K

#### Kelvin

- Jan 1, 1970

- 0

this is a question of 2 transistors mounted on the same heat sink...

Two 2N3055 transistors are mounted close together on a heasink

consisting of a square piece of bright aluminum 1/8 thick. the

transistors are electrically insulated from the heatsink by using MICA

washers coated with silicone grease between each transistor and the

heatsink.

for a 2N3055. Qjc = 1.5 C/W and Q of a greased MICA washer = 0.5 C/W

Tj Maximum of a 2N3055 is given as 200C.

a) If each transistor is producing 20W of heat at its collector junction,

calculate the maximum allowable value of Qsink if the ambient temp cam be

as high as 45C

b) if Qsink = 2.5 C/W, and one transistor is dissipating 30 Watts and the

other

is in cutoff, calculate the case temp of each transistor if T ambient = 50C.

State

any assumption made.

after filtering out the decorations... i figure the following are the useful

part of the question:

2 transistor with Qjc = 1.5 C/W each is mounted on a Qsink = unknown

heatsink

between the heatsink and two transistors, there is 2 MICA Q = 0.5 C/W

Tj(max) = 200C

Tambient = 45C

a) im looking for Qsink

b) i have no clue

I did Tj(max) - Tambient = 200C - 45C = 155C (which is the heat allowed on

my components)

Total Power dissipation = 2 x 20W = 40 W

155C/40W = 3.875 C/W --> total thermal resistance I can have in my circuit

then im stucked... lol...

two transistor mounted on the same heatsink... so the total thermal

resistance they have

should be the sum of parrallel of the two?? the same for two MICAs...

and for (b)... i have totally no clue about it...

thanks for any help...