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Modeling >> Behavioral Models >> Diode junction cap equation
https://designers-guide.org/forum/YaBB.pl?num=1410256291

Message started by ranjang on Sep 9^th, 2014, 2:51am

Title: Diode junction cap equation
Post by ranjang on Sep 9^th, 2014, 2:51am

module diode (a, c);
inout a, c;
electrical a, c;
branch (a, c) i_diode, junc_cap;
parameter real is = 1e-14, tf = 0, cjo = 0, imax = 1, phi = 0.7;
analog begin
I(i_diode) <+ is*(limexp(V(i_diode)/$vt) – 1);
I(junc_cap) <+
ddt(tf*I(i_diode) - 2*cjo*sqrt(phi*(phi*V(junc_cap))));
if (I(<a>) > imax)
$strobe( "Warning: diode is melting!" );
end
endmodule

In this what does the below equation signifies and how it can be derived
I(junc_cap) <+
ddt(tf*I(i_diode) - 2*cjo*sqrt(phi*(phi*V(junc_cap))));

Thanks

Title: Re: Diode junction cap equation
Post by Ken Kundert on Sep 9^th, 2014, 10:29am

It is the junction capacitance. You can derive it by taking the normal junction capacitance formula for C(v), which equals dq(v)/dv, integrating it with respect to v to get q(v), then applying the time derivative to q(v(t)) to get i(t).

-Ken

Title: Re: Diode junction cap equation
Post by ranjang on Sep 10^th, 2014, 5:16am

Thanks Ken.

Title: Re: Diode junction cap equation
Post by Geoffrey_Coram on Sep 23^rd, 2014, 2:16pm

Note that tf*I(i_diode) is a diffusion capacitance; the rest is the usual junction (depletion) capacitance.