PFC Circuit Halts Inrush Currents

by John Bottrill, Senior Applications Engineer, Texas Instruments, Manchester, N.H. , Power Electronics Technology, Jun 01 2008

Focus:
The front end of a power factor correction circuit is typically a boost converter that includes a PFC inductor and PFC capacitor. At startup, current flows through this inductor to charge the capacitor. Depending on when power is applied during the input ac cycle, the rapid charging of the capacitor can produce output voltage overshoot and a high input-current spike, both of which can potentially damage PFC components. This overcurrent condition can be controlled passively using either a positive temperature coefficient (PTC) resistor or a negative temperature coefficient (NTC) resistor. However, an active current-limiting circuit provides more accurate control of the inrush current and can lower power dissipation. A current limiting circuit described here limits inrush current to the maximum line current during post-start-up operation. The circuit consists of two transistors and a handful of passive components. Its operation is described and instructions are given on how to select its components to suit application requirements.

What you’ll learn:

• How to build an active current limiting circuit for tighter control of inrush current in PFC circuits
• How to understand the factors influencing inrush current in PFC circuits

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