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Hi, this fig is a model of a circuit, where the cap is representing the gate-to-source capacitance of a hi-side FET in a bridge circuit driving a coil. VD is the intrinsic diode in the FET from Source-to-Drain. The Drain is tied to 3.3V (vcc), the pulse driving the gate goes up to 12V (vdd is a 12V step).

I'm only modeling (only interested in) the instant when the gate of the FET is driven by the square pulse, prior to the FET turning on, and am interested in obtaining the transient current that will flow through the diode from vdd to vcc. I can't figure out how to compute the current through the diode during this time. Effectively, the full 12V from the pulse (vdd in my model) is applied to the anode of the diode, connecting vcc to vdd through the diode. I'm interested in calculating what this current will be, but don't know how to use the diode equation in this case (Vd is no =~0.7V, it's 8.7V). How do I treat the diode as an ohmic element? What parameters from the diode datasheet would I use to determine what it is/would be?

Thank you, Jorge

  • 1
    \$\begingroup\$ These will be non-linear differential equations. Best bet is to simulate imo. And you don't treat it like an "ohmic element" but use Shockley's equation. \$\endgroup\$ – Fizz Dec 2 '15 at 22:28
  • \$\begingroup\$ If C is the gate capacitance and the diode is the reverse diode from drain to source then why on earth are they connected? \$\endgroup\$ – Andy aka Dec 2 '15 at 23:19
  • \$\begingroup\$ @Andy aka -- the intrinsic diode goes from source (anode) to drain (cathode)-- it's an N-chan FET. The gate capacitance goes from gate to source, so the input step voltage charges the gate capacitance in my example through this intrinsic diode. \$\endgroup\$ – jrive Dec 3 '15 at 4:29

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