When choosing a lighting fixture that best suits your needs, it helps to understand its driver features. The most essential characteristic of an LED driver is that the power parameter is kept stable at the output.
Constant Current
The brightness of an LED depends on the current flowing through it. That's why constant current drivers are the most widely used. These drivers connect directly to a series of LEDs.
The driver maintains a constant current, provided the voltage drop across the LEDs remains within a specific range. This voltage drop depends on the type of LEDs used and the quantity of them in the circuit. To accommodate different lighting designs, manufacturers produce a wide variety of current-regulated drivers.
Constant Voltage
Sometimes, a lighting fixture must be assembled for a project "just like a little LEGO." In such designs, the fixture consists of multiple LED modules connected in parallel, each containing one or more LED strings. A resistor in each string sets the current. These modules require a driver with a stabilized output voltage.
A drawback of constant voltage drivers is power loss in the resistors wired in series with the LEDs. However, the additional electricity cost is offset by the lower price of the lighting components.
Constant voltage drivers are also used to power LED strips.
Basic models of constant voltage drivers are often called "power supplies." In fact, standard power supplies can be used in place of such drivers. The term "drivers" usually refers to more advanced models with features like brightness adjustment.
Constant Power
The luminous flux of an LED string is determined by:
Φ = H × P,
H is the luminous efficacy of the LEDs, and P is the power consumed by the string.
The power consumption of a series-connected LED string is:
P = N × U × I,
N is the number of LEDs, U is the voltage drop per LED, and I is the current through the LEDs.
The fixture's design determines the number of LEDs (N). When purchasing LEDs, manufacturers select types with a specific luminous efficacy (H). However, the voltage drop (U) varies depending on the LED manufacturer. If the supplier changes, a different constant current driver may be needed to maintain the same luminous flux.
A new type of driver was developed to free lighting manufacturers from dependence on specific LED suppliers: constant power drivers. These drivers measure the voltage drop in the circuit and automatically adjust the current to maintain a constant power output (P). Switching from constant current to constant power reduces the variety of driver models required.
Constant power drivers are relatively pricey. However, the ability to freely choose cost-effective LED suppliers and reduce the variety of drivers in stock not only offsets the extra expense but also increases profitability.
