LEDs are complex semiconductor devices whose intercorrelated electrical and thermal characteristics should be factored into system design. As current-driven devices, LEDs must operate under constant current regulation in order to maintain their consistent output. Every LED, however, has a maximum rated current. Overdriving what the LED is rated for will result in irreversible performance degradation and shortened lifespan. As the current density is increased beyond a certain threshold, the internal quantum efficiency (IQE) is dropped. The reduction of quantum efficiency at high operating currents is called efficiency droop. A loss in efficiency means an increase in waste heat production. The forward current across the semiconductor junction of the LED can rise above the maximum allowed limit when there's an overvoltage event, or a failure of another LED string connected in parallel configurations.
An LED driver which regulates the power to the LED array of a high mast luminaire is designed as a switched-mode power supply (SMPS). SMPS drivers use a switching regulator to transform power rectified from AC mains supply into a pulsed waveform, which is then smoothed using an energy storage device. Switching power supplies are the only viable option for high power applications as they are very efficient, allow advanced dimming control, and have universal input voltage capability. In particular, the efficiency of an SMPS LED driver can be as high as 97% which is way much better than linear power supplies. Linear regulators have the advantages of low cost, driver-on-board (DOB) capability, and absence in electromagnetic interference (EMI). These driver circuits are found in some low-end products. However, this type of driving mechanism requires an input voltage at least some minimum amount higher than the desired output voltage. The minimum voltage differential between the input and output required for regulation is simply thrown away as waste heat, which not only leads to a significant power loss of around 20% but also produces substantial thermal stresses to co-located semiconductor components.
Switched-mode LED drivers are technically complex in that they use reactive components, such as oscillating coils and electrolytic capacitors in order to convert and store the electrical energy. Switching regulation generates high frequency noise that has to be suppressed by EMI filters. EMI filters also use reactive components such as filtering coils and high voltage capacitors. Flicker can be a problem in sports lighting applications and outdoor nighttime events where television recording and broadcasting take place. A ripple suppressor can be added to the driver circuit to reduce the output current ripple so there are no stroboscopic effects caused by flicker from the light source as well as no perceived flicker at high camera frame rates. Another essential requirement for line-operated LED drivers is power factor correction (PFC) which shapes and time-aligns the input current into a sinusoidal waveform in phase with the line voltage. The PFC is also used to suppress total harmonic distortion (THD) caused by non-linear electrical loads.
An LED driver executes a number of sub-tasks sequentially or in parallel, including but not limited to overcurrent protection, overvoltage protection, over-temperature protection, zero-current detection (ZCD) and handling, peak-current detection and handling, analog or digital voltage compensator, and constant light output (CLO). High mast luminaires are exposed to transient overvoltages caused by lightning, industrial and switching surges, or electrostatic discharges (ESD). A single-pulse event will cause an immediate catastrophic failure of the LED. Accordingly, a surge protective device (SPD) should be used to suppress excessive surges.
