# why boost PFC topology is the most used?

While searching about power factor correction topologies, the topology I came across almost all the time was the Boost PFC topology, and I'd like to know why? why is it the most widely used one, in comparaison to say, buck PFC or flyback PFC.

Active power factor correction (PFC) is all about controlling the circuit's input current waveform and making it as close to input voltage waveform as possible (Sinusoidal in the case of AC mains).

Converter topologies such as Buck (Shown in next image) and Flyback work by quickly connecting and disconnecting themselves from supply source in order to modify their output voltage while keeping it continuous with stored energy from capacitors and inductors. Since they are completely disconnecting from power source while switching, input current is inherently discontinuous which is not desired if we are attempting to control it.

On the other hand, converters such as Boost (Shown in next image) and SEPIC don't directly disconnect themselves from power source. This causes input current to be continuous. Furthermore, its value can be smoothly controlled from the converter's switching components, making these topologies ideal for active PFC.

Active PFC methods for Buck-type topologies exist but tend to be less effective than Boost-type topologies. That said, this paper proposes a 2-stage Boost-Buck converter topology (as opposed to one-stage Boost-Buck), in order to control input current with the first stage and to reduce output voltage with the last one (Which could not be done with a Boost converter alone). Disclaimer: I'm one of the authors of this work.

• If i want to implement some (even not most effective) PFC in my device, that normally only comes with a rectifier. What is the minimal circuit for that? – Gregory Kornblum Jun 30 '17 at 3:55
• Do you have an open-access version of your paper posted somewhere? – pericynthion Jun 30 '17 at 4:53
• @JorgeC does a boost PFC improve system efficiency? What is the effect on electricity bills? – Gregory Kornblum Jun 30 '17 at 7:51
• A PFC does not exactly increase efficiency because it is another stage that you insert in the conversion chain. However, because the input current becomes more sinusoidal, the input rms current once the PFC is inserted reduces. For a given output power, you can use small gauge power cords with a PFC than without. You can connect more PFC-equipped devices to an ac outlet because the total rms current is less than without PFCs. You can optimize efficiency with a boost PFC by having Vout close to Vin,peak. Some systems adjust Vout, 400 V for 230 Vin and 250 V for 110 Vin: follow-boost technique. – Verbal Kint Jun 30 '17 at 8:32
• The non-linear input current of a bridge rectifier comes from the re-fuelling time of the "bulk" capacitor (the big filtering capacitor after the bridge) constrained at the input peak voltage. An active PFC will "spread" this re-fulling time along the sinewave input so that the absorbed current looks like it comes from a resistance. The power meter will measure a total rms current with some error though given the distortion. By multiplying $V_{in,rms}$ by $I_{in,rms}$ you obtain VAs but the "true" power in W is obtained by integrating the instantaneous power $p(t)$ over a mains period. – Verbal Kint Jun 30 '17 at 10:36