AN1542 datasheet(4/6 Pages) STMICROELECTRONICS

4 / 6 page

APPLICATION NOTE

4/6

STMicroelectronics has developed a Schottky

family dedicated to the OR-ing function. This “L”

family demonstrates very low forward voltage in

order to reduce conduction losses. Consequently,

the leakage current is relatively high.

For example, the “L15" family (VRRM=15V) is op-

timized for 3.3V, 5V and eventually 12V output

as OR-ing diode.

Due to the specific thermal runaway law of the

Schottky in OR-ing application, we can optimize

the device choice in order to improve the SMPS

efficiency, while keeping the risk of thermal run-

away under control.

For example, let’s take a 3.3V 35A output, with a

STPS40L15C as OR-ing diode. The two diodes

have to be considered like connected in parallel:

4. FROM THERMAL RUNAWAY TO PRODUCT

OPTIMIZATION

STPS40L15C

= 35A (=2I

)

out

fwd

= 3.3V

out

= 115.5W

out

Fig. 8: SMPS output synopsis

In the forward mode, the forward losses can be

calculated as:

()

fwd

=×

⋅

=×

−

0 18

8 010

(

atasheet

)

= 112

Theses losses decrease the global SMPS effi-

ciency about 9.7%.

The risk of thermal runaway can be evaluated by

calculating the maximum junction temperature

that must not be reached in forward mode to

avoid reverse losses being higher than forward

losses, thus avoiding thermal runaway.

()

diodes in parallel

rev

out

rev

out

=×

⋅

=×

⋅

23 3

(

)

()

(

)

thermal cons

rev

cTj

100

3 3

0 055

100

°×

≈°

−°

−

tan

()

per diode datasheet

rev

100

3 3

° ;.

(

)

Note that it is very important to use maximum re-

verse current values to evaluate reverse losses.

Actually, the worst case must be considered to

evaluate junction temperature in order to be sure

to avoid thermal runaway.

The limit of the thermal runaway criteria being de-

fined by Pfwd =Prev, the maximal junction tempera-

ture Tj max corresponds to Prev max =Pfwd:

()

(

)

rev

out

rev

max

=×

⋅

−°

100

3 3

100

()

100

3 3

137

°+

×⋅













=°

fwd

out

rev

The maximum junction temperature reachable in

forward mode before the risk of thermal runaway

occurs is so high, that we can consider a well

adapted device. This one will have a lower forward

voltage so a highest reverse current.

The same process can be applied to different

devices

STPS80L15C

Forward losses Pfwd = 9.0W

The efficiency loss about 7.8%.

Maximal junction temperature before thermal run-

away: Tjmax = 127°C

STPS120L15

Forward losses: Pfwd = 7.6W

The efficiency loss is about 6.6%

Maximal junction temperature before thermal run-

away: Tjmax = 100.3°C

STPS20L15 (the 20A average current specified is

only indicative value)

Forward losses: Pfwd = 16.1W

The efficiency loss is about 13.9%

Maximal junction temperature before thermal run-

away: Tjmax = 155.9°C

The comparison between the 4 parts considered

on the 3.3V 35A output can be summarized on the

following graph:

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