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AN1842 Datasheet(PDF) 9 Page - STMicroelectronics |
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AN1842 Datasheet(HTML) 9 Page - STMicroelectronics |
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9 / 17 page  9/17 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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Similar Description - AN1842 |
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