AN-7515 Datasheet, PDF (1/7 Pages) Fairchild Semiconductor – Combined Single-Pulse and Repetitive UIS Rating System

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AN-7515
Combined Single-Pulse and Repetitive UIS Rating System
Summary
A rating system for Unclamped Inductive Switching (UIS) in
PowerMOS transistors already widely accepted and
implemented on Fairchild PowerMOS transistor datasheets
can be applied to a wide range of applications and expanded
to cover repetitive UIS pulses by superposition. This allows
PowerMOS transistor users to determine if their application
lies within the rated capability of a power transistor. Two
examples are given of the analysis of UIS stress level in
representative applications. The ability of PowerMOS
transistors to withstand UIS has been recognized since 1985.
Although Blackburn has clearly shown^[1] UIS stress level is
not directly related to energy, many manufacturers of
PowerMOS transistors persist in rating their devices in terms
of energy capability. Since the energy capability varies with
the operating conditions, this rating is valid only at the
condition specified and the PowerMOS transistor user has no
way to calculate whether the particular application exceeds the
device rating. Ronan has defined a rating system^[3], herein
after called the UIS Rating System, that allows manufacturers
to specify the capability of PowerMOS transistors for single-
pulse UIS in such a way that users can determine if the
application exposes the device to more UIS stress than is
guaranteed in the datasheet.
Single-Pulse UIS Rating System
This UIS Rating System requires the user to determine only
the peak current through the PowerMOS transistor (I_AS), the
junction temperature at the start of the UIS pulse (T_J), and
the time the transistor remains in avalanche (t_AV). It allows
the determination of the conformance of any application to a
specified UIS capability where the worst-case conditions can
be simulated. It is also quite feasible to calculate the UIS
stresses for circuits not yet constructed or conditions not
easily simulated.
The UIS rating for a PowerMOS transistor (see Figure 1) is
presented as a chart with a vertical axis of (I_AS) maximum
avalanche current vs. (t_AV) time in avalanche as the horizontal
axis. Two lines are shown, one for 25^°C and one for a higher
junction temperature. It is fairly easy, in most applications, to
determine the avalanche current and time in avalanche in an
existing application by using a current probe. If the time in
avalanche and avalanche current plotted on the UIS rating
curve fall above and to the right of the 25^°C line, the
application is beyond the UIS rating of the device and the user
stands a risk of device failure. If the time and current plotted
© 2002 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 10/8/10
on the rating curve fall below and to the left of the maximum
junction temperature line, the application is within the UIS
rating of the device. In either case, no further analysis is
needed. If the time and current plotted on the rating chart falls
between the 25^°C and the maximum junction temperature
lines, further analysis is required.
To analyze those cases where the starting temperature and
time in avalanche fall between the 25^°C and maximum
temperature line, determine the junction temperature of the
PowerMOS transistor at the start of the UIS pulse. If the UIS
stress occurs after a long period in conduction, it may be
sufficient to measure the case temperature of the device and
calculate the temperature rise between the case and junction
from the dissipation and thermal resistance of the device. Any
other approach may be used. Once the junction temperature at
the start of the pulse has been determined, extrapolate between
the two published rating curves to determine the UIS
capability at that starting junction temperature.
Ronan^[3], Stoltenburg^[2] and Blackburn^[1] have all indicated
that the UIS capability I^2ASt_AV is a linear function of
temperature. Using this allows a straight line extrapolation
of the UIS capability of the device at the calculated junction
temperature. Then compare the calculated capability to the
stress to determine if the device is within ratings. This
simple approach allows users to decide if the application is
safe for any single UIS pulse.
Figure 1. FDB8444 Unclamped Inductive Switching
(Single-Pulse UIS)
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