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TOP242-249

Function

TOPSwitch-FX

TOPSwitch-GX

TOPSwitch-GX Advantages

P/G Package Current

Limits

Y/R Package Current

Limits

Thermal Shutdown

Maximum Duty Cycle

Reduction Threshold

Line Under-Voltage

Negative (turn-off)

Threshold

Soft-Start

Identical to Y

packages

TOP243P or G and

TOP244P or G internal

current limits reduced

100%

90% (for equivalent RDS )(ON)

(R package N/A*)

125 °C min.

130 °C min. 75 °C

70 °C hysteresis hysteresis

90 µA

60 µA

N/A*

40% of positive (turn-on)

threshold

• Matches device current limit to

package dissipation capability

• Allows more continuous design to

lower device dissipation (lower RMS

currents)

• Minimizes transformer core size

• Optimizes efficiency for most

applications

• Allows higher output powers in

high ambient temperature

applications

• Reduces output line frequency

ripple at low line

• DMAX reduction optimized for

forward design

• Provides a well defined turn-off

threshold as the line voltage falls

10 ms (duty cycle) 10 ms (duty cycle + current

limit)

• Gradually increasing current limit

in addition to duty cycle during soft-

start further reduces peak current

and voltage

• Further reduces component

stresses during start up

Table 5 (cont). Comparison Between TOPSwitch-FX and TOPSwitch-GX. *Not available

TOPSwitch-GX Design Considerations

Power Table

Datasheet power table represents the maximum practical

continuous output power based on the following conditions:

TOP242 to TOP246: 12 V output, Schottky output diode,

150 V reflected voltage (VOR) and efficiency estimates from

curves contained in application note AN-29. TOP247 to TOP249:

Higher output voltages used 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 VDC 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 cm2 of 610 g/m2 copper heat sink area in an enclosed

adapter, or 19.4 cm2 in an open frame.

TOPSwitch-GX Selection

Selecting the optimum TOPSwitch-GX depends upon required

maximum output power, efficiency, heat sinking constraints

anwd wcowst .gDoaalst.aWSihthetheet4opUti.ocnotomexternally 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.

Input Capacitor

The input capacitor must be chosen to provide the minimum

DC voltage required for the TOPSwitch-GXconverter to maintain

regulation at the lowest specified input voltage and maximum

output power. Since TOPSwitch-GX has a higher DCMAX 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 VOR

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 VORas this keeps the leakage spike conduction

time short. When using a Zener clamp in a universal input

application, a VOR 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.

August 8, 2000

29E

7/01