Datasheet

TOP242-250
29
I
4/03
Table 5 (cont). Comparison Between TOPSwitch-FX and TOPSwitch-GX. *Not available
TOPSwitch-GX Design Considerations
Power Table
Data sheet power table (Table 1) represents the maximum practi-
cal continuous output power based on the following conditions:
TOP242 to TOP246: 12 V output, Schottky output diode, 150 V
reflected voltage (V
OR
) and efficiency estimates from curves con-
tained in application note AN-29. TOP247 to TOP250: Higher
output voltages, with a maximum output current of 6 A.
For all devices, a 100 VDC minimum for 85-265 VAC and
250 VDC minimum for 230 VAC are assumed and sufficient heat
sinking to keep device temperature ≤100 °C. Power
levels shown in the power table for the R package device
assume 6.45 cm
2
of 610 g/m
2
copper heat sink area in an
enclosed adapter, or 19.4 cm
2
in an open frame.
TOPSwitch-GX Selection
Selecting the optimum TOPSwitch-GX depends upon required
maximum output power, efficiency, heat sinking constraints
and cost goals. With the option to externally reduce current
limit, a larger TOPSwitch-GX may be used for lower power
applications where higher efficiency is needed or minimal heat
sinking is available.
Function TOPSwitch-FX TOPSwitch-GX TOPSwitch-GX Advantages
P/G Package Current Identical to Y TOP243P or G and • Matches device current limit to
Limits packages TOP244P or G internal package dissipation capability
current limits reduced • Allows more continuous design to
lower device dissipation (lower RMS
currents)
Y/R/F Package Current 100% (R and F 90% (for equivalent R
DS (ON)
)•Minimizes transformer core size
Limits package N/A*) • Optimizes efficiency for most
applications
Thermal Shutdown 125 °C min. 130 °C min. 75 °C•Allows higher output powers in
70 °C hysteresis hysteresis high ambient temperature
applications
Maximum Duty Cycle 90 µA 60 µA•Reduces output line frequency
Reduction Threshold ripple at low line
•D
MAX
reduction optimized for
forward design
Line Under-Voltage N/A* 40% of positive (turn-on) • Provides a well defined turn-off
Negative (turn-off) threshold threshold as the line voltage falls
Threshold
Soft-Start 10 ms (duty cycle) 10 ms (duty cycle + current • Gradually increasing current limit
limit) in addition to duty cycle during soft-
start further reduces peak current
and voltage
• Further reduces component
stresses during start up
Input Capacitor
The input capacitor must be chosen to provide the minimum DC
voltage required for the TOPSwitch-GX converter to maintain
regulation at the lowest specified input voltage and maximum
output power. Since TOPSwitch-GX has a higher DC
MAX
than
TOPSwitch-II, it is possible to use a smaller input capacitor.
For TOPSwitch-GX, a capacitance of 2 µF per watt is possible
for universal input with an appropriately designed transformer.
Primary Clamp and Output Reflected Voltage V
OR
A primary clamp is necessary to limit the peak TOPSwitch-GX
drain to source voltage. A Zener clamp requires few parts and
takes up little board space. For good efficiency, the clamp
Zener should be selected to be at least 1.5 times the output
reflected voltage V
OR
as this keeps the leakage spike conduc-
tion time short. When using a Zener clamp in a universal input
application, a V
OR
of less than 135 V is recommended to allow
for the absolute tolerances and temperature variations of the
Zener. This will ensure efficient operation of the clamp circuit
and will also keep the maximum drain voltage below the rated
breakdown voltage of the TOPSwitch-GX MOSFET.