ON Semiconductor's complete set of key solutions for LED power supply design [Graphic Analysis]

Power Solutions for LED Lighting Applications

LED portable backlight application power supply solution

Compared with traditional light sources, light-emitting diodes (LEDs) have many advantages, such as low operating voltage, high energy efficiency, small size and directional light. They can provide a very wide range of colors and white light, do not produce infrared (IR) or ultraviolet (UV) radiation, and because they are solid-state devices, they are mechanically strong and mercury-free. When properly designed and used, they can have an operating life of more than 50,000 hours, which is much longer than the 1,000-hour life of standard incandescent lamps. In addition, they are fully dimmable. These advantages have made the application of LEDs more and more extensive. Today, they have many application markets, such as architectural landscape lighting, traffic lights, display screens, retail, small and medium-sized liquid crystal display (LCD) backlights, automobiles and solar energy, and have an increasing development space in street lighting, residential lighting and even medium and large-sized LCD backlights.

High-brightness LEDs are critical to enabling the solid-state lighting revolution, with significant implications for lighting design, global energy savings and innovative products. This revolution requires a holistic approach in which LEDs are integrated with power conversion and control electronics, as well as thermal management solutions and optics.

Power Solutions for LED Lighting Applications

As mentioned above, LEDs are inherently low voltage devices; their forward voltage can range from less than 2 V to 4.5 V, depending on color and current. In addition, LEDs need to be driven with a constant current to ensure the desired brightness and color. This requires a power conversion and control solution that can adapt to different power sources, whether it is the AC line, solar panels, 12 V car batteries, DC power or low voltage AC systems, or even alkaline and nickel-based batteries or lithium-ion batteries.

As a global leading supplier of energy-efficient power semiconductors, ON Semiconductor focuses on using its low-voltage and high-voltage technologies and expertise in power management solutions to address the challenges of LED lighting: whether it is portable display products, automotive interior lighting or ballasts for LED signal lights. In the following, we will discuss ON Semiconductor's corresponding driver power solutions in conjunction with a variety of different applications for LED lighting, such as architectural, industrial, automotive and portable applications.

1) Offline AC-DC switching power supply solution with integrated high-voltage FET up to 700 V

Figure 1(a): Functional block diagram of LED lighting solution with input power less than 10 W

ON Semiconductor has extensive experience in converting power from AC mains to usable energy with power levels ranging from a few watts to hundreds of watts for a wide range of applications. ON Semiconductor has developed very high voltage integrated circuits (VHVICs) that combine high voltage switching transistors and medium voltage analog circuits to provide integrated, cost-effective solutions for a wide range of power requirements. ON Semiconductor offers a variety of fixed frequency controllers and converters that integrate high voltage field effect transistors (FETs) up to 700 V into simple, economical, low component count solutions.

These solutions include three areas: Monolithic AC-DC constant current drivers ranging from 5 to 15 W, such as the NCP1013, NCP1014, and NCP1028. The NCP series of offline controllers, both isolated and non-isolated, up to 150 W. ON Semiconductor offers a reference design with a single-stage power factor corrector (PFC) for street lighting.

Figure 1(b): Functional block diagram of an LED lighting solution with an input power between 10 and 25 W

PFC Solutions

For lower power applications below 25 W (230 Vac), an integrated power switching regulator is most commonly used as it minimizes the number of components. Above this power range, a controller can be used as it provides the designer with the flexibility to choose the most appropriate high voltage FET for the application. Controllers can be used for both isolated and non-isolated applications, and ON Semiconductor offers a variety of different value-added features that allow designers to optimize the design for their specific design requirements.

Figure 2: NCP1351 offline LED driver reference design.

Figure 2 shows the ON Semiconductor NCP1351 offline LED driver reference design. This reference design has an input power range of 85 to 265 Vac, features small size, low cost, good line regulation, 80% high efficiency at 20 W load, and integrates safety features such as overload protection and short circuit protection, with a maximum size of only 125 × 37 × 35 mm.

Figure 3 (a): Traditional 2-stage PFC architecture.

The IEC1000-3-2 standard sets specifications for limiting harmonics injected into the AC line. For lighting applications, this specification applies if the input power is greater than 25 W (Class C). In addition, even in some regions where compliance with this IEC specification is not required, there may be requirements for a minimum power factor. This will require a front-end PFC controller in these applications. Adding such a stage will cause difficulties in meeting other system requirements such as energy efficiency and space unless a wise choice is made.

