TOP264VG datasheet(19/36 Pages) POWERINT

19 / 36 page

Rev. B 03/10

TOP264-271

www.powerint.com

6. Power levels shown in the power table for the V package

device assume 6.45 cm2 of 610 g/m2 copper heat sink area

in an enclosed adapter, or 19.4 cm2 in an open frame.

The provided peak power depends on the current limit for the

respective device.

TOP264-271 Selection

Selecting the optimum TOP264-271 depends upon required

maximum output power, efficiency, heat sinking constraints,

system requirements and cost goals. With the option to

externally reduce current limit, TOP264-271 may be used for

lower power applications where higher efficiency is needed or

minimal heat sinking is available.

Input Capacitor

The input capacitor must be chosen to provide the minimum

DC voltage required for the TOP264-271 converter to maintain

regulation at the lowest specified input voltage and maximum

output power. Since TOP264-271 has a high DC

MAX limit and an

optimized dual slope line feed forward for ripple rejection, it is

possible to use a smaller input capacitor. For TOP264-271, a

capacitance of 2

mF 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 TOP264-271 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 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 TOP264-271 MOSFET. A high V

OR is required to take full

advantage of the wider DC

MAX of TOP264-271. An RCD (or

RCDZ) 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 18). 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 TOP264-271,

along with an appropriate transformer turns ratio, can allow the

use of a 80 V Schottky diode for higher efficiency on output

voltages as high as 15 V.

Bias Winding Capacitor

Due to the low frequency operation at no-load, a bias winding

capacitance of 10

mF minimum is recommended. Ensure a

minimum bias winding voltage of >9 V at zero load for correct

operation and output voltage regulation.

Soft-Start

Generally, a power supply experiences maximum stress at

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

period of 17 ms, the on-chip soft-start linearly increases the

drain peak current and switching frequency from their low

starting values to their respective maximum values. 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 TOP264-271 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. Note that as soon as the

loop closes the soft-start function ceases even if this is prior to

the end of the 17 ms soft-start period.

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

TOPSwitch-HX vs. TOPSwitch-JX

Function

TOPSwitch-HX

TOPSwitch-JX

TOPSwitch-JX Advantages

CONTROL current I

C(OFF) at

0% duty cycle

C(OFF) = IB + 3.4 mA

(TOP256-258)

B = External bias current

C(OFF) = IB + 1.6 mA

(TOP266-268)

•

Reduced CONTROL current

•

Better no-load performance (<0.1 W)

•

Better standby performance

eDIP-12 package

Not available

Available

•

66/132 kHz frequency option for DIP style heatsink

less designs

•

Better thermal performance for increased power

capability over DIP-8 package

Breakdown voltage BV

DSS

Min. 700 V at T

J = 25 °C

Min. 725 V at T

J = 25 °C

•

Simplifies meeting customer derating requirements

(e.g. 80%)

•

Extended line surge withstand

Fast AC reset

3 External transistor circuits

using the V pin

1 External transistor circuit

using the X pin

•

Saves 5 components

Table 4.

Comparison Between TOPSwitch-HX and TOPSwitch-JX.

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