How to Select a Transistor For Switch and Linear Applications

There are few things to consider on how to select a transistor. Specific application requires a perfect match device so that expected functionality will be obtained. For instance, if the application is switching, a device that is more suited for switching must be selected. It follows with an active application. Below transistor selection guide is based on actual experience. Continue reading below to digest everything about how to select a transistor.

How to Select a Transistor – Non Continuous Switching

 

Non-continuous switching means that the transistor is used to operate as a switch but not continuously switching between high and low such in PWM converters. Example of these are relay driver of an automobile front wiper, headlamps, fog lamps, remote turn on and off switch and the likes. In using a the transistor as a switch like this, the main parameters to take note are:

 

1. Device Current Gain (ß)

Number one in the list on how to select a transistor for switch application is current gain or beta. Select a device with a higher minimum current gain or beta. A higher beta device can be easily driven into saturation. For example the available parts are having beta of 160-400, 100-300 and 200-400; select the 200-400 beta range. When configuring transistor as a switch, the circuit gain is no longer dependent to the device gain. To know why device beta is important to consider when operating transistor as a switch, read THIS.

 

2. Continuous Current Rating

Select a device with a DC current rating that is high enough compare to the actual load. You can consider the 70% stress allowance. For example the continuous current of the circuit (specifically collector current) is 200mA, select a device with a current rating of at least 285mA. In actual design for a circuit continuous current of 200mA the nearest standard value would be 500mA transistor.

It is also important to consider the continuous current rating with respect to ambient temperature. For power transistor, the current versus case temperature must be considered as well. For example, the maximum ambient or case temperature the circuit may expose to is 50’C, look for the equivalent current rating to this temperature and from this current rating provide a 70% stress margin as mentioned above.

 

3. Single Pulse Peak Current

If the circuit has transient or inrush current, include on your checklist on how to select a transistor the single pulse peak current rating. You have to estimate (or measure) the inrush current and the capability of the device must be higher than this. Maintain 70% stress.

 

4. Power Dissipation

Check the power dissipation. Compute for the transistor power dissipation and compare it to the rating. The power dissipation of the transistor is the sum of the dissipation of the base-emitter and collector-emitter drops.

 

PdissipationTotal=VBE\times Ib+VCE\times Ic

 

The power dissipation rating of any transistors is given in the datasheet. In most cases the given value is taken from nominal conditions such as nominal ambient temperature. If your circuit is subjected to high temperatures, get the equivalent power dissipation rating of the device that corresponds to the maximum temperature. Some manufacturers provided power dissipation versus temperature graph in their datasheet. In case there is none, you can use below equation.

For low power transistors

 

PdissCapability=\frac{Tjmax-Tamax}{Rthja}

 

For high power transistors

 

PdissCapability=\frac{Tjmax-Tcmax}{Rthjc}

 

Where;

  • PdissCapability is the power dissipation the device can handle on a specific operating temperature.
  • Tjmax is the maximum junction temperature of the device can operate
  • Tamax is the maximum operating ambient temperature
  • Tcmax is the maximum case temperature of the device
  • Rthja is the thermal resistance from junction to ambient
  • Rthjc is the thermal resistance from junction to case

For example, a certain transistor has Tjmax=150’C and Rthja=200K/W and it will be exposed to 120’C, compute for the power capability of the transistor.

 

PdissCapability=\frac{Tjmax-Tamax}{Rthja}=\frac{150'C-120'C}{200K/W}=150mW

 

The transistor power handling capability at 120’C is only 150mW. The computed actual power dissipation of the device must not exceed this. Consider a maximum power stress of 70%.

 

5. Collector-emitter voltage with open base (VCEO)

This is very important. The VCEO rating of the device must be higher than the collector supply voltage by 30% margin. Supposing the collector supply is 35V, the VCEO rating of the device must be around 45V. When the load is a relay coil, you cannot rely on the transistor VCEO rating because a relay coil will produce a kickback voltage by the moment the transistor is cut-off. This kickback voltage is very high and you may need additional clamping element such as free-wheeling diode or a transient voltage suppressor.

 

6. Operating Temperature Range

Do not forget this parameter. If your design is to be used in North America wherein the temperature is reaching below zero, you need to select a transistor that can operate at negative temperature. On the other hand if your design is exposed to very hot environment such as in Africa or installed under the hood of an automobile, you must consider using a transistor with a maximum operating temperature of higher than 100’C.

