The Power Factor Controller (PFC) is the most common block in modern AC/DC LED drivers. A boost converter is inserted between the bridge rectifier and the main input capacitors. This regulator can operate in three modes. In Discontinuous-Conduction Mode (DCM), the energy stored in the inductor (L) during the conduction interval of the switch is equal only to the energy required by the load for one switching cycle - see figure 3
. The energy in the inductor drops to zero before the end of each switching cycle, resulting in a period of no energy flow, or discontinuous operation. In Transition Mode (TM) – also called Boundary Conduction Mode (BCM) or Critical conduction Mode (CRM), the converter operates at the boundary between DCM and Continuous Conduction Mode (CCM), reducing the idle time of DCM to close to zero.
Figure 3: Peak and average current in the inductor (IL) in a) discontinuous conduction mode b) transition conduction mode and c) continuous conduction mode.
In CCM, the inductor has continuous current during the operation of the converter. The extra energy stored in the inductor during the conduction time of the switch is equal to the energy discharged into the output during the non-conductive time of the switch; at the end of the discharge interval, residual energy remains in the inductor.
During the next conduction interval of the switch, energy builds from that residual level to that required by the load for the next switching cycle. CCM has a lower peak-to-average current ratio; thus it has lower ripple currents, lower coil conduction and core losses, and lower electromagnetic emission levels. The drawbacks are that it requires a very fast boost diode (otherwise diode recovery current starts to dominate, resulting in increased power losses and additional electromagnetic emissions). Unfortunately, it also requires hard MOSFET switching, and this results in high switching losses, and these are the main source of electromagnetic emissions in a CCM system. The biggest advantage of TM or DCM operation is the absence of reverse recovery in the boost diode, which means that the circuit can use a low-cost diode with a low forward voltage. On the other hand, the cost of filters to block the electromagnetic emissions generated at the high peak currents might be excessive.
New components in the second DC/DC stage of the LED driver - see figure 4
- also offer new ways to reduce electromagnetic emissions. They often contain ‘resonant’ LC networks, of which the voltage and current waveforms are sinusoids. The turn-on or turn-off transitions of semiconductor devices can occur at zero crossings of the tank voltage or current waveforms, thereby reducing or eliminating some of the switching loss. This means that resonant converters can operate at higher switching frequencies than comparable PWM converters, leading to smaller inductor and capacitor values and costs. In addition, zero-voltage switching reduces converter-generated EMI as it has no current or voltage ripples during switch commutation.
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