SMPS Transformers

[Voltz]

Hi, this is a question that has confused me for some time now, I know SMPSs use different transformers but I can't pin down what kind of transformer and I've goggled many times but all the articles I click seem to obfuscate a simple answer - so if I wanted to build a SMPS what kind of transformer should I buy?

 

[Diver300]

Mains frequency transformers can be made in standard ranges because the input voltage and frequency is known. You can simply plug them into the mains and they will work. The only parameters are really output VA and voltage, so manufacturers can make a range of 10 or so sizes for different VA ratings, and then wind each size for 5 or so different voltages. That gives them 50 or so different transformers that need to be stocked.

SMPS transformers have so many parameters that they are usually made to order.

You can buy the components for transformers from the usual electronic suppliers. You need to chose a core, and then decide if you want it gapped (probably useful for a flyback SMPS) or not (more likely for a push-pull SMPS). The manufacturers who make the cores normally say how to calculate the inductance and the maximum magnetising current, and that lets you calculate the primary and secondary turns you need. You then have to decide if you can fit them in, and how the winding should be constructed. You can use wire or copper tape. Copper tape is used for higher currents. The winding can be secondary first, or primary first, or some of the secondary, the primary and then the rest of the secondary. You might need or want multiple thinner wires on some windings. Stray inductance is really important and the driving circuitry may have to be designed for the stray inductance of the transformer.

Many SMPS need more than two windings. There can be multiple outputs, and there can be an auxiliary winding to run the control circuitry.

From the same manufacturer as the core, you can usually buy a coil former and a clamp to hold the whole thing together.

I'm sorry if this doesn't help, but SMPS design is an art in itself. Unless you have a really good reason to build one, it is easier to buy a complete one.

 

[Voltz]

Mains frequency transformers can be made in standard ranges because the input voltage and frequency is known. You can simply plug them into the mains and they will work. The only parameters are really output VA and voltage, so manufacturers can make a range of 10 or so sizes for different VA ratings, and then wind each size for 5 or so different voltages. That gives them 50 or so different transformers that need to be stocked.

SMPS transformers have so many parameters that they are usually made to order.

You can buy the components for transformers from the usual electronic suppliers. You need to chose a core, and then decide if you want it gapped (probably useful for a flyback SMPS) or not (more likely for a push-pull SMPS). The manufacturers who make the cores normally say how to calculate the inductance and the maximum magnetising current, and that lets you calculate the primary and secondary turns you need. You then have to decide if you can fit them in, and how the winding should be constructed. You can use wire or copper tape. Copper tape is used for higher currents. The winding can be secondary first, or primary first, or some of the secondary, the primary and then the rest of the secondary. You might need or want multiple thinner wires on some windings. Stray inductance is really important and the driving circuitry may have to be designed for the stray inductance of the transformer.

Many SMPS need more than two windings. There can be multiple outputs, and there can be an auxiliary winding to run the control circuitry.

From the same manufacturer as the core, you can usually buy a coil former and a clamp to hold the whole thing together.

I'm sorry if this doesn't help, but SMPS design is an art in itself. Unless you have a really good reason to build one, it is easier to buy a complete one.

Thank you for your reply, though I don't understand all of it, it's cleared things up a bit - I want to build and SMPS Power supply not only for my own use but also to get a complete understanding of how they work - and nothing teaches you how something operates better than building one - so my next question would be, where to I go to learn how to wind my own transformer?

 

[Diver300]

I suggest that you take apart an unwanted SMPS. Any phone charger rated from 100 - 240 V will be an SMPS.

 

[crutschow]

What type of SMPS do you want to make -- buck, boost, fly-back, inverter, etc. ? The buck and boost are generally the easiest and only require an inductor, not a transformer.

 

[Voltz]

Which ever is simplest to make - i'm a newbie

 

[WTP Pepper]

I suggest that you take apart an unwanted SMPS. Any phone charger rated from 100 - 240 V will be an SMPS.

As he describes himself a newbie, then I wouldn't recommend anything mains related.

Download a copy of LTSpice from Linear Tech's website.

https://www.linear.com/designtools/software/#LTspice

There are also hundreds of SMPS examples. Have a play with something simple and low power like the LT1174.

https://www.linear.com/designtools/software/demo_circuits.php

Also take a look at this:

https://www.dos4ever.com/flyback/flyback.html