Temperature in transistors
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 6th May 2009, 07:57 PM #1 Rafael L   diyAudio Member   Join Date: Nov 2008 Location: Brazil Temperature in transistors Hello to All How do I know what temperature will reach the transistor in the circuit, not to damage the transistor. For exemple: In a circuit have 33V and 3.3 mA, power dissipated by the transistor ~110mW (LTspice DC operating point). the component is TO92 and Power given by the datasheet is 650mW (25 Celsius). What temperature would have the transistor in circuit with 33V and 3.3mA?
 6th May 2009, 09:34 PM #2 megajocke   diyAudio Member     Join Date: Jan 2003 It depends on a lot of things like airflow, lead length, copper area on card etc. There might be a thermal resistance value in the datasheet that you can use, but remember that it depends on lots of other factors than just the transistor. But anyways, 100mW is not going to be a problem for a TO92 even at elevated temperatures.
nigelwright7557
diyAudio Member

Join Date: Apr 2008
Location: Carlisle, England
Re: Temperature in transistors

Quote:
 Originally posted by Rafael.luc Hello to All How do I know what temperature will reach the transistor in the circuit, not to damage the transistor. For exemple: In a circuit have 33V and 3.3 mA, power dissipated by the transistor ~110mW (LTspice DC operating point). the component is TO92 and Power given by the datasheet is 650mW (25 Celsius). What temperature would have the transistor in circuit with 33V and 3.3mA?

If you look up the datasheet for the transistor it will tell you how many degrees the temperature raises per watt or millwatt

It will also tell you by how much to derate the transistor per degree.
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 7th May 2009, 02:41 AM #4 Rafael L   diyAudio Member   Join Date: Nov 2008 Location: Brazil I want to know to how many degrees will the transistor, with application of some power. What should I multiply with the power? Voltage the test of transistor ????
 7th May 2009, 05:21 AM #5 destroyer X   diyAudio Member     Join Date: Feb 2004 Location: Recife - Brasil Northeast Do not let the case go above 52 degrées celsius the best option is when you have around 42 degrées celsius, or 12 degrées above environment temperature. Hot fever temperature goes fine..... temperatura de febre alta vai bem..não deixe passar de 52 graus porque a junção vai estar mais quente e pode derreter. Do not operate with temperature higher than 52 degrées because some transistor junctions may melt... the junction use to be hotter than the case. Reducing stand by current to have less than 1 miliampere to each output transistor use to make an amplifier to operate cool. Carlos __________________ Dx unbeatable amplifiers; https://www.youtube.com/watch?v=u4tGBiqMnzQ
megajocke
diyAudio Member

Join Date: Jan 2003
Quote:
 Do not operate with temperature higher than 52 degrées because some transistor junctions may melt... the junction use to be hotter than the case.
Thermal resistance junction to ambient * power = 200*0.11: 22 degrees C temperature rise ambient to junction.

Even if the ambient was 75 degrees C (HOT!) the junction would still be below 100 degrees C. No danger for the transistor.

GK
Account disabled at member's request

Join Date: Jan 2006
Re: Do not let the case go above 52 degrées celsius

Quote:
 Originally posted by destroyer X Reducing stand by current to have less than 1 miliampere to each output transistor use to make an amplifier to operate cool. Carlos

Quote:
 Originally posted by destroyer X Reducing stand by current to have less than 1 miliampere to each output transistor use to make an amplifier to operate cool.

??

The BC547 is rated at 625mW at 25degC ambient with a derating factor of 5mW / degC above 25. This means that, at 70 degrees ambient (about as hot as you'd ever wan't it to be inside and amplifiers case) it can dissipate a maximum of

70-25*0.005mW
=225mW

625mW-225mW

=400mW.

Under these conditions (TA=70 degC, Pdiss=0.4W) the junction temperature will be 150 degC

Rafael L
diyAudio Member

Join Date: Nov 2008
Location: Brazil
Quote:
 Originally posted by megajocke Thermal resistance junction to ambient * power = 200*0.11: 22 degrees C temperature rise ambient to junction.
Thanks,
More applying a power 110mW of transistor, the temperature should increase in transistor above the ambient.
how many degrees I have in transistor, I believe that more than 25 C ? ...... (Ignoring external temperature).
I want to know if the HFE will not vary, I saw a amp with this problem, open loop varied with the level (negative feedback decreased in high level).

Quote:
 Under these conditions (TA=70 degC, Pdiss=0.4W) the junction temperature will be 150 degC
Thanks G.Kleinschmidt

Quote:
 temperatura de febre alta vai bem..não deixe passar de 52 graus porque a junção vai estar mais quente e pode derreter.
Carlos, you answered me in Portuguese, my English is difficult to understand?

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