TOP232G datasheet(23/36 Pages) POWERINT

23 / 36 page

TOP232-234

7/01

using a Zener clamp, 165 V when using an RCD clamp and

185 V when using RCD clamp with current limit feed forward.

For designs where operating current is significantly lower than

the default current limit, it is recommended to use an externally

set current limit close to the operating peak current to reduce

peak flux density and peak power (see Figure 17). In most

applications, the tighter current limit tolerance, higher switching

frequency and soft-start features of TOPSwitch-FX contribute

to a smaller transformer when compared to TOPSwitch-II.

Standby Consumption

Cycle skipping can significantly reduce power loss at zero load,

especially when a Zener clamp is used. For very low secondary

power consumption use a TL431 regulator for feedback control.

Alternately, switching losses can be significantly reduced by

switching from 132 kHz in normal operation to 66 kHz under

light load conditions.

TOPSwitch-FX Layout Considerations

Primary Side Connections

Use a single point (Kelvin) connection at the negative terminal

of the input filter capacitor for TOPSwitch-FX SOURCE pin

and bias winding return. This improves surge capabilities by

returning surge currents from the bias winding directly to the

input filter capacitor.

The CONTROL pin bypass capacitor should be located as close

as possible to the SOURCE and CONTROL pins and its

SOURCE connection trace should not be shared by the main

MOSFET switching currents.

All SOURCE pin referenced components connected to the

MULTI-FUNCTION pin should also be located close to

SOURCEandMULTI-FUNCTIONpinswithdedicatedSOURCE

pin connection. The MULTI-FUNCTION pin's trace should be

kept as short as possible and away from the DRAIN trace to

prevent noise coupling. Line sense resistor (R1 in Figures 29 and

30) should be located close to the MULTI-FUNCTION pin to

minimize the trace length on the MULTI-FUNCTION pin side.

Inadditiontothe47

µFCONTROLpincapacitor,ahighfrequency

bypass capacitor in parallel may be used for better noise immunity.

The feedback optocoupler output should also be located close to

the CONTROL and SOURCE pins of TOPSwitch-FX.

Y-Capacitor

The Y-capacitor should be connected close to the secondary

output return pin(s) and the primary DC input pin of the

transformer (see Figures 29 and 30).

Heat Sinking

The tab of the Y package (TO-220) is internally electrically

tied to the SOURCE pin. To avoid circulating currents, a heat

sink attached to the tab should not be electrically tied to any

nodes on the PC board.

When using P (DIP-8) or G (SMD-8) packages, a copper area

underneath the package connected to the SOURCE pins will act

as an effective heat sink.

In addition, sufficient copper area should be provided at the

anode and cathode leads of the output diode(s) for heat sinking.

Quick Design Checklist

As with any power supply design, all TOPSwitch-FX designs

should be verified on the bench to make sure that components

specifications are not exceeded under worst case conditions.

The following minimum set of tests is strongly recommended:

1. Maximum drain voltage – Verify that peak V

DS does not

exceed 675 V at highest input voltage and maximum overload

output power. Maximum overload output power occurs

when the ouput is overloaded to a level just before the power

supply goes into auto-restart (loss of regulation).

2. Maximum drain current – At maximum ambient temperature,

maximum input voltage and maximum output load, verify

drain current waveforms at start-up for any signs of

transformer saturation and excessive leading edge current

spikes. TOPSwitch-FX has a leading edge blanking time of

200 ns to prevent premature termination of the on-cycle.

Verify that the leading edge current spike is below the

allowed current limit envelope (see Figure 33) for the drain

current waveform at the end of the 200 ns blanking period.

3. Thermal check – At maximum output power, minimum

input voltage and maximum ambient temperature, verify

that temperature specifications are not exceeded for

TOPSwitch-FX, transformer, output diodes and output

capacitors. Enough thermal margin should be allowed for

the part-to-part variation of the R

DS(ON) of TOPSwitch-FX as

specified in the data sheet. The margin required can either

be calculated from the tolerances or it can be accounted for

by connecting an external resistance in series with the

DRAIN pin and attached to the same heatsink, having a

resistance value that is equal to the difference between the

measured R

DS(ON) of the device under test and the worst case

maximum specification.

Design Tools

1. Technical literature: Data Sheet, Application Notes,

Design Ideas, etc.

2. Transformer design spreadsheet.

3. Engineering prototype boards.

Up to date information on design tools can be found at Power

Integrations Web site: www.powerint.com

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