NPN transistor overheating even when used with appropriate parameters

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Discussion Overview

The discussion revolves around the overheating issues encountered when using an NPN TIP35C transistor in a current control circuit. Participants explore potential causes of the overheating, including the need for a heat sink, thermal calculations, and alternative components. The scope includes technical explanations, design considerations, and suggestions for circuit modifications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports overheating of the TIP35C transistor despite calculations suggesting power dissipation is within limits.
  • Another participant questions whether thermal calculations for a heat sink were performed, implying that a proper heat sink is necessary.
  • A suggestion is made to provide a schematic and describe the load resistance to better understand the issue.
  • Thermal calculations are discussed, indicating that without a heat sink, the transistor can only dissipate a limited amount of power, suggesting a need for a larger heat sink.
  • One participant proposes switching to an N-channel FET as a potentially better alternative for the application, citing high power dissipation concerns with the current transistor choice.
  • There is mention of design specifications and considerations for the circuit, including cooling management and efficiency.
  • Confusion arises regarding whether the circuit is intended as a linear regulator or a switching topology, with participants noting that different comments may apply based on the circuit type.
  • One participant emphasizes the importance of accounting for extreme temperature conditions when designing for heat dissipation.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of a heat sink, the appropriateness of the chosen transistor, and the overall design approach. No consensus is reached regarding the best solution or the exact cause of the overheating issue.

Contextual Notes

Participants highlight the need for detailed thermal calculations and the importance of understanding the circuit topology. There are unresolved questions about the load characteristics and the specific design requirements for the circuit.

Who May Find This Useful

Individuals working on transistor-based current control circuits, those interested in thermal management in electronic designs, and participants looking to understand the implications of component selection in high-power applications.

Erik_clifton102
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TL;DR
Using TIP31C NPN transistor to amplify a current from 0A to 10A with 10V, but the transistor is over heating, max loads are 100V and up to 25A if I have read the data sheet correctly.
I have been trying to build a simple Current control circuit using a NPN TIP35C transistor but have run into the problem of it constantly over heating and being destroyed. The transistor is the only component that heats up whilst it is on. The supply voltage is 10V, well below the Max and at the moment I am only running a max of 7A through the transistor. If I have also done my calculations a am well below the power dissipation for the transistor.
what could I possibly be doing wrong or what have a missed? or do I just need a heat sink to dissipate the heat?
I have attached the Datasheet for the transistor below.
 

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Erik_clifton102 said:
If I have also done my calculations a am well below the power dissipation for the transistor.
Did you do the calculations for the heatsink (you have a proper heatsink, right?) too?
 
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Please describe the load resistance. Or even better show us your schematic. We can't answer without that information.

Have you done the thermal calculation with the worst case power dissipation and using the thermal resistance parameters in the data sheet. Using TJ ≤ 130C, TA ≤ 50C (a fairly conservative design), this device can only dissipate PD ≤ 2.24W without a heat sink. At IC ≤ 7A, you are limited to VCE ≤ 0.32V. So, yes, you undoubtedly need a heatsink, probably a big one.

Read these if you aren't familiar with this sort of thermal calculation, it can be a bit complicated.

https://www.ti.com/lit/an/slva462
https://components101.com/articles/selecting-the-right-heatsink
https://cheever.domains.swarthmore.edu/Class/e12Code/HEAT-NOTE.pdf
 
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Erik[QUOTE=Erik_clifton102 said:
TL;DR Summary: Using TIP31C NPN transistor to amplify a current from 0A to 10A with 10V, but the transistor is over heating, max loads are 100V and up to 25A if I have read the data sheet correctly.
...
what could I possibly be doing wrong or what have a missed? or do I just need a heat sink to dissipate the heat?
I have attached the Datasheet for the transistor below.

With Ic=25A, Ib=5A Vce(sat)=4V thus 100W and now you need a forced air cooled 100W heatsink.
This is a poor choice. Consider a Nch FET switch instead. rated > 100V with flyback diode added and possibly rated at 50A so you can operate at the 50% of max rated temp for better reliability.

regards, Tony EE since'75
 
Rive said:
Did you do the calculations for the heatsink (you have a proper heatsink, right?) too?
Originally I didn't have a heat sink, looking into one now but from other advice I'm looking at using a FET instead.
 
DaveE said:
Please describe the load resistance. Or even better show us your schematic. We can't answer without that information.

Have you done the thermal calculation with the worst case power dissipation and using the thermal resistance parameters in the data sheet. Using TJ ≤ 130C, TA ≤ 50C (a fairly conservative design), this device can only dissipate PD ≤ 2.24W without a heat sink. At IC ≤ 7A, you are limited to VCE ≤ 0.32V. So, yes, you undoubtedly need a heatsink, probably a big one.

Read these if you aren't familiar with this sort of thermal calculation, it can be a bit complicated.

https://www.ti.com/lit/an/slva462
https://components101.com/articles/selecting-the-right-heatsink
https://cheever.domains.swarthmore.edu/Class/e12Code/HEAT-NOTE.pdf
I thought I could originally use it without a heatsink as it was going to only be used for about half of its capabilities. but definitely looking into one now.
thank you as well for the links
 
TonyStewart said:
With Ic=25A, Ib=5A Vce(sat)=4V thus 100W and now you need a forced air cooled 100W heatsink.
This is a poor choice. Consider a Nch FET switch instead. rated > 100V with flyback diode added and possibly rated at 50A so you can operate at the 50% of max rated temp for better reliability.

regards, Tony EE since'75Thanks for that advice, i did a bit more research and think that what you suggested would be better suited for my little project.
 
- start with all design specs, then design it.
- must haves, nice to haves, inputs control, output , power cooling management switching speed PWM? slew rate, diode recovery time, efficiency, max temp rise, etc
 
Last edited:
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I thought it was a linear regulator design, but now I don't know why. Maybe the use of the term "amplify". Anyway, some of the comments apply to switching topologies, some to linear topologies. Keep that in mind; know which sort of circuit you are asking about (extra credit: tell us too).
 
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  • #10
DaveE said:
I thought it was a linear regulator design, but now I don't know why. Maybe the use of the term "amplify". Anyway, some of the comments apply to switching topologies, some to linear topologies. Keep that in mind; know which sort of circuit you are asking about (extra credit: tell us too).
DaveE said:
I thought it was a linear regulator design, but now I don't know why. Maybe the use of the term "amplify". Anyway, some of the comments apply to switching topologies, some to linear topologies. Keep that in mind; know which sort of circuit you are asking about (extra credit: tell us too).
I'm trying to use the transistor to amplify and regulate the current for a load, I've been using a simple voltage divider paired with a pot to regulate the current. Anything over 5A at 10V starts to heat the transistor dramatically so I was wondering weather I need a heat sink or I need to change my way of regulating the current allowed through the transistor, or maybe even change the transistor to a MOSFET.
 
  • #11
1st you need to answer my questions in #8.

what is the load and voltage drop * current on every part in schematic. It may need to be redesigned.
 
  • #12
Belated, but please allow for 'Stinkin' Hot Days' with your heat-sinking.
Else, sure as Murphy, you'll get a day that's pushing past 40ºC in the shade...
 

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