TOP243F datasheet(30/52 Pages) POWERINT

30 / 52 page

TOP242-250

12/04

frequency radiated noise (for example, video noise sensitive

applications such as VCR, DVD, monitor, TV, etc.), operating

at 66 kHz will reduce snubber loss resulting in better efﬁciency.

Also, in applications where transformer size is not a concern,

use of the 66 kHz option will provide lower EMI and higher

efﬁciency. Note that the second harmonic of 66 kHz is still

below 150 kHz, above which the conducted EMI speciﬁcations

get much tighter.

For 10 W or below, it is possible to use a simple inductor in

place of a more costly AC input common mode choke to meet

worldwide conducted EMI limits.

Transformer Design

It is recommended that the transformer be designed for

maximum operating ﬂux density of 3000 Gauss and a peak ﬂux

densityof4200Gaussatmaximumcurrentlimit. Theturnsratio

should be chosen for a reﬂected voltage (V

OR) no greater than

135 V when using a Zener clamp, or 150 V (max) when using

an RCD clamp with current limit reduction with line voltage

(overload protection).

For designs where operating current is signiﬁcantly lower than

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

setcurrentlimitclosetotheoperatingpeakcurrenttoreducepeak

ﬂux density and peak power (see Figures 20 and 34). In most

applications,thetightercurrentlimittolerance,higherswitching

frequency and soft-start features of TOPSwitch-GX contribute

to a smaller transformer when compared to TOPSwitch-II.

Standby Consumption

Frequency reduction can signiﬁcantly reduce power loss at

light or no 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 changing from 132 kHz in normal

operation to 66 kHz under light load conditions.

TOPSwitch-GX

Layout Considerations

As TOPSwitch-GX has additional pins and operates at

much higherpowerlevels compared to previous TOPSwitch

families, the following guidelines should be carefully

followed.

Primary Side Connections

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

of the input ﬁlter capacitor for the TOPSwitch-GX 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, LINE-

SENSE or EXTERNAL CURRENT LIMIT pins should

also be located closely between their respective pin and

SOURCE. Once again, the SOURCE connection trace of these

components should not be shared by the main MOSFET

switching currents. It is very critical that SOURCE pin

switching currents are returned to the input capacitor negative

terminal through a seperate trace that is not shared by the

components connected to CONTROL, MULTI-FUNCTION,

LINE-SENSE or EXTERNAL CURRENT LIMIT pins. This

is because the SOURCE pin is also the controller ground

reference pin.

Any traces to the M, L or X pins should be kept as short as

possible and away from the DRAIN trace to prevent noise

coupling. LINE-SENSE resistor (R1 in Figures 47-49) should

be located close to the M or L pin to minimize the trace length

on the M or L pin side.

In addition to the 47 µF CONTROL pin capacitor, a high

frequency 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-GX.

Y-Capacitor

The Y-capacitor should be connected close to the secondary

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

the transformer.

Heat Sinking

The tab of the Y package (TO-220) or F package (TO-262)

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 primary ground/source nodes on the

PC board.

When using a P (DIP-8), G (SMD-8) or R (TO-263) package,

a copper area underneath the package connected to the

SOURCE pins will act as an effective heat sink. On double

sided boards (Figure 49), top side and bottom side areas

connected with vias can be used to increase the effective heat

sinking area.

In addition, sufﬁcient copper area should be provided at

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

sinking.

In Figures 47, 48 and 49, a narrow trace is shown between

the output rectiﬁer and output ﬁlter capacitor. This trace acts

as a thermal relief between the rectiﬁer and ﬁlter capacitor to

prevent excessive heating of the capacitor.

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