LED selection matches specific application circuit diagram

Crystal oscillator

Photocoupler

Today's LED lighting has a diverse range of applications, from simple incandescent or cold cathode fluorescent (CCFL) alternatives to new architectural, industrial, medical and other applications. In order to optimize matching lamps and light in applications, different LED lighting applications typically have corresponding performance standards.
In order to drive LEDs, engineers can choose from a wide range of drive architectures. However, each architecture has its own strengths and weaknesses, and the adaptability for specific applications is good or bad. There are many factors to consider when choosing a drive architecture, where cost is the primary priority, followed by isolation, dimming, flicker, color temperature, power factor, reliability, thermal management and more.
There are several basic LED driver architectures: secondary side control, primary side control, isolated/non-isolated. In addition, power factor control (PFC) is also a major performance consideration in many applications, with solutions ranging from two-stage or single-stage drivers with PFC functionality or single-stage drivers without PFC functionality (mainly for low power) Composition in 5W applications). As a result, the entire drive subsystem is the result of a series of trade-offs aimed at reducing bill of materials (BOM) costs for maximum efficiency while providing dimming capabilities to create a temperature-controlled, fail-safe product.
Basic Driver Architecture For optimal isolation and control, the secondary side control architecture monitors the output voltage/current and provides a feedback signal to the primary side driver through an optical isolation path (Figure 1). This feedback signal enables the secondary side controller to provide better current and voltage control accuracy. The simpler primary side control scheme eliminates the secondary side controller and optically isolated signal path, reducing system cost and reducing system size while improving system performance. In this scheme, the primary side driver determines the output current and voltage through primary side waveform analysis (Figure 1). Depending on the quality of the analysis, the primary side control can match or even exceed secondary side regulation and performance, making it a common solution for today's isolated LED drivers.


Figure 1: Two common LED driver schemes use secondary side control (top) and primary side control (bottom). Secondary side control has good current and voltage control accuracy, but primary side control reduces component count and system size while improving performance.
The basic primary side control circuit is isolated by an output stage transformer. However, in order to reduce component costs, the non-isolated solution uses an inductor instead of a transformer and can replace the primary side driver flyback circuit with a buck controller (Figure 2). In non-isolated schemes, the control mechanism is simplified, but to prevent short circuits between input and output, the circuit requires more complex physical isolation. Currently, most LED driver designs use an isolated architecture. In the next year or two, advances in circuit design will provide even further cost reduction solutions.

Figure 2: The primary side driver can be designed as an isolated configuration by using a transformer at the output stage, or by using an inductor instead of an output transformer and using a buck controller instead of a flyback circuit.