APPLICATION NOTE
A 500W HIGH POWER FACTOR WITH THE L4981A
CONTINUOUS MODE IC
The widespread use of passive AC/DC off-line converters causes low power factor and high line
current harmonic distortion. To reduce these phenomena and to comply with relevant regulatory
agency requirements , designers are employing active power factor correction in their off-line SMPS
applications. This paper describes a practical, low cost and easy to implement 500W power factor
corrected application that employs the L4981A Continuous Mode PFC IC.
INTRODUCTION
Reduction of line current harmonic distortion and
improvement of power factor is of great concern
to many designers of off-line switched mode
power supplies. This concern has been moti-
vated by present and impending regulatory re-
quirements regarding line current harmonics. The
reasons for improving power factor and reducing
line current harmonic distortion are well known
and understood. Active power factor correction
using the boost topology and operating in the
continuous inductor current control mode is an
excellent method to comply with these require-
ments and is well accepted in the industry.
This paper will present a practical power factor
corrected design for a 500 Watt output and uni-
versal mains input application. The detailed deri-
vations of all power, IC biasing and control com-
ponent values and types will be shown. The
evaluation results from an actual working de-
moboard will be presented as well as several
relevant oscillograms.
DESIGN SPECIFICATIONS
The design specifications given below are real-
ized by the implementation of a functional de-
moboard.
The design target specifications are as follows:
- Universal mains input AC voltage
V
irms
= 88Vac to 264Vac, 60/50Hz
- DC regulated output voltage V
out
= 400Vdc
- Full load output ripple voltage
∆V
ripple
=
±8V
- Rated output power P
out
= 500W
- Maximum output overvoltage V
omax
= 450V
- Switching frequency f
sw
= 80kHz
- Maximum inductor current ripple
∆I
L
= 23%
- Input power factor PF > 0.99
- Input line current total harmonic distortion <5%
To meet these specifications, the selection of
component values and material types is very im-
portant. The next sections will describe the com-
ponent selection criteria along with some critical
derivations. For detailed explanations on the con-
troller operation and pin description, refer to Ap-
plication Note AN628
Designing A High Power
Factor Switching Preregulator With The L4981
Continuous Mode [1]
and the corresponding
Datasheet
L4981A/B Power Factor Corrector [2].
POWER COMPONENTS SELECTION
The power component values and types are de-
rived and selected in the next section. Please re-
fer to Figure 2, 500 Watt Demoboard Schematic.
Input Diode Bridge
The input diode bridge, D1, can be a standard
slow-recovery type. The selection criteria include
the maximum peak reverse breakdown voltage,
maximum forward average current, maximum
surge current and thermal considerations.
Maximum peak reverse voltage:
V
prv
= V
irmsmax
⋅ √ ⋅
1.2 (safety margin) =
2
= 264V
⋅ ⋅
1.2 = 448V
√
2
Therefore use a 600V rated diode.
Maximum forward average current:
I
rmsmax
=
P
OUT
V
rms min
⋅
n
=
500
= 6.31A
88
⋅
0.9
I
fave
=
I
rmsmax
⋅ √
2
6.31
⋅ √
2
=
= 2.84A
π
π
The thermal considerations require the I
fave
rating
to be significantly higher than the value calcu-
lated. The part chosen has a I
fave
of 25A. Addi-
1/16
AN827/1297
STMICROELECTRONICS [ STMICROELECTRONICS ]
