Transition mode PFC increases efficiency to help meet the 80+ requirements

Countries around the world are now adopting standards to improve efficiency and to conserve energy. One of these standards is the 80+ initiative. This is where an offline power supply needs to have greater than 80 percent efficiency from 20 percent to 100 percent of the converter's output power. These new standards are forcing power supply designers to find innovative ways to improve efficiency on new designs. This article will discuss how transition mode power factor corrected (PFC) boost pre-regulators can help power supply manufacturers meet these new efficiency requirements.

Problem
To help remove unwanted losses from the power lines, the European Union has enacted EN61000-3-2 standards for offline power supplies. These standards consist of harmonic current content specifications that apply to offline power supplies greater than 75 W. The most popular way for switching power supplies to meet these specifications is with an active power factor corrected boost pre-regulator followed by a step down converter (see Figure 1). Overall total system efficiency (η _total) is the product of stage one's efficiency (η _Stage1) and stage two's efficiency (η _Stage1), which makes for an overall less efficient power supply.

Equation 1

Figure 1: Two Stage Offline Power Supply with PFC Front End