 

7. Collector-emitter saturation voltage

In using transistor as a switch, the collector-emitter voltage must be low enough so that the logic low requirement will easily meet. A very low collector-emitter saturation voltage will also reduce transistor power stress.

 

How to Select a Transistor – For Continuous or Repetitive Switching

 

Here are the important parameters to take note for application wherein there is continuous transitions between saturation and cut-off.

 

1. Device current gain

Select a device with a higher minimum current gain or beta. A higher beta device can be easily driven into saturation. For example the available parts are having beta of 160-400, 100-300 and 200-400; select the 200-400 beta range.

 

2. Continuous Current Rating

Select a device with a DC current rating that is high enough compare to the DC level of actual load. You can consider the 70% stress allowance. If the transistor is used in switching converter, the collector current is not pure DC; it can be triangular, trapezoidal, pulsating DC or the likes such as Figure 1 below. Get the DC equivalent of these waveforms and compare it to the continuous current rating of the transistor.

For example the continuous current of the circuit is 2A, select a device with a current rating of at least 2.85A.

It is also important to consider the continuous current rating with respect to ambient temperature. For power transistor, the current versus case temperature must be considered as well. For example, the maximum ambient or case temperature the circuit may expose to is 50’C, look for the equivalent current rating to this temperature and from this current rating provide a 70% stress margin as mentioned above.

 

3. RMS Current Rating

Some manufacturers give this data also in their datasheet. The same with item number 2, get the equivalent RMS of the non-DC waveform and compare it to this rating. Again maintain a maximum 70% stress.

 

4. Peak Repetitive Current

For application such as switching converter wherein the transistor is operating between cut-off and saturation repetitively, the peak repetitive collector current must be take note. Below figure shows the peak repetitive current.

 

How to Select a Transistor
Figure 1 – This is the collector current of a power transistor used as a switch in a CCM boost converter


5. Single Pulse Peak Current

This is different from the peak repetitive current in item 4. This is a single pulse current that caused by inrush or transient conditions. You have to estimate (or measure/simulate) the actual inrush current and the capability of the device must be higher than this. Maintain 70% stress.

 

6. Power Dissipation

Check the power dissipation. Compute for the transistor power dissipation and compare it to the rating. The power dissipation of the transistor is the sum of the dissipation of the base-emitter and collector-emitter drop plus the switching losses.

 

PdissipationTotal=VBE\times Ib+VCE\times Ic+Switching Losses

 

Switching losses is due to the output capacitance, turn on and off times or rise and fall times. The base capacitance is also a contributor on this.

The power dissipation capability of any transistors is given in the datasheet. In most cases the given value is taken from nominal conditions such as nominal ambient temperature. If your circuit is subjected to high temperatures, get the equivalent power dissipation capability of the device that corresponds to the maximum temperature. Some manufacturers provided power dissipation versus temperature graph in their datasheet. In case there is none, you can use below equation.

For low power transistors

 

PdissCapability=\frac{Tjmax-Tamax}{Rthja}

 

For high power transistors

 

PdissCapability=\frac{Tjmax-Tcmax}{Rthjc}

 

Where;

  • PdissCapability is the power dissipation the device can handle on a specific operating temperature.
  • Tjmax is the maximum junction temperature of the device can operate
  • Tamax is the maximum operating ambient temperature
  • Tcmax is the maximum case temperature of the device
  • Rthja is the thermal resistance from junction to ambient
  • Rthjc is the thermal resistance from junction to case

For example, a certain transistor has  Tjmax=150’C and Rthjc=20K/W  and the estimated maximum case temperature is  100’C, compute for the power capability of the transistor.

 

PdissipationCapability=\frac{Tjmax-Tcmax}{Rthjc}=\frac{150'C-100'C}{20K/W}=2.5W

 

The transistor power handling capability at 100’C case temperature is only 2.5W. The computed actual power dissipation of the device must not exceed this. Consider a maximum power stress of 70%.

 

7. Collector-emitter voltage with open base (VCEO)

This is very important. The VCEO rating of the device must be higher than the collector supply voltage by 30% margin. Supposing the collector supply is 35V, the VCEO rating of the device must be around 45V. When the load is a relay coil, you cannot rely on the transistor VCEO rating because a relay coil will produce a kickback voltage by the moment the transistor is cut-off. This kickback voltage is very high and you may need additional clamping element such as free-wheeling diode or a transient voltage suppressor.

 

8. Dynamic Characteristics

Select a transistor that has low output capacitance. Higher capacitance may slow down the response of the transistor and it may contribute to the total loss/dissipation.

