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VIPer100/SP - VIPer100A/ASP

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 supply

output of the converter), the external capacitor

discharges itself down to the low threshold voltage

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 VIPer100/100A 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

used. It mixes a high performance compensation

network together with a separate high value soft

start capacitor. Both soft start time and regulation

loop bandwidth can be adjusted separately.

If the device is intentionally shut down by putting

the COMP pin to ground, the device is also

This voltage can be used for supplying external

functions, provided that their consumption doesn’t

exceed 0.5mA. Figure 17 shows

a typical

application of this function, with a latched shut

down. Once the "Shutdown" signal has been

activated, the device remains in the off state until

the input voltage is removed.

C

VDD

I

DD

t

SS

V

DDhy st

--------------------------

>

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

Ref.

UNDERVOLTAGE

LOCK OUT LOGIC

15 mA

1 mA

3 mA

2 mA

15 mA

VDD

DRAIN

SOURCE

VIPer100

Auxiliary primary

winding

VDD

t

VDDoff

VDDon

Start up duty cycle ~ 12%

FC00100