draw[red]
    (igbt1.G)   to[short,red] ++ (0,-0.5) node[below,red] {$S_1$};

    ctikzset{bipoles/length=6mm}
    draw       (igbt2.S) [red] -- ++ (0,-0.3) coordinate (D2)
    to [Do,a=D2,color = red] (D2 -| igbt2.D) -- (igbt2.D);
    draw[gray] (igbt2.G)  -- ++ (-0.5,0) node[left,gray] {$S_2$}
    (igbt1.S) -- ++ (0.3,0)  coordinate (D1)
    to [Do,a=D1,color=gray] (D1 |- igbt1.D) -- (igbt1.D);
    draw[B]    (in+)  -- node[ left=5pt] {$V_1$} (in-);
    draw[B]    (out-) -- node[right=5pt] {$V_2$} (out+);
end{circuitikz}