LED Driver General Requirements

Driving LEDs faces many challenges. For example, the forward voltage will change with changes in temperature and current, and the forward voltage of LEDs from different individuals, batches, and suppliers will also vary. In addition, the "color point" of the LED will also drift with changes in current and temperature.

In addition, multiple LEDs are usually used in applications, which involves the issue of how to arrange multiple LEDs. Among various arrangements, driving a single string of LEDs in series is preferred because this method provides excellent current matching performance regardless of how the forward voltage changes or how the output voltage (Vout) "drifts". Of course, users can also use other arrangements such as parallel, series-parallel combination and cross connection for applications that require "mutually matched" LED forward voltages and obtain other advantages. For example, in a cross connection, if one of the LEDs is open due to a fault, the drive current of only one LED in the circuit will be doubled, thereby minimizing the impact on the entire circuit.

Figure 1: Common LED arrangements

The arrangement of LEDs and the specifications of LED light sources determine the basic driver requirements. The main function of an LED driver is to limit the current flowing through the LED under a certain range of operating conditions, regardless of how the input and output voltages change. The basic operating circuit diagram of an LED driver is shown in Figure 2, where the so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (i.e., input and output). The most common method is to use a transformer for electrical isolation, while "non-isolation" does not use a high-frequency transformer for electrical isolation.

Figure 2: Schematic diagram of the basic operating circuit of an LED driver.

It is worth mentioning that in LED lighting design, the two parts of the circuit, AC-DC power conversion and constant current drive, can be configured in different ways: 1) Integral configuration, that is, the two are integrated together and are both located in the lighting fixture. The advantages of this configuration include optimizing energy efficiency and simplifying installation; 2) Distributed configuration, that is, the two exist separately. This configuration simplifies safety considerations and increases flexibility.

Depending on the application requirements, LED drivers can operate with constant voltage (CV) output, that is, the output is a voltage clamped under a certain current range; they can also operate with constant current (CC) output, and the output design can strictly limit the current; they may also operate with constant current constant voltage (CCCV) output, that is, providing constant output power, so the forward voltage of the LED as a load determines its current.

In general, LED lighting design needs to consider the following factors:

lOutput power: involves LED forward voltage range, current and LED arrangement, etc.

lPower supply: AC-DC power supply, DC-DC power supply, directly driven by AC power supply

l Functional requirements: dimming requirements, dimming methods (analog, digital or multi-level), lighting control

Other requirements: energy efficiency, power factor, size, cost, fault handling (protection characteristics), standards to be followed, reliability, etc.

lMore considerations: mechanical connection, installation, repair/replacement, life cycle, logistics, etc.