I'm an amateur at this, and I'm planning on building my first bench power supply and need some help checking my homework. The design I'm working on is for an adjustable low voltage and current supply (1.5-12VDC, 0-1.0A). Before getting into the finer details of the circuit design, I first want to make sure the voltage regulators I've chosen can handle the power dissipation and won't go into thermal shutdown under max conditions since the circuit is built around them.
I'm planning on using an 18V transformer (9-0-9) rated for 2A max, and I want to have adjustable, regulated DC output (after full-wave rectification and smoothing) of 1.5-12VDC and adjustable current of 0-1A max.
Two questions:
1. I have chosen to use the LM138 in the TO-3 pkg. due to its better heat dissipation properties over a LM317 and will be using 2, one for current regulation and one for voltage. I calculate that at either max current output of 1.0A or min voltage output of 1.5V I will need to dissipate between 16.9 and 22.7 Watts in either or both regulators (depending on the voltage and current settings), and depending on whether I’m using the rated transformer voltage of 18V or peak voltage of 23.8V. Planning for worst case, I used the peak voltage of 23.8V in the following calculations:
As a current regulator:
P = (Vmax-Vref) * (max current + 10mA) {10mA being sunk current necessary for stable regulation}
(23.8 – 1.25) * 1.01 = 22.77W
As voltage regulator:
P = (Vmax – Vmin) * (max current + 10mA)
(23.8 – 1.5) * 1.01 = 22.52W
Have I done this correctly?
2. To determine the heat the regulators would be dissipating with a heat sink on each one under the above conditions, I attempted to calculate the junction temperature for the 2 LM138’s under my max current and minimum voltage conditions, using information from the LM138 data sheet http://www.ti.com.cn/cn/lit/ds/symlink/lm338.pdf as follows:
Rjc (thermal resistance junction to case) = 1.0 deg. C/W (TO-3 pkg.)
Rch (thermal resistance case to heat sink) = .25 deg. C/W (based upon published avg. thermal resistance properties of various thermal paste materials)
Rha (thermal resistance of heat sink) = 2.70 deg C/W (from data sheet of heat sink)
Ta (Ambient temp) = 25 deg. C
P = 22W
Tj = P(Rjc+Rch+Rha) + Ta
= 22(1.0+.25+2.7) + 25
= 111.9 deg. C
While this is hot enough to boil water, it shows the Tj temp to be under the 150 deg. C max operating temperature of the LM138. I plan to add a cooling fan to pull air through the chassis case to improve the cooling efficiency and keep the temps down.
Have I calculated this correctly and does anyone see any problems with what I’m proposing to do? What is not clear to me from the data sheet (and I freely admit I barely understand them) is what effect input-output voltage differentials above 15V will have on power dissipation per note 1 at the bottom of page 2 of the data sheet. Any thoughts here would be appreciated as well.
Thanks.
I'm planning on using an 18V transformer (9-0-9) rated for 2A max, and I want to have adjustable, regulated DC output (after full-wave rectification and smoothing) of 1.5-12VDC and adjustable current of 0-1A max.
Two questions:
1. I have chosen to use the LM138 in the TO-3 pkg. due to its better heat dissipation properties over a LM317 and will be using 2, one for current regulation and one for voltage. I calculate that at either max current output of 1.0A or min voltage output of 1.5V I will need to dissipate between 16.9 and 22.7 Watts in either or both regulators (depending on the voltage and current settings), and depending on whether I’m using the rated transformer voltage of 18V or peak voltage of 23.8V. Planning for worst case, I used the peak voltage of 23.8V in the following calculations:
As a current regulator:
P = (Vmax-Vref) * (max current + 10mA) {10mA being sunk current necessary for stable regulation}
(23.8 – 1.25) * 1.01 = 22.77W
As voltage regulator:
P = (Vmax – Vmin) * (max current + 10mA)
(23.8 – 1.5) * 1.01 = 22.52W
Have I done this correctly?
2. To determine the heat the regulators would be dissipating with a heat sink on each one under the above conditions, I attempted to calculate the junction temperature for the 2 LM138’s under my max current and minimum voltage conditions, using information from the LM138 data sheet http://www.ti.com.cn/cn/lit/ds/symlink/lm338.pdf as follows:
Rjc (thermal resistance junction to case) = 1.0 deg. C/W (TO-3 pkg.)
Rch (thermal resistance case to heat sink) = .25 deg. C/W (based upon published avg. thermal resistance properties of various thermal paste materials)
Rha (thermal resistance of heat sink) = 2.70 deg C/W (from data sheet of heat sink)
Ta (Ambient temp) = 25 deg. C
P = 22W
Tj = P(Rjc+Rch+Rha) + Ta
= 22(1.0+.25+2.7) + 25
= 111.9 deg. C
While this is hot enough to boil water, it shows the Tj temp to be under the 150 deg. C max operating temperature of the LM138. I plan to add a cooling fan to pull air through the chassis case to improve the cooling efficiency and keep the temps down.
Have I calculated this correctly and does anyone see any problems with what I’m proposing to do? What is not clear to me from the data sheet (and I freely admit I barely understand them) is what effect input-output voltage differentials above 15V will have on power dissipation per note 1 at the bottom of page 2 of the data sheet. Any thoughts here would be appreciated as well.
Thanks.