TOP232 datasheet(20/38 Pages) POWERINT

20 / 38 page

Rev. D 10/20

TOP232-234

www.power.com

TOPSwitch-FX Design Considerations

TOPSwitch-FX Selection

Selecting the optimum TOPSwitch-FX depends upon required

maximum output power, efficiency, heat sinking constraints

and cost goals. With the option to externally reduce current

limit, a larger TOPSwitch-FX 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-FX converter to

maintain regulation at the lowest specified input voltage and

maximum output power. Since TOPSwitch-FX has a higher

MAX than TOPSwitch-II, it is possible to use a smaller input

capacitor. For TOPSwitch-FX, a capacitance of 2

µF per watt

is usually sufficient 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-FX

drain to source voltage. A Zener clamp (see Figure 26, VR1)

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 conduction 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-FX MOSFET.

A high V

OR is required to take full advantage of the wider DCMAX

of TOPSwitch-FX. An RCD clamp provides tighter clamp

voltage tolerance than a Zener clamp and allows a V

OR as high

as 165 V. The V

OR can be further increased in continuous

mode designs up to 185 V by reducing the external current

limit as a function of input line voltage (see Figure 18). The

RCD clamp is more cost effective than the Zener clamp but

requires more careful design (see quick design checklist).

*Not available

Table 3 (cont). Comparison Between TOPSwitch-II and TOPSwitch-FX.

Function

TOPSwitch-II

TOPSwitch-FX

Figures Advantages

Remote ON/OFF

N/A*

Single transistor

8, 19,

• Fast on/off (cycle by cycle)

or optocoupler

20, 21,

• Active-on or active-off control

interface or manual

22, 23,

• Low consumption in remote off state

switch

• Active-on control for fail-safe

• Eliminates expensive in-line on/off

switch

• Allows processor controlled turn on/off

• Permits shutdown/wake-up of

peripherals via LAN or parallel port

Synchronization

N/A*

Single transistor

• Synchronization to external lower

or optocoupler

frequency signal

interface

• Starts new switching cycle on demand

Thermal Shutdown

Latched

Hysteretic (with

• Automatic recovery from thermal fault

°C hysteresis)

• Large hysteresis prevents circuit

board overheating

Current Limit Tolerance

±10% (@25

°C)

±7% (@25

°C)

• 10% higher power capability due to

-8% (0

°C to100 °C)

-4% (0

°C to 100 °C)

tighter tolerance

DRAIN

DIP

0.037” / 0.94 mm

0.137” / 3.48 mm

• Greater immunity to arcing as a

Creepage at

SMD

0.037” / 0.94 mm

0.137” / 3.48 mm

result of build-up of dust, debris and

Package

TO-220 0.046” / 1.17 mm

0.068” / 1.73 mm

other contaminants

DRAIN Creepage at

0.045” / 1.14 mm

0.113” / 2.87 mm

• Preformed leads accommodate

PCB for TO-220

(preformed leads)

large creepage for PCB layout

• Easier to meet Safety (UL/VDE)

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