AN-9731 Datasheet, PDF (1/22 Pages) Fairchild Semiconductor – LED Application Design Guide Using BCM Power Factor Correction (PFC) Controller for 100W Lighting System

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AN-9731
LED Application Design Guide Using BCM Power Factor
Correction (PFC) Controller for 100W Lighting System
1. Introduction
This application note presents practical step-by-step
design considerations for a Boundary-Conduction-Mode
(BCM) Power-Factor-Correction (PFC) converter
employing Fairchild PFC controller, FL7930. It includes
designing the inductor and Zero-Current-Detection (ZCD)
circuit, selecting the components, and closing the control
loop. The design procedure is verified through an
experimental 140W prototype converter.
Unlike the Continuous Conduction Mode (CCM)
technique often used at this power level, BCM offers
inherent zero-current switching of the boost diodes (no
reverse-recovery losses), which permits the use of less-
expensive diodes without sacrificing efficiency.
The FL3930B provides an additional OVP pin that can be
used to shut down the boost power stage when output
voltage exceeds the OVP level due to damaged resistors
connected at the INV pin. The FL7930C provides a PFC-
ready pin can be used to trigger other power stages when
PFC output voltage reaches the proper level (with
hysteresis). This signal can be used as the V_CC trigger
signal for another power stage controller after PFC stage or
be transferred to the secondary side to synchronize the
operation with PFC voltage condition. This simplifies the
external circuit around the PFC controller and saves BOM
cost. The internal proprietary logic for detecting input
voltage improves the stability of PFC operation. Together
with the maximum switching frequency clamping at
300kHz, FL7930 can limit inductor current to within pre-
designed ranges at one or two cycles of the AC-input-
absent test to simulate a sudden blackout. Due to the
startup-without-overshoot design, audible noise from
repetitive OVP triggering is eliminated. Protection
functions include output over-voltage, over-current, open-
feedback, and under-voltage lockout.
An Excel^®-based design tool is available with this
application note and the design result is shown with the
calculation results as an example.
Figure 1. Typical Application Circuit
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 3/24/11
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