AN1513 datasheet(10/13 Pages) STMICROELECTRONICS

10 / 13 page

Short circuit protection

AN1513

10/13

Short circuit protection

The SMPS is protected against short circuit on both outputs. The short circuit test has been

done with an active load for the complete input voltage range. As shown in Figure 7 and

Figure 8, taken at worst case at 374 Vdc, during short circuit on output1 or output 2, the

SMPS works in “hiccup” mode keeping the mean output current at a safe value for the

rectifiers, respectively at 3.3 A for output 1 and 2.2 A for output 2. Also the drain voltage

remains at a safe level during short circuit. To achieve these results a Zener diode has been

connected between pin Comp and Source of the VIPer. This Zener clamps the voltage on

pin Comp, limiting the maximum primary peak current. During short circuit the auxiliary

voltage is low because it is proportional to the output voltage. VDD drops under the low

threshold (8 V) blocking the VIPer and beginning the start up cycle. When C12 is charged at

11 V, the VIPer turns on again. It works as long as C12 is discharged to 8 V because the

auxiliary voltage, thanks to the reduced duty-cycle, is not capable of supplying the VIPer.

Overvoltage protection

Thanks to the VDD regulation capability, this SMPS is protected against secondary feedback

failure. If the secondary feedback loses control, the voltages increase. As the auxiliary

voltage is proportional to the outputs, VDD increases up to 13 V, which is the typical

regulation value for VIPer50A-E, so the VIPer takes control avoiding any damage to the

output capacitors and rectifiers. Table 5 gives the measurements in standby and at full load

with R6 removed from the board to simulate secondary feedback failure.

Figure 7.

Short circuit (Vout1)

Figure 8.

Short circuit (Vout2)

Table 5.

Overvoltage measurements

Operation mode

Vout1

Vout2

Standby

9.5V

22.5 V

Full load

7.2V

11.2 V

AN1513 Datasheet(PDF) 10 Page - STMicroelectronics