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Critical Conduction Mode (CRM) Boost (<300 W). Critical conduction mode (sometimes called borderline conduction mode or transition mode) occurs at the boundary between discontinuous conduction mode (DCM) and continuous conduction mode (CCM). CRM is a variable frequency topology in which the next driver on-time is initiated when the boost inductor current reaches zero. It is an ideal choice for medium power PFC boost stages because it combines the lower peak currents of CCM operation with the zero current switching of DCM operation. This topology is effective for up to 300 W and is very popular for consumer devices, notebook adapters, and light ballast.

Continuous Conduction Mode (CCM) Boost (>300 W). Continuous conduction mode is a fixed frequency topology in which the cycle-by-cycle inductor current never drops to zero. The main benefit to CCM operation is the low inductor current slope and lower peak currents. The result is lower conduction losses and reduced EMI. However, the design is often more complicated and requires more external components in comparison to CRM. Therefore, the topology is mainly reserved for high output power applications (>300 W) or very EMI sensitive applications.

Single Stage Flyback. Traditional PFC controllers utilize two stage conversion in which a PFC boost pre-regulator charges a bulk storage capacitor and then relies on a downstream converter to regulate at the required output voltage. The single stage flyback topology uses a single controller to provide the required regulated output voltage. Single stage flyback PFC is ideal for applications in the 75 W to 150 W output power range where size and component count are key. These devices are designed to operate in continuous conduction (CCM) or discontinuous conduction (DCM) modes and therefore share many of the tradeoffs of these topologies as well as the additional tradeoffs below.

Interleaved. Interleaved PFC uses two parallel small stages in lieu of one bigger stage. For example, two 150 W PFC stages can be combined to form a 300 W PFC stage. Since only half of the total power is going through each stage, smaller components can be used and the heat dissipation is better distributed. This makes it a popular solution in applications with a slim form factor such as notebook adapters and LCD TVs.

Fixed Frequency DCM/CRM. The Fixed frequency DCM/CRM solution provides the best of both modes with few drawbacks. These devices typically operate in a fixed frequency discontinuous conduction mode (DCM) during light loading, but transfer to critical conduction mode (CRM) for high loading. This solves the poor light load efficiency experienced by CRM solutions. Since the circuit’s maximum frequency is limited, there is a smaller range of frequencies to filter than in CRM, which reduces EMI concerns.