AN-8035 Datasheet, PDF (1/22 Pages) Fairchild Semiconductor – Boundary Conduction Mode Power Factor Correction (PFC)

www.fairchildsemi.com
AN-8035
Design Consideration for Boundary Conduction Mode
Power Factor Correction (PFC) Using FAN7930
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, FAN7930. 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 200W 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.
FAN7930 has a PFC-ready pin to acknowledge when
PFC output voltage reaches stable operation range. 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 total BOM cost. The
internal proprietary logic for detecting input voltage
greatly improves the stability of PFC operation. Together
with the maximum switching frequency clamping at
300kHz, FAN7930 can limit inductor current within pre-
designed value 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.
Visit http://www.fairchildsemi.com/design_tools/ to
download the design tool.
L_BOOST
V_OUT
V_CC
FAN7930
8 _VCC
5 _ZCD
3
Comp
2
RDY
7
Out
CS ⁴
INV ¹
GND
6
Q1
V_OUT acknowledge
Figure 1. Typical Application Circuit
Excel^® is a registered trademark of Microsoft Corporation.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 5/3/10
www.fairchildsemi.com