User manual

ECE 477 Final Report Spring 2004
For a Schottky Power Diode, base failure rate λ
b =
0.0030. According to the graphs in the
design specs, we can expect our diode to run around 85C with the current draw estimated during
design time, making π
T
= 5.7. For Voltage stress π
S
, since the Voltage applied isn’t near the
maximum rated voltage, the value of 0.054 can be used. An appropriate Quality factor was 5.5,
or commercial grade, and contact construction looked like 2.0. And while we frequently feel that
C
L
(factor listed in the RPEE as “Launched from a Cannon”) is an appropriate environment, G
M
(Ground, Mobile) will be the environment for all components, so π
E
= 9.0.
λ
p
= 0.0030 * 5.7 * 0.054 * 2.0 * 5.5 * 9.0 = 0.091 failures/ 10
6
hour.
MTTF = 1/ λ
p
= 1.1
7
hours, or around 1250 years.
Si9435 Power MOSFET
The equation for transistors is very similar to that for diodes, with a few different values
for constants.
λ
p
= λ
b
π
T
π
A
π
Q
π
E
Base rate λ
b
= 0.012 for MOSFETs. I couldn’t find appropriate information on operating
temperature, so I will assume it to be near that of the power diode (it should be close, and
hopefully not too much over), so π
T
= 3.0. Since 5W is the maximum power of the power
MAX776, we will use that value and get 2.0 for π
A
, the application factor. The quality and
environment values of 5.5 and 9.0 remain the same, so
λ
p
= 0.012 * 3.0 * 2.0 * 5.5 * 9.0 = 3.6 failures/ 10
6
hour.
MTTF = 1/ λ
p
= 2.8x10
5
hours, or about 32 years. This might be a problem, so it would
be wise to try to find a solution to this. First, we could run tests to find out more accurately how
hot this component is getting, and lower it with a heat sink if needed. Perhaps a component with
a higher quality rating could be used as well, at some expense. This may be the component that
causes the most complaint.
Power Resistor
The equation for a power resistor is:
-14-