AN2782 Application Note: Designing a 400 W Fixed-Off-Time PFC Preregulator with L6562A

About this manual

The document introduces Fixed-Off-Time (FOT) control as a cost-effective alternative to Transition Mode and Fixed-Frequency Continuous Conduction Mode for Power Factor Correction (PFC) pre-regulators. It explains how FOT combines the simplicity of transition mode with high-current capability, suitable for mid-range power levels around 150-300W or higher.

The note provides detailed design equations and a practical example for a 400W board using the L6562A controller. It covers circuit implementation, component selection (bridge rectifier, capacitors, inductor, MOSFET, diode), biasing, and includes an Excel spreadsheet tool for calculations. Test results from an EVL6562A-400W demonstration board are presented to validate compliance with standards like EN61000-3-2.

What's inside

• Introduction to FOT control
• Operation of an FOT-controlled PFC pre-regulator
• Circuit implementing line-modulated fixed off-time
• Designing a fixed off-time PFC
• Input specification and operating conditions
• Power section design components
• L6562A biasing circuitry
• EVL6562A-400W demonstration board test results

Specifications

Frequently asked questions

What is the main advantage of FOT control over other PFC methods?

It couples the simplicity and affordability of transition mode operation with the high-current capability of fixed-frequency continuous conduction mode.

What is the recommended power range for FOT?

It is difficult to apply strictly, but it is often used in mid-range power levels around 150-300W.