How to select a transistor?

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In summary, the conversation discusses selecting a suitable npn transistor for a circuit with a 1000V and 4mA peak inductance current in the collector side with the emitter grounded. The parameters to consider are Vbe and Vce, with Vce needing to be 1000V. The calculation for Vce (Sat) is needed for both maximum and minimum test conditions. The current should be evaluated in the worst case scenario. The conversation also mentions the importance of considering the Safe Operating Area (SOA) and the signals that will be handled in the circuit. Additionally, heat-sinking the transistor will be necessary. A circuit schematic is attached for reference and the project is based on a HVDC power supply using a DC
  • #1
Khalid
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Say you want to choose a npn transistor which works for 1000V and 4mA (Peak inductance current is around 1A) in the collector side while emitter is grounded.

Which parameters should I look into to select a suitable transistor ? for the voltage should I be looking into Vbe ? or is Vce enough ? I'm thinking in this case Vce should be 1000V. For Vce (Sat) how would I calculate it when it's given for test condition (max or minimum sometimes both). For the current I should be looking into the worst case (peak current) ?

Will there be a problem if transistor which is designed for much higher voltage/current used for much lower values other than cost ?
 
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  • #2
Khalid said:
Say you want to choose a npn transistor which works for 1000V and 4mA (Peak inductance current is around 1A) in the collector side while emitter is grounded.
Can you post the circuit schematic? With transient loads, things like Safe Operating Area (SOA) come into play.

https://www.sciencedirect.com/topics/engineering/operating-area
Also, if you can give us an idea of the signals that will be handled (bandwidth, rise/fall time, etc.), that will also help.

Will you be heat-sinking this transistor?
 
  • #3
berkeman said:
Can you post the circuit schematic? With transient loads, things like Safe Operating Area (SOA) come into play.

https://www.sciencedirect.com/topics/engineering/operating-area
Also, if you can give us an idea of the signals that will be handled (bandwidth, rise/fall time, etc.), that will also help.
I have attached the diagram. I haven't designed the circuit so i do not fully understand every part of it but I get the jest of it, its from a project I found online which is HVDC power supply using DC to DC converter. The project is based on using a 12V battery to get 500V 2mA DC. The project also had a calculator which can measure the value of the components based on the output you desire. (You need to enter Vce sat of T1,Vin,Vout,Current,Diode voltage drop, and timing capacitor value). However I would like to adjust it to 1000V 4mA which means that I would have to change the transistor (T1) and find its Vce (sat). Diode used in the diagram will change too however I don't think will affect the calculations much if at all since the small voltage drop is small. I can link the project page if its allowed here.

berkeman said:
Will you be heat-sinking this transistor?
Yes.
 

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1. How do I determine the correct type of transistor for my circuit?

To select the right transistor for your circuit, you need to consider factors such as the required voltage and current ratings, the type of signal being amplified, and the frequency of operation. You can refer to transistor datasheets or consult with an electronics expert for guidance.

2. What is the difference between a bipolar junction transistor (BJT) and a metal-oxide-semiconductor field-effect transistor (MOSFET)?

BJTs are current-controlled devices, while MOSFETs are voltage-controlled. This means that BJTs require a small current to control a larger current, while MOSFETs require a small voltage to control a larger voltage. Additionally, BJTs are typically used for low-power applications, while MOSFETs are better suited for high-power applications.

3. How do I calculate the power dissipation of a transistor?

The power dissipation of a transistor can be calculated by multiplying the voltage drop across the transistor by the current passing through it. It is important to ensure that the power dissipation does not exceed the maximum rating specified in the transistor datasheet to avoid damaging the device.

4. Can I use a transistor with a higher voltage or current rating than what is required for my circuit?

It is generally not recommended to use a transistor with higher ratings than necessary for your circuit. This can lead to increased power dissipation and potential damage to the transistor. It is best to select a transistor with ratings that closely match the requirements of your circuit.

5. How do I determine the gain of a transistor?

The gain of a transistor can be calculated by dividing the output current by the input current. The gain is typically specified in the transistor datasheet and can vary depending on the operating conditions. It is important to consider the gain when selecting a transistor for amplification applications.

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