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enter image description here

In the MOSFET switching application of an Inductive load, I have a question.

Why can't we place the flyback diode in a position as shown in the above image? Why can't it be placed across the MOSFET so that the diode will conduct even when the MOSFET is off?

Why should the diode be placed parallel to the inductor rather than parallel to the MOSFET?

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    \$\begingroup\$ It's kind of important to be able to turn the load off, otherwise you're not really switching.... \$\endgroup\$
    – Hearth
    Jun 6, 2023 at 4:40
  • \$\begingroup\$ @Hearth, thank you for the clarification \$\endgroup\$
    – Freshman
    Jun 6, 2023 at 5:11

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As pointed out, the diode would conduct and prevent the load from turning off. If you flip the diode it won't prevent the voltage at the drain from getting very high (which is what you want it for).

There's a situation where a diode can be used in that position, and that's with a Zener diode:

schematic

simulate this circuit – Schematic created using CircuitLab

This allows the current through the inductor to be reduced much faster than with a diode, however it's harder on the MOSFET.

enter image description here

The voltage increases until the Zener diode breaks down, protecting the MOSFET (and dissipating the stored energy in the inductance partially in the Zener diode). As you can see, the current through the coil drops much faster in the circuit with the zener.

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  • \$\begingroup\$ Thank you for the answer. What do the yellow and blue curves mean? \$\endgroup\$
    – Freshman
    Jun 6, 2023 at 5:14
  • \$\begingroup\$ There is a legend in the upper right hand corner of each graph. They represent drain voltage (top graphs) in case 1 and 2 (blue and yellow) and coil current (bottom graphs) in case 1 and 2 (blue and yellow). \$\endgroup\$ Jun 6, 2023 at 5:15
  • \$\begingroup\$ Thank you for the clarification. But the high spike in voltage in the blue waveform is very high because the selected zener has a higher clamping voltage, right? If we selected a zener with lower clamping voltage, then it would be fine right? \$\endgroup\$
    – Freshman
    Jun 6, 2023 at 5:34
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    \$\begingroup\$ @Freshman The point is to make it as high as practical so as to collapse the magnetic field quickly. You can make it lower, but it must never be lower clamping voltage than the highest possible power supply voltage or it will conduct with the MOSFET off and that will cause problems. If it's close to the supply voltage, you may as well just use a diode across the coil. \$\endgroup\$ Jun 6, 2023 at 6:23
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Why should the diode be placed parallel to the inductor rather than parallel to the MOSFET?

In very basic terms, the flyback effect (a high voltage spike) is generated across the inductor as the magnetic field collapses, so the diode parallels the inductor to shunt (short out) the high voltage spike.

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