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EETE NOVEMBER 2012

RC = Resonant Converter, energy remains in the induc- Qr = Quasi-resonant con- tor. during the next conduc- verter, PFC = Power Factor tion interval of the switch, Controller, CC = Constant energy builds from that Current control, CV = Con- residual level to that required stant Voltage control, CCM by the load for the next = continuous-conduction switching cycle. Mode, CRM = Critical Con- ccm has a lower peak- duction mode to-average current ratio; thus The pFc is the most Fig. 1: a typical AC/DC LED driver design. The H-field is the result of it has lower ripple currents, common block in mod- winding leakage, the primary loop area and the secondary loop area. lower coil conduction and ern Ac/dc Led drivers. A The E-field is the result of high dV/dt on conductive surfaces and of core losses, and lower boost converter is inserted high-frequency ripple in cables. electromagnetic emission between the bridge rectifier levels. The drawbacks are and the main input capaci- that it requires a very fast tors. This regulator can boost diode (otherwise operate in three modes. in diode recovery current starts discontinuous-conduction to dominate, resulting in Mode (DCM), the energy increased power losses and stored in the inductor (L) dur- additional electromagnetic ing the conduction interval emissions). Unfortunately, it of the switch is equal only to also requires hard mosFeT the energy required by the switching, and this results in load for one switching cycle high switching losses, and - see figure 3. The energy these are the main source of in the inductor drops to electromagnetic emissions in zero before the end of each a ccm system. switching cycle, resulting in The biggest advantage of a period of no energy flow, Tm or dcm operation is the or discontinuous operation. absence of reverse recovery in Transition mode (Tm) in the boost diode, which – also called Boundary con- means that the circuit can duction mode (Bcm) or criti- Fig. 2: The evolution of components for AC/DC LED drivers. use a low-cost diode with a cal conduction Mode (CRM), low forward voltage. on the the converter operates at other hand, the cost of filters the boundary between dcm to block the electromagnetic and continuous conduction emissions generated at the Mode (CCM), reducing the high peak currents might be idle time of dcm to close to Fig. 3: Peak and average current in the inductor (IL) in a) discontinuous excessive. zero. conduction mode b) transition conduction mode and c) continuous new components in the In CCM, the inductor has conduction mode. second dc/dc stage of the continuous current during LED driver - see figure 4 - the operation of the converter. The extra energy stored in the also offer new ways to reduce electromagnetic emissions. They inductor during the conduction time of the switch is equal to the often contain ‘resonant’ LC networks, of which the voltage and energy discharged into the output during the non-conductive current waveforms are sinusoids. The turn-on or turn-off transi- time of the switch; at the end of the discharge interval, residual tions of semiconductor devices can occur at zero crossings of Cabling Systems the company you trust Standard Power Supplies Customized Power Supplies 8608 Bubikon. Switzerland Tel. +41 (0)55 253 31 90 Hall B2 . Booth 118 www.fabrimex.com Visit us: _Fab_Ins_electronica_CPS_190x66_e.indd 1 11.10.12 09:32 www.electronics-eetimes.com Electronic Engineering Times Europe November 2012 47


EETE NOVEMBER 2012
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