TOP243F datasheet(29/54 Pages) POWERINT

29 / 54 page

Rev. R 10/20

TOP242-250

www.power.com

Input Capacitor

The input capacitor must be chosen to provide the minimum

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. For TOPSwitch-GX, a capacitance

of 2

µF per watt is possible for 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 reflected voltage V

OR, as this

keeps the leakage spike conduction time short. When

using a Zener clamp in a universal input application, a

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.

A high V

OR is required to take full advantage of the wider

MAX 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 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

heat sinking, 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 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.

-20

-10

0.15

Frequency (MHz)

EN55022B (QP)

EN55022B (AV)

EN55022B (QP)

EN55022B (AV)

-20

-10

0.15

Frequency (MHz)

TOPSwitch-HX (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.

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