AN1842 Datasheet, PDF (9/17 Pages) STMicroelectronics – 60W WIDE-RANGE POWER SUPPLY

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 V_AC -50Hz - full load
Figure 17. Short circuit on 5V @90 V_AC - 60Hz -
full load
CH1:
CH2:
CH4:
VQ4 - Drain
VPIN8 (Vcc)
+5V short circuit current
Figure 18. Short circuit on 5V @265 V_AC - 50Hz
- full load
CH1: V_C13
CH3: +12V Vout
CH4: +5V Vout
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 BV_DSS 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
BV_DSS 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.
CH1:
CH2:
CH4:
VQ4 - Drain
VPIN8 (Vcc)
+5V short circuit current
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.
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