Design example

AN4209

14/19

DocID023949 Rev 2

3

Design example

The case study corresponds to protection of a MOSFET device. Because of stray

inductance in series with the power switch STP50NF25, fast transient over voltages appear

across it at each switching period. A STRVS device is therefore inserted in parallel to protect

the switch.

Applications conditions are shown in Table 3:

3.1

STRVS pre-selection

Equation 13

Considering the criteria 1 and 2, several STRVS can be pre-selected as shown in Table 4

3.2

Clamping voltage assessment

Since the application requires a surface mount package (SMD) protection, STRVS185X02B

is primarily selected. Based on the methodology, the current will be sensed in application

with a DO-15 package to minimize the thermal issue. Current and voltage waveforms across

the device are shown below during the clamping time.

Table 3. Worst case application conditions (maximum values)

Package

120 V

100 kHz

Unknown

50 °C

250 V

SMD

Table 4. Possible pre-selected devices

Order code

[1 µA, 25 °C]

[1 mA, 25 °C]



[1 mA]

Package

STRVS182X02F/C

128 V

158 V

DO-201/SMC

STRVS185X02B/E

128 V

158 V

SMB/DO-15

STRVS222X02F

154 V

189 V

DO-201

STRVS225X02E

154 V

189 V

DO-15

V

V

V

DSS

P

212

85

.

0

max

=

·

=