LED Driver: Select the circuit design that matches your application

　　Today's LED lighting has a variety of applications, from simple incandescent or cold cathode fluorescent lamp (CCFL) replacements to new architectural, industrial, medical and other applications. In order to optimize the matching of lamps and light in the application, different LED lighting applications usually have corresponding performance standard requirements.

　　To drive LEDs, engineers can choose from a wide variety of driver architectures, but each architecture has its own advantages and disadvantages, and its adaptability to specific applications varies. There are many factors to consider when choosing a driver architecture, among which cost occupies the first position, followed by isolation, dimming, flicker, color temperature, power factor, reliability, thermal management and other issues.

　　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, and its solutions consist of two-stage or single-stage drivers with PFC function, or single-stage drivers without PFC function (mainly used in applications with power less than 5W). Therefore, the entire driver subsystem is the result of a series of trade-offs, with the aim of reducing bill of materials (BOM) cost and achieving maximum efficiency while providing dimming functions and creating a temperature-controlled and fault-protected product.

　　Basic drive architecture

　　To achieve 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 optically isolated path (Figure 1). This feedback signal enables the secondary-side controller to provide better current and voltage control accuracy. Simpler primary-side control schemes eliminate the secondary-side controller and the optically isolated signal path, thereby reducing system cost and improving system performance while reducing system size. 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, primary-side control can match or even exceed secondary-side regulation and performance, making it a common solution for isolated LED drivers today.

　　Figure 1: Two common LED driver schemes use secondary-side control (top) and primary-side control (bottom). Secondary-side control provides better current and voltage control accuracy, but primary-side control reduces component count and system size while improving performance.

　　The basic primary-side control circuit achieves isolation through the output stage transformer. However, to reduce component cost, non-isolated solutions use an inductor instead of a transformer and can use a buck controller instead of a primary-side driver flyback circuit (Figure 2). In non-isolated solutions, the control mechanism is simplified, but the circuit requires more complex physical isolation to prevent short circuits between the input and output. Currently, most LED driver designs use isolated architectures. In the next year or two, advances in circuit design will provide further cost reduction solutions.

　　Figure 2: The primary-side driver can be designed in an isolated configuration by using a transformer in the output stage, or in a non-isolated configuration by replacing the output transformer with an inductor and selecting a buck controller instead of the flyback circuit.