TOP261YN datasheet(26/49 Pages) POWERINT

26 / 49 page

Rev. F 01/09

TOP252-262

www.powerint.com

TOPSwitch-HX Design Considerations

Power Table

The data sheet power table (Table 1) represents the maximum

practical continuous output power based on the following

conditions:

12 V output.

Schottky or high efﬁciency output diode.

135 V reﬂected voltage (V

OR) and efﬁciency estimates.

A 100 VDC minimum for 85-265 VAC and 250 VDC mini-

mum for 230 VAC.

Sufﬁcient heat sinking to keep device temperature ≤100

°C.

Power levels shown in the power table for the M/P 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.

TOPSwitch-HX Selection

Selecting the optimum TOPSwitch-HX depends upon required

maximum output power, efﬁciency, heat sinking constraints,

system requirements and cost goals. With the option to

externally reduce current limit, an Y, E/L or M package

TOPSwitch-HX 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 TOPSwitch-HX converter to

maintain regulation at the lowest speciﬁed input voltage and

maximum output power. Since TOPSwitch-HX has a high

MAX limit and an optimized dual slope line feed forward for

ripple rejection, it is possible to use a smaller input capacitor.

For TOPSwitch-HX, 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-HX

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

TOPSwitch-HX MOSFET. A high V

OR is required to take full

advantage of the wider DC

MAX of TOPSwitch-HX.

An RCD

clamp provides tighter clamp voltage tolerance than a Zener

clamp and allows a VOR 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 23 and 36).

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-HX, 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 (see Figure 41).

Bias Winding Capacitor

Due to the low frequency operation at no-load, a 10

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 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 TOPSwitch-HX 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 45, the beneﬁts of jitter increase

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

frequency deviation. Devices in the P, G or M package and

TOP259-261YN operate at a nominal switching frequency of

66 kHz. The FREQUENCY pin of devices in the TOP254-258 Y

and E packages offer a switching frequency option of 132 kHz or

66 kHz. In applications that require heavy snubber on the drain

node for reducing high frequency radiated noise (for example,

video noise sensitive applications such as VCRs, DVDs, monitors,

TVs, 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 using a Zener clamp or 150 V (max) when

using an RCD clamp with current limit reduction with line

voltage (overload protection). For designs where operating

current is signiﬁcantly lower than the default current limit, it is

recommended to use an externally set current limit close to the

operating peak current to reduce peak ﬂux density and peak

power (see Figures 22 and 35). In most applications, the tighter

current limit tolerance, higher switching frequency and soft-start

features of TOPSwitch-HX contribute to a smaller transformer

when compared to TOPSwitch-GX.

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