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EETE MAY 2013

POWER COMPONENTS controlled current source, or a highly resistive–looking input impedance. The peak current through the primary side of the transformer is directly proportional to the ontime of the primary drive. This inherently has a high PF , near 1. While the constant on-time architecture offers the benefit of high power factor, the trade-offs are often times unacceptable. Normally operated in DCM mode, the peak currents are very high and have two major consequences, high stress on the passive components and high current ripple on the LEDs. The high stress on the input capacitors can degrade the lifetime of these key components and lead to reduced operating lifetime on the bulb. The excessive current ripple on the output reduces the quality of the light output by adding flicker to the light output as well as degrading efficiency and increasing self-heating due to the ripple current through the output capacitors. Using a constant current mode operation instead of constant ontime alleviates the ripple current on the output and reduces the stress on the passive components in the circuit, but due to the inherently reactive nature of the input impedance, the power factor diminishes significantly. Finding an alternative approach that combines high PF and low ripple current while minimizing the impact on the passive external components is key to finding the optimal low-cost solution in a single-stage LED driver. The iW3626 shown in figure 3 is an example of a singlestage, Fig. 2: Single-stage (top) and two-stage (bottom) PFC architectures. high PF LED driver that combines a high PF driver with the ability to minimize the output ripple. The technology to accomplish this resides in the core of digital engine that monitors input voltage and current along with the output conditions by monitoring the primary side of a power transformer. The digital core allows for modulating the drive to the main power transistor, in this instance a power bipolar junction transistor. The proprietary modulation technique in this example allows the end user to program the desired minimum power factor at 0.7, 0.8 or 0.9, or no power factor at all. Along with the minimum power factor comes a corresponding output ripple. This flexibility allows the designer to optimize the circuit for either output ripple (with no PF C) or for PF (with moderate output ripple), or for a balanced, high PF and low output ripple design. An additional LED driver characteristic important in solid state lighting, and also regulated by the international lighting standards shown in table 1, is total harmonic distortion (THD). As a general rule, when THD is low, power factor tends to be high (>0.9). But, with single-stage conversion technology, normally there is a tradeoff between output ripple, THD and PF . The iW3626 uniquely combines low output ripple and high PF with THD at an acceptable level for most applications. Table 1: Regional standards for power factor and total harmonic distortion 22 Electronic Engineering Times Europe May 2013 www.electronics-eetimes.com


EETE MAY 2013
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