Consider the turn on and turn off time as well. Faster turn on and turn off times is better for some applications. However some applications do not need a very fast turn on and off times.

Select a transistor that has a fast forward recovery time for the body diode.

 

9. Operating Temperature Range

Do not forget this parameter. If your design is to be used in North America wherein the temperature is reaching below zero, you need to select a transistor that can operate at negative temperature. On the other hand if your design is exposed to very hot environment such as in Africa or installed under the hood of an automobile, you must consider using a transistor with a maximum operating temperature of higher than 100’C.

 

10. Collector-emitter saturation voltage

Since the application is a switch, you may need a very low saturation voltage so that logic low level is near zero. A very low collector-emitter saturation voltage will also reduce transistor power stress.

 

How to Select a Transistor – Linear/Active Operation

These are the main parameters to take note on how to select a transistor for linear operation.

 

1. Device current gain

Select a transistor with a higher gain and with a tight range. A higher gain is good for amplification and linear operation. A tight gain range prevents the output from varying a lot, so the operation is stable.

 

2. Consider the Bandwidth and Frequency Specifications for high frequency applications

 

3. Continuous Current Rating

Select a transistor with a DC current rating that is high enough compare to the actual load. You can consider the 70% stress allowance. For example the continuous current of the circuit (specifically collector current) is 200mA, select a device with a current rating of at least 285mA. In actual design for a circuit continuous current of 200mA the nearest standard value would be 500mA transistor.

It is also important to consider the continuous current rating of the transistor with respect to ambient temperature. For power transistor, the current versus case temperature must be considered as well. For example, the maximum ambient or case temperature the circuit may expose to is 50’C, look for the equivalent current rating to this temperature and from this current rating provide a 70% stress margin as mentioned above.

 

4. RMS Current Rating

If the collector current waveform is non-DC, get its RMS value and compare it to the rating of the device you want to use. Always consider a 70% maximum stress.

 

5. Single Pulse Peak Current

If the circuit has transient or inrush current, select a transistor that has inrush capability. You have to estimate (or measure the actual inrush current) the inrush current and the capability of the device must be higher than this. Maintain 70% stress.

 

6. Power Dissipation

Check the power dissipation of transistor. Compute for the transistor power dissipation and compare it to the rating. The power dissipation of the transistor is the sum of the dissipation of the base-emitter and collector-emitter drops.

 

PdissipationTotal=VBE\times Ib+VCE\times Ic

 

The power dissipation of any transistors is given in the datasheet. In most cases the given value is taken from nominal conditions such as nominal ambient temperature. If your circuit is subjected to high temperatures, get the equivalent power dissipation of the device that corresponds to the maximum temperature. Some manufacturers provided power dissipation versus temperature graph in their datasheet. In case there is none, you can use below equation.

For low power transistors

 

PdissipationCapability=\frac{Tjmax-Tamax}{Rthja}

 

For power transistors

 

PdissipationCapability=\frac{Tjmax-Tcmax}{Rthjc}

 

Where;

  • PdissCapability is the power dissipation the device can handle on a specific operating temperature.
  • Tjmax is the maximum junction temperature of the device can operate
  • Tamax is the maximum operating ambient temperature
  • Tcmax is the maximum case temperature of the device
  • Rthja is the thermal resistance from junction to ambient
  • Rthjc is the thermal resistance from junction to case

7. Collector-emitter voltage with open base (VCEO)

This is very important. The VCEO rating of the device must be higher than the collector supply voltage by 30% margin. Supposing the collector supply is 35V, the VCEO rating of the device must be around 45V. When the load is a relay coil, you cannot rely on the transistor VCEO rating because a relay coil will produce a kickback voltage by the moment the transistor is cut-off. This kickback voltage is very high and you may need additional clamping element such as free-wheeling diode or a transient voltage suppressor.

 

8. Operating Temperature Range

Do not forget this parameter. If your design is to be used in North America wherein the temperature is reaching below zero, you need to select a transistor that can operate at negative temperature. On the other hand if your design is exposed to very hot environment such as in Africa or installed under the hood of an automobile, you must consider using a transistor with a maximum operating temperature of higher than 100’C.

 

9. Collector-emitter saturation voltage

Since the application is a switch, you may need a very low saturation voltage so that logic low level is near zero. A very low collector-emitter saturation voltage will also reduce transistor power stress.

 

Aside from all the things mentioned above on how to select a transistor; there are some other parameters that you need to take care in dealing with transistors. Always refer to the datasheet for complete coverage of the parameters to comply when using a transistors.

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