Magnetizing Inductance

[hithesh123]

Why is magnetizing inductance represented in parallel to the actual inductance?

In a flyback transformer, the magnetizing inductance in series with the leakage inductance together represent the real primary side?

Also, why does the secondary voltage get reflected to the primary side?
If Mosfet is the switch, then the voltage will be Drain Voltage+reflected voltage.

 

[chuckey]

The magnetizing inductance is the actual inductance, the drawing shows the transformer to have a "perfect" inductance, these other bits represent "real life" transformer.

As said the circuit represents a "perfect" transformer, so you would expect to see idealized voltages on every windings. Then you put in all the losses and start doing the maths. . .

Voltage = Drain voltage + di/dt X L
Frank

 

[hithesh123]

Voltage = Drain voltage + di/dt X L
Frank

Here, L is the primary inductance? and di refers to primary current?

I still don't get why secondary voltage gets reflected to the primary side.

 

[chuckey]

Yes .
Its generated across the primary by the change of current through it.
Frank

 

[RCinFLA]

It is just a model representation of an unload transformer. As the secondary is loaded, more and more, it creates a larger and larger counter magnetic field that lowers the primary side inductance. In the limit case where the secondary is shorted, the leakage inductance becomes the inductance measured across the primary.

A power transformer's purpose is to tightly couple primary to secondary for maximum power transfer. Its core is not designed to store energy.

A flyback is a coupled inductor designed to store energy and transfer the energy on the discharge cycle. Its core is designed to store energy.

 

[chuckey]

"magnetic field that lowers the primary side inductance." The inductance is a physical property of the number of turns, permeability of core, magnetic path, etc. What a perfect transformer does is to step up/down the primary voltage to the secondary, it also steps down/up the primary current to the secondary. This is what it does, if it does not do this it is not a perfect transformer. To move from the theoretical to the practical transformer we have to add primary inductance, leakage inductance, secondary inductance, resistance of windings, capacitive effects, magnetic losses. . . In real life you select the parameters that will most effect your application.
"magnetic field that lowers the primary side inductance." As the secondary is loaded more, this impedance is reflected as a load on the primary winding, as being in series with the leakage inductance, and as the reflected load gets lower and lower, the leakage inductance become the dominant impedance. The opposite is the case as the secondary is unloaded, as the reflected impedance gets higher and higher, the primary inductance becomes dominant.
Frank

 

[eem2am1]

The point is, if you have a load of current carrying coils wrapped round the same core....................then the core does not know which winding is afflicting a magnetic field into it, and it doesn't care.

If one winding is running current , and has a volts/turn to it.

...then that same volts/turn will be on other windings too....cuzz they are coupled with it...they are "in bed" with each other, they cannot help but affect each other.