TOP232G datasheet(21/36 Pages) PAM

21 / 36 page

TOP232-234

7/01

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

heat sinking, air circulation, etc.). The higher DC

MAX of

TOPSwitch-FX 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 24. A

12 V, 30 W design using a 60 V Schottky for the output diode).

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, which causes the

primary current and 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-FX 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 28, the benefits of jitter increase

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

frequency deviation.

The FREQUENCY pin of TOPSwitch-FX 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

2nd

3rd

4th

5th

Switching Harmonic

Quasi-Peak

Average

-20

-10

0.15

Frequency (MHz)

VFG243B (QP)

VF646B (AV)

TOPSwitch-II (no jitter)

VFG243B (QP)

VF646B (AV)

-20

-10

0.15

Frequency (MHz)

TOPSwitch-FX (with jitter)

(a)

(b)

(c)

Figure 28. (a) Conducted noise improvement for low frequency

harmonics due to jitter, (b) TOPSwitch-II full range EMI

scan (100kHz, no jitter), (c) TOPSwitch-FX full range

EMI scan (132 kHz, with jitter) with identical circuitry

and conditions.

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