TOP243FN datasheet(29/52 Pages) POWERINT

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TOP242-250

12/04

Input Capacitor

Theinputcapacitormustbechosentoprovidetheminimum DC

voltage required for the TOPSwitch-GX converter to maintain

regulation at the lowest speciﬁed input voltage and maximum

output power. Since TOPSwitch-GX has a higher DC

MAX than

TOPSwitch-II

, it is possible to use a smaller input capacitor.

ForTOPSwitch-GX,acapacitanceof2µFperwattispossiblefor

universal input with an appropriately designed transformer.

Primary Clamp and Output Reﬂected Voltage V

A primary clamp is necessary to limit the peak TOPSwitch-GX

drain to source voltage. A Zener clamp requires few parts and

takes up little board space. For good efﬁciency, the clamp

Zener should be selected to be at least 1.5 times the output

reﬂectedvoltageV

OR, as this keeps the leakage spike conduction

time short. When using a Zener clamp in a universal input

application, a V

OR of less than 135 V is recommended to allow

for the absolute tolerances and temperature variations of the

Zener. This will ensure efﬁcient operation of the clamp circuit

and will also keep the maximum drain voltage below the rated

breakdown voltage of the TOPSwitch-GX MOSFET.

AhighV

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

as150V. RCDclampdissipationcanbeminimizedbyreducing

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

Figures 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 Schottky 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

periodof10ms,theon-chipsoft-startlinearlyincreasestheduty

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

transformersaturationduringstart-up. Also,soft-startlimitsthe

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

-20

-10

0.15

Frequency (MHz)

EN55022B (QP)

EN55022B (AV)

TOPSwitch-II (no jitter)

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).

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

Jitter) with Identical Circuitry and Conditions.

TOP243FN 数据表(PDF) 29 Page - Power Integrations, Inc.

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