OVERVOLTAGE PROTECTION

If the output voltage accuracy is not a concern,

but only a limitation is desired, the internal

overvoltage protection can be used. In this case,

five components can be taken out from the

schematics of figure 10 (R3-R10-R6-C4-C5) and

the input pin FB of the error amplifier is simply

grounded. The internal overvoltage protection will

29 V, by turning off the power mosfet switch. An

hysteresis of about 3 V will enable again the

switching of the device at a lower voltage level on

limitation, in a burst mode operation type, with

some ripple on the output. Case by case

experiments will define the correct value of output

capacitor C3, according to the loading current in

low output power condition.

STANDBY MODE

The standby mode is represented by a very low

output current, corresponding to a full loaded

battery in a battery charger application. The

output voltage is limited by either the overvoltage

protection or the error amplifier, according to the

design. This results into different situations :

burst mode operation as described previously

takes place, governed by the hysteresis of the

overvoltage comparator.

occur, depending on the compensation network

foreseen by the designer. These situations can

range from a normal continuous operation, to

burst mode. In any case, the output voltage will

be regulated to the desired value.

Note that the burst operation is providing a very

low input power consumption, because it reduces

the switching frequency, and thus commutation

losses. Less than 1 W of input power can be

observed in this operative mode, with a few

hundreds of mW delivered to the secondary load.

This is far compliant with standby standards, like

the ”Blue Angel” one.

VIPer31SP

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