MIC2588_05 Datasheet, PDF (16/21 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
the drain-source breakdown voltage of the MOSFET must
be greater than V_IN(MAX), or V_DD – V_EE(min).
The second breakdown voltage criterion that must be met
is the gate-source voltage. For the MIC2588/MIC2594, the
gate of the external MOSFET is driven up to a maximum of
11V above VEE. This means that the external MOSFET must
be chosen to have a gate-source breakdown voltage of 12V
or more; 20V is recommended. Most power MOSFETs with
a 20V gate-source voltage rating have a 30V drain-source
breakdown rating or higher. For many 48V telecom applica-
tions, transient voltage spikes can approach, and sometimes
exceed, 100V. The absolute maximum input voltage rating
of the MIC2588/MIC2594 is 100V; therefore, a drain-source
breakdown voltage of 100V is suggested for the external
MOSFET. Additionally, an external input voltage clamp is
strongly recommended for applications that do not utilize
conditioned power supplies.
Power MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum continuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
•The value of I_{LOAD(CONT, MAX.)} for the output in question
(see Sense Resistor Selection).
•The manufacturer’s datasheet for the candidate MOS-
FET.
•The maximum ambient temperature in which the device
will be required to operate.
•Any knowledge you can get about the heat sinking avail-
able to the device (e.g., can heat be dissipated into the
ground plane or power plane, if using a surface-mount
part? Is any airflow available?).
The datasheet will almost always give a value of on resistance
for a given MOSFET at a gate-source voltage of 4.5V and
10V. For MIC2588/MIC2594 applications, choose the gate-
source ON resistance at 10V and call this value R_ON. Since
a heavily enhanced MOSFET acts as an ohmic (resistive)
device, almost all that’s required to determine steady-state
power dissipation is to calculate I²R. The one addendum to
this is that MOSFETs have a slight increase in R_ON with in-
creasing die temperature. A good approximation for this value
is 0.5% increase in R_ON per °C rise in junction temperature
above the point at which R_ON was initially specified by the
manufacturer. For instance, if the selected MOSFET has a
calculated R_ON of 10mΩ at aT_J = 25°C, and the actual junc-
tion temperature ends up at 110°C, a good first cut at the
operating value for R_ON would be:
R_ON ≈ 10mΩ[1 + (110 – 25)(0.005)] ≈14.3mΩ
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in °C/W) as that with which the MOSFET’s
performance was specified by the manufacturer. Here are
a few practical tips:
1. The heat from a TO-263 power MOSFET flows
almost entirely out of the drain tab. If the drain tab
can be soldered down to one square inch or more,
the copper will act as the heat sink for the part. This
copper must be on the same layer of the board as
Micrel
the MOSFET drain.
2. Airflow works. Even a few LFM (linear feet per-
minute) of air will cool a MOSFET down substan-
tially. If you can, position the MOSFET(s) near
the inlet of a power supply’s fan, or the outlet of a
processor’s cooling fan.
3. The best test of a candidate MOSFET for an ap-
plication (assuming the above tips show it to be a
likely fit) is an empirical one. Check the MOSFET’s
temperature in the actual layout of the expected
final circuit, at full operating current. The use of a
thermocouple on the drain leads, or infrared pyrom-
eter on the package, will then give a reasonable
idea of the device’s junction temperature.
Power MOSFET Transient Thermal Issues
If the prospecitve MOSFET has been shown to withstand the
environmental voltage stresses and the worse-case steady-
state power dissipation is addressed, the remaining task is to
verify if the MOSFET is capable of handling extreme overcur-
rent load faults, such as a short circuit, without overheating.
A power MOSFET can handle a much higher pulsed power
without damage than its continuos 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 Impedence”, or Z_θ, which
is a real number (variable factor) used as a multiplier of the
thermal resistance (R_θ). The multiplier is determined using
the given “Transient Thermal Imepedence Graph”, normalized
to R_θ, that displays curves for the thermal impedence versus
power pulse duration and duty cycle. The single-pulse curve
is appropriate for most hot swap applications. Z_θ is specified
from junction-to-case for power MOSFETs typically used in
telecom applications.
The following example provides a method for estimating the
peak junction temperature of a power MOSFET in determin-
ing if the MOSFET is suitable for a particular application.
V_IN (VDD – VEE) = 48V, I_LIM = 4.2A, and the power MOSFET
is SUM110N10-09 (TO-263 package) from Vishay-Siliconix.
This MOSFET has an R_ON of 9.5mΩ (T_J = 25°C), the junc-
tion-to-case thermal resistance (R_θ(J-C)) is 0.4°C/W, junc-
tion-to-ambient thermal resistance (R_θ(J-A)) is 40°C/W, and
the Transient Thermal Impedence Curve is shown in Figure
7. 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
MIC2588/MIC2594 controller to adjust the GATE output
voltage and regulate the load current for 400µs at the pro-
grammed current limit value, 4.2A in this example. During this
short-circuit load condition, the dissipation in the MOSFET
is calculated by:
P_D(short) = V_DS × I_LIM
V_DS = 0V – (-48V) = 48V
P_D(short) = 48V × 4.2A = 201.6W for 400µs.
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,
September 2005
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M9999-083005