Hello. I'm looking to construct a digitally controlled power supply. I have a circuit drawn up with a PWM voltage control circuit of my own design, and the current control design that Dave drew on his PSU series hooked up to the input of the second LM317.
After being plugged into a circuit simulator, it seems to have really weird behavior even though both circuits work great on their own. Any help??? Schematic below.
(The +5v inputs are where the PWM will be inputted. An RC filter will be added in the final design. The 1ohm resistor to the far right will also be removed.)
Hello,
A couple points to be made here.
[1]
That 220 Ohm resistor is not considered to be low enough anymore, 120 Ohms is the preferred value now. That's to load the LM317 with at least 10ma.
[2]
When you drive the ADJ terminal directly with a voltage source, you essentially give up the voltage regulation offered from the LM317. The external voltage source appears in series with the 1.25 volt reference source internal to the LM317. That means that your external voltage source is now a large part of the reference source, which means it must be well regulated if the output is to be well regulated. Another side issue is that the LM317 now becomes just a driver, similar to an amplifier with a gain of 1 with an added reference of around 1.25v, and with the external voltage source Vs the reference becomes Vs+1.25v, and that is 'amplified' by the gain of 1 amplifier and then the output of that amplifier drives the output load.
That's probably not a problem, but the external source has to be regulated itself, which for your circuit could be a problem. More about that later.
[3]
The input circuit, ideally, should really be a buck switching regulator that automatically adjusts to the minimum requirements of the input of the LM317. If the so-called drop out voltage was 2.5 volts for example, then the output of the buck should be 2.5 volts plus whatever the output voltage has to be. This isn't too hard to do, but the buck circuit front end is more important because that lowers power dissipation by a very large amount. It means the circuit can go from instead of 30 watts of dissipation for a given load, down to 3 watts of dissipation for that same load. That's a lot and that is why switching regulators make good front ends for linear regulators.
So this means drop the linear front end and go with a buck front end. If you can't do that, then you're going to be stuck with pure linear operation which sometimes will dissipate a large amount of power and require a very large heat sink.
Now a little more about the external voltage source.
Because you will be using PWM, you might have to incorporate some feedback so that you can get the PWM to output the right source voltage so you can get decent accuracy on the output voltage setting. This means sensing the output or the filtered PWM signal and adjusting the pulse width accordingly. This is because the LM317 can no longer regulate the output voltage like it does when only resistors connect to the ADJ terminal. That means you have to do all the regulation with a separate feedback system. This brings in the question of if an LM317 is really needed at all if you have to incorporate your own feedback system anyway and why put up with the unusually large voltage drop of the LM317 that comes with it. With a power transistor and some feedback you can get the voltage drop down to at least 1v or even less.