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The Continuous Conduction Mode of PFC Boost Converters

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The main objective of the power factor correction (PFC) technique is to make the power factor as close to unity as possible. 

  • DC-DC boost converter topology is the most widely used PFC converter.

  • A PFC boost converter is in continuous conduction mode when the switching device turns on before the inductor current drops to zero. 

Power Factor Correction graphic

DC-DC boost converter topology is the most widely used PFC converter

The power factor is a figure of merit that describes how much real power an electrical device draws from the main supply. The power factor is given by the cosine of the angle between the mains voltage and the input current. Ideally, the power factor should be unity so that all the power drawn from the mains is utilized to do useful work. However, this is not the case in most electrical systems; as long as there is a phase difference between the voltage and current waveforms, the power factor is less than unity. Power factor correction is required in such electrical systems to synchronize the input current with the mains voltage.

Power factor correction (PFC) converters are usually used for improving the power factor. The DC-DC boost converter is an example of one such PFC converter. A PFC boost converter is either operated in continuous conduction mode or discontinuous conduction mode. In this article, we will discuss PFC boost converter modes of operation and their advantages.      

Low Power Factor: Causes and Effects

There are regulations on the minimum power factor allowed in an electrical device, just like there are harmonic content regulations. To sustain in the market, electrical devices must cater to the power factor standards devised by regional committees or international organizations. A low power factor is mainly due to:

  1. Reactive loads - When a load is resistive, the current and voltage waveforms are in-phase. Whenever a load turns reactive, the current lags or leads the voltage waveforms, depending on the reactive element. The displacement of the input current waveform from the voltage due to inductive and capacitive loads results in a low power factor. 
  2. Non-linear loads - When non-linear loads are connected to the mains supply, it draws distorted input current with high harmonic content. The current THD distorts the power grid voltage as well. 

A poor or low power factor decreases the efficiency of electrical systems, increases heating losses, and subsequently leads to power failure in extreme cases. Power factor correction is essential for improving the power factor.  

Power Factor Correction (PFC)

The main objective of the power factor correction (PFC) technique is to make the power factor as close to unity as possible. PFC can be done in two ways: passive PFC or active PFC.

In passive PFC, passive filters improve the power factor by filtering out harmonics. PFC converters are utilized for improving the power factor in active PFC. PFC converters make the input current follow the mains voltage waveform, just like in a purely resistive circuit.

In systems utilizing AC grid power to obtain DC power, DC-DC switching converters are typically used as active PFC for shaping the input current closer to a sinusoidal waveform. Various topologies of DC-DC converters are used in active PFC applications. However, DC-DC boost converter topology is the most widely used one.   

PFC Boost Converters

The boost converter is the most popular PFC converter. In most high voltage DC applications, stepping up the DC voltage obtained from the rectifier connected to the AC power grid is necessary. The inclusion of the DC-DC boost converter serves this purpose, and PFC also comes along with it. A buck-boost converter is also an option, considering the non-isolated DC-DC converter topologies for boosting the DC voltage and PFC. However, the high switching stress in buck-boost converters offsets its PFC application and highlights the advantages of the PFC boost converter.

Another reason to choose a PFC boost converter is the filter inductor in the input side of the boost converter topology. The inductor smoothens the continuous current and eases the filtering action. Incorporating a PFC boost converter eliminates the need for additional filters and reduces the cost as well as weight. 

A PFC boost converter can be operated in three modes: continuous conduction mode, critical continuous conduction mode, and discontinuous conduction mode. We will discuss the continuous conduction mode of a PFC boost converter in the upcoming section. 

The Continuous Conduction Mode of a PFC Boost Converter

A PFC boost converter is in continuous conduction mode when the switching device turns on before the inductor current drops to zero. The inductor current is continuous in the continuous conduction mode of a PFC boost converter. In this mode, there are two states in one switching cycle: the ON state and the OFF state.

  1. When the switching device is turned on (ON state) - During the ON state, the switching device is turned on and the inductor charges from the input voltage to the boost converter. The diode is reverse biased during this state and the capacitor supplies the output current.
  2. When the switching device is turned off (OFF state) - During the OFF state, the switching device is turned off and the inductor starts to discharge via the forward-biased diode to the load. Before the inductor completely discharges, the switching device is turned on. 

In power grid AC-DC conversion applications utilizing PFC boost converters, the input current also becomes continuous, as it is the same as the converter inductor current. Usually, continuous conduction mode PFC boost converters are switched at constant frequency to achieve a sinusoidal input current.

Continuous conduction mode in PFC boost converters is preferred over critical conduction mode and discontinuous conduction mode due to low values of peak current, switching device conduction losses,  turn-off losses, and high-frequency ripple amplitude.

The design of a PFC boost converter is important to ensure smooth operation under any conduction mode. The PCB design and layout tools from Cadence can help you to build the best PFC boost converter layout possible.

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