Fishbone diagram

AN4250

10/15

DocID024230 Rev 1

Short-circuit of the load

When the load is in short-circuit condition, if the short protection circuit is not present or if its

operative time is too long, the following failures can happen:

1.

If the “pre-charge” circuit is not present, the switch works in start-up phase with a huge

quantity of current (refer to the start-up critical condition)

2.

the “pre-charge” circuit is present till it works, then current flows onto the switch like the

previous case

Switching diode recovery

In CCM as above written, during the commutation, the current, which flows through the

diode, is not zero. In this case, both the coil current inside the switch and the diode current

flow (current spike depends on the reverse recovery time t

rr

). In relation to the intensity of

the spike, the overall absorbed current could cause the failure.

Figure 9. Recovery current inside a diode

Causes linked to the material

Due to the intrinsic characteristics of the Power MOSFET device (V

th

, R

g

, C

rss

), a spurious

turn-on can happen during the turn-off commutation.

In detail, during the turn-off, spurious noise on the drain (like a fast d

v

/d

t

) generates with the

C

rss

capacitance a spurious current:

Equation 9

High values of C

rss

can cause the undesired turn-on of the switch.

ID

ID nom

T

TIRM

25%

IRM

IRM

TRR

AM17357v1

dt

d

v

C

i

DS

rs s