MIC2593 datasheet(23/26 Pages) STMICROELECTRONICS

23 / 26 page

April 2004

M9999-042204

MIC2593

Micrel

50ms before the MIC2593 circuit breaker trips. During that

time, the dissipation in the MOSFET is given by:

P = E

× I; E

MOSFET = [5V–5A(0.6Ω)] = 2V

MOSFET = (2V × 5A) = 10W for 50ms

At first glance, it would appear that a really hefty MOSFET is

required to withstand this sort of fault condition. This is where

the transient thermal impedance curves become very useful.

Figure 13 shows the curve for the Vishay (Siliconix) Si4430DY,

a commonly used SO-8 power MOSFET.

Taking the simplest case first, we’ll assume that once a fault

event such as the one in question occurs, it will be a long time,

several seconds, before the fault is isolated and the channel

is reset. In such a case, we can approximate this as a “single

pulse” event, that is to say, there’s no significant duty cycle.

Then, reading up from the X-axis at the point where “Square

Wave Pulse Duration” is equal to 0.1sec (=100msec), we see

that the Zθ(J-A) of this MOSFET to a highly infrequent event of

this duration is only 7% of its continuous Rθ(J-A).

This particular part is specified as having an Rθ(J-A) of

°C/W for intervals of 10 seconds or less. Thus:

Assume T

A = 55°C maximum, 1 square inch of copper at the

drain leads, no airflow.

Recalling from our previous approximation hint, the part has

an R

ON of (0.014/2) = 7mΩ at 25°C.

Assume it has been carrying just about 5A for some time.

When performing this calculation, be sure to use the highest

anticipated ambient temperature (T

A(MAX)) in which the

MOSFET will be operating as the starting temperature, and

find the operating junction temperature increase (

∆T

J) from

that point. Then, as shown next, the final junction temperature

is found by adding T

A(MAX) and ∆TJ. Since this is not a closed-

form equation, getting a close approximation may take one or

two iterations, but it’s not a hard calculation to perform and

tends to converge quickly.

Then the starting (steady-state)T

J is:

J ≅ TA(MAX) + ∆TJ

≅ T

A(MAX) + [RON + (TA(MAX) – TA)(0.005/°C)(RON)]

× I2 × Rθ(J-A)

J ≅ 55°C + [7mΩ + (55°C–25°C)(0.005)(7mΩ)]

× (5A)2 × (35°C/W)

J ≅ (55°C + (0.20125W)(35°C/W)

≅ 62.0°C

Iterate the calculation once to see if this value is within a few

percent of the expected final value. For this iteration we will

start with T

J equal to the already calculated value of 62.0°C:

J ≅ TA + [7mΩ + (62.0°C-25°C)(0.005)(7mΩ)]

× (5A)2 × (35°C/W)

J ≅ ( 55°C + (0.21008W)(35°C/W) ≅ 62.35°C

So our original approximation of 62.0

°C was very close to the

correct value. We will use T

J = 62°C.

Finally, add (10W)(35

°C/W)(0.07) = 24.5°C to the steady-state

J to get TJ(TRANSIENT MAX.) = 86.5°C. This is an acceptable

maximum junction temperature for this part.

10-4

10-3

10-2

10-1

100

600

0.1

0.01

0.2

0.1

0.05

0.02

Single Pulse

Duty Cycle = 0.5

Normalized Thermal Transient Imperance, Juction-to-Ambient

1. Duty Cycle, D =

2. Per Unit Base = RθJA = 67°C/W

3. T

JM – TA = PDMZ

θJA

(t)

4. Surface Mounted

Notes:

PDM

Square Wave Pulse Duration (sec)

Figure 13. Si4430DY MOSFET Transient Thermal Impedance Curve

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