Background Research

Magnetics Design for High Performance Power Converters:

https://www.youtube.com/watch?v=45kTQwZSLHc

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Flux lines in an air coil are very leaky (hard to control), so use magnetic core to control it

high permeability material allows control of where flux flows
if the primary flux flows to secondary flux, can get transformer voltage conversion

S is magnetic reluctance

if you have a core with high permeability (μr = 1000), then only the reluctance of the gap really matters (since μ0 = 4pi*10^-7

B can be defined as Bmax, saturation flux density

energy is stored in volume of air gap (B = Area * Length of Air Gap)
if you need a tranformer that stores a certain amount of energy, then u can set B = Bmax and then solve for volume of air gap

Focusing on L = N^2/S, we see that as S increases, we need greater N to get the desired amount of inductance

Power Loss Mechanisms:

Hysteresis in magnetic material (energy in graph is energy lost per switch)
Eddy Currents (since the core itself is conductive)

Datasheets provide core power loss, in Watts, as a function of frequency

Ferrite has lowest core loss, but has lower Bsat and worse under high temperature conditions (but we don’t need high so let’s use Ferrite)

Skin Effect: When in AC, the current ends up flowing around the skin of the conductor, not the whole area

So if we plot it for copper, we see that, for a given frequency, you shouldn’t use a wire with a greater diameter than the skin depth (e.g. at 100kHz, don’t use conductor with >0.42mm dia)

Proximity loss: conductors flowing next to each other will repel or attract

reduces area used to conduct, so impedance increases
so stacking lots of coils next to each other will increase impedance

Fringing loss: fringing magnetic fields leaking from the core will hit the primary coils and start generating eddy currents, which will increase heat! (and thus also impedance)

Rule of thumb: Keep windings >3 airgap distances from the airgap

Designing a Transformer for a Flyback Converter:

Notes:

E = 0.5*Ipk*L^2

Questions:

How to calc Ipk?
What is current density J? Why is it 3A/mm?
What is a reasonable packing factor to assume?

Building the transformer:

Use Tex-E wire for secondary wire so there’s no shorts and no additional insulation needed

Notes:

Measuring Properties:

Primary Leakage Inductance can be calculated by shorting the secondary coils together and then powering the primary side
Primary/Secondary resistance: AC has much higher resistance than DC (cuz losses above)
Using RMS current, switchin freqneyc and resistance, can find winding loss (W)