220V to 12V converter help

[Achu]

Hi,
I need a circuit diagram for a 230V to 12V converter.Ineed to use it as the auxiliary supply in an smps I intend to make.

Also an explanation on its working will be really helpful.

Thanks

[DannyTheGhost]

Almost every auxiliary power supply you will see in SMPS is using flyback topology for its simplicity (you need to concentrate on design of much beefier SMPS)
If you need to understand basics of flyback operation - see for chips like UCx48x
If you need something like 'put it all together ASAP' - see for chips like VIPERx series, which has embedded high-voltage MOSFET and fixed operation frequency for given chip type
P.S. my suggestions are a little bit outdated and kinda old, but surely easily obtainable

[strawberry]

cheaper option is to use nonisolated BUCK converter LNK3202 usually used for LED power supply

[Achu]

I'm looking for a more discrete version.
Also is RC (Ringing Choke) converters suited for my purpose?

[mariush]

How much current? How efficient?

If the current is low enough, you could use a single chip, a linear regulator.... here's LR8K4 : https://uk.farnell.com/microchip/lr8k4-g/linear-volt-reg-0-02a-440v-to/dp/2448524

It takes up to 440v in, outputs up to 20mA ... though keep in mind it will dissipate difference as heat... so maybe 2-3w max of heat. Can't just give it 350v DC (full bridge rectified 230v AC) and expect 12v at 20mA cause that would mean chip needs to dissipate (350v -  12v ) x 0.02 = ~6.8w

So for example :  diode for half wave rectification of AC voltage, small capacitor, big enough to guarantee minimum 24v at around output current + a few mA to the regulator, regulator, capacitor on output -> 12v

[Achu]

I wanna be able to power the SG3525 and drive the base of the main transformers.
As in any ATX style smps.

[DannyTheGhost]

Like in any power supply application, calculate how much power you need from your auxiliary supply to feed control of your main SMPS
Make assumptions about current consumption of pwm controller chip, add to it switch driver consumption etc.
Maybe you want to add cooling fan powering from it
Your decision mainly depends on estimated power output of future auxiliary power supply

[Achu]

Say 10Watts .Almost an Amp at 12v.

[DannyTheGhost]

With this kind of power output things like capacitive dropper and linear regulation have too little effiency
You should definetely look into flyback topology, which is the most easiest in implementation and has the least quantity of needed parts to build it in compare with other isolated topologies
There are a lot of chips and documents about all calculations required and theory of operation in the internet, just try to search for it
In case of when you don't want to design your own 'flyback transformer' you can buy one and make your auxiliary power supply around it

[Achu]

I would very mu.ch like make one on my own, but I lack in practice.I mean I have learned the theory from books but I have no idea how to design the actual circuit.

[DannyTheGhost]

For the start, use algorithm that I'm using right now in my projects involving SMPS
Maybe it's not very correct, but until now got me succesful flyback designs
1.Decide with output power (what you've already done)
2.Decide with operating frequency (lower frequency - bigger core, higher frequency - higher core, winding and switching losses. As for me, in my last design I used 115kHz for 20ish watt flyback PSU without using any heatsinks)
3.BUT, in the case when you want to buy assembled flyback transformer, you should adjust frequency as stated in its datasheet.
Deciding with core size and its air gap - it is rather complicated, because it will almost definitely require iterations of calculation
With some given core, you get desired primary inductance, if according to calculations during full power output core saturates -> make air gap bigger -> get more turns to achieve its L -> find out if windings will actually fit in core window, if no -> get bigger core......repeat until you get the best mix 'lowest possible core size and number of windings and no core saturation achieved'
4.Flyback topologies, expecially off-line ones, will definitely require snubber circuit for protecting your main switch. Calculate it too
BTW, it is just safer to use DCM in off-line flybacks
Somewhere between this steps decide with your PWM controller (which should based on current-mode control). Examples of such chips I already gave you earlier.
Finally, don't afraid to look for reference designs and PCB layouts
P.S. Also search how to wind flyback transformers to minimize its parasitics (interwinding capacitance, leakage inductance etc.)

[Achu]

I found these two circuits on the web.Now I have to find optimum component.Any advice?

[NiHaoMike]

Have you considered just buying a ready made module?

[Achu]

I am a fan of homebrewing.

[bob91343]

Are you eager to spend a serious amount of development time to reinvent the wheel?  Or to say it differently, why not just use an old ATX power supply as is?  Those things are available free and are likely to meet the need easily.  In fact I have several laying around only because it's not emotionally possible for me to discard them.

I have torn apart a number of them and put the parts in my junk box but I still have some working units.  They are small and light and usually include a cooling fan along with an IEC socket.  Typically about a six inch cube.  You just have to find the odd color wire and ground it to enable the unit.  And most require a minimum load but you can easily take care of that.

A bonus is that you have nothing home made connected to the power line just waiting to invalidate a homeowners insurance claim.

[BravoV]

I think the real question to OP that so passionate to DIY is, whats your experience in building power switching circuit ?

Have you successfully built simpler cheap IC based DC to DC switching power supply ?

What measurement tools you own currently ? DMM ? Scope ? etc ?

This is to have a realistic expectation, especially for people who want to help, as this involves AC mains high voltage down to low volt DC, which is totally different level of difficulty compared to simpler DC to DC switcher.

[strawberry]

I guess P1 have to be 5V and half of S1 turns . C4 ~10nF/400V, R8~100k/1W, D3~SUF4007, Q1~2N3904, Q2~MJE18003



 

[Achu]

I think the real question to OP that so passionate to DIY is, whats your experience in building power switching circuit ?

Have you successfully built simpler cheap IC based DC to DC switching power supply ?

What measurement tools you own currently ? DMM ? Scope ? etc ?

This is to have a realistic expectation, especially for people who want to help, as this involves AC mains high voltage down to low volt DC, which is totally different level of difficulty compared to simpler DC to DC switcher.

I have no experience in that arena, to be honest.My major experience is in radio/audio electronics, and arduino projects.

[BravoV]

I think the real question to OP that so passionate to DIY is, whats your experience in building power switching circuit ?

Have you successfully built simpler cheap IC based DC to DC switching power supply ?

What measurement tools you own currently ? DMM ? Scope ? etc ?

This is to have a realistic expectation, especially for people who want to help, as this involves AC mains high voltage down to low volt DC, which is totally different level of difficulty compared to simpler DC to DC switcher.

I have no experience in that arena, to be honest.My major experience is in radio/audio electronics, and arduino projects.

Good, thats all needed to clear things out.