Buck converter. (adjustable)

[TheMrbunGee]

Hey There!

I have now spent more than a week of my free time to get into buck converters and stepping down DC voltage. I now know how they reduce the voltage, but it is not clear what controls the
MOSFET, what decides when it turns on and when off.

I started doing this because I need a DC power supply, which ideally is adjustable (2-12V) and can handle 500 W or even more.

I have fixed 48V 30A power supply (if it is easier to convert from 48VDC than 230VAC). I have quite high power MOSFETs, diodes, bridge rectifiers, capacitors, inductors and other stuff, what can be salvaged from DC supply. Can such buck converter be made by beginner? Voltage does not have to be smooth, very constant or precise, so there is no need for complex smoothing circuitry.

Can voltage be adjusted without complex controllers?

Thanks, sorry for lack of some basics, but i can't sleep at night if I won't resolve this.

Also - English is not my main language, so it was not easy to say what I need to get done, I might sound more stupid then I am!

Thanks!

[Zero999]

The duty cycle should theoretically be equal to the ratio of the output vs input voltage, so if you have 12V out and 48V in, that's a ratio of 12:48, giving a duty cycle of 12/48 = 25%.  In reality, there are losses in the inductor, switching transistor and diode, so the actual duty cycle will be slightly higher.

The controller circuit should adjust the duty cycle automatically, by monitoring the output voltage and making the required adjustments.

As the ratio between the output and input voltages gets higher, the simple buck converter circuit becomes less efficient. If you want 2V out and 48V in, then it's better to use a transformer or a multiphase buck, rather than a standard buck converter.

See links below:
http://www.ti.com/lit/an/slyt293/slyt293.pdf
http://www.ti.com/lit/an/slyt449/slyt449.pdf
https://en.wikipedia.org/wiki/Buck_converter
https://www.mouser.de/pdfdocs/BuckConverterDesignNote.pdf

[TheMrbunGee]

Ok, I got that, but how do I control duty cycle? Can it be done without timers? Can I use 555 timer for that? I have those..

[Zero999]

Ok, I got that, but how do I control duty cycle? Can it be done without timers? Can I use 555 timer for that? I have those..

It's possible to use a 555 timer, but not recommended, since the efficiency will be poor. The easiest way is to use a switched mode power supply, such as the TL594.
http://www.ti.com/lit/ds/symlink/tl594.pdf

[TheMrbunGee]

Ok, I got that, but how do I control duty cycle? Can it be done without timers? Can I use 555 timer for that? I have those..

It's possible to use a 555 timer, but not recommended, since the efficiency will be poor. The easiest way is to use a switched mode power supply, such as the TL594.
http://www.ti.com/lit/ds/symlink/tl594.pdf

So I use TL594 to drive MOSFET, which then regulates voltage. and control the chip with potentiometer, is that correct? Sounds doable for a beginner like me. Thanks!!