# Understanding a switched-mode power supply datasheet

I'm trying to make an AC/DC converter (230V AC to 5V DC). As part of this converter, for the first stage, I've decided to use a switched-mode power supply like https://www.digikey.com/product-detail/en/wurth-electronics-midcom/7508170312/1297-1174-1-ND/5700118

If you take a look to the datasheet, you'll see this:

As I understand, I can apply any AC voltage from 78V to 375V, and I will get two fixed outputs: 15V (DC) and 16V (DC).

Is that right?

• This is not a switching converter, this is a part of a switching converter. What you have is just a transformer intended to be used as part of a flyback converter. In other words, no; what you get out will be AC at a mostly unpredictable voltage because this transformer is only spec'ed for operation at 50kHz and you likely intend to apply 50 or 60 Hz to it. Oct 28 '18 at 12:39
• Link to the english digikey page in case anyone else wants it: digikey.com/product-detail/en/wurth-electronics-midcom/… Oct 28 '18 at 12:42

What you have here is not a switched-mode power supply, but only a part of one. This is a transformer intended to be used as part of a flyback converter. If you apply 50/60 Hz AC to the primary winding, you won't damage it, but what you get out will be AC again at an unpredictable voltage. This device is designed to work with high-frequency AC from a switching circuit, and its output is again high-frequency AC that needs to be rectified and regulated. The coil marked "aux" is a feedback coil, part of the control mechanism of this type of converter.

I'm not familiar enough with flyback converters to properly explain how this is meant to be used, though, so if anyone wants to do that go right ahead.

• Connecting a 50kHz transformer to 50Hz will make the core saturate after 1ms or so. But the half-wave is 10ms long. The result is not much different from a short on the primary. Oct 28 '18 at 14:50

User Felthry has said already many important facts. This transformer is designed for flyback DC to DC converters. The winding inductances, core size and the used materials are designed to fit certain voltages and 50kHz switching frequency. The center tap in the primary gives a possiblity to more complex than basic applications for competent engineers who can do the needed calculations without application notes.

AUX is for the regulation circuit.It needs power , too. There's a kickstart circuit for start, but long time consumption needs a supply. This way the circuit feeds itself. Feedback from the output is generally made optically to retain the isolation.

To use this you should learn how flyback converters work and are designed. Converter IC producers have good application notes. The general principle is to charge some energy from input DC to the primary winding (=to its magnetic field) and when the current has grown high enough, the primary current is turned off. The resulted inductive spike around the magnetic core is directed through diodes to capacitors in the outputs. It appears in all windings around the same core, not only in the primary.

The regulator IC decides when the output voltage in the capacitor has dropped enough for a new pulse. The output voltage really swings few millivolts up and down.