With LED lighting applications operating under 35W, such as interior light bulbs, there exist a tough set of challenges for the LED driver designer. The design must be small enough to fit in a standard socket, it may need to support dimming and there are various regulations for safety and efficacy to consider – all while trying to, in the end, meet low cost targets.
A single-stage isolated flyback driver solution is typically the best approach, but it is important to pick the best control architecture. For example, one of the more common choices for low-power applications is the flyback controller with secondary-side regulation (SSR), however this control method alone may not be great for LED lighting if it lacks power factor correction (PFC) and if it regulates as a constant voltage supply versus a constant current supply. Additionally, it requires extra feedback circuitry to cross the isolation boundary; most common is the use of an opto-isolator.
Adding passive PFC, might seem attractive because it limits power losses and reduces voltage stress on components. However, the technique requires high-voltage capacitors and this may not be good enough for an LED driver’s power factor, life time, reliability, or size requirements.
Unsatisfied with these control options, we looked at our power management options used in other applications. We leveraged our technique for load control, TRUECURRENT® technology, developed for battery charging. Figure 1 highlights the TRUECURRENT technology block as it is implemented in a pulse width modulation (PWM) controller. TRUECURRENT technology performs the measurements and comparisons necessary to deliver precise control from the primary-side of the isolation boundary for a constant current output without the need for additional feedback circuitry from the secondary-side.
Using TRUECURRENT technology, we then added two features for LED lighting — power factor correction and dimming control. The result is a single-stage flyback topology with PSR that eliminates the SSR feedback circuitry and doesn’t require an HV input capacitor.
The architecture is available in two new PWM controllers, the dimmable FL7730 PWM controller and the non-dimmable FL7732 PWM controller. Figure 2 is a simplified application schematic showing the FL7730 used for TRIAC dimming.
These controllers also apply input line compensation by receiving information about the line voltage from the VS pin and using it to modify the peak current circuit. This allows for extremely tight constant current regulation over a wide input voltage range.
For dimming, a simple resistor divider network, working with an RC filter, converts the duty cycle of the AC line voltage into a DC voltage, which is then placed on the DIM pin. A two-angle control block is used to offset the current sense measurement and to provide input to the TRUECURRENT technology calculation block. This simple technique is compatible with nearly every form of dimming, including analog and TRIAC-based dimming.
The four key features of the FL7730/FL7732 LED drivers are: TRUECURRENT technology control, power factor correction, line compensation, and dimming. These features enable a single-stage topology that effectively optimizes power condition, power conversion, and load control. With TRUECURRENT technology as the starting point, we’ve created a better way to design compact, cost-effective LED drivers for low-power applications.
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