The linear scheme refers to the use of a linear regulator to achieve control of the LED current. The linear regulator, as its name suggests, is based on the use of a balanced section of the switching tube to balance the input voltage to effect the constant current regulator controller (conventional switching control is the switching control state), somewhat similar to LDO. LED linear drive technology is a new generation of power technology, no high-frequency switching action, so there is no hidden worry of EMC; can also save the magnetic components such as transformers, some applications can also save electrolytic capacitors, achieve high PF, low THD. Considering power consumption, the main difficulty of IC now is how to achieve a wider input voltage and achieve high efficiency. In the region where the grid is relatively stable, single-stage control has its advantages of low cost and high performance, and it has gained a large application. Applications include flat bulbs, filament lamps, thyristor dimming, etc.
(a) (planar) bulb lamp application
Bulbs, as the name implies, are bulb-shaped lamps. Since Edison invented the tungsten bulb, its shape has been deeply rooted in people's hearts. There is not much development in appearance, but the content is changing with each passing day. Energy-saving lamps (scientific name CFL compact fluorescent lamps) have been leading the way for years, energy-saving and high efficiency is synonymous with it; now, the LED era has come, LED is far superior to traditional CFL lighting products in terms of lighting principle, energy saving and environmental protection. In the case of the same light effect, the energy consumption has dropped to 1/8 of the incandescent lamp and 1/2 of the energy-saving lamp, and it is naturally suitable for dimming.
The biggest market for LED bulb lighting is the civilian market. To prevent glare, the outer casing is usually made of frosted glass or acrylic. It can be driven directly by the mains. The following picture shows the LED bulbs of the commonly used plastic aluminum structure.
Figure 1 LED bulbs with plastic aluminum structure
The traditional meaning bulb bulb is SMT type, so we refer to the plane bulb lamp for short. Now, how a typical LED bulb is driven to light up.
Figure 2 shows an 800lm bulb lamp solution made with PT4515. As can be seen from Figure 2, the line is extremely simple, REXT sets the LED output current, and OUT is the regulator output, which is used to regulate the fluctuation caused by the input voltage. PowTechPT4515 only has 3pin. In addition to the traditional current regulation, the constant power output characteristics can greatly expand the application range, making the system more stable and reliable.
Figure 2 An 800lm bulb lamp solution made by PT4515
The linear benefit of the linear lamp drive is that there is no need to use electrolytic capacitors and magnetic components. The whole lamp electronic components are completely composed of patch components, which can realize automatic production, improve reliability and improve production efficiency and reduce the efficiency. cost. Figure 3 is a photo of the photovoltaic engine consisting of this scheme, the entire system is all patch components. Very simple. Figure 4 is a thermal imaging diagram of this photovoltaic engine when it is illuminated, showing that the heat distribution is uniform. Figure 5 is an LED bulb consisting of this photovoltaic engine.
(2) Filament lamp application
The filament lamp is another form of the bulb lamp, and the LED lamp bead is a wire shape similar to a matchstick type. The filament lamp was first introduced in 2008. The traditional incandescent lamp made of LED lamp beads made by the Japanese oxtail light source has attracted great attention in the industry and mass production. Then, the candle lights and bulbs with LED filaments as the light source began to appear in large numbers and were accepted by more and more consumers. At that time, the applicable occasions were mainly in indoor lighting places such as five-star business hotels and high-end luxury houses. In people's psychology, it is a high-end synonym. Because it has two spirits, retro and innovation, but it has not been widely used. With the advancement of technology, the price of LED filaments has been reduced, and the filament lamps have gradually become civilians and have begun to fly into the homes of ordinary people.
The shape of the filament lamp is very similar to that of the tungsten lamp. Several filaments are cleverly connected to form a variety of classic shapes, and the full-circumference design makes it safe and dead. The basic structure of the filament lamp is shown in Figure 6. The LED lamp bead is bound to the transparent substrate, and then the phosphor is coated. The upper electrode is used as the upper electrode, and the power is applied to emit light.
Figure 6 LED filament light
The filament lamp generally considers the self-heating condition, and the design is high voltage and small current. The LED driver needs precise constant current control. The filament lamp is limited by the volume in many cases, and the linear solution has special requirements. Usually for the two commonly used lamp holders, E26/E27 and E12/14 models, PT4515 has a corresponding solution.
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