Bipolar junction transistors (BJTs) look like old-fashioned electronic components, but they solve many problems due to their low cost and excellent parameters. We can find new applications that were not possible in the past due to the high cost of these components. For example, we can replace more power transistors (with or without heat sinks) with multiple parallel small power transistors in some cases, and Many benefits are gained from it.
In general, small power transistors are faster, have higher operating frequencies, lower noise, less total harmonic distortion, and their higher power, especially for transistors with bulk heat sinks. The package is more convenient for manual and automatic soldering.
Many transistors with a power consumption of up to about 1W are packaged in a TO-92-like package. Most of these transistors are relatively inexpensive and can be purchased in large quantities, and the TO-92 package is easy to use.
The heat generated from these packages is easily dissipated efficiently by cooling fans or even normal air convection. In addition, we can take advantage of the large copper surface area around these transistors to increase their power consumption. For the different packages of these electronic components, a large amount of heat dissipation information and calculation methods are recorded in their data sheets and literatures, so we will not discuss them in detail here.
Power transistor packages such as the TO-126 and TO-220 are large and heavy, are difficult to mount on the PCB, and additional heat sinks are needed to maximize the performance and reliability of these power transistors.
These packages and heat sinks block the flow of cooling air, and the use of additional heat sinks creates mechanical and electrical problems. For example, in vibration equipment, the heat sinks are not very stable, they require electrical isolation.
Transistor circuit
Let us consider the following NPN/PNP transistor pairs that are often 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 these parameters may vary from manufacturer to manufacturer, and some may not be labeled by all manufacturers.
We can see that the power consumption of the two parallel BC639 is about 1.6W, which exceeds the power consumption of a single BD135/137/139 1.25W.
In addition, the guaranteed conversion frequency of the BC639/BC640 is much higher than the Ft of the BD139/BD140 pair (not always guaranteed in the data sheet). The DC gain of a small power transistor is typically much higher than the gain of a larger transistor. So we can try to replace two more power transistors with or without a small heat sink with two or more small power transistors.
Figure 1 shows an audio amplifier circuit with one op amp (OA) and six low-power transistors, which can be replaced by an op amp and a pair of larger power transistors without a heat sink (such as BD135/BD136). Amplifier circuit.
Figure 1 Circuit of one op amp and six low-power transistors instead of one op amp plus two larger power transistors
It is necessary to connect the equalizing resistors R6 to R11 of the emitter. These resistors can reduce the difference between the parallel transistors to some extent. Their resistance is typically between 2% and 10% of the amplifier's common load. To ensure proper distribution of the output current between all parallel transistors, the voltage drop across these resistors should be monitored.
Resistor R5 is also necessary and should have a minimum applicable value. It reduces the crossover distortion of the amplifier.
IC1 can be any suitable amplifier, such as the NE5534/A. It is best to use an op amp that can drive a load of at least 600Ω. If you need to adjust the amplifier's output offset, you can use an op amp with an offset adjustment pin.
The entire supply voltage range of the op amp can be obtained without overloading the op amp and transistor.
We should note that many op amps have large quiescent currents that can cause the op amp to heat up. for example:
The maximum quiescent current of the NE5534/A is Iqmax = 8mA.
The maximum quiescent current of the LF355 is Iqmax = 4mA.
The maximum quiescent current of the LF356 is Iqmax = 10mA.
The maximum quiescent current of the NE5532 is Iqmax = 16mA.
The maximum quiescent current of the RC4560 is Iqmax = 5.7mA.
If we use these and similar op amps at a supply voltage of ±15V or higher, these op amps will also have significant power consumption without any input signals. This is especially bad for op amps with surface-mount packages. For example, the power consumption of the NE5532 will be 30V*16mA = 540mW, which should be carefully considered.
The newly added high-gain small-power transistor requires a small current output from the op amp, thus reducing the risk of heat dissipation of the op amp IC. In fact, these new transistors can also be used to reduce the power consumption of the op amp IC with the maximum peak-to-peak voltage of the op amp because it provides less output current to the load.
