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 crucial role in determining the basic driver requirements. The primary function of an LED driver is to maintain a stable current through the LED under varying operating conditions, regardless of fluctuations in input and output voltages. One of the most common methods involves the use of transformers for electrical isolation. This paper delves into the factors that must be considered when designing LED lighting solutions.

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Firstly, let’s discuss the general requirements for LED drivers:

Driving LEDs presents numerous challenges. For instance, the forward voltage of an LED varies with temperature and current. Additionally, the forward voltage of LEDs from different manufacturers or batches can differ significantly. Furthermore, the color of the LED may shift due to changes in current and temperature.

In practical applications, multiple LEDs are often utilized, necessitating careful consideration of their arrangement. Among various configurations, arranging LEDs in a single series string is generally preferred because it ensures excellent current matching, irrespective of changes in forward voltage or output voltage (Vout). However, users can opt for alternative arrangements like parallel, series-parallel combinations, or cross-connections depending on specific requirements such as mutual matching of forward voltages and increased reliability in case of a fault. For example, in a cross-connected setup, if one LED fails due to an issue, only one LED in the circuit would experience a doubling of its drive current, thereby minimizing the impact on the overall performance.

The layout of the LEDs and the specifications of the LED source dictate the fundamental driver requirements. The core function of an LED driver is to regulate the current flowing through the LED under certain operating conditions, irrespective of input and output voltages. The basic working circuit diagram of an 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). Transformers are commonly used for electrical isolation. Conversely, "non-isolated" designs do not employ high-frequency transformers for isolation.

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

1) Integrated configuration, where the two are combined within the lighting fixture. This approach offers optimized energy efficiency and simplified installation.

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

LED drivers can operate with constant voltage (CV) outputs, clamping the output voltage within a certain current range. Alternatively, they can operate with constant current (CC) outputs, precisely limiting the current. Some drivers even offer constant current constant voltage (CCCV) outputs, providing a constant output power, where the current is determined by the forward voltage of the load's LED.

In general, LED lighting design must take into account several factors:

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

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.

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

Secondly, let’s explore the LED drive power topology:

In LED lighting applications utilizing AC-DC power supplies, the power conversion building blocks include discrete components such as diodes, switches (FETs), inductors, capacitors, and resistors to perform their respective functions, while pulse width modulation (PWM) regulators are employed to control power conversion. The isolated AC-DC power conversion, typically including a transformer in the circuit, encompasses topologies such as flyback, forward, and half-bridge. As seen in Figure 3, the flyback topology is the standard choice for medium and low-power applications with power below 30W. The half-bridge structure is ideal for achieving higher energy efficiency and power density. For transformers in isolated structures, the size of the transformer is influenced by the switching frequency, and most isolated LED drivers essentially use "electronic" transformers.

In LED lighting applications employing DC-DC power supplies, LED drivers can be implemented using resistors, linear regulators, or switching regulators. Refer to Figure 4 for a basic application. In the resistive drive mode, the forward current of the LED can be regulated by adjusting the current-sense resistor in series with the LED. This drive mode is simple to design, low-cost, and free from electromagnetic compatibility (EMC) issues. However, it depends on the voltage and requires filtering. (Binning) LEDs exhibit lower energy efficiency.

Linear regulators are also straightforward to design and free from EMC issues. They support current regulation and fold-back, and provide an external current set point. However, they suffer from insufficient power dissipation and a high input voltage, resulting in low energy efficiency. Switching regulators continuously control the opening and closing of the switch (FET) via the PWM control module to regulate the flow of current.