TOP242 datasheet(25/54 Pages) POWERINT

25 / 54 page

Rev. R 10/20

TOP242-250

www.power.com

The power supply could also be turned on remotely

through a local area network or a parallel or serial port by

driving the optocoupler U4 input LED with a logic signal.

Sometimes it is easier to send a train of logic pulses

through a cable (due to AC coupling of cable, for example)

instead of a DC logic level as a wake up signal. In this

case, a simple RC filter can be used to generate a DC

level to drive U4 (not shown in Figure 45). This remote

on feature can be used to wake up peripherals such as

printers, scanners, external modems, disk drives, etc., as

needed from a computer. Peripherals are usually designed

to turn off automatically if they are not being used for a

period of time, to save power.

In addition to using a minimum number of components,

TOPSwitch-GX provides many technical advantages in this

type of application:

1. Extremely low power consumption in the off mode:

80 mW typical at 110 VAC and 160 mW typical at

230 VAC. This is because, in the remote OFF mode,

the TOPSwitch-GX consumes very little power and the

external circuitry does not consume any current (either

M, L or X pin is open) from the high voltage DC input.

2. A very low cost, low voltage/current, momentary

contact switch can be used.

3. No debouncing circuitry for the momentary switch

is required. During turn-on, the start-up time of the

power supply (typically 10 ms to 20 ms) plus the

microprocessor initiation time act as a debouncing filter,

allowing a turn-on only if the switch is depressed firmly

for at least the above delay time. During turn-off, the

microprocessor initiates the shutdown sequence when

it detects the first closure of the switch and subsequent

bouncing of the switch has no effect. If necessary, the

microprocessor could implement the switch debouncing

in software during turn-off, or a filter capacitor can be

used at the switch status input.

4. No external current limiting circuitry is needed for the

operation of the U4 optocoupler output due to internal

limiting of M pin current.

5. No high voltage resistors to the input DC voltage rail are

required to power the external circuitry in the primary.

Even the LED current for U3 can be derived from the

CONTROL pin. This not only saves components and

simplifies layout, but also eliminates the power loss

associated with the high voltage resistors in both ON

and OFF states.

6. Robust design: There is no ON/OFF latch that can be

accidentally triggered by transients. Instead, the power

supply is held in the ON-state through the secondary-

side microprocessor.

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