Bipolar Junction Transistors (BJTs) look like old-fashioned electronic components, but they can solve many problems due to their advantages of 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 powerful transistors (with or without heat sinks) with multiple low-power transistors in parallel in some cases. Gain many benefits from it.
In general, small power transistors are faster, have higher operating frequencies, lower noise, and less total harmonic distortion than higher power transistors, especially those with large heat sinks, and their The package is more convenient for manual and automatic soldering.
Many transistors that consume up to about 1W consume a similar TO-92 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 can be easily dissipated through cooling fans and even normal air convection. In addition, we can use the larger 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 literature, so we will not discuss them in detail here.
Power transistor packages such as the TO-126 and TO-220 are large and heavy and difficult to mount on the PCB. In order to realize the full performance and reliability of these power transistors, an additional heat sink is required.
These packages and heat sinks can block the flow of cooling air, and the use of additional heat sinks can cause mechanical and electrical problems. For example, in a vibration device, the heat sinks are not very stable and 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 the parameters of these transistors may vary from manufacturer to manufacturer, and some parameters may not be labeled by all manufacturers.
We can see that the power consumption of two parallel BC639s is about 1.6W, which exceeds the power consumption of a single BD135/137/139 1.25W.
In addition, BC639/BC640 has a guaranteed conversion frequency much higher than that of the BD139/BD140 pair (which is not always guaranteed in the data sheet). The DC gain of a small power transistor is usually much higher than the gain of a larger transistor. So we can try to use two or more small power transistors instead of one more power transistor with or without a small heatsink.
Figure 1 shows an audio amplifier circuit using one op amp (OA) and six low-power transistors, which can replace an op amp and a pair of larger power transistors (such as BD135/BD136) without a heat sink Amplifier circuit.
Figure 1 Circuit of one op amp and six low-power transistors instead of one op amp plus two more power transistors
It is necessary to connect the equalizing resistors R6 to R11 of the emitter. These resistors can reduce the difference between parallel transistors to some extent. Their resistance is usually between 2% and 10% of the amplifier's common load. To ensure that the output current is properly distributed across all parallel transistors, the voltage drop across these resistors should be monitored.
Resistor R5 is also necessary and should have the smallest applicable value. It can reduce 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 capable of driving a load of at least 600Ω. If you need to adjust the output offset of the amplifier, you can use an op amp with an offset adjustment pin.
The entire supply voltage range of the op amp can be obtained under conditions that the op amp and the transistor are not overloaded.
We should note that many op amps have a large quiescent current that causes the op amp to heat up. for example:
The maximum quiescent current Iqmax of the NE5534/A is 8mA.
The maximum quiescent current Iqmax of the LF355 is 4mA.
The maximum quiescent current Iqmax of the LF356 is 10mA.
The maximum quiescent current Iqmax of the NE5532 is 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 signal. This situation is especially bad for op amps with surface-mount packages. For example, the power consumption of the NE5532 will reach 30V*16mA = 540mW, which should be carefully considered.
The newly added high-gain, low-power transistor requires the op amp to output a very small current, thus reducing the thermal risk of the op amp IC. In fact, these new transistors can also be used to reduce the op amp IC's power consumption by using the op amp's maximum peak-to-peak voltage, because it provides a smaller output current to the load.
The low-power transistor is faster and the threshold voltage in the base-emitter junction is also lower. They are usually designed for preamplifiers and use them for lower total harmonic distortion (THD) and intermodulation distortion (IMD) than with higher power transistors. Low-power transistors also typically have higher gains, ranging from 400 to 800, which is one reason for lower THD and IMD. Paralleling a lower power linear regulator instead of a single high power regulator
There are many advantages to using a lower power linear regulator in parallel. The above method of paralleling small power transistors instead of a single high power transistor (with or without a heat sink) is also applicable to linear regulators such as the 78xx, 79xx, LM317x, LM337x, and the like.
Figure 2 shows four parallel circuits of the 78Lxx in a TO-92 package that can replace a single 78Mxx circuit using a TO-220 or similar package. It is not necessary to use all C1 to C8 capacitors 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 regulators. 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. The four 78Lxx parallel circuits in the 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 regulator tolerance, the number of regulators, and the average output current and maximum output current of each regulator.
From this point of view, it is best to use a regulator with a tolerance of ±2% or better.
In this case, the standard R1 to R4 equalizer resistance calculation process can be used. For example, if we use two 78L15 regulators with output voltage of 15V±2% in parallel, the output voltage of these two regulators may be 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 less than 100mA per regulator.
If the equalization resistance is 10Ω and the maximum output current of the regulator is 100mA, then the first regulator will generate 100mA when the output voltage is 14.3V, and the second regulator will output the same 14.3V when it outputs the same voltage. 40mA current is generated on the load. (For reference only, we can assume 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. Therefore, 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 do not overload, and we are using the 78L15, a smaller, lower-cost regulator. 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.
This article summary
This short paper suggests using multiple low-power transistors or low-power regulators or other low-power components in parallel instead of a single high-power transistor or high-power regulator that requires an extra heat sink.
This parallel solution has advantages when using op amps. There are many advantages to using multiple op amps in parallel, which can also 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 mentioned above, such as:
* Easier assembly of smaller, cheaper and faster components
* can get better assembly resistance
*Equilibrium resistance can be used for diagnostic purposes, such as measuring the current between parallel componentsWe can use parallel Darlington transistors when needed.
There are often one or more long-running cooling fans in the system chassis. In this case, using a large number of smaller packages without a heat sink like TO-92 is also more advantageous because smaller packages have less hindrance to the flow of cooling air. We can also dissipate the heat generated by electronic components from all sides of the package, thereby increasing the cooling efficiency of all components on the PCB.
The development and production of PCBs with smaller packages is much easier than the development and production of larger and heavier packages with and without heat sinks.
Installing a smaller package PCB can better withstand mechanical vibrations and shocks, which is especially important for mobile devices.
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