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In Flyback , what is voltage across leakage inductance of transformer?

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treez

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Hello,

In a Flyback SMPS, when the FET switched off, the drain nide voltage shoots up to the clamp level, and then the clamp voltage appears across the primary. Throughtout this time, what proportion of the primary coil voltage is across the leakage, and what proportion is across the coupled-in part of the primary coil.?

Is it just going by the rule of inductors in series?

....if so, then that would mean theres hardly any voltage across the leakage, and it would therefore take ages to discharge.

....so evidently, the rule of inductors in series cannot apply here.....but why not?
 

A coil keeps current flowing when the switching device shuts off. This is the definition of an inductor.

The high impedance shortens the time constant (calculated as L / R ). As a result the coil will generate however high voltage is necessary in order to keep current flowing. The discharge will be brief.

When continuity is broken the coil may create a spark to keep current flowing. It may destroy sensitive components in the process.

When the field takes a long time to collapse, it is when the coil is in a low impedance loop.
 
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Thanks Brad.

Though i am wondering how much of the primary voltage appears across the leakage inductance as the leakage inductance discharges into the RCD clamp?
 

I know that you are basically familar with transformer equivalent circuits. You should visualize the role of leakage inductance when going through the phases of flyback operation.

Curiously, you are only talking about primary voltage in this thread. As long as you don't consider the relation of primary and secondary voltage, in other words the transformer specific behaviour, leakage inductance doesn't exist, respectively would be only part of the primary inductance and can't be distinguished from it.

The voltage across the leakage inductance is the voltage difference between primary and (transformed) secondary voltage. That's true for any transformer, also a flyback one.
 
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