What happens at the load of Buck Converters? Really Confused.

So I had designed a Buck Converter on LT Spice IV.

Input 220VAC

Output - 24 Volt, 350mA,

So after I finished designing I checked voltage across the output and it is close 24volts, but when I place a resistor as a load of a value that should give me 350mA everything changes. Output voltage becomes 80mv across the load and current resistor is drawing becomes 1.2mA.

I am not being able to understand that.

Also I have one major confusion or doubt or lack of knowledge about one thing which is: If I design a converter for specific voltage and current, say for example 24volt and 2 amps, that means that load should have a resistance of 6ohms but if I replace thatr resistor with say 2 ohms, would the current become 24/2 = 12 Amps? Could I get an explanation for that please?

What should I design if I wish to convert 220Ac voltage to say 24 volt but the current drawn depends on the load. what i mean is that I should get a fixed 24 volts at output and if I want 2 Amps I connect a 12ohm Resistor, if I want 1 Amps I connect a 24 ohm resistor, if I want 500mAmps I connect a 48ohm resistor etcetera.

Thank you 

Hi Harsh Chandola,

Sorry, I think you designed more of a power supply rectifier [not exactly a buck converter strictly speaking].

If you isolate the right side of the circuit starting with the MOS, you get the output side of an asynchronous buck converter. But, you can't just use a pulsing waveform as the gate's driver because then, it won't regulate well. If you want a simple circuit like that, then you're looking for an LDO. A buck/boost is more complicated because the switching depends on how much the load decreases the output voltage [voltage mode control] or the inductor current induced [current mode control].

Cheers!
Justin

Hello Justin,
Thank you so much for your reply.
Could you shed some light on a couple of things or refer me to links or books that'd teach me that :
# Could you explain me how is regulation done ?
# What is LDO?
See I designed it for the sake of simulating and seeing what is happening at the output.
So far I realized that my converter would be either constant-current or constant-voltage, I am assuming if it were constant voltage I would have constant voltage across the load and current would vary depending on resistance of the load. Now my question is how do I regulate a constant voltage across the output?
What I had in mind was, first i rectified the voltage to dc, then to decide the duty cycle I simply divided the required Output voltage by the incoming rectified DC voltage, I decided the value of capacitor such that ripples are minimum. And now I think I am wrong by a long shot!
I would really appreciate if you could tell me how to calculate the inductance and capacitor values in a buck converter and how is duty cycle chosen if not the way I chose it like. it'd be totally fine if you can just give me an idea only that we must keep in mind while designing constant current or constant voltage buck converter and how the values of inductor capacitor, duty cycle and switching frequency is selected.
I'd equally appreciate if you could refer me to a book or a link that explains these things nicely.
Thank you so much for your patience and sharing your knowledge.
Harsh Chandola

https://en.wikipedia.org/wiki/Buck_converter
has a basic article plus a lot of references at the end, which you should look at.
It depends on how your maths and physics are, if you can manage the maths in the Wiki article, then you should be OK, if not, then you need a more "cookbook" approach, where you can type values into a design sheet, and have the answers produced for you. Companies that produce the controller chips often have design sheets that do just that.
See http://www.ti.com/tool/buck-convcalc for example.
I have used Excel spreadsheets of my own making for designing ZETA converters, and it works just fine.
If you are using LT spice, then there are a large number of controller ICs for Buck converters in the library. Additionally, They have some software called LT PowerCad II as well, which has support for some of their buck converters. http://www.linear.com/designtools/software/#LTPower
LDO is a low dropout linear regulator, and is not related to buck converters.
Buck converters use a "high side switch", a PMOS fet is easier to use if you are simulating circuits, or possibly a voltage controlled switch, which are in spice libraries.
NMOS fets normally require additional circuitry to make them work.
The output is usually constant voltage (constant current is used to power things like a chain of LEDs). In either case, a controller chip must be used if the load resistance or input voltage changes, by means of feedback it alters the duty cycle of the switch, there is a limit to the duty cycle ratio, the inductor value is a compromise, which sets the span over which the output can be regulated.
Have a look at the articles and see what you can make of them.
Incidentally, in a practical circuit you would not use the buck topology with your input voltage, the duty cycle is very low, and a failure of the switch could put that voltage on the output. It is more common to use transformer isolation, for example, as in a flyback converter, but that is another story,
cheers,
Richard