DocID024230 Rev 1

9/15

AN4250

Fishbone diagram

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4

Fishbone diagram

In this section most common critical conditions, affecting the Power MOSFET mounted as

switch on the Boost converter, are described. Besides, they are reported in a fishbone

diagram.

Causes linked to the application

Short-circuit of the startup

In the Boost topology above described, the “pre-charge” circuit has not been presented; this

circuit works during the start-up phase when the output capacitor is not charged and the V

IN

is greater than V

OUT

. This condition forces the switch to absorb more current than the

steady-state operation, therefore the switch could work with current exceeding the

maximum ratings.

Using the “pre-charge” circuit, the current doesn't flow through the switch since it flows

through the diode of the “pre-charge” circuit until V

IN

> V

OUT

.

Figure 8. Pre-charge circuit

Choke dimensions

Since coil saturation depends on the current flowing through it, if the coil is not dimensioned

correctly, coil saturation can occur (the coil behaves as a simple short-circuit); a huge

quantity of current can flow through the switch causing an overheating that, in some cases,

causes the failure of the device.

Losses during the switching

If the MOSFET is not driven correctly, an overheating can occur. This phenomenon can

cause the failure of the device.

V1

V

L1

L

D1

D

C1

C

R1

R

R2

R

D2

D

SWITCH

AM17356v1