APEX MICROTECHNOLOGY CORPORATION • TELEPHONE (520) 690-8600 • FAX (520) 888-3329 • ORDERS (520) 690-8601 • EMAIL prodlit@apexmicrotech.com

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This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

MSA260U REV B JULY 2004 © 2004 Apex Microtechnology Corp.

OPERATING

CONSIDERATIONS

MSA260

CALCULATING INTERNAL POWER DISSIPATION

Detailed calculation of internal power dissipation is complex

but can be approximated with simple equations. Conduction

loss is given by:

where I = output current

Switching loss is given by:

W = 0.00046 * I * Vsupply * Fswitching

Combine these two losses to obtain total loss. Calculate

heatsink ratings and case temperatures as would be done for

a linear amplifier. For calculation of junction temperatures,

assume half the loss is dissipated in each of two switches:

Tj = Ta + Wtotal * RØhs + 1/2Wtotal * RØjc, where:

RØhs = heatsink rating

RØjc = junction-to-case thermal resistance of the

MSA260.

The SOA typical performance graphs below show perfor-

mance with the MSA260 mounted with thermal grease on the

Apex HS26. The Free Air graph assumes vertical orientation

of the heatsink and no obstruction to air flow in an ambient

temperature of 30°C. The other two graphs show performance

with two levels of forced air. Note that air velocity is given

in linear feet per minute. As fans are rated in cubic delivery

capability, divide the cubic rating by the square area this air

flows through to find velocity. As fan delivery varies with static

pressure, these calculations are approximations, and heatsink

ratings vary with amount of power dissipated, there is no sub-

stitute for temperature measurements on the heatsink in the

center of the amplifier footprint as a final check.

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