LED driver power supply design basis for lighting - Power Circuit - Circuit Diagram

The arrangement of the LEDs and the specifications of the LED source play a critical role in determining the basic driver requirements. The primary function of an LED driver is to regulate the current flowing through the LED under specific operating conditions, regardless of fluctuations in input and output voltages. One of the most common methods involves using transformers for electrical isolation. This paper explores the key factors that must be considered when designing LED lighting solutions.

Isolated LED driver power solution: [Link to Solution]

First, the general requirements for LED drivers:

Driving LEDs presents several challenges. For instance, the forward voltage changes with temperature and current. Additionally, the forward voltage of different LEDs from various manufacturers or batches can vary significantly. Furthermore, the color of the LED tends to drift with changes in current and temperature.

In many applications, multiple LEDs are used, necessitating careful consideration of their arrangement. Among various configurations, a single string of LEDs connected in series is generally preferred due to its ability to maintain excellent current matching irrespective of variations in forward voltage and output voltage (Vout).

Of course, users can opt for alternative arrangements like parallel, series-parallel combinations, or cross-connections, especially in cases where mutual matching of LED forward voltages is required. For example, in a cross-connection setup, if one LED fails due to a fault, only one LED in the circuit would experience a doubling of its drive current, minimizing the overall impact on the circuit.

The arrangement of the LEDs and the specifications of the LED source define the basic driver requirements. The core function of an LED driver is to limit the current flowing through the LED under specific operating conditions, regardless of input and output voltages. The fundamental working circuit diagram of the LED driver is illustrated in Figure 2. By "isolated," we mean there is no physical electrical connection between the AC line voltage and the LED (i.e., input and output). The most common method involves using a transformer for electrical isolation. "Non-isolated" configurations do not employ high-frequency transformers for electrical isolation.

It's important to note that in LED lighting design, AC-DC power conversion and constant current drive can be configured in different ways:

1) Integrated configuration, where both functionalities are combined within the lighting fixture. This approach offers advantages such as optimized energy efficiency and simplified installation.

2) Distributed configuration, where they exist separately. This setup simplifies safety considerations and enhances flexibility.

An LED driver can operate with either constant voltage (CV) output, clamping the output voltage within a certain current range, or constant current (CC) output, strictly limiting the current. Alternatively, it can work in a constant current constant voltage (CCCV) mode, providing a constant output power, where the current is determined by the forward voltage of the LED load.

In general, LED lighting design requires consideration of the following factors:

Output power: related to the LED forward voltage range, current, and LED arrangement.

Power supply: AC-DC power supply, DC-DC power supply, or direct AC power supply.

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

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

Additional considerations: mechanical connections, installation, repair/replacement, lifecycle, logistics, etc.

Second, the LED drive power topology:

In LED lighting applications with AC-DC power supplies, the power conversion building blocks include discrete components such as diodes, switches (FETs), inductors, capacitors, and resistors to perform their functions, while pulse width modulation (PWM) regulators are used to control power conversion.

The isolated AC-DC power conversion typically includes a transformer in the circuit and comprises topologies such as flyback, forward, and half-bridge. As shown in Figure 3, the flyback topology is the standard choice for medium and low power applications with power less than 30 W. The half-bridge structure is best suited for achieving higher energy efficiency and power density. For transformers in isolated structures, the size of the transformer is related to the switching frequency, and most isolated LED drivers essentially use "electronic" transformers.

In LED lighting applications using DC-DC power supplies, LED drivers can be implemented in the form of resistors, linear regulators, and switching regulators. A basic application is illustrated in Figure 4. In resistive drive mode, the forward current of the LED can be controlled by adjusting the current sense resistor in series with the LED. This drive mode is easy to design, low in cost, and has no electromagnetic compatibility (EMC) issues. However, it depends on the voltage and requires filtering. (binning) LEDs with low energy efficiency.

Linear regulators are also easy to design, have no EMC issues, support current regulation and fold-back, and allow an external current set point. However, they suffer from insufficient power dissipation, as the input voltage is always higher than the forward voltage, leading to lower energy efficiency. Switching regulators continuously control the opening and closing of the switch (FET) through the PWM control module to control the flow of current.