TOP232 datasheet(21/38 Pages) POWERINT

21 / 38 page

Rev. D 10/20

TOP232-234

www.power.com

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

-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

(100 kHz, no Jitter), (c) TOPSwitch-FX Full Range EMI Scan

(132 kHz, with Jitter) with Identical Circuitry and Conditions.

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