AN557 datasheet(51/52 Pages) STMICROELECTRONICS

51 / 52 page

51/52

AN557 APPLICATION NOTE

Figure 71.

Figure 72.

Figure 73.

But since the aim of this section is not that of stud-

ing the transistors, the circuit can be further re-

duced as shown in figure 74.

Figure 74.

If we now consider the ground potential as ambi-

ent temperature, we have:

Tj = Ta + (Rjc + Rth) Pd

Thermal contact resistance depends on various

factors such as the mounting, contact area and

planarity of the heatsink. With no material between

the device and heatsink the thermal resistance is

around 0.5°C/W;; with silicone grease roughly

0.3°C/W and with silicone grease plus a mica insu-

lator about 0.4°C/W. See fig. 75. In application

where one external transistor is used together, the

dissipated power must be calculated for each

component. The various junction temperature can

be calculated by solving the circuit shown in fig.

75. This applies if the dissipating elements are fair-

ly close with respect to the dissipator dimensions,

otherwise the dissipator can no longer be consid-

ered as a concentrated constant and the calcula-

tion becomes difficult. This concept is better

explained by the graph in fig.77 which shows the

case (and therefore junction) temperature varia-

tion as a function of the distance between two dis-

sipating elements with the same type of heatsink

and the same dissipated power. The graph in fig.

77 refers to specific case of two elements dissipat-

ing the same power, fixed on a rectangular alumin-

ium plate with a ratio of 3 between the two sides.

The temperature jump will depend on the total dis-

sipated power and on the devices geometrical po-

sitions. We want to show that there exists an

optimal position between the two devices:

· side of the plate

Figure 75.

Figure 76.

–

jc Pd

–

---------------------------------------

---

AN557 датащи(PDF) 51 Page - STMicroelectronics

AN557 датащи(HTML) 51 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

датащи скачать

ссылки URL