TOP250F datasheet(30/54 Pages) POWERINT

30 / 54 page

Rev. R 10/20

TOP242-250

www.power.com

The FREQUENCY pin of TOPSwitch-GX offers a

switching frequency option of 132 kHz or 66 kHz. In

applications that require heavy snubbers on the drain

node for reducing high 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 density of 4200 Gauss at maximum current

limit. The turns ratio 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 set current limit close to the operating peak

current to reduce peak flux density and peak power

(see Figures 20 and 34). In most applications, the tighter

current limit tolerance, higher switching 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 higher power levels 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.

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