Questions about building SMPS

I'd like to get into understanding and building SMPS for audio stuff, but I can't understand a few things.
First of all, this supply would be used to step down the mains voltage to something that can be used in an amp (30-40V dual rail) and it should be as simple as possible.
The controller that I've got so far is SG3525, but I guess even a 555 would do fine.
The transformer core is ETD39 (or 49), something easy to wind anyway.

So here it is:
- how do you calculate the turns ratio and the optimum number of turns per winding if you know the core size and you have the liberty of choosing the switching frequency?
- how do you design it in such a way that it has good transient response? Does more capacitance on the secondary side work as in linear supplies?
- can the controller (or oscillator) alone drive the mosfets or do you need a driver?
- is the regulation actually that important? I've never used it in linear power supplies, but I've heard that switching supplies are much more sensitive when loaded and that their transient response is not that good if not regulated
 
SMPS design is a black art.
You start getting involved with things like wire skin effect.
I used litz wire for my transformers.

If you are using push/pull then you need a gate driver for low and high sides.
I used a IR2113.

I designed an LLC SMPS using a PIC. The PIC gtes a feedback signal from the output if volts is low and outputs a high or low frequency to keep output regulated.
Its all good clean fun, I blew a few mosfets on my way to getting it right.
In the end I added an over current sensor and that made the job easier.
If I got an over current event the PIC shut down the mosfets and flashed an LED.
 
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29285

Disabled Account
2010-01-17 9:20 pm
I am fiddling with LLC-converters for some time. The black art is in the transformer. You have to learn to calculate it. You need a collection of enamelled wires, litz wires, insulating tapes, bobbins, ferrite cores and silicon sleeves. All that stuff is not easy available for the hobbyist not living in Shenzhen.
And then you see that the transformer is much hotter than expected. The learning curve begins....
 
I am fiddling with LLC-converters for some time. The black art is in the transformer. You have to learn to calculate it. You need a collection of enamelled wires, litz wires, insulating tapes, bobbins, ferrite cores and silicon sleeves. All that stuff is not easy available for the hobbyist not living in Shenzhen.
And then you see that the transformer is much hotter than expected. The learning curve begins....

I made a huge mistake with one of my transformers.
I used multiple enamelled wires on my secondary.
However I didn't get them all exactly the same length and the transformer drew masses of current and got hot.
Litz wire is better as it is has all the same lengths connected together.
 
As far as I know, litz wire has thick plastic insulation. Is there another type?
Great idea with microcontrollers, it's actually easier to change things with them.
How about the mosfets? Does a TO-220 type work? Like IRF540...

Please do NOT do anything with dangerous voltage as long as you don't understand the basics! You have to learn many things!
 
I'm professionally trained to work with high voltages. I'm also not dumb.

OK, then you only have to learn about the basics of SMPSs. It starts at power stage topologies, basic equations, not at choosing semiconductor nor at turn number calculation.

Litz wire never has thick insulation. But this is the last step of building.
 
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Eldam

Banned
2012-09-13 12:25 am
France
Is it a good idea if we know the spec of the amp, load to buy one of the shelf or does a SMPS should be made exatly ( a little like choosing the better VA value of a traffo for linear suplly for instance if) at the spec of the load ?

We can see prices are very different between an Hypex SMPS or an italian one or even chineese ?

Black magic ? brands advised, better than some other (factory of noise who will waste the result of tha amp or load !) !

Sorry, a little OT, but as there are many snake oïl around SMPS I would like to understand more : Better than a well made Linear or regulated PS because more rapid, or just less expensive to produce because less copper ? :confused:

When I see for Instance a design here which seems to deserve good testimonials as the FC-100 from Roender member, could it be sound even nicer with a SMPS supply like some on shelf amp : Chord, I2C amps, ... ?

regards,

Eldam
 
There are several kinds of SMPS topologies. Every type has different drawbacks and benefits. Some are simple to build but difficult to design, some are easy to control, but efficiency is low, some are especially good for audio, but needs fine tuning, etc... No ultimate solution, and every type have to be calculated, designed, controlled differently.

There are logic level MOSFETs, but even then reference potential consideration is a big task.

There are many many things you have to know.