Figure 3(b): Improved single-stage PFC architecture.

Fortunately, ON Semiconductor deeply understands these challenges and has an innovative PFC controller product lineup that meets designers' expectations for simple, compact and rugged solutions. ON Semiconductor not only supports traditional 2-stage applications, but also introduces several unique solutions that further simplify the design, such as the NCP1651 single-stage flyback controller.

The NCP1651 integrates power factor correction and converter in a single chip solution with an external FET, allowing a scalable solution to be implemented depending on the power required. The NCP1651 has an isolated buck function, supports continuous or discontinuous mode operation, and supports average current mode control (ACMC) and fixed frequency control, and its high-precision multiplier can be used to reduce total harmonic distortion (THD). It also has functions such as over-temperature shutdown and external shutdown. This single-stage PFC controller is suitable for applications such as electronic ballasts, street lights, traffic lights and lighting.

For LED lighting applications with input power between 25 W and 60 W, a PWM controller such as NCP1351 can be used; for input power in the range of 60 W to 150 W, a PWM controller such as NCP1271 can be used; for input power in the range of 150 W to 300 W, a PWM controller such as NCP1396 can be used. For input power in the range of 25 W to 300 W, a critical conduction mode (CRM) PFC controller NCP1606 can be used.

2) DC-DC power supply solution for medium voltage LED applications with wide input range

In addition to portable power applications, there is 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 application is numerous, such as event lighting, landscape and road lighting, automotive and traffic lighting, solar-powered lighting, and display cabinet 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 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.

For LED applications with an input voltage less than 40 V, if the output voltage is less than the input voltage, a buck topology is selected. On this basis, the output current is selected. If the output current is greater than 1.2 A, the 1.5 A constant current switching regulator NCP3065 designed to power high-brightness LEDs can be used. This device has a very low feedback voltage rating of 235 mV, which is suitable for regulating the average current of the LED string. It has a wide input voltage range of up to 40 V, allowing it to operate from a 12 Vac or 12 Vdc power supply. The NCP3065 also offers a version suitable for automotive applications - the NCV3065. With only a few external components (such as MOSFETs or low VCEsat switches), the NCP3065 switching regulator can be configured as a buck, boost or SEPIC topology. This allows it to be used in applications with currents less than 1.2 A. For applications with currents less than 1.2 A, the NCP1215+MOSFET can also be used to form a buck converter. For applications where the current is less than 500 mA and the input voltage is close to the output voltage, a constant current linear driver such as the NUD4001 can be used.

If the output voltage is greater than the input voltage, the boost topology is selected. On this basis, if it is a low-voltage battery-powered application, low-voltage LED drivers such as NCP5005, NCP5604, NCP5608 and NCP5050 can be selected; if it is not a low-voltage battery-powered application, look at its output voltage value. If it is greater than 40 V, the NCP3065 switching regulator can also be used. At this time, the NCP3065 is configured as a boost controller structure in combination with an external NMOS MOSFET; otherwise, it depends on the switch current. If the switch current is greater than 1.3 A, use the NCP3065 configured as a boost controller in combination with a MOSFET or a low VCEsat switch; if it is less than 1.3 A, use the NCP3065 configured as a boost converter.

In addition to pure buck or boost topologies, a buck-boost topology is also required in some environments, such as driving an LED string from a standard power input; in addition, it is also common to use this topology in situations where the input voltage and the LED load voltage overlap. The NCP3065 can also be configured as a buck-boost controller. This architecture requires two power switches. Although the NCP3065 itself also contains a power switch, we can use a low VCEsat PNP/NPN pair of tubes to obtain higher conversion efficiency. This is a scalable solution, and both the rectifier and the power switch can be adjusted according to the specific input and output voltages and current levels. It is worth mentioning that low VCEsat transistors such as the NSS40500UW3 provide excellent performance in a small 2×2 mm package.

Figure 4 shows the NCP3065 buck-boost circuit for up to 12 W. This circuit is designed for applications up to 0.7 A and an input voltage range of 8 to 26 Vdc. The efficiency is 72% to 80% for an output voltage Vout of 16 Vdc @ 700 mA and an input voltage Vin of 13-26 Vdc.

