TOP232-234
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
(guaranteedminimumvalueof75%vs. 64%forTOPSwitch-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
jitterprovidesimprovedmarginforconductedEMImeetingthe
CISPR 22 (FCC B) specification.
Application Examples
A High Efficiency, 30 W, Universal Input Power Supply
ThecircuitshowninFigure24takesadvantageofseveralofthe
TOPSwitch-FXfeaturestoreducesystemcostandpowersupply
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
approximately70%ofthedefaultcurrentlimit. Thisallowsuse
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
signalthatreducesthecurrentlimitwithincreasinglinevoltage,
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 ofTOPSwitch-FX, allows the
use of a low cost RCD clamp (R3, C3 and D1) with a higher
A simple Zener sense circuit is used for low cost. The output
voltage is determined by the Zener diode (VR2) voltage and the
voltagedropsacrosstheoptocoupler (U2)LEDandresistorR6.
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
L3
3.3 µH
12 V
@ 2.5 A
R3
68 kΩ
2W
C3
4.7 nF
1KV
C12
220 µF
35 V
D8
MBR1060
C10
560 µF
35 V
C11
560 µF
35 V
BR1
600 V
2A
RTN
D1
UF4005
D2
1N4148
R1
4.7 MΩ
1/2 W
R6
150 Ω
L1
20 mH
R8
C6
100 nF
150 Ω
T1
C1
U2
LTV817A
CX1
100 nF
68 µF
D
S
M
TOPSwitch-FX
400 V
U1
CONTROL
250 VAC
C
TOP234Y
R5
6.8 Ω
R2
F
F1
3.15 A
VR2
1N5240C
10 V, 2%
9.09 kΩ
J1
L
C5
47 µF
10 V
N
PI-2525-040401
Figure 24. 30 W Power Supply using External Current Limit.
B
7/01
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PAM [ POWER ANALOG MICOELECTRONICS ]
