In phase-cut dimming systems, a key technical issue has been mostly TRIAC holding current maintenance with low power LED lamps. In order to continue TRIAC conduction until the line voltage zero crossing, there have been several trials to flatten the driver input current higher than the holding current. In the previous compensation methods to provide constant input current, the input current is not precisely constant with a compensation error.
Power factor (PF) is also required in case that dimmer is not connected. When dimmer absence is detected, several phase-cut dimming LED driver solutions change the operating mode for better PF and THD. However, PF is worse due to additional high voltage bleeder capacitors. Various technical approaches have been introduced to improve PF, especially when buck and buck-boost topologies are designed in boundary conduction mode. However, there has not been a general-purpose switching method to optimize PF and THD at all conduction modes such as DCM, BCM and CCM.
To overcome the limitation of constant input current control for dimmer compatibility and the degradation of PF and THD in boundary conduction mode, a novel input current calculation switching technique for high PF and stable TRIAC dimmer compatibility has been constructed in 8 ~ 40W universal line voltage LED driver systems. The input current calculation (ICC) switching is implemented by calculating the average switching current to accurately control the input current shape without the conventional compensation methods. For high TRIAC dimmer compatibilities, ICC switching is utilized to shape the constant TRIAC conduction current to stabilize TRIAC conduction, which allows for universal line system designs. In order to optimize PF and THD, ICC switching controls the excellent sinusoidal line current in all conduction mode such as DCM, BCM and CCM. The proposed switching mode shows high TRIAC dimmer compatible performance and significant improvement of the PF and THD.
Drop by Fairchild’s booth (#1717) at APEC to see our live demonstration of the universal line voltage phase-cut dimmable single-stage LED driver based on the FL7734, a highly integrated LED controller solution for cost sensitive and highly reliable LED lighting solutions from 8 W to 40 W. Learn how the FL7734 eliminates visible flicker and shimmer symptoms as well as delivers over 90% dimmer compatibility with a wide range of dimmers.
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