Figure 4: NCP3065 buck-boost circuit up to 12 W

LED portable backlight application power supply solution

White LEDs and RGB LEDs are widely used in small-size LCD panels and keyboard backlights and indicator light applications. In mobile phones and digital cameras, high-brightness LEDs can also be used as flash power sources. These applications require optimized solutions that can not only maximize battery life, but also minimize PCB footprint and height.

In this type of LED backlight application, either linear regulators or switching regulators can be used, each with its own advantages. Linear regulators have a relatively simple structure, simple design, low cost, low noise, small size, and low quiescent current. Switching regulators have a higher energy efficiency, which can reach 70~85%. ON Semiconductor provides a variety of solutions based on the user's application needs. In terms of switching regulator solutions, both inductive and charge pump types are provided. Specifically, the inductive boost driver NCP5005 and NCP5050 have the advantages of high energy efficiency and high output voltage, with an energy efficiency of more than 90%, and most of them can drive up to 5 series LEDs (output voltage up to 21 V). In addition, since the LEDs are connected in series, this type of device also has ideal lighting/current matching characteristics. Since no inductor is required, the charge pump driver NCP5602, NCP5612, and NCP5623 help create a small and compact solution, which is particularly important for portable products that use small and medium-sized LCDs. This type of driver is equipped with low-cost capacitors and resistors, which can create a lower cost solution. This driver can drive 1 to 3 LEDs and is suitable for backlighting applications in small devices.

In addition to LCD, the emerging display technology of organic light-emitting diode (OLED) has also been applied in portable devices and even medium and large-sized flat-panel TVs in recent years. The first to appear on the market was passive matrix OLED (PMOLED), which was limited to 1.8 inches and below due to its driving topology, and was often used in MP3 players and mobile phone sub-screens. The newer active matrix OLED (AMOLED) technology has no display size restrictions and has several important advantages over LCD, such as wide viewing angles, high contrast, extremely fast response time and thin thickness, thanks to the elimination of the need for backlight. AMOLED panels are used for the main display of mobile phones and MP3 players, etc.

ON Semiconductor is the first to market with a dedicated active matrix (AMOLED) panel regulated power supply IC, the NCP5810D. The NCP5810D dual-output DC-DC converter has an overall power supply efficiency of up to 83% at a 1.75 MHz oscillator frequency. It has excellent line transient suppression, with a line transient voltage of 5 mV at 25 mA current. To accommodate the slim form factor of AMOLED displays, the converter can be converted to a high frequency of 1.75 MHz, allowing the use of smaller inductors and ceramic output capacitors. Its ultra-thin 0.55mm thick package makes the NCP5810D suitable for the thinnest portable product designs. In addition, its power-off function in shutdown mode limits the leakage display current to 1 microamp, saving battery power in the shutdown state. The NCP5810D also has cycle-by-cycle peak current limiting and thermal shutdown protection. The NCP5810D is available in an ultra-thin 3.0mm×3.0mm×0.55mm LLGA-12 package.

Figure 5: Functional block diagram of the ON Semiconductor AMOLED regulated power IC NCP5810D.

In addition to LCD backlight and AMOLED power applications, ON Semiconductor also launched LED driver ICs for camera flash applications, such as NCP5680. Take NCP5680 as an example. This is a dual LED flash driver with supercapacitor that can drive 2 high-brightness LEDs at 2.5 A or higher. It uses I2C control and has a built-in flash sequence. Its light-sensing function can limit the flash time in bright environments, thereby saving power . This device uses supercapacitors to support other circuits with high peak currents such as audio, and also has protection functions such as short-circuit protection. This device began mass production in July 2008.

Figure 6: Application diagram of ON Semiconductor’s supercapacitor LED driver NCP5680.

Summarize

With many advantages such as low operating voltage, high energy efficiency, rich colors, directional light, and mercury-free, LED technology has been increasingly widely used in the fields of architecture, transportation, automobiles, solar energy, LCD backlight, automobile and residential lighting. As a leading global supplier of energy-efficient power solutions, ON Semiconductor provides a rich product line of LED driver power solutions to meet the wide range of needs from high-voltage offline AC-DC switching power supplies to medium-voltage LED power supplies with a wide input range, as well as portable product backlight and flash drive applications, and provides some related reference designs to help customers shorten development time and accelerate product launch.