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I was reading a book, "switching power supply design - Abraham Pressman", specifically the section about flyback converters. One part I'm not quite sure I understand is where he says that the saturation of the primary coil could destroy the driving transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

I went back to school on how inductors work and I think what will happen is this. At time zero when the transistor turns on, current will begin to flow slowly and the magnetic field will build up in the inductor. Current flow will increase as time goes on and this will increase the size of the magnetic field. At some point the core material will become saturated, meaning that even though you pump more current into it the magnetic field does not increase very much.

It was this build up of the magnetic field that was protecting the FET in the first place, but now that it's saturated its inductance will begin to drop. So effectively the impedance of the coil is dropping and more and more current now wants to flow through the transistor. Eventually that current times the RDS on of the FET will exceed the power rating of the package and the FET will melt down.

Am I understanding this correctly?

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  • \$\begingroup\$ In a nutshell: yes. \$\endgroup\$ – Adam Lawrence Nov 30 '15 at 15:41
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    \$\begingroup\$ Interesting fact: These are flyback converter designs in which the drive voltage for the primary FET is generated from a feedback winding on the core. As soon as the core saturates, the feedback voltage breaks down and the FET is shut off, preventing damage. Actually, your cheap cellphone charger might work that way. Regulation is done by adjusting pulse delay. \$\endgroup\$ – Michael Karcher Nov 30 '15 at 18:09
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Yes, you understood correctly. At saturation it is like the inductor "loses" its inductance. What you're left with is only the Rs (ESR) of the inductor. For a quality inductor Rs will be very low resulting in a large current and possible damage to inductor and/or FET.

An inductor stores electrical energy as magnetic energy, when the inductor saturates it cannot store more energy since it is "full". The core material of the inductor (if there is any) has reached its maximum magnetic flux and cannot accept more.

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Pulse width and current waveform: -

enter image description here

A few more technicalities....

For small pulse widths, the rate of change of current (di/dt) is applied voltage divided by primary coil inductance. As saturation starts to kick in the rate of change of current rises dramatically as primary coil inductance falls off.

Saturation is, (with all other things being equal), directly proportional to current so, even though the core material may have a fairly shallow hysteresis curve, once the current starts increasing its rate of change, saturation can happen really fast and be devastating.

This is why you find that fly-back designs switch the duty cycle off when current exceeds a certain preset value.

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