Hi all,
I've looked at many posts in this forum about lm317 and its applications but never found the answer to the following thought:
According to the datasheet, the lm317 is rated to provide up to 1.5A for a Vin-Vout <= 15V. Above 15V (approx) the maximum current that the lm317 can provide decreases.
So, if we have Vin-Vout = 15V and Iout = 1.5A ; the maximum power through the lm317 will be 15x1.5 = 22.5W.
For a thermal resistance calculation for the heatsink I've got the following standard inputs:
Ta = 25°C
Tjmax = 125°C
Rth(jc) = 4°C/W (datasheet)
Rth(heatsink) = to be calculated
With the data given above, the maximum thermal resistance must not exceed 4.44 °C/W = (125-25)/22.5 = Rth(jc) + Rth(heatsink)
Therefore Rth(heatsink) must be under 0.44 °C/W. Is this value possible to have?
The result is even worst if using lm350, which allows up to 3A instead of 1.5A. In this case, the power through the lm350 will be 15x3 = 45W, which leads to a total thermal resistance of 2.22 °C/W but with a Rthjc = 4 °C/W (according to datasheet), the minimum value for Rth(heatsink) will never be reached ! >> the only solution is to share the power with additional device (two lm350 in parallel or additinal power transistor).
Why the lm350 is rated to provide 3A when Vin-Vout = 15 if it cannot support a single configuration layout with heatsink ?
Thanks in advance for your support,
Efron
I've looked at many posts in this forum about lm317 and its applications but never found the answer to the following thought:
According to the datasheet, the lm317 is rated to provide up to 1.5A for a Vin-Vout <= 15V. Above 15V (approx) the maximum current that the lm317 can provide decreases.
So, if we have Vin-Vout = 15V and Iout = 1.5A ; the maximum power through the lm317 will be 15x1.5 = 22.5W.
For a thermal resistance calculation for the heatsink I've got the following standard inputs:
Ta = 25°C
Tjmax = 125°C
Rth(jc) = 4°C/W (datasheet)
Rth(heatsink) = to be calculated
With the data given above, the maximum thermal resistance must not exceed 4.44 °C/W = (125-25)/22.5 = Rth(jc) + Rth(heatsink)
Therefore Rth(heatsink) must be under 0.44 °C/W. Is this value possible to have?
The result is even worst if using lm350, which allows up to 3A instead of 1.5A. In this case, the power through the lm350 will be 15x3 = 45W, which leads to a total thermal resistance of 2.22 °C/W but with a Rthjc = 4 °C/W (according to datasheet), the minimum value for Rth(heatsink) will never be reached ! >> the only solution is to share the power with additional device (two lm350 in parallel or additinal power transistor).
Why the lm350 is rated to provide 3A when Vin-Vout = 15 if it cannot support a single configuration layout with heatsink ?
Thanks in advance for your support,
Efron