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o/t - thermal impedance

 
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dazza
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o/t - thermal impedance
PostPosted: Sat Feb 14, 2004 3:48 am     Reply with quote
Hi,

I have +28V DC feeding a +5V linear regulator (L4940D2T5) via a 56 ohms 1/4 watt resistor.

Current being passed is 40mA (fourty milliamps).

Regulator thermal imp. is:

Rthj - case = 3 DegC/Watt
Rthj - amb = 62.5 DegC/Watt

Drop across resistor is 56 x 2.24 Volts. Thus voltage at input is 28 - 2.24 = 25.76 Volts.

Drop across reg. is 25.76 - 5 = 20.76 Volts.

Power in reg. is 20.76 * 40mA = 0.83 Watts.

Max. ambient is 50 Deg C, therefore max. temp. at regulator junction is:

(Rthj - amb) x 0.83 Watts = 62.5 x 0.83 = 51.875 Deg C

add 50 Deg C ambient giving:

51.875 + 50 = 101.875 Deg C.

This is without any heatsinking. Being a D2Pack there will be some heatsinking provided by pcb copper. Maybe reducing the temp. by 10/20 Deg C.

Given that the max. junction temp. of the reg. is 150 Deg C, I am well within limits... that's if my calculations are correct!?

Can someone be so kind as to confirm my calculations.

I thank you in advance.

Regards,
Darren
dazza
Guest
oops
PostPosted: Sat Feb 14, 2004 3:49 am     Reply with quote
Of course the above line "Drop across resistor is 56 x 2.24 Volts. Thus voltage at input is 28 - 2.24 = 25.76 Volts."

is supposed to say:

Drop across resistor is 56 x 40mA = 2.24V. Thus voltage at input is 28 - 2.24 = 25.76 Volts.
Ttelmah
Guest
Re: oops
PostPosted: Sat Feb 14, 2004 4:41 am     Reply with quote
dazza wrote:
Of course the above line "Drop across resistor is 56 x 2.24 Volts. Thus voltage at input is 28 - 2.24 = 25.76 Volts."

is supposed to say:

Drop across resistor is 56 x 40mA = 2.24V. Thus voltage at input is 28 - 2.24 = 25.76 Volts.


Your basic 'premises', are OK, as far as they go. You should try to work out how much the temperature will be reduced by the PCB tracking. Also the assumption made, that is not 'proved', is 'free air'. If the board is enclosed, you will be dealing with a lot more heat build up. You also have to worry about the temperature rating of your PCB material. FR4 (the commonest board substrate), has a maximum continuous working temp of 130C. This however may have to be reduced, depending on the bonding agent used between the copper and the laminate. Remember also that other parts 'nearby', may have a much lower temperature rating, and may may experience problems (or generate them if heat is being generated by other parts nearby...).

Best Wishes
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