Digital Implementation of the Feedforward Loop of the Asymmetrical Half-Bridge Converter for LED Lighting Applications

Abstract

Abstract.-The Asymmetrical Half Bridge converter (AHBC) has proven to be a promising candidate for LED lighting applications. It provides high efficiency, galvanic isolation and it can be easily built without electrolytic capacitor. On the other hand, its main drawback is its poor attainable bandwidth. In any two-stage ac-dc LED driver based on the AHBC, the first stage is a Power Factor Corrector (PFC) converter which has to be also implemented without electrolytic capacitor. As a consequence, its output voltage (input voltage of the AHBC) presents a low-frequency ripple. Due to the poor bandwidth of the AHBC, this voltage ripple will be transferred to the converter output voltage, leading to flickering. Due to the complex and non-linear transfer function of the AHBC, any analog feedforward loop has to be tuned for a given operating point, leading to a poor performance when the AHBC moves away from that point. In this paper, a digital feedforward loop is proposed in order to solve this problem. The digital implementation allows the feedforward loop to perfectly cancel the ripple under any condition (e.g., output voltage variation due to dimming). Besides, this digital feedforward loop has been designed and simplified considering the specific purpose of cancelling flickering in the emitted light. In this way, it can be easily implemented in small-size microcontrollers. Experimental results with a 40-W prototype prove the usefulness of the proposed feedforward loop.

Optimized Design of a High Input-Voltage-Ripple-Rejection Converter for LED Lighting

“…This problem of the traditional AHB converters with the feedforward loop and the low-frequency ripple has been addressed in [29] and [36]. In the first one, the authors propose a double feedforward loop, so it has a different gain for the positive and the negative part of the ripple.…”

“…In the first one, the authors propose a double feedforward loop, so it has a different gain for the positive and the negative part of the ripple. In [36], the authors propose a digital feedforward loop based on a low-cost microcontroller. Although both solutions lead to an improvement in the feedforward loop, the ripple cancellation is still no perfect and the cost and complexity of the proposed systems are considerably increased.…”

“…In Fig. 13, the currents obtained with the complete equations ((32), (33), (38), (39), (41) and (42)) and with the simplified ones ((35), (36), (38), (40), (43) and (44)) are represented for the ZAHB converter proposed in section IV under nominal conditions. As can be seen, the differences are small and they do not significantly affect the results obtained.…”

“…Nonetheless, cancelling the low-frequency ripple may require a bandwidth higher than the one that can be reached without compromising stability. To overcome this problem, a feedforward loop may be implemented in the control of the AHB converter [29], [36]. In this way, the feedforward loop attenuates the ripple and the feedback loop bandwidth can be tailored as much as required to reach stability and a reasonable emitted light regulation.…”

An E-capless AC-DC CRM Flyback LED Driver with Variable On-time Control

“…The first type is adopting energy storage elements to handle the instantaneous input and output power difference [7]- [9]. The second type is working on the control loop of the converters in order to lower the output current ripple without the need for increasing the capacitor size [10], [11]. The third type is using a relatively new approach in which the majority of the input power reaches the LEDs through one stage of conversion while the rest of the power is processed in some other stages after passing the first stage toward the LEDs [12].…”