Hi All,
My on going project is to build a bench power supply with these specs :
- Two CV Outputs
- Adjustable Output voltage from 0V to 24V DC (fine+coarse)
- Rated Output current of 1 A
The circuit is currently composed of :
-22V/100VA double secondary transformer (2.27A each secondary)
For one channel :
-one LM338K in TO-3 (improved LM317) for current limiting
in serie with
-one LM338K in TO-3 (with heat sink) for voltage adjust
The problem that i have is, of course, heat dissipating...
For example : output 12V @ 1A, the heat dissipated by the last LM338K is
P=(30V-12V)x1A=18W
30V corresponds to 22V*sqr(2) - 1V (diodes bridge drop)
Junction temp is given by
Tj=(rthjb+rthra)*p+ta=(1+10)*18+25=223°C > 125°C -> the IC will thermal shutdown
where rthjb is the thermal resistance of the LM338K
and rthra thermal resistance of the heat sink (no air flow)
So what i want to do is keeping constant dropout voltage across the linear regulator in order to remove the thermal problem.
To do this, the best solution is to use a tracking pre-regulator which keep constant voltage between its output (connect to linear regulator input) and the final output.
I bought recently a SMPS adjustable step-down module based on LM2596S, see link below
2pcs LM2596 DC-DC Step Down Adjustable Power Supply Module | eBay
This IC (and others) has a FEEDBACK pin which is used to program output voltage with a resistor divider between output and ground.
Now the final question is : what is the best schematic you can show me to track final output voltage of the linear regulator with this feedback pin?
Maybe OPAMP circuit...
(you can show me another kind of step-down IC)
(see attached my current design + link to LM2596 datasheet)
http://www.ti.com/lit/ds/symlink/lm2596.pdf
Thanks in advance
My on going project is to build a bench power supply with these specs :
- Two CV Outputs
- Adjustable Output voltage from 0V to 24V DC (fine+coarse)
- Rated Output current of 1 A
The circuit is currently composed of :
-22V/100VA double secondary transformer (2.27A each secondary)
For one channel :
-one LM338K in TO-3 (improved LM317) for current limiting
in serie with
-one LM338K in TO-3 (with heat sink) for voltage adjust
The problem that i have is, of course, heat dissipating...
For example : output 12V @ 1A, the heat dissipated by the last LM338K is
P=(30V-12V)x1A=18W
30V corresponds to 22V*sqr(2) - 1V (diodes bridge drop)
Junction temp is given by
Tj=(rthjb+rthra)*p+ta=(1+10)*18+25=223°C > 125°C -> the IC will thermal shutdown
where rthjb is the thermal resistance of the LM338K
and rthra thermal resistance of the heat sink (no air flow)
So what i want to do is keeping constant dropout voltage across the linear regulator in order to remove the thermal problem.
To do this, the best solution is to use a tracking pre-regulator which keep constant voltage between its output (connect to linear regulator input) and the final output.
I bought recently a SMPS adjustable step-down module based on LM2596S, see link below
2pcs LM2596 DC-DC Step Down Adjustable Power Supply Module | eBay
This IC (and others) has a FEEDBACK pin which is used to program output voltage with a resistor divider between output and ground.
Now the final question is : what is the best schematic you can show me to track final output voltage of the linear regulator with this feedback pin?
Maybe OPAMP circuit...
(you can show me another kind of step-down IC)
(see attached my current design + link to LM2596 datasheet)
http://www.ti.com/lit/ds/symlink/lm2596.pdf
Thanks in advance
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