Micrel
MIC2589/MIC2595
higher pulsed power without damage than its
continuous power dissipation ratings imply due to an
inherent trait, thermal inertia. With respect to the
specification and use of power MOSFETs, the
parameter of interest is the “Transient Thermal
Impedance”, or Zθ, which is a real number (variable
factor) used as a multiplier of the thermal resistance
(Rθ). The multiplier is determined using the given
“Transient Thermal Impedance Graph”, normalized to
Rθ, that displays curves for the thermal impedance
versus power pulse duration and duty cycle. The
single-pulse curve is appropriate for most hot swap
applications. Zθ is specified from junction-to-case for
power MOSFETs typically used in telecom
applications.
following equation based on the highest ambient
temperature of the system environment.
TC(max) = TA(max) + PD × (Rθ(J-A) – Rθ(J-C)
)
(2)
Let’s assume a maximum ambient of 60°C. The
power dissipation of the MOSFET is determined by
the current through the MOSFET and the ON
resistance (I2RON), which we will estimate at 17mꢀ
(specification given at TJ = 125°C).
Using our
example information and substituting into Equation 2,
TC(max)
= 60°C+[((3A)2×17mꢀ)×(40–0.4)°C/W]
= 66.06°C
Substituting the variables into Equation 1, TJ is
determined by:
T(steady-state) ≅TC(max)+[RON+(TC(max)–TC)(0.005)
J
× (RON)][I2×(Rθ(J-A)–Rθ(J-C))]
≅66.06°C+[17mꢀ+(66.06°C–25°C)(0.005/°C)
×(17mꢀ)][(3A)2×(40–0.4)°C/W]
≅66.06°C + 7.30°C
The following example provides
a method for
estimating the peak junction temperature of a power
MOSFET in determining if the MOSFET is suitable for
a
particular
application.
VIN (VDD – VEE) = 48V, ILIM = 4.2A, tFLT is 20ms, and
the power MOSFET is the SUM110N10-09 (TO-263
package) from Vishay-Siliconix. This MOSFET has an
RON of 9.5mꢀ (TJ = 25°C), the junction-to-case
thermal resistance (Rθ(J-C)) is 0.4°C/W, junction-to-
ambient thermal resistance (Rθ(J-A)) is 40°C/W, and the
Transient Thermal Impedance Curve is shown in
Figure 8. Consider, say, the MOSFET is switched on
at time t1 and the steady-state load current passing
through the MOSFET is 3A. At some point in time
after t1, at time t2, there is an unexpected short-circuit
applied to the load, causing the MIC2589/MIC2595
controller to adjust the GATE output voltage and
regulate the load current for 20ms at the programmed
current limit value, 4.2A in this example. During this
short-circuit load condition, the dissipation in the
MOSFET is calculated by:
≅73.36°C
Since this is not a closed-form equation, getting a
close approximation may take one or two iterations.
On the second iteration, start with TJ equal to the
value calculated above. Doing so in this example
yields;
TJ(steady-state)
≅
66.06°C+[17mꢀ+(73.36°C
-25°C)×(0.005/°C)
×(17mꢀ)][(3A)2×(40–0.4)]°C/W
≅
73.62°C
Another iteration shows that the result (73.63°C) is
converging quickly, so we’ll estimate the maximum
T
J(steady-state) at 74°C.
The use of the Transient Thermal Impedance Curves
is necessary to determine the increase in junction
temperature associated with a worst-case transient
PD(short) = VDS × ILIM ; VDS = 0V – (-48V) = 48V
PD(short) = 48V × 4.2A = 201.6W for 20ms.
condition.
From our previous calculation of the
At first glance, it would appear that a very hefty
MOSFET is required to withstand this extreme
overload condition. Upon further examination, the
calculation to approximate the peak junction
temperature is not a difficult task. The first step is to
determine the maximum steady-state junction
temperature, then add the rise in temperature due to
the maximum power dissipated during a transient
overload caused by a short circuit condition. The
equation to estimate the maximum steady-state
junction temperature is given by:
maximum power dissipated during a short circuit
event for the MIC2589/MIC2595, we calculate the
transient junction temperature increase as:
TJ(transient) = PD(short) × Rθ(J-C) × Multiplier
(3)
Assume the MOSFET has been on for a long time –
several minutes or more – and delivering the steady-
state load current of 3A to the load when the load is
short circuited. The controller will regulate the GATE
output voltage to limit the current to the programmed
value of 4.2A for 20ms before immediately shutting off
the output. For this situation and almost all hot swap
applications, this can be considered a single pulse
event as there is no significant duty cycle. From
Figure 8, find the point on the X-axis (“Square-Wave
Pulse Duration”) for 25ms, allowing for a 25% margin
TJ(steady-state) ≅ TC(max) + ∆TJ
(1)
TC(max) is the highest anticipated case temperature,
prior to an overcurrent condition, at which the
MOSFET will operate and is estimated from the
23
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
