AN1542 datasheet(3/6 Pages) STMICROELECTRONICS

3 / 6 page

APPLICATION NOTE

3/6

losses

Pfwd

Forward

mode

SMPS

break down

Reverse

mode

Thermal

runaway

time

Monotonous variation

Fig. 4: Typical loss variation in the OR-ing before

and after the SMPS failure

In these more complicated cases, device thermal

behavior can be simulated with tools like PSPICE.

The following analogies have to be used:

Forward

mode

continuous

variation

SMPS

break down

Reverse

mode

Monotonous variation

time

Thermal

runaway

Tamb

Fig. 5: Typical junction temperature variation in

the OR-ing before and after the SMPS failure

More complicated cases, where the assumptions

of §3.2 do not exist, can be considered.

For example:

OR-ing diode not on its own heatsink.

The OR-ing diode can be mounted on common

heatsink with other dissipative devices. In this

case, the junction temperature of the OR-ing di-

ode can be influenced by the other devices,

thanks to coupling thermal resistance.

Non constant thermal resistance system:

The convection can be forced by a fan con-

nected to the Anode side of the OR-ing diode. In

case of SMPS failure, the fan will stop and the

Rth(j-a) will increase. In this case, the junction

temperature variation will not be monotonous.

3.3. Results in more complicated cases

Forward

mode

continuous

variation

SMPS

break down

Fan switch

OFF

Reverse

mode

Thermal

runaway

Tamb

time

NON-monotonous variation

Fig. 6: Example of junction temperature variation

in non-constant thermal resistance system

Rth(c-a)

P (losses)

Rth(j-c)

Tamb

Tcase

Tjunction

Junction

Case

Ambiant

Fig. 7: Thermal / electric analogy for simulation

This analogy can be use to analyze any complex

thermal problem.

Thermal:

Electrical:

Temperature

Voltage

Power

Current

Resistance

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