电子设备热设计(第七章)Thermal Design of Electronic Equipment(Ralph Remsburg).pdf
0082-07.fm Page 343 Wednesday, August 23, 2000 10:12 AM
Combined Modes of
7 Heat Transfer for
Although we have examined the three modes of heat transfer in great detail, in
engineering practice we usually see cases where modes are combined simultaneously.
Radiation is not usually a signiﬁcant factor, but to achieve the best result we should
calculate the importance of each mode. In a computer chip, for example, heat is conducted
in parallel paths from the junction to the case and leads. Heat is then conducted from
the leads to the circuit board, and from the case to a heat sink. Simultaneously, the
heat in the leads and in the heat sink is convected to the air and radiated to the ambient
Table 7.1 is a reminder of the equations used for heat transfer and thermal resistance
in the three modes.
The simplest way to solve most problems of combined modes is to set up a
resistance network. In this manner we can graphically examine the paths of each
mode for simultaneous, parallel, and series heat transfer.
7.2 CONDUCTION IN SERIES AND IN PARALLEL
Because combined modes of heat transfer often occur in parallel, reviewing the