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Old 6th May 2009, 07:57 PM   #1
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Default 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?
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Old 6th May 2009, 09:34 PM   #2
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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.
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Old 6th May 2009, 11:09 PM   #3
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Default 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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Old 7th May 2009, 02:41 AM   #4
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I want to know to how many degrees will the transistor, with application of some power.
What should I multiply with the power?

Click the image to open in full size.

Voltage the test of transistor ????

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Click the image to open in full size.
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Old 7th May 2009, 05:21 AM   #5
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Default 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
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Old 7th May 2009, 12:10 PM   #6
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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.
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Old 7th May 2009, 12:21 PM   #7
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Default 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
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Old 7th May 2009, 05:59 PM   #8
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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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