5 Op eration Descr iption VIPer100A-E/ASP-E
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As soon as the power goes below this limit, the auxili ary secondary voltage starts to increase
abov e the 13V regulation level , forcing the output volta ge of the transconduc tance amplifier to
low state (V COMP < VCOMPth). This situat i on leads to the shutdown mode wher e the power
swit ch is maint ained in the Of f state, resulting in missi ng c ycles and zero duty cycle. As soon as
VDD gets back to the regula tion level and the VCOMPth threshold is reached, the device
operates agai n. The above cycle repeats indefinitely, providing a burst m ode of which the
effective duty cycle is mu ch lower than the minimum one when in normal operation. The
equivale nt switch ing frequency is also lower th an the normal one, leading to a reduce d
consum ption on the input main supply lines. This mode of operation al l ows the VI Per100A-E/
ASP-E to meet th e n e w G e r man "Blu e Angel" Norm w it h less than 1W total power consumption
for the system when working in stand-by m ode. The output voltage remains regu lated around
the normal level, with a low f requency ripple corresponding to the burst mode. The ampl itude of
this ripple is low, because of the output capacitors and lo w output curren t drawn in su ch
con ditions.The norm al operation resum es automat ically when the power gets back to high er
levels than PSTBY.
5.3 High Voltage Start-up Current Source
An integrated high voltage current source provides a bias current from the DRAIN pin during
the start-up phase. This current is partially absorbed by internal control circuits which are
placed into a st andby mode with reduced consumption and also provided to th e external
capacitor connected to the V DD pin. As soon as t he voltage on this pin reaches the high v oltage
threshold V DDon of the UVLO logic, the device becomes active mode and starts switching. T he
start-up current generator is switched off, and the convert er should normally provide the
needed c urrent on the VDD pin through the auxiliary winding of the transf ormer, as shown on
(see Figure 11).
In case there are abnormal conditions where the auxiliary winding is unable to provide the l ow
voltage supply curre nt to the VDD pin (i.e. short circui t on the output of the converter), the
external capacitor discharges to the low thre shold voltage VDDoff of the UVLO logic, and the
devi ce goes back to the inactive state where the internal circuit s are in st andby mode and the
start-up current source is activated. The converter en ters a endless start-up cycle, with a start-
up duty cycle defined by the ratio of charging current towards discharging when the devic e tries
to start. T his ratio is fixed by design to 2A to 15A, which gives a 12% start-up duty cycle while
the power dissipation at start-up is approximately 0.6W, for a 230Vrms input voltage.
This low value start-up duty cycle prevents the appl ication of stress to the output rectifiers as
wel l as the transformer when a short circuit occurs.
The external capaci tor C VDD on the VDD pin must be sized according to the time needed by the
con verter to start up, when the device starts switching. This time tSS depend s on many
parameters, amo ng which transforme r design, output capacitors, soft start feature, and
compensation network implemented on the COMP pi n. The f ol lowing formula can be used for
defining th e minimu m capacitor needed:
where:
IDD is the consumption current on the VDD pin when switching. Refer to specified IDD1 and IDD2
values.
tSS is the start up time of the converter when the device begins to switc h. Worst case is
generally at full l oad.
CVDD
IDDtSS
VDDhyst
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