AN3432

Optimized bypass diode for a given solar panel or junction box

Doc ID 019041 Rev 1

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Step 3: Eliminate the risk of thermal runaway

As explained in the previous section, the most critical step is the thermal runaway risk that

occurs at the diode switching off. In the worst case, the diode will conduct a current very

The power conduction losses calculation gives:

With:

The power reverse losses are given by the following equation:

Then we apply the thermal law in order to find the junction temperature at switching time.

Thermal law and power conduction losses curves cross gives the junction temperature at

switching time in Figure 16.

Figure 16.

STPS3045C thermal runaway risk analysis

The junction temperature at switching off is 164 °C. At this junction temperature, the reverse

losses are below the conduction losses. In conclusion, there is no risk of thermal runaway

with STPS3045CGC.

sc

j

sc

F

j

cond

I

)·

T

,

I

(

V

)

T

(

P

=

)

25

T

(

10

1

2

.

0

)

T

(

R

)

25

T

(

10

3

.

2

51

.

0

)

T

(

V

I

)

T

(

R

)

T

(

V

)

T

,

I

(

V

j

4

j

d

j

3

j

0

T

sc

j

d

j

0

T

j

sc

F

−

•

×

+

=

−

•

×

−

=

•

+

=

−

−

)

125

Tj

(

06

.

0

1

j

rev

e

10

51

.

1

)

T

(

P

−

−

•

•

=

j

Thermal

) =

T

(

P

(Tj - 85 °C)

30

0

1

2

3

4

5

6

7

8

9

10

-40

-20

0

20

40

60

80

100

120

140

180

200

P(W)

Pthermal(Tj )

Tj@t 0=164 °C

No thermal runaway risk

Prev(Tj@t 0)< Pcond (Tj@t 0)

Tj (°C)

160