Small power transistors are faster and have lower threshold voltages in the base-emitter junctions. They are typically designed for preamplifiers that can achieve lower total harmonic distortion (THD) and intermodulation distortion (IMD) than larger power transistors. Small power transistors typically also have higher gains with gains ranging from 400 to 800, which is one reason for lower THD and IMD. Parallel smaller power linear regulators replace single high power regulators
There are many benefits to using a smaller power linear regulator in parallel. The above method of replacing a small high-power transistor with a single high-power transistor (with or without a heat sink) is equally applicable to linear regulators such as the 78xx, 79xx, LM317x, LM337x, and similar devices.
Figure 2 shows four 78Lxx parallel circuits in a TO-92 package that can replace a single 78Mxx circuit in a TO-220 or similar package. It is not necessary to use all of the capacitors from C1 to C8 in each case. As long as we design the correct PCB layout, we can use a single electrolytic capacitor and a single high frequency capacitor at the input and output of all shunt regulator banks. However, the use of these capacitors depends on the requirements of the parallel IC. In some cases we should place these capacitors close to each IC.
Figure 2 Four 78Lxx parallel circuits in a TO-92 package can replace a single 78Mxx circuit in a TO-220 package.
Resistors R1 to R4 are necessary. The actual resistance of these resistors depends on the tolerance of the regulator, the number of regulators, and the average output current and maximum output current of each regulator.
From this perspective, it is best to use a regulator with a tolerance of ±2% or better.
In this case, the standard R1 to R4 equalization resistance calculation process can be used. For example, if we use two 78L15 regulators with an output voltage of 15V ± 2% in parallel, the output voltage of these two regulators may range from 14.7V to 15.3V.
In the worst case, the output of the first regulator is 15.3V and the output of the other regulator is 14.7V.
We want the output currents of both regulators to be below their respective maximum currents, such as below 100mA per regulator.
If the equalization resistance is 10Ω and the regulator's maximum output current is 100mA, then the first regulator will generate 100mA at an output voltage of 14.3V, and the second regulator will output the same 14.3V. A 40 mA current is generated across the load. (For reference purposes only, we can think that 15V ± 10% is from 13.5V to 16.5V, and 15V ± 5% is from 14.25V to 15.75V.)
In addition, the first regulator will generate more heat and its output voltage will drop because the output voltage has a negative temperature coefficient. So the first regulator will generate less than 100mA and the second regulator will generate more than 40mA. In summary, the two regulators will generate at least 140mA at 14.3V or higher.
Although the two regulators do not have the same output current, this is not a problem because they are not overloaded, and we are using a 78L15 regulator that is smaller and cheaper than the 78M15. Another advantage is that if one of the regulators has an open circuit fault for some reason, the other regulator can still work for a while.
Summary of this article
This short article suggests considering the use of multiple parallel small power transistors or low power regulators or other low power components to replace a single high power transistor or high power regulator that requires an additional heat sink.
This parallel scheme is more advantageous when using an op amp. There are many benefits to using multiple op amps in parallel, and you can reduce the power consumption of each op amp.
This method is also suitable for linear regulators such as the 78xx, 79xx, LM317x, LM337x and similar devices.
This method has many advantages, some of which are mentioned above, such as:
* Easier to assemble smaller, cheaper and faster components,
* Can get better assembly resistance
* Equalization resistors can be used for diagnostic purposes, such as measuring current between parallel components, etc.
We can use Darlington transistors in parallel when needed.
There are often one or more long-running cooling fans in the system chassis. In this case, it is more advantageous to use a large number of smaller packages such as TO-92 without a heat sink, since smaller packages have less obstruction to the flow of cooling air. We can also dissipate the heat generated by the electronic components from all sides of the package, thereby increasing the cooling efficiency of all components on the PCB.
PCB development and production with smaller packages is much easier to develop and produce than larger and more heavily packaged PCBs with or without heat sinks.
Installing a smaller package PCB is better able to withstand mechanical vibration and shock, which is especially important for mobile devices.
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