FAN501A Datasheet, PDF (9/16 Pages) Fairchild Semiconductor – Offline DCM / CCM Flyback PWM Controller for Charger Applications

Functional Description
FAN501A is an offline flyback converter controller that
offers constant output voltage (CV) regulation through
opto-coupler feedback circuitry and constant output
current (CC) regulation with primary-side control.
Advanced output current estimation technology allows
stable CC regulation regardless of the power stage
operation mode: Continuous Conduction Mode (CCM)
or Discontinuous Conduction Mode (DCM).
Dual-switching-frequency operation adaptively selects
the operational frequency between 85 kHz and 140 kHz
according to the line voltage. As a result, the
transformer can be fully utilized and high efficiency is
maintained over entire line range. A frequency-hopping
function is incorporated to reduce EMI noise.
Line voltage information through transformer auxiliary
winding is used for dual-switching frequency selection
and line voltage CC correction.
mWSaver® technology, including high-voltage startup
and ultra-low operating current in Burst Mode, enables
system compliance with Energy Star’s 5-star
requirement of <30 mW standby power consumption.
while COMI is saturated to HIGH level. During CC
regulation, COMI determines the duty cycle while
COMV is saturated to HIGH level.
VBLK
Vo
PWM Control
Block
GATE
Slope
Compenastion
ZCOMP
COMV
CS
AV
VSAW
Σ
FB
COMI
IO Estimator
VS
Zero Current
Detector
Figure 19. Simplified CV / CC PWM Control Circuit
CV
CC
COMI
Protections such as VDD Over-Voltage Protection (VDD
OVP), VS Over-Voltage Protection (VS OVP), internal
Over-Temperature Protection (OTP), and brownout
protection improve reliability.
All these innovative technologies allow the FAN501A to
offer low total cost, reduced component counts, small
size / weight, high conversion efficiency, and high power
density for compact charger / adapter applications
requiring CV / CC control.
CV / CC PWM Operation Principle
Figure 19 shows a simplified CV / CC PWM control
circuit of the FAN501A. The Constant Voltage (CV)
regulation is implemented in the same manner as the
conventional isolated power supply, where the output
voltage is sensed using a voltage divider and compared
with the internal reference of the shunt regulator to
generate a compensation signal. The compensation
signal is transferred to the primary side through an opto-
coupler and scaled down by attenuator AV to generate a
COMV signal. This COMV signal is applied to the PWM
comparator to determine the duty cycle.
The Constant Current (CC) regulation is implemented
internally with primary-side control. The output current
estimator calculates the output current using the
transformer primary-side current and diode current
discharge time. By comparing the estimated output
current with internal reference signal, a COMI signal is
generated to determine the duty cycle.
These two control signals, COMV and COMI, are
compared with an internal sawtooth waveform (VSAW) by
two PWM comparators to determine the duty cycle.
Figure 20 illustrates the outputs of two comparators
,combined with an OR gate, to determine the MOSFET
turn-off instant. Of COMV and COMI, the lower signal
determines the duty cycle. As shown in Figure 20,
during CV regulation, COMV determines the duty cycle
COMV
VSAW
GATE
Figure 20. PWM Operation for CV / CC Regulation
Primary-Side Constant Current Operation
Figure 21 and Figure 22 show the key waveforms of a
flyback converter operating in DCM and CCM,
respectively. The output current of each mode is
estimated by calculating the average of output diode
current over one switching cycle:
𝐼_𝑂 =< 𝐼_𝐷 >_{𝑡𝑆} =
𝑡_𝑆
0
𝐼_𝐷
𝑡
𝑡_𝑆
𝑑𝑡
=
[𝐼_𝐷 ]_{𝐴𝑅𝐸𝐴}
𝑡_𝑆
(1)
The area of output diode current in both DCM and CCM
operation can be expressed in a same form, as a
product of diode current discharge time (tDIS) and diode
current at the middle of diode discharge (ID-Mid), such as:
[𝐼_𝐷]_{𝐴𝑅𝐸𝐴} = 𝐼_{𝐷−𝑀𝑖𝑑} ∙ 𝑡_{𝐷𝐼𝑆}
(2)
In steady state, ID_Mid can be expressed as:
𝐼_{𝐷−𝑀𝑖𝑑}
=
𝐼_{𝐷𝑆 _𝑀𝑖𝑑}
∙
𝑁_𝑃
𝑁_𝑆
(3)
where IDS_Mid is primary-side current at the middle of
MOSFET conduction time and NP/NS is primary-to-
secondary turn ratio.
© 2014 Fairchild Semiconductor Corporation
FAN501A • Rev. 1.0.0
9
www.fairchildsemi.com