Depending on the application, the 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 1 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 1: Schematic diagram of the application 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 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 2: Comparison of different architectures in LED driver applications that require 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 3 shows the application design of the ON Semiconductor NCP1216 PWM current mode controller.
Figure 3: NCP1216 non-isolated off-line 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
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.
Table 1: DC-DC LED applications with wide input range.
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 4: Schematic of the ON Semiconductor NCP3065 in LED constant current buck 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 5 is a schematic diagram of the application and principle of such a shunt protection solution.
Figure 5: Schematic diagram of the application 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.
Compact Fluorescent Bulbs,Energy Saving Lamp Spira,17Mm 5500K Bulbs For Studio
f.s.c. Technology Corp., Limited , http://www.cfl-manufacturer.com