placed into a standby mode with reduced

consumption and also provided to the external

the voltage on this pin reaches the high voltage

turns into active mode and starts switching. The

start up current generator is switched off, and the

converter should normally provide the needed

winding of the transformer, as shown on figure

15.

In case of abnormal condition where the auxiliary

winding is unable to provide the low voltage

the

output

of

the

converter), the

external

capacitor discharges itself down to the low

the device get back to the inactive state where

the internal circuits are in standby mode and the

start up current source is activated. The converter

enters a endless start up cycle, with a start-up

duty cycle defined by the ratio of charging current

towards discharging when the VIPer100B/BSP

tries to start. This ratio is fixed by design to 2 to

15, which gives a 12% start up duty cycle while

the power dissipation at start up is approximately

0.6 W, for a 230 Vrms input voltage. This low

value of start-up duty cycle prevents the stress of

the output rectifiers and of the transformer when

in short circuit.

be sized according to the time needed by the

converter to start up, when the device starts

parameters, among which transformer design,

output

capacitors,

soft

start

feature

and

compensation network

implemented

on

the

COMP pin. The following formula can be used for

defining the minimum capacitor needed:

>

where:

values.

device begins to switch. Worst case is generally

at full load.

logic. Refer to the minimum specified value.

Soft start feature can be implemented on the

COMP pin through a simple capacitor which will

be also used as the compensation network. In

this case, the regulation loop bandwidth is rather

low, because of the large value of this capacitor.

In case a large regulation loop bandwidth is

mandatory, the schematics of figure 16 can be

Figure 15: Behaviour of the high voltage current source at start-up

FC001 00B

Ref.

UNDERVOLTAGE

LOCK OUT LOGIC

15 mA

1mA

3mA

2mA

15 mA

VDD

DRAIN

SO URCE

VIP er100B

Auxiliary prima ry

winding

VDD

t

VDDoff

VDDon

Sta r t up duty cycle ~ 10%

VIPER100B/BSP

13/20