DC/DC buck/bost mudules vs LM317 comparing for Bench power supply circuit

Question

I Have this two circuit, one of them based of lm317 like this:

and have some Circuit based of XL4005 or XL4015 IC like this:

Input: main using a 18V 2A transformer Output: 0-18V Current: 0-1.5A Current Limiting controlled by a potentiometer Voltage output controlled by a potentiometer

But i want the same output/input parameters for both of them like this :

Input: main using a 24V 5A transformer or AC/DC power supply (almost 75W)

Output: 0-18V Current: 0-1.5A

Current Limiting controlled by a potentiometer

Voltage output controlled by a potentiometer

In this post said about advantage of dc/dc boost/buck modules :

We used linear regulators like LM317, LM2940 etc. As I said before these are so inefficient and can't be used for a battery powered setup. So what you can do is, find one of those cheap DC-DC buck modules from any online shops and replace the linear regulators with them. They're more efficient (>90%), has better load regulation, more current capability, current limiting, short circuit protection and all. LM2596 is one of that kind. The buck (step down) modules will have a precision potentiometer on top. You can replace it with a "multi-turn potentiometer" and use it at the front panel instead of normal linear pots. That'll give you more control over the output voltage.

One of the important thing is having max current for 0- max voltage without considerable decrease,but i have seen this current voltage output chart for XL4005 or XL4015 base:

but don't know that chart for LM317 based!

So i like to know your suggestion about which one of this two schematic or system or better for bench power supply?

And can solve the current drop based of boot/buck converter modules (like XL4005) for this purpose?

Thanks.

Answer 1

To answer you main question about LM317 performance.

The current vs voltage output chart for LM317 is much worse than for the switching module. You're limited to less-than or equal the input current at all voltages, and are limited to less if it starts to overheat because the heatsink is too small. (so the slope may actually be backwards compared to what you posted. and all the numbers are worse)

some comments on your drawing.

You have drawn a "boost converter" where you probably want a buck converter

If the current is less than 10A you can use a voltmeter-ammeter module that has an integrated shunt.

you may have to place the shunt in the negative side of the output

If you connect the ground end of the voltage setting potentiometer to a small voltage instead of to ground you will be able to adjust the voltage down to zero. (eg use a resistive divider off the 12V supply to make 1.5V )

Answer 2

A power supply project like that is a great learning experience. Go for it!

You won't get 24V input, 2V output @ 1.5A from the LM317. That's 22V x 1.5A = 33W. The Rja of an LM317 on a heatsink will be far too high. You'll have to derate the current at lower output voltages or parallel up some LM317s or use external pass devices (a la some 2N3055's). You can search the internet and find many examples of such things.

If you did solve the thermal problem then you'll have the benefit of a very low broadband noise supply and a supply with nearly zero output voltage ripple. This is what most of the good bench supplies are doing and why they have big, heavy heat sinks on the back.

A switcher is going to be pretty easy - just buy a module, hook it up and use it. A super low cost module may not have the best load transient response, it may be quite noisy (output voltage ripple) under some conditions. It may even behave very poorly under some conditions. But unless you are doing work that is very sensitive to noise then that's fine. If you do run into noise sensitivity issues then you can add a linear regulator to that circuit to clean things up.

In either case, you can probably only get to ~1.2V or so, not 0V, without some additional circuit tricks because these devices have internal references of that voltage, usually.

As mentioned earlier, you want a buck converter instead of a boost converter. It would be a great learning experience to build both, test them and gain some insight and appreciation!