Why is my transistor overheating?

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Discussion Overview

The discussion centers around the overheating issue of a transistor in a motor control circuit, specifically involving a TIP3055 transistor driven by a PWM signal from an MSP430 microcontroller. Participants explore potential causes and solutions related to circuit design, component selection, and thermal management.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the base current may be insufficient and suggests using a power buffer, a Darlington pair, or a MOSFET depending on the PWM frequency.
  • Another participant questions whether a heat sink is being used on the transistor, implying that it may be necessary for thermal management.
  • A participant calculates that the transistor could be dissipating around 1 watt, suggesting that it may not be driven into saturation, which could lead to inefficiency and overheating.
  • It is mentioned that the datasheet for the TIP3055 includes characteristic curves that indicate the transistor may not be adequately driven at certain collector currents.
  • One participant proposes using a gate drive N-channel FET for potentially better performance compared to bipolar transistors.

Areas of Agreement / Disagreement

Participants express differing views on the cause of the overheating, with some suggesting insufficient base current and others emphasizing the need for proper thermal management. There is no consensus on a single solution, and multiple approaches are proposed.

Contextual Notes

Participants reference specific datasheets and characteristic curves, indicating that the discussion is grounded in technical specifications. There are unresolved questions regarding the adequacy of the transistor's drive and the necessity of a heat sink.

Who May Find This Useful

This discussion may be useful for individuals working on motor control circuits, particularly those using transistors in PWM applications, as well as those interested in thermal management strategies for electronic components.

Adrian Collings
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Okay so I am having this problem with a transistor overheating in a motor control circuit and I am not exactly sure why it is overheating. There is a PWM signal being used to power the transistor from an MSP430 microcontroller. I am using a standard 1N004 kickback diode for the motor and an external power source that does not go above 5V or 3A for the purpose it's being used.

This is the datasheet for the motor I am using go to the RE-380 model for specs.
http://docs-europe.electrocomponents.com/webdocs/0032/0900766b800320a6.pdf

The transistor that is overheating is a TIP3055 apparently rated to way more than what we should be needing, the datasheet is here too, we are using the NPN:
http://docs-europe.electrocomponents.com/webdocs/0c65/0900766b80c65646.pdf

And here is a basic picture of the circuit I am trying to implement:
https://www.dropbox.com/s/dj5gnzo1aagnv66/20160409_213437.jpg?dl=0

It shouldn't be this difficult, I have literally tried everything!
 
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The base current seems insufficient! I would use a power buffer with a much smaller resistor or a Darlington pair/transistor or a good MOSFET. The choice depends on the PWM frequency.
 
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AlexCaledin said:
The base current seems insufficient! I would use a power buffer with a much smaller resistor or a Darlington pair/transistor or a good MOSFET.

Brilliant advice, thank you very much.
 
Adrian Collings said:
Okay so I am having this problem with a transistor overheating in a motor control circuit and I am not exactly sure why it is overheating. There is a PWM signal being used to power the transistor from an MSP430 microcontroller. I am using a standard 1N004 kickback diode for the motor and an external power source that does not go above 5V or 3A for the purpose it's being used.

This is the datasheet for the motor I am using go to the RE-380 model for specs.
http://docs-europe.electrocomponents.com/webdocs/0032/0900766b800320a6.pdf

The transistor that is overheating is a TIP3055 apparently rated to way more than what we should be needing, the datasheet is here too, we are using the NPN:
http://docs-europe.electrocomponents.com/webdocs/0c65/0900766b80c65646.pdf

And here is a basic picture of the circuit I am trying to implement:
https://www.dropbox.com/s/dj5gnzo1aagnv66/20160409_213437.jpg?dl=0

It shouldn't be this difficult, I have literally tried everything!
Do you have a big heat sink on the transistor?
 
tech99 said:
Do you have a big heat sink on the transistor?
I don't, I did think of doing this but I kept thinking to myself that the transistor really shouldn't be overheating anyway.
 
Adrian Collings said:
I don't, I did think of doing this but I kept thinking to myself that the transistor really shouldn't be overheating anyway.
Well it is delivering 2 amps and will have a saturation voltage of perhaps 1 volt, so that is 2 watts dissipation for half the time, say 1 watt total. Maybe it is not being driven into saturation - that will make it inefficient and run hot - so try increasing drive first as mentioned previously.
 
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This datasheet includes some characteristic curves for the transistor:
http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

Have a look at the curves for collector-emitter voltage vs base current. At just 1 A collector current and ~20 mA drive, it's hardly even on. At more collector current, the majority of the power delivered to your circuit is probably just being dissipated in the transistor.

As already mentioned, you need a driver circuit or another transistor.
 
You could also use a gate drive N-channel FET. That would give you even better performance than any bipolar you could find.

BoB
 
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