I have a basic switching circuit question. I'm confused about the currents. I'm trying to understand how does the current loop through the diode in the below circuit:

enter image description here

I get the following plots from simulation:

(Blue plot is diode current, red plot is MOSFET current)

enter image description here

Can someone explain in detail(step by step in slow motion) what happens during the pulse_in goes ON from OFF state?

It looks like in the plots, the moment the pulse_in goes ON(MOSFET should start turning on), the diode and MOSFET current both makes a short huge peak. But when the MOSFET is ON how come the diode conducts here? The plots confuses my understanding.

Another thing is where does the diode peak current(during flyback operation) flows into(from and through where)?

  • \$\begingroup\$ Looks a bit weird to me, but maybe it's because of something to do with the initial circuit parameters? Maybe you could have the OFF->ON transition at 3uS, instead of 0uS? \$\endgroup\$
    – BeB00
    Oct 19 '17 at 22:51

The 50A spike is being caused by the turn off switching time of that diode ~250nS. It is still turned on when the MOSFET turns on. So initially you have almost a short to the rail.

Use a much faster diode.

Once you get rid of that the auto-scale should allow you to see the actual MOSFET, source and fly-back currents.


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