LEDs have been used in electronic systems for many years and are mainly used as indicator lights for electronic devices. The recent significant advances in brightness and color depth mean that LEDs can now be used in a wider range of applications, from fun lighting in mobile phones and multimedia players, to replacing traditional light sources in commercial and home lighting applications.
The key driving force for the development of the LED lighting market is the emergence of high-brightness LEDs and intelligent LED controllers. Product designers using high-brightness LEDs face many design challenges, including thermal management, drive solutions, topologies, and existing infrastructure.
To replace the existing dimmable incandescent or halogen light source, such an electronic lamp drive system must be realized, which can not only work with the existing dimmer switch, but also can replicate the dimming performance of the existing light source. NXP (NXP) SSL2101 IC can meet the above performance requirements, in addition, it is also an efficient power converter. This is the industry's first IC that integrates these capabilities. It enables high-brightness LED lamp / module designers to integrate electronic circuits in a cost-effective and volume-saving manner and benefit from the best 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, and LEDs require a current source. The luminous flux generated 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 LED forward voltage (VF) during operation is very large. This voltage is related to the energy (eV) generated when electrons are converted into photons, which is directly related to the color of light. In addition, the VF values ​​between different production batches can vary greatly.
Serial / parallel configuration
In most applications that replace existing light sources with LEDs, multiple LEDs need to be connected to the driver because a single LED cannot produce sufficient luminous flux. The LEDs can be connected in series or 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 current on all LEDs is equal).
If the LEDs are connected in parallel, the current will be distributed to each branch. However, since the forward voltage of an LED will decrease as the temperature rises, this configuration is inherently unstable. As the temperature rises, more and more current will flow to the branch with lower forward voltage, these branches will become brighter, and those with higher forward voltage will become Darker.
However, one reason for the parallel configuration (or series-parallel combination) is that it allows a large number of LEDs to be combined 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.
The parallel configuration also provides redundancy advantages. If there is a problem with one LED or connection on a serially connected LED chain, it will cause a disconnection and all LEDs on the entire chain will not emit light. But if the parallel configuration is adopted, this situation will not happen. When using a parallel configuration, it is recommended to add a current adjustment mechanism to each branch to prevent thermal runaway and unequal current or light. All in all, the power converter has the highest efficiency when the output and input voltage difference is the smallest. When an LED driver with mains input is used, 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 more advanced models can be used with transformers connected to halogen lamps. At present, the commercial power dimmers specifically for LED loads are still very rare.
The cost of buying and installing a new dedicated dimmer can easily exceed the light source itself. Therefore, an LED lighting system that can work with the existing dimming infrastructure (such as the system built on NXP SSL2101) will open the door to this huge potential and space market.
Since existing dimmers are designed for incandescent lamps (approximately resistive loads, power is mostly between 20W and 50W), some additional circuits are required to allow them to be used in LED lighting systems. At present, there is no standard for incandescent lamp dimmers, so there are a large number of commercial power dimmers with different performance and parameter values ​​on the market.
SSL2101 is a multi-chip module (MCM) in SO16 package. It contains a high-efficiency power converter and internal circuits to achieve mains dimming compatibility. Compared with existing solutions, it has the following advantages:
· Integrated power switch. This reduces component costs and ensures optimal drive and switch protection.
· Valley detection. This feature can reduce converter losses because the switch can be closed at the optimal time.
· Integrated bleeder switch and comparator, which can reduce the number and size of components.
· Smart bleeder operation. The SSL2101 can detect when no need for the bleeder operation (such as the LED chain provides sufficient load), which can reduce power consumption and increase system efficiency.
· Enhanced heat dissipation pin design. This can increase the life of SSL2101 and allow it to work properly at higher ambient temperatures. For alternative solutions, the lifetime of electronic devices at higher temperatures can be a key parameter.
Dimming based on duty cycle and converter switching frequency. This allows the designer more freedom to define parameter values. This allows designers to adjust the parameters of the dimming system to achieve more accurate low-brightness dimming and eliminate audio transformer noise.
· Logarithmic dimming correction. This allows the dimming behavior of LEDs to be identical to incandescent or halogen lamps.
· Built-in overheat protection, overcurrent protection, short circuit detection and maximum duty cycle limitation. These features ensure the high reliability of SSL2101, even under extreme operating conditions.
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