An LED driver, also called power supply, is a very important component in an LED system, and plays an important role in the overall design of the lighting. It not only regulates the power output, but also takes away some workload from the microcontroller in used in the LED system. The driver converts AC power to low voltage DC power required by the LEDs, and protects them from voltage fluctuations.
LEDs are usually arranged in an array, and their brightness depends on the operating current. The LED array has to be powered by a steady current source, and the current has to be regulated accurately so that all the LEDs have the same brightness. And this steady power supply is given by the LED drivers, despite power variations.
LED drivers that are usually compact in size and fits inside a junction box, are either constant voltage types (usually 10V, 12V and 24V) or constant current types (350mA, 700mA and 1A). While some LED drivers can operate LED devices or arrays, others can operate only the LEDs.
Constant current driver and constant voltage driver
These are two choices commonly offered by the manufacturers. A constant current LED driver can efficiently drive a number of strings of LEDs used in series. However, if strings of LEDs are connected in parallel, the challenge is to match the current in all the strings. So a constant voltage driver is used.
The number of LEDs that can be operated in series is determined by the maximum output voltage of an LED driver. When the voltage is high enough to supply it to a desired number of LEDs, buck drivers are used. But when the voltage is too low, boost drivers are used. However, in some cases, a combined driver type is needed.
Dimming the LEDs
An important function of the LED driver is its capability to dim the LEDs. LED drivers can dim the LED light output from 100% to 0%. Two popular methods for dimming LEDs in switched mode driver circuits are pulse width modulation (PWM) dimming and analogue dimming. Both the methods control the time averaged current through the LED or LED string, but there are some differences between the two.
With the PWM method, the frequency could range from a hundred modulations per second to as high as hundreds of thousands of modulations per second, so that the LEDs light up continuously without a flicker. Analogue dimming of LEDs, on the other hand, is the adjustment of the constant LED current level. It can be achieved by making an adjustment of the current sense resistor RSNS, or by driving an analogue voltage on some DIM function pin of the IC.
Application of colour LEDs are becoming more and more popular in portable devices like mobile phones, portable media players, gaming and navigation devices, etc. They are used for indication lights, which could be an RGB LED where the colour changes according to the activity like
SMS or incoming calls.
LED drivers can be used for changing colour or the sequence of the colour. This can be achieved by dimming a mix of coloured LEDs in an array to change colours. Another option is that the driver can work with a colour sequencer, which receives the 10V or 24V LED driver output and converts it into three channel output?usually red, blue and green?that can be mixed to create a wide, dynamic range of colours.
Choosing the right LED driver
Since the LED driver plays the most important role in running the LED system, special attention needs to be given while selecting or using a driver. Choosing the right LED driver/power supply is key to ensuring that you get the best performance from your LED system.
Right load: Usually, problems erupt in the LED system if the driver is overloaded more than its maximum load. If too many LED strings are connected in series, it may result in low voltage and the last string(s) in the chain may get very low voltage.
Quality of the driver: It is necessary to pay attention to the quality of the driver to maximise the light output from the LEDs without overstressing them. For this the driver needs to give out a constant DC current. Usually, it is seen that the remote mounting of a driver results in the voltage drops and power losses on the DC wiring. Therefore, precaution should be taken in such cases.
Right voltage: Care should be taken to choose the right LED driver with right voltage. If a wrong voltage driver is used, the LEDs will either not light up or may light up at higher current than required. So it is necessary to check the voltage rating of the LED load being used against the rated output voltage of the driver. If a wrong voltage driver is used, it can result in shorter life of the LED system.
Short circuit protection: Drivers should have short circuit protection, and should be able to handle temperature extremes. Off-the-shelf DC power drivers usually fail to perform in extreme conditions.
Compatibility with ambient temperatures: If the LED driver is not compatible with the ambient temperature of the application, LEDs do not work properly. LEDs start at -40°C temperature, so in a colder place they start showing operating problems. In such a situation, LEDs draw higher power and this leads to system malfunction. So, in outdoor applications where the power supply is mounted remotely, the maximum LED load on the driver should be de-rated by 10-20% to avoid such failures.
Also, since drivers are made for dry areas, if they are installed in an outdoor application, they should be installed in an enclosure.