LED Drivers—Match architectures with applications
To power LEDs, designers have a choice of several driver architectures, and each architecture has its own plusses and minuses that make it a good or poor fit for a particular application. The various factors that should be considered when selecting a driver architecture span a wide range, with cost being the elephant in the room. Not far behind, though, are issues such as isolation, dimming, flicker, color temperature, power factor, reliability, thermal management, and still other concerns.
There are several basic LED driver architectures – Secondary side control, Primary side control, and Isolated/Non-isolated. Additionally, power-factor control (PFC) is a major performance concern in many applications, with solutions consisting of either a two-stage driver with PFC or a single stage with PFC, or a single stage with no PFC (common for power levels of less than 5W). The entire driver subsystem is thus one big set of tradeoffs to reduce bill of materials (BOM) cost, to get the best efficiency while achieving dimmer compatibility, and to build a reliable product that keeps thermals under control and will fail safely if something goes wrong.
Basic Driver Architectures
To provide the best isolation and control, the Secondary-Side control architecture monitors the output voltage/current and provides feedback to the driver on the primary side via an optically isolated signal path (Figure 1a). The feedback enables the secondary-side controller to provide good current and voltage control accuracy. The simpler Primary-Side control approach eliminates the secondary-side controller and the optically-isolated feedback path, thus lowering system cost and reduces the system size while improving performance. In this approach, the primary-side driver determines the output current and voltage by waveform analysis on the primary side (Figure 1b). Depending on the quality of this analysis, the primary side control can match or exceed the secondary-side regulation and performance and is thus the popular solution in isolated LED drivers today.
Figure 1. Two popular LED driver approaches employ Secondary-Side control (a) and Primary Side control (b). Secondary-Side control provides good current and voltage control accuracy, but Primary-Side control lowers component count and size while improving performance.
The basic Primary-Side control circuit is isolated thanks to the transformer in the output stage. However, to lower component cost, a non-isolated approach replaces the transformer with an inductor and can replace the Primary-Side driver flyback circuit with a buck controller (Figure 2). In the non-isolated approach, the control mechanism is simplified but the circuit requires more complex physical isolation to prevent input-to-output short circuits. Today, a majority of LED driver designs employ an isolated architecture. However, advances in circuit design may offer some additional lower-cost options in the next year or two.
Power Factor Basics
The ideal power factor of 1 occurs when input voltage and current are in phase and the input voltage and current waveforms track. When the phase difference between the voltage and current waveforms increases, the power factor decreases and system efficiency suffers. However, boost converters inherently have a current waveform envelope that tracks the input voltage waveform and thus maintain a power factor near unity.
To improve the power factor, a two-stage power-factor correction (PFC) boost circuit can be added in front of the Primary-Side driver and control circuits (Figure 3). The PFC circuit also eliminates the 2× line frequencies that can cause flicker. In the example shown the output stage uses a flyback converter to provide isolation for the driver circuits in the iW3616. The driver chip uses primary-side sensing technology to achieve excellent line voltage and LED load current regulation without a secondary-side feedback circuit and it eliminates the need for the optocoupler feedback loop. In addition, cycle-by-cycle waveform analysis technology allows fast dimmer setting response. The digital control loop maintains stability of overall operating conditions without the need for loop compensation components.
Figure 2: A Primary-Side driver can be set up to operate 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 optionally replacing the flyback circuit with a buck controller.
Figure 3: A two-stage power-factor correction boost circuit added to the iW3616 digital power controller lets the driver circuit deliver flickerless dimming and high power factors (>0.95).