TOP243Y datasheet(31/48 Pages) POWERINT

31 / 48 page

TOP242-249

August 8, 2000

7/01

switching frequency and soft-start features of TOPSwitch-GX

contribute to a smaller transformer when compared to

TOPSwitch-II.

Standby Consumption

Frequency reduction can significantly reduce power loss at

light or no load, especially when a Zener clamp is used. For very

low secondary power consumption use a TL431 regulator for

feedback control. Alternately, switching losses can be

significantly reduced by changing from 132 kHz in normal

operation to 66 kHz under light load conditions.

TOPSwitch-GX Layout Considerations

As TOPSwitch-GX has additional pins and operates at much

higher power levels compared to previous TOPSwitch

families, the following guidelines should be carefully

followed.

Primary Side Connections

Use a single point (Kelvin) connection at the negative terminal

of the input filter capacitor for TOPSwitch-GX source pin and

bias winding return. This improves surge capabilities by returning

surge currents from the bias winding directly to the input filter

capacitor.

The CONTROL pin bypass capacitor should be located as

close as possible to the SOURCE and CONTROL pins and its

SOURCE connection trace should not be shared by the main

MOSFET switching currents. All SOURCE pin referenced

components connected to the MULTI-FUNCTION, LINE-

SENSE or EXTERNAL CURRENT LIMIT pins should also be

located closely between their respective pin and SOURCE.

Once again the SOURCE connection trace of these components

should not be shared by the main MOSFET switching currents.

It is very critical that SOURCE pin switching currents are

returned to the input capacitor negative terminal through a

seperate trace that is not shared by the components connected

to CONTROL, MULTI-FUNCTION, LINE-SENSE or

EXTERNAL CURRENT LIMIT pins. This is because the

SOURCE pin is also the controller ground reference pin.

Any traces to the M, L or X pins should be kept as short as

possible and away from the DRAIN trace to prevent noise

coupling. LINE-SENSE resistor (R1 in figures 47-49) should

be located close to the M or L pin to minimize the trace length

on the M or L pin side.

In addition to the 47

µF CONTROL pin capacitor, a high

frequency bypass capacitor in parallel may be used for better

noise immunity. The feedback optocoupler output should also

be located close to the CONTROL and SOURCE pins of

TOPSwitch-GX.

Y-Capacitor

The Y-capacitor should be connected close to the secondary

output return pin(s) and the positive primary DC input pin of the

transformer.

Heat Sinking

The tab of the Y package (TO-220) is internally electrically

tied to the SOURCE pin. To avoid circulating currents, a heat

sink attached to the tab should not be electrically tied to any

primary ground/source nodes on the PC board.

When using a P (DIP-8), G (SMD-8) or R (TO-263) package,

a copper area underneath the package connected to the SOURCE

pins will act as an effective heat sink. On double sided boards

(Figure 49), top side and bottom side areas connected with vias

can be used to increase the effective heat sinking area.

In addition, sufficient copper area should be provided at the

anode and cathode leads of the output diode(s) for heat sinking.

In Figures 47, 48 and 49 a narrow trace is shown between the

output rectifier and output filter capacitor. This trace acts as a

thermal relief between the rectifier and filter capacitor to

prevent excessive heating of the capacitor.

Quick Design Checklist

As with any power supply design, all TOPSwitch-GX designs

should be verified on the bench to make sure that components

specifications are not exceeded under worst case conditions.

The following minimum set of tests is strongly recommended:

1. Maximum drain voltage – Verify that peak V

DS does not

exceed 675 V at highest input voltage and maximum overload

output power. Maximum overload output power occurs

when the output is overloaded to a level just before the

power supply goes into auto-restart (loss of regulation).

2. Maximum drain current – At maximum ambient temperature,

maximum input voltage and maximum output load, verify

drain current waveforms at start-up for any signs of

transformer saturation and excessive leading edge current

spikes. TOPSwitch-GX has a leading edge blanking time of

220 ns to prevent premature termination of the on-cycle.

Verify that the leading edge current spike is below the

allowed current limit envelope (see Figure 52) for the drain

current waveform at the end of the 220 ns blanking period.

3. Thermal check – At maximum output power, minimum

input voltage and maximum ambient temperature, verify

that temperature specifications are not exceeded for

TOPSwitch-GX, transformer, output diodes and output

capacitors. Enough thermal margin should be allowed for

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