TOP243F datasheet(30/52 Pages) POWERINT

30 / 52 page

TOP242-250

1/02

Figure 46b. TOPSwitch-GX Full Range EMI Scan (132 kHz,

with jitter) with Identical Circuitry and

Conditions.

A high V

OR is required to take full advantage of the wider DCMAX

of TOPSwitch-GX. An RCD clamp provides tighter clamp

voltage tolerance than a Zener clamp and allows a V

OR as high as

150 V. RCD clamp dissipation can be minimized by reducing

the external current limit as a function of input line voltage (see

Figure 21 and 35). The RCD clamp is more cost effective than

the Zener clamp but requires more careful design (see quick

design checklist).

Output Diode

The output diode is selected for peak inverse voltage, output

current, and thermal conditions in the application (including

heatsinking, air circulation, etc.). The higher DC

MAX of

TOPSwitch-GX along with an appropriate transformer turns ratio

can allow the use of a 60 V Schoktty diode for higher efficiency

on output voltages as high as 15 V (see Figure 41. A 12 V, 30 W

design using a 60 V Schottky for the output diode).

Bias Winding Capacitor

Due to the low frequency operation at no-load a 1

µF bias

winding capacitor is recommended.

Soft-Start

Generally a power supply experiences maximum stress at

start-up before the feedback loop achieves regulation. For a

period of 10 ms the on-chip soft-start linearly increases the duty

cycle from zero to the default DC

MAX at turn on. In addition, the

primary current limit increases from 85% to 100% over the same

period. This causes the output voltage to rise in an orderly

manner allowing time for the feedback loop to take control of the

duty cycle. This reduces the stress on the TOPSwitch-GX

MOSFET, clamp circuit and output diode(s), and helps prevent

transformer saturation during start-up. Also soft-start limits the

amount of output voltage overshoot, and in many applications

eliminates the need for a soft-finish capacitor.

EMI

The frequency jitter feature modulates the switching frequency

over a narrow band as a means to reduce conducted EMI peaks

associated with the harmonics of the fundamental switching

frequency. This is particularly beneficial for average detection

mode. As can be seen in Figure 46, the benefits of jitter increase

with the order of the switching harmonic due to an increase in

frequency deviation.

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

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

EN55022B (QP)

EN55022B (AV)

-20

-10

0.15

Frequency (MHz)

TOPSwitch-GX (with jitter)

Figure 46a. TOPSwitch-II Full Range EMI Scan

(100 kHz, no jitter).

-20

-10

0.15

Frequency (MHz)

EN55022B (QP)

EN55022B (AV)

TOPSwitch-II (no jitter)

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