MIL-PRF-19500/359K
6.5 Applications data.
6.5.1 Half-sine-wave application with 1N4942 through 1N4948. For a PCB mounting example with FR4 material
where the full 1 amp IO rating (half-sine-wave) is used at a TJ of 175ºC and ambient temperature of 55ºC, the
following steps guide the user in what the PCB pad size will need to be with 1 oz, 2 oz, and 3 oz copper for 1N4942
to 1N4948. For axial-leaded, the lead length for mounting will be .187 inch (4.76 mm) or less from body to entry point
on PCB surface.
a. Use the IO verses Po curve on figure 3 to look up 1 amp (X-axis) and follow up to the TJ =175ºC curve (lower)
for 1.04 watts.
b. Calculate maximum thermal resistance needed (175ºC - 55ºC) /1.04 W = 115 ºC/W.
c. Look up thermal resistance of 115ºC/W on Y-axis using a thermal resistance verses pad area plot on one of
the three curves on figure 4 for different weights of copper cladding and then intersect curve horizontally to
get answer. These curves assume still air, horizontal position.
2
2
2
2
d. In this example, the answer is: 1 oz PCB = .042 in (1.07 mm ), 2 oz PCB = .025 in (0.64 mm ), 3 oz PCB =
2
2
.014 in (0.36 mm ) for each pad.
e. Add a conservative guard-band to the pad size (larger) to keep TJ below 175ºC.
6.5.2 Square-wave application with 1N4942 through 1N4948. For a PCB mounting example with FR4 material to
support a 0.5 amp IO square wave switching at a 0.50 duty factor (50 percent duty cycle) at TJ = 125ºC and ambient
temperature of 55ºC, the following steps guide the user in what the PCB pad size will need to be with 1 oz, 2 oz, and
3 oz copper.
a. Find size of copper pads on standard FR4 PCB to support operation at 0.5 amp IO square wave switching at a
0.50 duty factor (50 percent duty cycle) at TJ = 125ºC with TA = 55ºC.
b. Calculate peak IF = 0.5 A / 0.50 duty factor = 1 amp.
c. Use the VF verses IF curve on figure 5 to look up IF = 1 A (Y-axis) and follow across to the TJ = 125ºC curve
(middle) for VF = 0.96 V.
d. Calculate power = IF * VF * duty factor = 1 x 0.96 x 0.50 = 0.48 W.
e. Calculate maximum thermal resistance needed (125ºC - 55ºC) / 0.48 W = 146ºC/W.
f. Look up thermal resistance of 146ºC/W on the Y-axis using a thermal resistance verses pad area plot on one
of the three curves on figure 4 for different weights of copper cladding and then intersect curve horizontally to
get answer. Curves assume still air, horizontal position.
2
2
2
2
g. In this example, the answer is: 1oz PCB = .02 in (0.51 mm ), 2oz PCB = .012 in (0.30 mm ), 3oz PCB =
2
2
.008 in (0.20 mm ) for each pad.
h. A conservative pad guard-band is optional since TJ is only 125ºC. NOTE: Multilayer PCB s, forced air
cooling, will improve performance. Closed confinement of the PCB will do the opposite. Use sound thermal
management.
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