TOP242-250
TOPSwitch-GX (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
powersupplyefficiency.Thefrequencyreductionfeatureofthe
TOPSwitch-GX 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.
Application Examples
A High Efficiency, 30 W, Universal Input Power Supply
The circuit shown in Figure 41 takes advantage of several of
the TOPSwitch-GX features to reduce system cost and power
supply size and to improve efficiency. This design delivers
30 W at 12 V, from an 85 VAC to 265 VAC input, at an ambient
of 50 °C, in an open frame configuration. Anominal efficiency
of 80% at full load is achieved using TOP244Y.
The current limit is externally set by resistors R1 and R2 to a
value just above the low line operating peak DRAIN 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-GX power dissipation, while at the same
time avoiding transformer core saturation during startup and
output transient conditions. The resistors R1 & R2 provide a
signalthatreducesthecurrentlimitwithincreasinglinevoltage,
which in turn limits the maximum overload power at high input
line voltage. This function in combination with the built-in
soft-startfeatureofTOPSwitch-GX,allowstheuseofalowcost
RCD clamp (R3, C3 and D1) with a higher reflected voltage,
by safely limiting the TOPSwitch-GX drain voltage, with
adequate margin under worst case conditions. Resistor R4
provides line sensing, setting UV at 100 VDC and OV at
450 VDC. The extended maximum duty cycle feature of
Output regulation is achieved by using a simple Zener sense
circuit 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.
AHigh Efficiency, Enclosed, 70 W, UniversalAdapter Supply
The circuit shown in Figure 42 takes advantage of several of the
TOPSwitch-GX features to reduce cost, power supply size and
PERFORMANCE SUMMARY
CY1
2.2 nF
Output Power:
Regulation:
Efficiency:
Ripple:
30 W
± 4%
≥ 79%
≤ 50 mV pk-pk
C14 R15
1 nF
150 Ω
L3
3.3 µH
12 V @
2.5 A
R3
68 kΩ
2 W
C3
4.7 nF
1 kV
C12
220 µF
35 V
D8
MBR1060
C10
560 µF
35 V
C11
560 µF
35 V
BR1
600 V
2A
D1
UF4005
RTN
D2
1N4148
R4
2 MΩ
1/2 W
R6
150 Ω
L1
20 mH
R8
150 Ω
C6
0.1 µF
R1
4.7 MΩ
1/2 W
T1
C1
U2
68 µF
400 V
CX1
100 nF
250 VAC
TOPSwitch-GX
LTV817A
D
S
L
U1
TOP244Y
CONTROL
C
R5
6.8 Ω
X
F
F1
3.15 A
VR2
1N5240C
10 V, 2%
J1
R2
9.09 kΩ
L
C5
47 µF
10 V
N
PI-2657-081204
Figure 41. 30 W Power Supply using External Current Limit Programming and Line Sensing for UV and OV.
M
20 12/04
POWERINT [ Power Integrations ]
