MIC2593
Micrel
50ms before the MIC2593 circuit breaker trips. During that
time, the dissipation in the MOSFET is given by:
MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase (∆T ) from
J
thatpoint.Then,asshownnext,thefinaljunctiontemperature
P = E × I; E
= [5V–5A(0.6Ω)] = 2V
MOSFET
isfoundbyaddingT
and∆T . Sincethisisnotaclosed-
A(MAX)
J
P
= (2V × 5A) = 10W for 50ms
MOSFET
formequation, gettingacloseapproximationmaytakeoneor
two iterations, but it’s not a hard calculation to perform and
tends to converge quickly.
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.
Figure13showsthecurvefortheVishay(Siliconix)Si4430DY,
a commonly used SO-8 power MOSFET.
Then the starting (steady-state)T is:
J
T
T
T
+ ∆T
J
J
A(MAX)
+ [R + (T
– T )(0.005/°C)(R )]
A ON
Taking the simplest case first, we’ll assume that once a fault
eventsuchastheoneinquestionoccurs, itwillbealongtime,
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
A(MAX)
ON
A(MAX)
2
× I × R
θ(J-A)
T
T
55°C + [7mΩ + (55°C–25°C)(0.005)(7mΩ)]
J
J
2
× (5A) × (35°C/W)
(55°C + (0.20125W)(35°C/W)
62.0°C
that the Z
of this MOSFET to a highly infrequent event of
θ(J-A)
Iterate the calculation once to see if this value is within a few
percent of the expected final value. For this iteration we will
this duration is only 7% of its continuous R
.
θ(J-A)
This particular part is specified as having an R
35°C/W for intervals of 10 seconds or less. Thus:
of
start with T equal to the already calculated value of 62.0°C:
θ(J-A)
J
T
T + [7mΩ + (62.0°C-25°C)(0.005)(7mΩ)]
× (5A) × (35°C/W)
J
A
2
Assume T = 55°C maximum, 1 square inch of copper at the
A
drain leads, no airflow.
T
( 55°C + (0.21008W)(35°C/W) 62.35°C
J
Recalling from our previous approximation hint, the part has
So our original approximation of 62.0°C was very close to the
an R of (0.014/2) = 7mΩ at 25°C.
ON
correct value. We will use T = 62°C.
J
Assume it has been carrying just about 5A for some time.
When performing this calculation, be sure to use the highest
Finally, add (10W)(35°C/W)(0.07) = 24.5°C to the steady-state
T to get T
= 86.5°C. This is an acceptable
J
J(TRANSIENT MAX.)
anticipated ambient temperature (T
) in which the
maximum junction temperature for this part.
A(MAX)
Normalized Thermal Transient Imperance, Juction-to-Ambient
2
1
Duty Cycle = 0.5
0.2
0.1
Notes:
P
DM
0.1
t
1
0.05
t
2
t
t
1
1. Duty Cycle, D =
2. Per Unit Base = R2θJA = 67°C/W
0.02
(t)
3. TJM – TA = PDMZθJA
4. Surface Mounted
Single Pulse
0.01
10-4
10-3
10-2
10-1
1
10
100
600
Square Wave Pulse Duration (sec)
Figure 13. Si4430DY MOSFET Transient Thermal Impedance Curve
April 2004
23
M9999-042204
STMICROELECTRONICS [ ST ]
