APPLICATION NOTE

5/6

Efficiency

loss

9.7%

Forward

losses

16.1W

11.2W

13.9%

9.0W

7.6W

STPS

L15

20

STPS

L15C

40

STPS

L15C

80

STPS

L15

120

T

= 156°C

jmax

T

= 137°C

jmax

T

= 127°C

jmax

T

= 100.3°C

jmax

7.8%

6.6%

V

= 3.3V

I

= 35A

out

out

Fig. 9: Comparison between 4 parts, forward

Using the specific thermal runaway law, the SMPS

designer can optimize the OR-ing diode choice in

order to improve the global efficiency.

The risk of thermal runaway is controlled by limiting

the junction temperature during the forward mode

below the maximum value evaluated.

STMicroelectronics is developing “L” family diodes

dedicated to the OR-ing application. This family

shows very low forward voltage in order to reduce

conduction losses and to improve efficiency:

STPSXXL15,

STPSXXL25,

STPSXXL30,

STPSXXL45, and STPSXXL60.

With the very simple law presented, it becomes

straightforward to optimize devices choice by

evaluating the risk of thermal runway in Schottky

used in OR-ing function in SMPS.

This reliable and accurate law allows the optimi-

zation of the devices used in order to improve

converter efficiency while controlling the risk of

thermal runaway risk.

CONCLUSION

To evaluate the limit before thermal runaway, the

maximum value of the reverse current has to be

considered. Actually, this parameter is critical for

thermal runaway and the worst case must be con-

sidered.

To evaluate the maximum reverse current of a

power Schottky, take the typical value given in

figure. Apply the ratio between typical and maxi-

get the expected junction temperature.

Example: STPS80L15C (twin diode in parallel)

under 3.3V @125°C

Figure 5 of the STPS80L15C datasheet gives the

typical value of the reverse current @100°C for

3.3V (per diode):

ANNEXE: EVALUATION OF MAXIMUM RE-

VERSE CURRENT FROM DATASHEET

3.3V

220mA

0123456789 10 11 12 13 14 15

1E-1

1E+0

1E+1

1E+2

1E+3

IR(mA)

Tj=25°C

Tj=75°C

VR(V)

Tj=100°C

Fig. 5: Reverse leakage current versus reverse

voltage applied (typical values, per diode).

The static electrical characteristics table gives the

ratio between typical and maximum values (per

diode):

Symbol

Parameter

Tests conditions

Min.

Typ.

Max.

Unit

I

Reverse leakage

current

Tj=25

°CV

4

mA

Tj = 100

°C

280

400

Tj=25

°CV

11

Tj = 100

°C

0.44

1.1

A

Tj=25

°CV

16

mA

Tj = 100

°C

0.53

1.3

A

Pulse test :* tp = 380

µs, δ <2%

STATIC ELECTRICAL CHARACTERISTICS (per diode).