AN-6921 Datasheet, PDF (1/16 Pages) Fairchild Semiconductor – Integrated Critical Mode PFC / Quasi-Resonant Current Mode PWM Controller

www.fairchildsemi.com
AN-6921
Integrated Critical Mode PFC / Quasi-Resonant
Current Mode PWM Controller FAN6921
1. Introduction
This application note presents practical step-by-step design
considerations for power supply system employing
Fairchild’s FAN6921 PFC/PWM combination controller,
which combines a Boundary Conduction Mode (BCM)
Power Factor Correction (PFC) controller and Quasi-
Resonant (QR) PWM controller. Figure 1 shows the typical
application circuit, where the BCM PFC converter is in the
front end and the Quasi-Resonant flyback converter is in the
back end.
FAN6921 achieves high efficiency with relatively low cost
for 75~200W applications where BCM and QR operation
with a single switch show best performance. BCM boost
PFC converter can achieve better efficiency with lower cost
than continuous conduction mode (CCM) boost PFC
converter. These benefits result from the elimination of the
reverse-recovery losses of the boost diode and zero-voltage
switching (ZVS) or near ZVS (also called valley switching)
of boost switch. The QR flyback converter for the DC/DC
conversion achieves higher efficiency than the conventional
hard-switching converter with valley switching.
Moreover, FAN6921 has variable PFC output voltage
function that can improve the overall efficiency by reducing
the conduction loss of PFC converter and switching loss of
DC/DC converter stage at low-line condition.
Figure 1. Typical Application Circuit
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 8/24/10
www.fairchildsemi.com