Mains input dimmable LED lighting universal design

LEDs have been used in electronic systems for many years and are primarily used as indicators for electronic devices. Recent advances in brightness and color depth mean that LEDs can now be used in a wider range of applications, from fun lighting in cell phones and multimedia players, to traditional light sources in commercial and home lighting applications.

The key driver for the development of the LED lighting market is the emergence of high-brightness LEDs and smart LED controllers. Product designers using high-brightness LEDs face many design challenges, including thermal management, drive solutions, topology, and existing infrastructure.

To replace existing dimmable incandescent or halogen light sources, an electronic light drive system must be implemented that not only works with existing dimmer switches, but also replicates the dimming performance of existing light sources. The NXP (Enhanced) SSL2101 IC meets these performance requirements and is an efficient power converter. This is the industry's first IC to integrate these capabilities, enabling high-brightness LED lamp/module designers to integrate electronic circuits in a cost- and volume-efficient manner and benefit from optimal thermal trade-offs.

LED characteristics

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

Serial/parallel configuration

In most applications where LEDs are used in place of existing sources, multiple LEDs need to be connected to the driver because a single LED does not produce sufficient luminous flux. LEDs can be connected in series or in parallel.

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

If the LEDs are connected in parallel, current will be distributed to each branch. However, since the forward voltage of an LED drops as the temperature rises, this configuration is inherently unstable. As the temperature increases, more and more current will flow to the branch with a lower forward voltage, these branches will become brighter, and those with higher forward voltage will become Darker.

However, one reason for using a parallel configuration (or a series-parallel combination) is that it allows a large number of LEDs to be combined together with a safe supply voltage, and if you want to use a serial connection to achieve the same brightness, you may need a high Unacceptable voltage.

Parallel configuration also provides redundancy benefits. If there is a problem with an LED or connection on a serially connected LED chain, it will cause an open circuit and all LEDs on the entire chain will not illuminate. However, this situation will not be possible if a parallel configuration is used. When using a parallel configuration, it is recommended to add a current regulation mechanism to each branch to prevent thermal runaway and unequal current or light. In summary, the power converter is most efficient when the output and input voltage differences are minimal. When using a mains input LED driver, a higher output voltage is allowed, allowing more LEDs to be connected in series.

Mains dimming

Standard industrial and household dimmers are designed for incandescent lamps. Some of the more advanced models can be used with transformers connected to halogen lamps. Current commercial dimmers specifically for LED loads are still rare.

The cost of purchasing and installing a new dedicated dimmer can easily exceed the source itself. So an LED lighting system that works with existing dimming infrastructure (such as those built on NXP's SSL2101) will open the door to this huge potential and space market.

Since existing dimmers are designed for incandescent lamps (approximately resistive loads, most of which are between 20W and 50W), some extra circuitry is required to allow them to be used in LED lighting systems. There are currently no incandescent dimmer standards on the market, so there are a large number of commercial dimmers with different performance and parameter values ​​on the market.

The SSL2101 is a multi-chip module (MCM) in an SO16 package. It includes a highly efficient power converter and internal circuitry for commercial dimming compatibility. It has the following advantages over existing solutions:

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

· Valley detection. This feature reduces converter losses because the switch can be closed at the optimum point in time.

· Integrated bleeder switches and comparators, which reduce component count and size.

· Intelligent bleeder operation. The SSL2101 detects when no bleeder operation is required (such as the LED chain provides sufficient load), which reduces power consumption and increases system efficiency.

· Enhanced thermal pin design. This increases the life of the SSL2101 and allows it to work at higher ambient temperatures. For alternative solutions, the lifetime of an electronic device at higher temperatures can be a key parameter.

• Dimming based on duty cycle and converter switching frequency. This allows the designer to have more freedom to define parameter values. This allows the designer to adjust the parameters of the dimming system to achieve more precise low-brightness dimming and eliminate audio transformer noise.

· Logarithmic dimming correction. This allows the dimming behavior of the LED to be exactly the same as an incandescent or halogen lamp.

Built-in overtemperature protection, overcurrent protection, short circuit detection and maximum duty cycle limit. These features ensure the high reliability of the SSL2101, even under extreme operating conditions.


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