# Flyback transformer selection

I am designing a flyback converter using the PA3855.004NL transformer. Now, the datasheet screenshot is as shown below - .

There is a value of 4.33 in the primary of the transformer. Is it the number of turns in the primary ? Now, suppose the number of turns of the primary is required to be 1.41, is it safe if I use one with 4.33 ? If not what will be the impact if I use one of higher or lesser turns.

Also, the datasheet gives only the primary DCR ,saturation current and inductance. Will these parameters of the secondary have no impact ?

• How about a link to the part? Mar 22, 2016 at 8:47
• @Andyaka mouser.com/ds/2/336/-242298.pdf Mar 22, 2016 at 8:51
• The primary inductance is specified there. If you can find the core's own datasheet you can calculate the actual primary turns from L = N^2 * Al where Al in the specific inductance in the datasheet. Now the figures above can be scaled from that N by their respective turns ratios . Mar 22, 2016 at 10:10

There is a value of 4.33 in the primary of the transformer. Is it the number of turns in the primary ?

It's the ratio of primary turns to secondary turns (pin 9 and pin 6) - notice the "1" alongside this secondary.

Primary inductance is stated at 21 uH therefore the secondary inductance (should you need to know it) is the square of 4.33 times lower at 1.12 uH. That's how transformer coil inductances are related to turns ratios.

Now, suppose the number of turns of the primary is required to be 1.41

No, you have to stick with integers for number of turns.

Now, suppose the number of turns of the primary is required to be 1.41, is it safe if I use one with 4.33 ?

A winding with more turns can have a massively different inductance and, because you are designing a fly-back power regulator, this is going to be a likely problem. Inductance is proportional to turns squared so if there are 4 times the turns there will be 16 times the inductance (on the same core). You need the primary inductance to be what you have designed it to be.

Also, the datasheet gives only the primary DCR ,saturation current and inductance. Will these parameters of the secondary have no impact ?

The picture in your question shows the DC resistance of all windings. As for inductance you calculate secondary values based on turns ratio and primary values (as shown above).

Saturation is a primary coil thing and not related to the secondary or its load.

• Thank you for the response. In the transformer datasheet nothing is mentioned about pins 2 and 8. How are they connected/used in this ? Mar 22, 2016 at 11:15
• I suspect they are not used for anything. Mar 22, 2016 at 12:08