I'd like to isolate, with high-side and low-side switches, a RLC circuit after the inductor has been energized with a current of 2-3A from a 12V source (see sketch). RLC circuit

When the RLC circuit is isolated quickly, the AC voltage in the RLC circuit will go to 200V due to the back-emf of the inductor (400uH). The sense resistor is small (22 mOhm).

Because I'd like low resistance in the switch and because of the fast switching speed required (<100ns) my thought was a back-to-back mosfet switch. Can anyone recommend how the high-side and low-side mosfet switches should look like to effectively isolate the AC circuit from the DC supply, and how to best drive them from 3.3V logic? Any advice would be much appreciated.

For Andy aka: I include the spice simulation of a (not working) single mosfet circuit as you suggested. {B} is 400uH. spice

Voltages at gate (blue) and drain (green) of mosfet.


and zoom-in


and just to show that's not the capacitor (or anything else) that causes the problem, here's the same trace when the IRF740 is replaced by a voltage controlled switch (which is of course not practical).


  • \$\begingroup\$ Do you know of any MOSFETs that have a drain-source capacitance that is significantly less than the value of C1 in your circuit? \$\endgroup\$ – Andy aka Nov 23 '18 at 10:11
  • \$\begingroup\$ C1 is >100nF, so most mosfets will have smaller Qgs. IRF740 is cheap and cheery. I'm more interested in the switch configuration rather than switching time. \$\endgroup\$ – HBua Nov 23 '18 at 22:48
  • \$\begingroup\$ Why do you need both high side and low side isolation? \$\endgroup\$ – Andy aka Nov 24 '18 at 9:17
  • \$\begingroup\$ I don't want to stress the battery (voltage source) with the excess energy in the inductor. But I'd be happy for your answers on how you would isolate either high-side or low-side. \$\endgroup\$ – HBua Nov 24 '18 at 10:35
  • 1
    \$\begingroup\$ You might also consider having a forward biased fast and high voltage diode in series with the MOSFET drain so that it conducts the current to charge the inductor then blocks any recoil voltage. Try that idea in the sim. \$\endgroup\$ – Andy aka Nov 25 '18 at 13:35

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