TOP234G-TL Datasheet, PDF (14/36 Pages) Power Integrations, Inc. – nullDesign Flexible, EcoSmart, Integrated Off-line Switcher

TOP232-234
Application Examples
A High Efficiency, 30 W, Universal Input Power Supply
The circuit shown in Figure 24 takes advantage of several of the
TOPSwitch-FX features to reduce system cost and power supply
size and to improve efficiency. This design delivers 30 W at
12 V, from an 85 to 265 VAC input, at an ambient of 50 ˚C, in
an open frame configuration. A nominal efficiency of 80% at
full load is achieved using TOP234.
The current limit is externally set by resistors R1 and R2 to a
value just above the low line operating peak current of
approximately 70% of the default current limit. This allows use
of a smaller transformer core size and/or higher transformer
primary inductance for a given output power, reducing
TOPSwitch-FX power dissipation, while at the same time
avoiding transformer core saturation during startup and output
transient conditions. The resistor R1 provides a feed forward
signal that reduces the current limit with increasing line voltage,
which, in turn, limits the maximum overload power at high
input line voltage. The feed forward function in combination
with the built-in soft-start feature of TOPSwitch-FX, allows the
use of a low cost RCD clamp (R3, C3 and D1) with a higher
reflected voltage, by safely limiting the TOPSwitch-FX drain
voltage, with adequate margin, under worst case conditions.
The extended maximum duty cycle feature of TOPSwitch-FX
(guaranteed minimum value of 75% vs. 64% for TOPSwitch-II)
allows the use of a smaller input capacitor (C1). The extended
maximum duty cycle and the higher reflected voltage possible
with the RCD clamp also permit the use of a higher primary to
secondary turns ratio for T1 which reduces the peak reverse
voltage experienced by the secondary rectifier D8. As a result,
a 60 V Schottky rectifier can be used for up to 15 V outputs,
which greatly improves power supply efficiency. The cycle
skipping feature of the TOPSwitch-FX eliminates the need for
any dummy loading for regulation at no load and reduces the no
load/standby consumption of the power supply. Frequency
jitter provides improved margin for conducted EMI meeting the
CISPR 22 (FCC B) specification.
A simple Zener sense circuit is used for low cost. The output
voltage is determined by the Zener diode (VR2) voltage and the
voltage drops across the optocoupler (U2) LED and resistor R6.
Resistor R8 provides bias current to Zener VR2 for typical
regulation of ±5% at the 12 V output level, over line and load
and component variations.
CY1
2.2 nF
C14 R15
1 nF 150 Ω
BR1
600 V
2A
L1
20 mH
CX1
100 nF
250 VAC
C1
68 µF
400 V
F1
J1 _{3.15 A}
L
N
C3
4.7 nF
1KV
R3
68 kΩ
2W
D8
MBR1060
C10
560 µF
35 V
D1
UF4005
R1
4.7 MΩ
1/2 W
D2
1N4148
T1
C6
100 nF
D
U1
TOP234Y
R2
9.09 kΩ
S
M TOPSwitch-FX
CONTROL
C
R5
F
6.8 Ω
C5
47 µF
10 V
Figure 24. 30 W Power Supply using External Current Limit.
L3
3.3 µH
12 V
@ 2.5 A
C11
560 µF
35 V
C12
220 µF
35 V
RTN
R6
150 Ω
R8
150 Ω
U2
LTV817A
VR2
1N5240C
10 V, 2%
PI-2525-040401
14 B
7/01