Universal design scheme for mains input dimmable LED lighting

　　推动LED照明市场发展的关键推动力是高亮度LED和智能LED控制器的出现。采用高亮度LED的产品设计师面临著许多设计挑战，包括散热管理、驱动方案、拓扑架构和已有的基础设施。

　　To replace existing dimmable incandescent or halogen light sources, it is necessary to implement an electronic lamp driver system that can not only work with existing dimming switches, but also replicate the dimming performance of existing light sources. The NXP SSL2101 IC meets the above performance requirements, and it is also an efficient power converter. This is the first IC in the industry to integrate these capabilities, which enables high-brightness LED lamp/module designers to integrate electronic circuits in a cost-effective and volume-saving way and benefit from the best thermal trade-offs.

　　LED Characteristics

　　LEDs require a completely different type of driver than incandescent or halogen lamps. Whereas incandescent lamps behave as pure resistive loads with self-stabilizing characteristics, LEDs require a current source. The luminous flux produced by an LED is approximately proportional to the current flowing through the device. The forward voltage of an LED increases with increasing current, but decreases with increasing temperature. In this respect, LEDs behave like diodes. However, during operation, the forward voltage (VF) of an LED is very large. This voltage is related to the energy (eV) generated when electrons are converted into photons, which in turn is directly related to the color of the light. In addition, VF values ​​can vary greatly between different production batches.

　　Serial/Parallel Configuration

　　In most applications where LEDs are used to replace existing light sources, multiple LEDs need to be connected to a driver because a single LED cannot produce enough luminous flux. LEDs can be connected in series or in parallel.

　　If the LEDs are connected in series, the total voltage across the LED chain is equal to the sum of the forward voltages (the current in all LEDs is equal).

　　如果LED并行连接，电流将分配到各支路中。不过，由于一个LED的正向电压会随著温度上升而下降，因此这一配置本质上是不稳定的。随着温度的升高，越来越多的电流将流到具有更低正向电压的支路上去，这些支路将变得更亮，而那些具有更高正向电压的支路将变得更暗。

　　However, one reason to use a parallel configuration (or series-parallel combination) is that it allows a large number of LEDs to be connected together at a safe supply voltage, whereas if you were to connect them in series to achieve the same brightness, you would need a voltage that would be unacceptably high.

　　The parallel configuration also provides the advantage of redundancy. If a problem occurs in one LED or connection in a serially connected LED chain, it will cause a short circuit and all LEDs in the entire chain will not emit light. However, this situation will not happen if a parallel configuration is used. When a parallel configuration is used, it is recommended to add a current regulation mechanism to each branch to prevent thermal runaway and unequal current or light. In summary, the efficiency of the power converter is the highest when the difference between the output and input voltage is minimized. When an LED driver with a mains input is used, a higher output voltage is allowed, allowing more LEDs to be connected in series.

　　Mains dimming

　　Standard industrial and domestic dimmers are designed for use with incandescent lamps. Some more advanced models can be used with transformers connected to halogen lamps. Mains dimmers specifically for LED loads are still rare.

　　The cost of purchasing and installing a new dedicated dimmer can easily exceed the cost of the light source itself, so an LED lighting system that works with existing dimming infrastructure, such as one built around the NXP SSL2101, will open the door to a market with huge potential and potential.

　　Since existing dimmers are designed for incandescent lamps (approximately resistive loads, mostly between 20W and 50W), some additional circuitry is needed to allow them to be used in LED lighting systems. There is no standard for incandescent lamp dimmers on the market, so there are a large number of mains dimmers with different performance and parameter values ​​on the market.

　　The SSL2101 is a multi-chip module (MCM) in an SO16 package. It contains a high-efficiency power converter and internal circuitry to achieve mains dimming compatibility. Compared with existing solutions, it has the following advantages:

　　Integrated power switch. This reduces component cost and ensures optimal drive and switch protection.

　　Valley detection. This feature reduces converter losses because the switch can be closed at the optimal time.

　　Integrated bleeder switch and comparator, which reduces component count and size.

　　Intelligent bleeder operation. The SSL2101 can detect when bleeder operation is not needed (such as when the LED chain provides sufficient load), which can reduce power consumption and increase system efficiency.

　　Enhanced heat sink pin design. This increases the life of the SSL2101 and allows it to operate normally at higher ambient temperatures. For alternative solutions, the life of electronic devices at higher temperatures can be a critical parameter.

　　Dimming based on duty cycle and converter switching frequency. This allows designers greater freedom to define parameter values. This allows designers to achieve more accurate low-brightness dimming and eliminate audio transformer noise by adjusting the parameters that control the dimming system.

　　Logarithmic dimming correction. This allows the LED to dim in the same way as an incandescent or halogen lamp.

　　Built-in over-temperature protection, over-current protection, short-circuit detection and maximum duty cycle limit. These features ensure the high reliability of SSL2101 even under extreme working conditions.