Magnetizing Inductor vs Transformer

Question

I am a bit confused by the diagram - are there two inductors in parallel on the primary side of the flyback transformer?

How does no current flow through v1 when the switch is closed?

Answer 1

There is an endless debate about the flyback converter regarding the primary-to-secondary transfer mechanism: transformer or coupled inductors? When you look at the magnetic structure, it certainly looks like a transformer but in the simplified view, there is no current simultaneously circulating in the primary and the secondary.

Look at the below picture, excerpted from my seminar The Dark Side of the Flyback Converter:

When the main switch closes, the input voltage is applied across the primary side and a current circulates: the primary-side inductance magnetizes and stores energy. The current "enters" the transformer by the dot and would like to "leave" by the opposite dot. However, the reverse-biased diode forbids this to happen and the secondary is decoupled from the primary side during this moment: there is no current circulation in the secondary.

When the PWM controller instructs the switch to open - because the peak current has reached the setpoint for instance - then the magnetic field inside the core collapses and the voltage across the magnetizing inductance reverses in an attempt to keep the amperes-turns. The secondary diode now conducts with a current and the energy stored in the primary feeds the output capacitor and the load. The secondary current at this moment is the primary peak current at the switch opening scaled by the transformer turns ratio:

The output voltage \$V_{out}\$ now appears across the secondary winding (because the diode conducts) but it also flies back to the primary side scaled by the turns ratio. The voltage on the blocked transistor during this time is thus the input source plus the reflected voltage. In reality, the switching mechanisms are more complicated (there are parasitics such as the leakage inductance, the various capacitances etc.) and, during the transitions, a current certainly circulates in the primary and the secondary at the same time during a short moment. You can find details about all these effects in my book on power supplies.