AN4191

Doc ID 023815 Rev 1

13/20

Looking at the green trace (Ch2, drain voltage) in Figure 13, the input voltage (5 V) and the

output voltage (- 5 V) are clearly evident, together with the oscillations due to the converter

fact, the converter works in “overcurrent protection mode” when the 5 V is pulled down to 3.5

V and the output voltage has an incorrect value. In Figure 15, more switching cycles for the

“bad” part are shown.

Figure 15.

High Rg device steady-state waveforms

Analyzing the drain voltage waveform, the slow drain falling edge indicates that the FET is

not turned off although the gate voltage is high (P-channel device). It is evident that the slow

falling-down of the FET drain voltage becomes more dangerous during steady-state

conditions, when the device turns on and off with a certain fsw. In fact, the device cannot be

turned-off effectively at each switching cycle. So, this slow MOSFET turn-off causes an

undesired switching loss increase, and therefore, a power dissipation and temperature rise,

finally producing the device failure.

Figure 13.

Figure 14.

AM16420V1

AM16421V1

AM16422V1