TOP243F datasheet(29/52 Pages) POWERINT

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TOP242-250

1/02

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 (V

OR) and efficiency estimates from curves

contained in application noteAN-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

and cost goals. With the option to externally reduce current

limit, a larger TOPSwitch-GX may be used for lower power

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

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-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

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.

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