Hi,

The formula that was given by the original poster has its roots in Faraday's Law of Induction:

V=N dPhi/dt

where:

V= voltage

N = Number of turns

Phi (should be Greek 'Phi') = flux density

t=time

The total flux Phi = flux density x Area

Flux density is B

Since the area of the core doesn't change with time.

We get V=NA dB/dt

The Si units are:

Volts

Turns (no units)

A in square metres.

B flux density in Tesla

t in seconds.

**Ae is different than Amin**

Ae is the effective Area of the core

Amin in the minimum area of area of the core.

But, in most core geometry Ae is approximately equal to Amin.

The formula that was given by the original poster gives the minimum number of turns to for a given peak flux.

How do you choose B?

Every time you magnetize and de-magnetize the core you travel around a B-H loop. The core losses are proportional to the area enclosed within the B-H multiplied by the frequency that you go around the loop.

The core loss is given in the ferrite material datasheet. The units may be kW/m^{2} for given peak-peak flux change.

Since the area inside the B-H loop changes approximately with B^{2} then the core loss is approximately proportional to B^{2}.

So we have the core loss

Core loss = k x core volume x Fsw x B^{2}

where k is a constant based on the material properties and Fsw = switching frequency.

**Consider the power supply topology**

Some power supplies, like discontinuous mode flytback converters, the flux density goes from 0 to Bpeak each cycle. In other topologies the flux may go from -Bpeak to +Bpeak (Full-bridge). So you have to understand the flux waveform when considering what value of Bpeak to use in the calculation.

Once you have determined Bpeak you can then calculate the number of turns.

Once the turns have been calculated, the core chosen, the construction of the transformer is another topic in its own right.

If you post you circuit and your requirements, I might be able to help with a transformer design.

Regards,

Jay_Diddy_B