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Paralleled transistors, regulators eliminate heat sinks

25 Sep 2014  | Petre Petrov

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Bipolar junction transistors (BJT) are not the most advanced electronic components out there but they remain useful for a great many tasks due to their low cost and good parameters. We can find new applications which were impossible in the past due to the previous higher cost of these components. For example, we can replace in some cases more powerful transistors (with or without heat sinks) with several paralleled, low-power transistors and we can reap the advantages of those replacements.

Generally speaking low power transistors are faster, have higher working frequencies, lower noise, lower THD, have packages which are much easier to solder manually and automatically if compared to the more powerful transistors especially with the additional mass of added heat sinks, etc.

Many transistors with power dissipation up to around 1W are in packages similar to TO-92. Most of these transistors are low cost, they can be acquired in large quantities and the packages similar to TO-92 are easy to use.

The heat from these packages can be easily removed effectively with a cooling fan or even normal convection. Additionally we can use larger copper surfaces around these transistors to increase their power dissipation. For each of the different packages of these electronic components there are a great deal of thermal information and calculation methods in the data sheets and literature so we will omit them here because there is no need to repeat them.

The packages for the power transistors such as TO-126, TO-220 and similar are larger, heavier, and more difficult to mount on the PCB and should be used with additional heat sinks in order to have full performance and reliability of these power transistors.

These packages and the heat sinks block the moving of the cooling air. The usage of additional heat sinks introduces mechanical and electrical issues, e.g. the heat sinks are not very stable in vibrating equipment, they may need electrical isolation, etc.

Figure 1: Circuit of the OA with six low power transistors instead of OA with two more powerful transistors with or without heat sinks.

Circuits with transistors
Let us consider the following pairs of NPN/PNP transistors frequently used in audio drivers:

TIP29/TIP30 (NPN/PNP, 40V, 1A, 2W, Ftmin = 3MHz, TO-220),

BD139/BD140 (NPN/PNP, 80V, 1.5A, 1.25W, Ftmin >3MHz or not specified, TO-126)

BC639/BC640 (NPN/PNP, 80V, 1A, 0.8W, Ft=130MHz/50MHz, TO-92)

BC327/BC337 (NPN/PNP, 45V, 0.8A, 0.625W, Ft(typ). 100MHz/100MHz, TO-92)

BC550/BC560 (NPN/PNP, 45V, 0.1A, 0.5W, Ftmin =100MHz/100MHz, TO-92)

Some of the parameters of these transistors may differ from different manufacturers or may be not specified from all manufacturers.

We can see that the power dissipation of two paralleled BC639 is around 1.6W and that is above the power dissipation of single BD135/137/139 with power dissipation of 1.25W.

Also the guaranteed transitional frequency of the BC639/BC640 pair is much higher than the Ft (which is not always guaranteed in the data sheet ) of the BD139/BD140 pair. The DC gain of the low power transistors is usually much higher than the gain of the more powerful transistors. Consequently we may try to use two or more low power transistors instead of one more powerful transistors with or without a small heat sink.

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