconductor's NCP3065/3066 is a multi-mode LED controller with integrated 1.5 A switch that can be configured as a buck, boost, invert (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 up to 250 kHz. Other features include: cycle-by-cycle current limit, no control loop compensation, all ceramic output capacitor operation, analog and digital PWM dimming capability, and internal thermal shutdown during hysteresis.
Figure 6: Schematic of ON Semiconductor NCP3065 in LED constant current step-down control applications
Protect LEDs
As mentioned earlier, LEDs are an extremely long-lasting light source (up to 50,000 hours). In addition to the need to select the right LED driver solution for a specific LED application, it is also necessary to provide proper protection for the LED, as occasionally the LED will fail. There are many reasons for this, either because of early LED failure, or because of local assembly defects or failure due to transients. Precautions must be provided for these possible failures, especially because certain applications are critical applications (high downtime costs) or safety-critical applications (such as headlights, lighthouses, bridges, aircraft, airstrips, etc.), or It is an application that is difficult to access geographically (difficulty in maintenance).
In this regard, ON Semiconductor's NUD4700 LED shunt protection solution can be used. Figure 7 is a schematic diagram of the application and principle of such a shunt protection solution.
Figure 7: Application diagram of ON Semiconductor's NUD4700 LED open-circuit shunt protector
When the LED is working normally, the leakage current is only nearly 100 μA; when it encounters transient or surge conditions, the LED will open, and the shunt channel where the NUD4700 shunt protector is activated will only be activated. 1.0 V, the effect brought to the circuit is reduced as much as possible. The device is housed in a small, space-saving package designed for 1 W LEDs (rated at 350 mA @ 3 V) and supports operation greater than 1 A if properly handled.
to sum up
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 the general lighting market. The application of LED in the general lighting market involves various requirements, such as light source, power conversion, LED control and driving, heat dissipation and optics. Focusing on LED drivers, this paper analyzes the challenges faced by LED drivers in the general lighting market, and combines the high-performance LED driver solutions from ON Semiconductor to explore different LED driver application examples, such as powering LEDs through AC isolated power supplies. Powering LEDs through a wide input range DC-DC power supply; finally, an ON Semiconductor LED shunt protection solution for LED applications that require high reliability and continuity.
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