I want to design a switching power supply with 12v input and 5v output, with a buck circuit, how do choose the MOSFET model? Or other chips?

 

SMPS' are a pretty broad and very technical subject...
Switch-Mode Power Supply Spice Cookbook-Basso | Sanders, Roger E., | download (1lib.us)

 

This is not a good idea for a beginner with rudimentary design skills. There are many component design choices that are driven by the actual design specifications. Listing the input and output voltages is a good start. A better start would look at the required power in and power out so that you can establish an efficiency goal to shoot for. Then you need to look at things like output ripple voltage, input filtering and then there is the whole control loop to specify, compensate and optimize. Are you up to the challenge?

I recommend that you become familiar with SPICE simulation methodology since that will allow you to experiment with parts and concepts without the expense and frustration of letting the magic smoke out of one or more parts. There is plenty of time for that later. I would expect that if you are a crackerjack analog design engineer and this is your first SMPS (Switch Mode Power Supply) design that you might spend up to a year on this project. More time will be required if you need to brush up on the basics of how various components affect the results. These include but are not limited to the inductors, the capacitors, the semiconductors, the voltage references, the error amplifiers, the comparators, the signal generators, waveform shapers, flip-flops and logic gates.

 

There are some semiconductor company websites that provide power supply design software to do such a design. But it is still not a trivial task by any means. Papabravo is completely correct in his post, a switching supply is actually a big deal project because even after you have a very good circuit design the layout arrangement and connections are still critical elements.
A linear regulator supply is far less efficient but very much simpler to create and get working.

 

Are you a beginner? if so, start with something really simple, like simple switcher which is easy to set up. Look for internal "mosfet" section from TI, Maxim, Analog etc.

 

Are you a beginner? if so, start with something really simple, like simple switcher which is easy to set up. Look for internal "mosfet" section from TI, Maxim, Analog etc.

This is actually pretty good advice, particularly if you use the manufacturer's reference design(s) and component choices. At least you will be able to say that your design will not be your first and you will gain valuable experience by doing the layout, and troubleshooting the first dozen prototypes.

 

Are you a beginner? if so, start with something really simple, like simple switcher which is easy to set up. Look for internal "mosfet" section from TI, Maxim, Analog etc.

Is that a generic expression?

How is defined a simple switcher? Sorry but I know basically nothing of all this.

 

Is that a generic expression?

How is defined a simple switcher? Sorry but I know basically nothing of all this.

Excellent question. It is a sort of a "trade name" for a line of parts that are particularly easy to apply to get specific results. For example there may be a part with a "cookie-cutter" design to give 3.3V @ 200 mA from 7V-12V input. The datasheet will even give part numbers and supplier information for the complete bill of materials. They may even have a board layout that you can copy.

 

Like has been said already, it all depends... 12v to 5v buck at 100mA is relatively trivial, using a simple chip like, for example, the LT1072, which has a comprehensive design guide and is easily simulated on LTSpice. Its an older design but quite user friendly. If you have a reasonable grounding in electronics its not hard to pick up. Of course, if you don't know your resistors from your capacitors and inductors are a total mystery then you might struggle....

 

I'd recommend the MC33063 as a starting point.
https://www.onsemi.com/pdf/datasheet/mc34063a-d.pdf
The datasheet has lots of applications information, and more importantly, the 33063 has a control loop that can't go unstable!
Loop stability is the hardest thing to get right on a switching regulator, and it can be very frustrating for a beginner in switched-mode designs.
The ubiquitous UC3842 is not for the faint-hearted.

 

Unless this is some kind of school assignment, Don't think you will ever make one as small or as cheap as just buying one. Look for something called an "isolated DC-DC convertor". Murata Electronics is a good one.

https://www.digikey.com/en/articles...s-with-embedded-transformers-to-ease-assembly

https://www.mouser.com/Power/DC-DC-Converters/Isolated-DC-DC-Converters/_/N-brwkv