Bode plot in Matlab
Ming Sun / November 28, 2022
9 min read • ––– views
The following code first does the Bode plot based on a s domain transfer function. Then it reads the csv data from Simplis POP and AC simulation results. After that, it compares the Bode plot between s domain transfer function and Simplis simulation results.
buck.m
clc; clear; close all;
L = 1e-6;
C = 1e-6;
R = 1;
Vg = 5;
s = tf('s');
Gvd = Vg/(1+s*L/R+L*C*s^2);
h = bodeplot(Gvd); % Plot the Bode plot of G(s)
setoptions(h, 'FreqUnits', 'Hz'); % change frequency scale from rad/sec to Hz
set(findall(gcf,'type','line'),'linewidth',2)
grid on;
hold on;
% read simplis simulation results from csv file
data = csvread("simplis.csv", 1, 0);
freq = data(:,1);
mag = data(:,2);
phase = data(:,3);
ax = findobj(gcf, 'type', 'axes');
phase_ax = ax(1);
mag_ax = ax(2);
% append simplis plot to bode plot
plot(phase_ax, freq, phase, 'r--', 'LineWidth', 2);
plot(mag_ax, freq, mag, 'r--', 'LineWidth', 2);
legend('Math', 'Simplis')
Fig. 1Bode plot in Matlab
Query y axis range
clc; clear; close all;
s = tf('s');
Rtot = 1e6;
Vref = 1.2;
Vout = 1.8;
ratio = Vref/Vout;
R2 = ratio*Rtot;
R1 = (1-ratio)*Rtot;
C3 = 10e-12;
R3 = 100e3;
C4 = 50e-12;
R4 = 100e3;
C5 = 100e-15;
p1 = 1/2/pi/R3/C3;
p2 = 1/2/pi/R4/C5;
z1 = 1/2/pi/R1/C3;
z2 = 1/2/pi/R4/C4;
H = -(1+s*R1*C3)*(1+s*R4*C4)/s/R1/C4/(1+s*R3*C3)/(1+s*R4*C5);
h = bodeplot(H); % Plot the Bode plot of H1(s)
setoptions(h, 'FreqUnits', 'Hz'); % change frequency scale from rad/sec to Hz
set(findall(gcf,'type','line'),'linewidth',2)
grid on;
hold on;
ax = findobj(gcf, 'type', 'axes');
phase_ax = ax(1);
mag_ax = ax(2);
xlim([100, 100e6]);
yrange = ylim(mag_ax);
plot(mag_ax, [p1,p1], yrange, ':', 'LineWidth', 2, "Color", "#3B82F6");
plot(mag_ax, [z1,z1], yrange, ':', 'LineWidth', 2, "Color", "#EF4444");
plot(mag_ax, [p2,p2], yrange, '--', 'LineWidth', 2, "Color", "#3B82F6");
plot(mag_ax, [z2,z2], yrange, '--', 'LineWidth', 2, "Color", "#EF4444");
legend(mag_ax, "Bode plot","pole1","zero1","pole2","zero2");
Fig. 2Bode plot in Matlab
Enable phase wrapping
comparision.m
clc; clear; close all;
Ts = 1e-6;
wn = pi/Ts;
Q = -2/pi;
s = tf('s');
H1 = exp(-s*Ts)*s*Ts/(1-exp(-s*Ts));
H2 = 1+s/wn/Q + (s/wn)^2;
opts = bodeoptions('cstprefs');
opts.PhaseWrapping='on';
h = bodeplot(H1, opts);
hold on;
h = bodeplot(H2, opts);
setoptions(h, 'FreqUnits', 'Hz'); % change frequency scale from rad/sec to Hz
set(findall(gcf,'type','line'),'linewidth',2)
grid on;
hold on;
xlim([10, 10e6]);
legend("exponential", "simplified");
Fig. 3Enable phase wrapping
Phase wrapping and phase matching
comparison.m
clc; clear; close all;
Vg = 3.8;
V = 20;
L = 1e-6;
C = 10e-6;
fsw = 3e6;
Rsns = 0.3;
R = 20;
Dp = Vg/V;
D = 1-Dp;
Ts = 1/fsw;
Se = (V-Vg)/L*Rsns;
Sr = Vg/L*Rsns;
s = tf('s');
Gvc0 = R*Dp/(2*Rsns + Dp^3*R*Se*Ts/Vg);
wp = 2/R/C;
wn = pi*fsw;
Q = 1/pi/(Dp*(1+Se/Sr)-0.5);
wrhpz = R*Dp^2/L;
Gvc = Gvc0*(1-s/wrhpz)/(1+s/wp)/(1+s/wn/Q+s^2/wn^2);
h = bodeplot(Gvc); % Plot the Bode plot of G(s)
setoptions(h, 'FreqUnits', 'Hz'); % change frequency scale from rad/sec to Hz
set(findall(gcf,'type','line'),'linewidth',2);
p = getoptions(h);
p.PhaseMatching = 'on';
p.PhaseMatchingFreq = 1;
p.PhaseMatchingValue = 0;
setoptions(h,p);
grid on;
hold on;
% read simplis simulation results from csv file
data = csvread("simplis.csv", 1, 0);
freq = data(:,1);
mag = data(:,2);
phase = data(:,3);
ax = findobj(gcf, 'type', 'axes');
phase_ax = ax(1);
mag_ax = ax(2);
% append simplis plot to bode plot
plot(phase_ax, freq, phase, 'r--', 'LineWidth', 2);
plot(mag_ax, freq, mag, 'r--', 'LineWidth', 2);
legend('Math', 'Simplis')
xlim([100, 1.67e6]);
Fig. 4Gvc transfer function Bode plot comparison between Mathematics derivation and Simplis