In recent years significant advances in high-brightness LEDs (HB-LEDs) have given designers the opportunity to replace conventional incandescent, fluorescent and halogen technologies with more reliable and energy-efficient LED-based alternatives. As a result, solid-state lighting has seen considerable uptake in automotive, digital signage, and architectural applications, as well as the illumination of our city streets.
However, when it comes to controlling and driving these LEDs there has been considerable inconsistency in the approaches employed. Many lighting system designs, for example, have used or modified existing solutions that did not take into account the specialist needs of HB-LEDs. If designers are to optimize the benefits that these devices can bring, careful consideration must be given to the techniques used for driving and controlling these devices.
A system perspective
Driving LED emitters from a voltage source without some form of power conversion is to be avoided, as normal fluctuations in the voltage can result in dramatic differences in LED current. Factors such as the very steep V/I curve and a wide variation in forward voltage from lot-to-lot with LED devices necessitates the inclusion of an isolated or non-isolated power conversion stage.
Regulating the current
The three basic driver/regulator topologies are buck (step-down), boost (step-up) and buck-boost (also known as single-ended primary inductor conversion or SEPIC). With buck circuits the minimum input voltage (Vin) is always greater than the maximum voltage of the LED string under all operating conditions, while boost circuits are used when the maximum Vin is always less than the minimum voltage of the LED string. SEPIC techniques are used where the input and output voltages overlap.
Likewise, LED current regulation solutions can be broadly categorized as follows:
1) Resistors: These represent the simplest, lowest cost approach to current regulation. In reality they do not deliver a practical solution as they are voltage-dependent, resulting in fluctuation of LED brightness. They also necessitate the costly and time-consuming practice of binning of LEDs, as well as leading to designs with poor efficiency levels.
2) Linear regulators: Easy to design in, these can provide effective current regulation/fold back. With an external current set-point, linear regulator ICs act as a ‘mid-range’ solution to current regulation in HB-LED lighting designs. However, they are often viewed as being too power consuming and having efficiency levels which are too low. The poor efficiency of linear regulators will normally mean that there are thermal management issues to contend with too. This typically results in the inclusion of a heatsink mechanism that takes up space and adds to the bill of materials for the design.
3) Switching regulators: These are the most costly and technically complex solution for LED current control. Unlike linear regulators and simple resistors, they are susceptible to electromagnetic interference (EMI), giving the designer an additional design hurdle to negotiate. Nevertheless, they are highly efficient, totally voltage independent and bring brightness control functionality to the application. Switching regulators are the only feasible option for medium to high power applications or in cases where wide input voltage ranges are involved.
4) Constant current regulators provide a simpler and lower cost solution compared to linear and switching regulators, yet offer significant performance benefits when compared to resistors.
Constant current regulation
As with linear and switching regulators, constant current regulators can maintain constant brightness over a wide voltage range. They can also protect the LEDs from over drive at higher input voltages and significantly reduce or completely eliminate the need for costly and often problematic binning. And because the latest integrated CCR devices also offer wide input ranges, they can provide the headroom to accommodate a wide range of fluctuating supply voltages to support deployment across a diverse variety of applications.
As the emitted light from an LED is proportional to the average current passing through it, constant current regulators can also deliver the capability to dim the light output. Dimming is achieved by either analog or digital pulse width modulation (PWM) techniques. The analog approach combines an input PWM signal with the feedback voltage resulting in a reduced average output current. The digital approach uses the input PWM signal to inhibit switching of the regulator and reduce the average output current. The typical dimming frequency is somewhere between 200 Hz and 1000 Hz, as the human eye cannot see slight variations over 200 Hz.
The latest constant current regulator ICs, such as those from ON Semiconductor, can accommodate supply voltages in excess of 40V and down to 3V, offer current outputs from 20mA to 1.5A, and are fully compatible with step-down (buck), step-up (boost), step-up/down (buck/boost) or SEPIC topologies. Figure 2, for example, shows a typical SEPIC application circuit based on the NCP3066 device.
Furthermore, if the LED string being driven has a current requirement beyond the capability of a single regulator, such devices can also be used as a controller with an external switch to deliver higher output current.
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