diode calculations

Download source: diodes.zip

Contents

Diode equation
Solve example 1
load line
Example 2 - assume diode conducts.
assume diode does not conduct

Diode equation

IS = 3e-15;
VT = .026;
vd = linspace(0,0.79,201);
id = IS*(exp(vd/VT)-1)*1e3;
plot(vd,id,'k','LineWidth',2);
xlabel('V_d (volts)');
ylabel('I_d (mA)');
axis([0 1 0 30]);
grid;

Solve example 1

Von = 0.7;
Vs = 4;
Rs = 500;
id = (Vs - Von)/Rs;
vd = fzero(@diode_fcn,Von);
fprintf('approximate id %g mA\n',id*1e3);
fprintf('exact id %g mA\n',(Vs-vd)/Rs*1e3);

approximate id 6.6 mA
exact id 6.5228 mA

load line

Vs = 4;
Rs = 500;
Ix = Vs/Rs*1e3;
vd = linspace(0,0.79,201);
id = IS*(exp(vd/VT)-1)*1e3;
plot(vd,id,'k','LineWidth',2);
hold on;
plot([0 Vs],[Ix 0],'r','LineWidth',2);
hold off;
xlabel('V_d (volts)');
ylabel('I_d (mA)');
axis([0 5 0 10]);
grid;

Example 2 - assume diode conducts.

R = [5 10 10];
Rp = 1/sum(1./R);
Vs = 14;
Vb = 9;
v1 = Rp*(Vs/R(1)+(Vb+0.7)/R(3));
fprintf('v1 %g V\n',v1);
i3 = (v1-Vb-0.7)/R(3);
fprintf('i3 %g A\n',i3);
if (i3<0)
    fprintf('diode is not conducting\n');
end

v1 9.425 V
i3 -0.0275 A
diode is not conducting

assume diode does not conduct

v1 = R(2)/(R(1)+R(2))*Vs;
v2 = Vb;
vd = v1 - v2;
fprintf('vd %g V\n',vd);
if (vd<0.7)
    fprintf('diode is not conducting\n');
end

vd 0.333333 V
diode is not conducting