AN1842 datasheet(9/17 Pages) STMICROELECTRONICS

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AN1842 APPLICATION NOTE

Turn-Off and Hold-up time

In figure 16 the most salient waveforms at turn-off

are shown. Even at turn off the transition is clean,

without abnormal behaviour like restarting at-

tempts or loss of control by the loop. The Hold-up

time, in evidence between the vertical lines, is 13.6

ms at 115Vac-50Hz, full load and becomes around

67ms at 230Vac.

Figure 16. Hold-up @115 VAC -50Hz - full load

Short-Circuit Tests at Full Load

The short circuit tests have been done under sev-

eral conditions, all over the input mains range. To

avoid any catastrophic failure of the circuit due to

overstress, a deep check of component stress has

been done. The main circuit parameters that have

been checked are the Mosfet drain voltage and the

mean value of the output current. The drain volt-

age is an important parameter to be monitored

during short circuits because an excessive in-

crease due to the transformer leakage inductance

and the overcurrent flowing in the primary can

cause the MOSFET BVDSS to be exceeded. In the

proposed circuit, thanks to the low leakage induc-

tance of the transformer and to the start-up com-

ponent correct dimensioning, in case any output is

shorted, the MOSFET drain voltage is below its

BVDSS and the mean value of the output current is

low too, thus preventing component overheating

because of the excessive power dissipation. In this

case the reliability against long term-shorts is not

affected even at 264Vrms. The auto-restart at

short removal has been also checked and it is cor-

rect in all conditions.

Figure 17. Short circuit on 5V @90 VAC - 60Hz -

full load

Figure 18. Short circuit on 5V @265 VAC - 50Hz

- full load

As clearly indicated by the waveforms in figure 17

and 18, the circuit works in hiccup mode, keeping

the mean value of the current at levels compatible

with the component rating. As visible the circuit

working time is constant because it depends only

on the auxiliary capacitor value, while the dead

time is inversely proportional to the input mains

voltage. In the following figures 19 and 20 the

waveforms relevant to the 12V output in short cir-

cuit at both the nominal input mains voltage are re-

ported: as visible, the circuit protects itself and the

load.

CH1:

VC13

CH3:

+12V Vout

CH4:

+5V Vout

CH1:

VQ4 - Drain

CH2:

VPIN8 (Vcc)

CH4:

+5V short circuit current

CH1:

VQ4 - Drain

CH2:

VPIN8 (Vcc)

CH4:

+5V short circuit current

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