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AN1317 - APPLICATION NOTE

increased. If no protection is foreseen, it is possible to apply the input voltage directly on the output,

with large overvoltages.

• Once started, overvoltages may also occur at the output, mainly for low input voltage values.

The root cause of the last phenomenon resides in the duty cycle increase at low input voltage, together

with a low output load. Fig. 2 shows the drain current shape for two input voltages. The lower is the input

voltage, and the higher is the turn on. As a consequence, the turn off phase during which the energy is

sent to C2 through D2 is reduced, and the device is increasing its drain current to maintain a correct

during the on phase, overvoltage is resulting on the output.

Figure 2: Drain current for two input voltages in low load conditions

Fig. 3 shows an extreme case where the phenomenon reaches its critical phase, with a continuous mode

of operation. The following computation demonstrates the risk of overvoltage and/or overcurrent on the

output.

Vin = 200V

Iout = 5mA

Vin = 50V

Iout = 5mA

50mA/Div - 10µs/Div