The transistor referred here is the bipolar junction transistor or BJT. Here is the complete guide on how to select a transistor. If you follow these, you can assure a rock solid design.
12 Key Items on How to Select a Transistor
The transistor part used in this article is from Diodes Incorporated. They are offering great options for your products. Visit them at https://www.diodes.com/.
1. Collector Current
From the transistor datasheet, look for the collector current rating (IC). For instance the transistor MJD2873Q-13 from Diodes Incorporated, the collector current is specified in below table as Continuous Collector Current. The maximum limit is 2A. Thus, in your design, do not exceed the actual collector current higher to this level. Set the actual collector current to only 50% of the maximum rating and your design will be fine. You can set to higher than 50% actually but be careful and ensure your actual current calculation is accurate enough.
With a high collector current of operation, the transistor will have a shorter life that expected. Exceeding the maximum collector current will damage the transistor immediately.
2. Peak Pulse Collector Current (ICM)
This rating is important when the transistor is used in application in which the collector current is not straight or pure DC, for example, in switching converter, PSU and inverters. Below current waveform is a perfect example. As you can see, the current is not a pure straight line but there is a triangular component.
From the datasheet, look for the Peak Pulse Collector Current and ensure that the actual peak current of you design is not exceeding the rating. Limiting the value to 50% is still a very good idea. Higher limits will shorten the life of the transistor.
3. Collector-Emitter Voltage (VCEO)
The first two important ratings above on how to select a transistor are both current. Another equally important rating is the Collector-Emitter Voltage. Actually, this is the voltage seen by the transistor when the base is open. To measure this, simply get a volt meter. Put the positive probe to the collector while the negative probe to the emitter.
Same with the current ratings, setting to 50% limit is a good idea. However, in high voltage and high power applications, 50% limit is expensive. Why is this? For example in an inverter circuit, the actual voltage seen in the collector is 1000V (this is the maximum value). 50% limit means the transistor voltage rating must be 2000V. 2000V is not common and very expensive so making the design not practical. Instead, the next abundantly available voltage rating is 1200V so use it. This is still okay as long as you have conducted robustness and reliability testing.
For low voltage application on the other hand, there are lot of transistor options that do not increase cost, so this is not an issue.
4. Emitter-Base Voltage (VEBO)
This is the voltage across the emitter to base junction while the collector is open. The base-emitter of a transistor is basically a diode. In other words, the emitter-base voltage is the maximum reverse voltage that can be applied across to this diode.
Take note not to exceed this value. Otherwise, the transistor will get damage right away.
5. Collector-Base Voltage (VCBO)
This is the voltage across the collector to base junction when the emitter is open. The base-collector of a transistor is a diode. So, the collector-base voltage is the maximum reverse voltage that can be applied across to this diode. Take note not to exceed this value. Otherwise, the transistor will get damage right away.
6. Saturation Voltage
Another parameter that is important is the saturation voltage. The collector – emitter saturation voltage is needed in order to compute the actual power dissipation of transistor. The ideal case is that this power dissipation is low. In order to attain it, the collector – emitter saturation voltage must be very low.
The base – emitter saturation voltage is another important thing in selecting a transistor. This is still related to the actual power dissipation. The lower this value, the lower the power dissipation due to the base current. The base – emitter saturation voltage is also an indicative value that needs to be satisfied by the circuit bias. Otherwise, the transistor will not turn on properly.
7. Power Dissipation
The next very important rating of a transistor is the power dissipation. It is given in the datasheet like below.
However, the provided value in the table is taken at nominal temperature which is usually at 25’C. This means, when the operating temperature is not anymore 25’C, the values in the table above is no longer valid. In the table above, the power dissipation is specified in Note 5, 6 and 7.
Compute the actual power dissipation of the circuit and do not let it exceeds the values specified in the table above. A 50% limit is a good value. For instance at Note 5, the power dissipation rating is 2.6W, thus limit the actual power dissipation to 1.3W.
8. Thermal Resistance
When the transistor is use to operate at temperature more than the typical value, thermal resistance is needed to get the maximum power rating of the transistor. This is also called the de-rated power. Thermal resistance is could be defined as junction to ambient or junction to case. In below table for instance, it is junction to ambient.
For instance the actual temperature of operation is 50’C; the power dissipation of the transistor is not anymore the same as specified in the table above. Continue reading below to learn on how to compute the new power dissipation.
9. Operating Temperature
Operating the transistor outside the operating range will lead to immediate failure. In below table, both operating and storage temperatures are provided.
Always follow the datasheet. However, it does not mean that you can operate the transistor always at +100’C since it is way lower than maximum limit of +150’C. You need to do little math to check if it is possible. The next topic will discuss this, so keep reading.
How to Compute the De-rated Power Dissipation
The datasheet usually provide the power rating at 25’C like in the table above. This is not useful anymore when the transistor is operating above 25’C.
The de-rated power is can be computed using below equation.
Power de-rated = (Tjmax – Tambient) / Rthja
Tjmax is the maximum junction temperature specified in the datasheet, Tambient on the other hand is the actual ambient temperature of the transistor while Rthja is the thermal resistance from junction to ambient. So, using the values provided above; Tjmax = +150’C and Rthja = 48’C/W at note 5 and the actual operating ambient temperature is 50’C.
Power de-rated = (Tjmax – Tambient) / Rthja = (150 – 50) / 48 = 2.08 W
The resulting power rating or the de-rated power is 2.08W. This is lower than the power dissipation provided in the table above which is 2.6W at Note 5.
Given the actual power dissipation, the maximum junction temperature that the transistor could operate is can be derived.
Actual Power Dissipation = (Tjmax – Tambient) / Rthja
Tjmax = Actual Power Dissipation X Rthja + Tambient
Assuming the actual power dissipation is 3W, the actual maximum junction temperature at an ambient temperature of 50’C is
Tjmax = Actual Power Dissipation X Rthja + Tambient = 3W X 48’C/W + 50’C = 194’C
The maximum junction temperature specified in the datasheet is only 150’C, thus the transistor will surely get damaged.
10. Gain and Bandwidth
All considerations above on how to select a transistor are about robustness and reliability related. On the other hand, gain and bandwidth are both needed when the transistor is used as an amplifier. Gain or DC current gain, beta of sometimes called HFE is needed to compute the collector current. From the term gain, it will amplify a small base current and convert to a big collector current.
The bandwidth is also important when operating as an amplifier. The amplifier will not give quality audio when the bandwidth is not sufficient.
11. Dynamic Parameters
When the transistor is used as a high speed switch like those in switching converters, power supplies, inverters and the likes, the dynamic parameters like output capacitance, input capacitance, delay and rise time and fall time is important. Imagine how fast the transistor is switching on and off and the rise and fall times are long, this will result to a very high switching losses. Switching losses is just another form of power dissipation but due to the continuous switching of the transistor.
12. Transistor Type, NPN or PNP
The last thing to take note on how to select a transistor is the type. You must know if you need a NPN or PNP type. These two are totally different. NPN is best to use for low side driving and switches while PNP is best to use as high side driving and switch. The electrical analysis is common for both type however if you are not usually used the PNP, it is little confusing.
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