Why Do I Need an LED Driver?
LEDs run on low voltage, direct current electricity, yet outlets typically supply electricity that is high voltage with alternating current. The main purpose of an LED driver is to rectify this higher voltage alternating current into the low voltage direct current electricity that LEDs are designed to handle.
LEDs are constant current devices with a forward voltage drop. This means the voltage supply must exceed that drop in order to allow current to flow, and by controlling the current, you control the intensity. Too much or too little current can cause the light output to vary or degrade at a faster rate due to higher temperatures within the LED. An LED driver responds to the changing needs of an LED circuit by providing a constant quantity of power (within a rated current range) to the LED as its electrical properties change with temperature.
There are several different kinds of external LED drivers, but they can be broken down into two main types: constant current (CC) drivers and constant voltage (CV) drivers.
When To Use Constant Current Drivers
A constant current LED driver varies the voltage across the electronic circuit in order to maintain a constant electrical current. This guarantees that no matter the fluctuation in voltage, the current driven to the LED will be maintained at the specified level. Constant current drivers are designed for LEDs that require a fixed output current and a range of voltages. Constant current drivers usually list their specifications on the device, with only one output current rating and a range of voltages that vary based on the wattage of the LED.
If you’re building your own fixture or working with high powered LEDs, constant current LED drivers are the way to go because they avoid burnout or thermal runaway by never exceeding the max specified current for the LED. Designers usually find these drivers easier to control in applications, and they enable a more consistent level of brightness. Control of the current as opposed to the voltage provides more precise control of the power dissipated in the LED and helps designers project failure rates much more accurately than when constant voltage supplies are used.
Constant-voltage drivers come in many different forms, from a conventional power supply to those that are enclosed depending on their target application. Constant voltage drivers have a fixed voltage that is typically either 12VDC or 24VDC. They are used for LEDs that require one stable voltage and have a current that is already regulated either via simple resistors or an internal constant current driver located inside the LED module.
If you have already confirmed your LED or array is rated to take a certain voltage, constant voltage LED drivers are typically more intuitive for design engineers, and they are often a cheaper solution compared to constant current drivers.
Driving Strings of LEDs
With LEDs connected in a series configuration, the forward voltage drop of each LED in the string is additive. This is why constant-current drivers always identify the output voltage range of which it is capable. Multiple strings of series-connected LEDs can be placed in parallel and driven by multi-output constant-current drivers to limit the drive voltage.
When many strings of LEDs are used in series, the most efficient way to drive them is with a constant-current power supply, which connects the LEDs directly across the terminals of the power supply. However, if strings are connected in parallel, matching the current in all the strings may be difficult. In this case, an external component to control current is used, resulting in a less efficient overall number of lumens per watt.
What you choose for your LED driver type can depend greatly on the intended end-use and other constraints. For signage and other applications that rely on vibrant or contrasting colors, a constant voltage supply can be more cost effective and easier to design. Additionally, any color shifts tend to be relative and will have a minimal impact on the effectiveness of the viewed light. Efficiency is also less important in signage, and additional heat sinking is more economical when the application is a single installation as opposed to lighting distributed throughout a site.
When the intended end-use is for illumination, however, constant current drivers can be a better choice. These allow for more uniform control of the light quality and brightness, and systems can be easily tuned to keep LEDs operating in their most efficient range. Running LEDs in their most efficient range typically requires less heat sinking and the metal from the lighting fixture is usually sufficient to spread the heat and keep the LEDs operating in an ideal and efficient state.