Paralleled transistors and regulators eliminate need for heat sinks
Bipolar junction transistors (BJT) seem like old-fashioned electronic components but they can solve 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.
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.
Figure 1 presents the circuit of an audio amplifier built around an Op Amp (OA) with six low power transistors instead of an amplifier built with one OA and a couple of more powerful transistors such as BD135/BD136 without heat sinks.
Figure 1: Circuit of the OA with six low power transistors instead of OA with two more powerful transistors with or without heat sinks.
The equalization resistors in the emitters R6 to R11 should be considered obligatory. These resistors reduce to some degree the differences between the paralleled transistors. They are usually between 2 and 10% of the common load of the amplifier. The voltage drop over these resistors should be measured in order to be sure that the output currents are appropriately divided between all paralleled transistors.
The resistor R5 is also obligatory and should have the minimum applicable value. It reduces the crossover distortion of the amplifier.
IC1 can be any appropriate amplifier, e.g. NE5534/A. It is preferable to use an OA capable of driving a load of at least 600 ohms. We may use an OA with offset adjust pins if we are in need of adjusting the output offset of the amplifier.
The full power supply range of the OA is available under the condition that the OA and the transistors are not overloaded.
We should mention than many OAs have significant quiescent current which heats up the OA. For example:
NE5534/A has Iqmax = 8mA,
the LF355 has Iqmax = 4mA,
the LF356 has Iqmax = 10mA,
NE5532 has Iqmax = 16mA,
RC4560 has Iqmax = 5.7mA,
If we use these and similar OAs at a power supply of +/-15V or greater we may have significant power dissipation in the OA without any input signal. This is especially bad for the OA in SMD packages, e.g. for NE5532 we have 30V*16mA = 540mW and that should be taken into consideration.
The additional high gain, low power transistors require very small output current from the OA and this reduces the thermal risk for the IC. In fact the usage of the additional transistors is also a way to use the maximum peak-to-peak voltage from the OA and reduces the power dissipation in the IC because it will supply less output current to the load.
Low power transistors are faster with lower threshold voltage in the base-emitter junction. They are usually designed for preamplifiers and we can obtain lower THD and IMD with them as compared to more powerful transistors. Also low power transistors usually have higher gain which can be in the range of 400 to 800 and that also can lead to lower THD and IMD.
Paralleling lower power linear regulators instead of one single powerful regulator
Paralleling lower power linear regulators can be advantageous. The approach described above of the paralleling the low power transistors instead of a single more powerful transistor (with or without heat sink) is applicable also to linear voltage regulators as 78xx, 79xx, LM317x, LM337x and similar.
Figure 2 presents the circuit of four paralleled 78Lxx in TO-92 packages instead of a single 78Mxx in a TO-220 or similar package. There is no need to use all capacitors C1 to C8 in every case. We may use single electrolytic and single high frequency capacitors at the input and the output of the group of all paralleled regulators under the condition that we have proper PCB layout. Nevertheless the usage of these capacitors depends on the requirements of the paralleled ICs. In some cases we should put these capacitors next to each IC.
Figure 2: Circuit of the four paralleled 78Lxx in TO-92 instead of single 78Mxx in package TO-220
The resistors R1 to R4 are obligatory. The exact values of these resistors depend on the tolerance of the regulators, the number of the regulators, the average and the maximum output current from each regulator.
From that point of view it is preferable to use regulators with a tolerance of +/-2% or better.
The standard procedure of calculating the equalization resistors R1 to R4 is applicable in this case. For example if we have two 78L15 regulators in parallel with an output voltage of 15V +/-2%, the output voltage of the regulators can be from 14.7V to 15.3V.
In the worst case from the first regulator we will have 15.3V and from the other regulator we will have 14.7V.
We may wish to have both regulator output currents below the maximum current for each regulator, e.g. below 100mA per regulator.
If we have equalization resistors of 10 ohms and maximum output current of 100mA, the first regulator will produce 100mA at the output of 14.3V and the second regulator will produce 40mA at the load with the same voltage of 14.3V. (Just for reference we will say that 15V +/-10% is from 13.5V to 16.5V and 15V+/-5% is from 14.25V to 15.75V.)
Moreover the first regulator will heat up more and its output voltage will drop because the output voltage has a negative temperature coefficient. As consequence the first regulator will produce less than 100mA and the second regulator will produce more than 40mA. In total both regulators will produce at least 140mA at voltage of 14.3V or higher.
Although both regulators do not have the same output current, that is not a problem because they are not overloaded and we use smaller and with lower cost regulators 78L15, compared to 78M15. One more advantage that if one of the regulators gives and open circuit due to some reason, the other regulator will operate for some time after the failure of the first regulator.
This short article proposes to take into consideration the usage of a large number paralleled, low-power transistors or low power regulators or other low power components instead of a single more powerful transistor or a more powerful regulator with an additional heat sink.
This is especially advantageous when we work with operational amplifiers. Also it can be a profitable option to the solution with the paralleling many OAs and can reduce the power dissipation of the OAs,
This approach can be also used with linear voltage regulators as 78xx, 79xx, LM317x, LM337x and similar.
This approach has a great many advantages and some of them are mentioned above, e.g.
* easier montage of the smaller, cheaper and faster components,
* better mechanical resistance of the montage,
* usage of equalization resistors for diagnostic purposes to measure the current between the paralleled elements, etc.
In case of need we may use paralleled Darlington transistors.
Frequently in the system enclosure we will have one or more running cooling fans. In that case the usage of a large number of smaller packages as TO-92 without additional heat sinks can be also more profitable because the smaller packages present smaller barriers for the moving cooling air. We can also remove the heat from the electronic components from all sides of the package and that increases the effectiveness of the cooling of all components on the PCB.
The development and the production of the PCB for smaller packages can be easier than the development and the production of the PCB with more powerful and heavier packages with or without heat sinks.
PCBs with smaller packages are more resistant to mechanical vibrations and shocks and that is important for mobile equipment.