Dieser Code ist mindestens wirr, höchstens chaotisch. Es gibt kein Gesetz, die gesamte Schaltung mit Gewalt in ein einziges `\draw[]... ... ... ` zu quetschen. Das macht solche späteren Änderungen schwierig bis unmöglich.
Bastel Dir ein [Schaltbrett][1] und zeichne Deine Schaltungen sinnvoll aufgeteilt in Schaltkreise.
Das Problem hier ist, dass `node (mosfet3) [nigfete];` sein eigenes Koordinatensystem mitliefert, etwa `(mosfet3.S)`, was nicht mehr nutzbar ist, wenn man diese node einfach löscht.
Um diesen durcheinander strukturierten Code noch halbwegs einfach zu hacken, sehe ich nur die Möglichkeit, die Änderung
% Alt ==========================
% node (mosfet3) [nigfete,right=13mm,lbl=$Q3$] {} % This is the only variable that needs to be changed to adjust the distance from Q1/Q2 to Q3/Q4
% (mosfet1.S)
%% Neu ========================
node[white] (mosfet3) [nigfete,right=13mm,lbl=$$] {} % This is the only variable that needs to be changed to adjust the distance from Q1/Q2 to Q3/Q4
(mosfet1.S)
% ============================
<br> auf `color=white` vorzunehmen; und dann die Ergänzung
\coordinate[label={[red]right:X}](X) at (mosfet3.D);
\coordinate[label={[red]right:Y}](Y) at (mosfet3.S);
\draw[red] (X) to[C=$C_\text{XY}$] (Y);
<br> `X` und `Y` hier, um das Ganze etwas hervorzuheben.
[![alt text][2]][2]
\documentclass[margin=5pt, multi=circuitikz]{standalone}
\usepackage[siunitx,european,fetbodydiode,smartlabels]{circuitikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{circuitikz}[lbl/.style = {label={[label distance=4mm]above right:#1}}] % this line defines a new style to alter the distance of labels, mosfets due to diode and transformer label
\draw[]
% top part of switch legs
(0,0) coordinate (s1)
to ++ (0,-0.4)
%node (mosfet1) [nigfete,below,anchor=D] {$Q_1$} % old, label distance incorrect
node (mosfet1) [nigfete,below,anchor=D,lbl=$Q_1$] {} % new, label distance ok
(mosfet1)
% Alt ==========================
% node (mosfet3) [nigfete,right=13mm,lbl=$Q3$] {} % This is the only variable that needs to be changed to adjust the distance from Q1/Q2 to Q3/Q4
% (mosfet1.S)
%% Neu ========================
node[white] (mosfet3) [nigfete,right=13mm,lbl=$$] {} % This is the only variable that needs to be changed to adjust the distance from Q1/Q2 to Q3/Q4
(mosfet1.S)
% ============================
to [short,current/distance =
0.1,i=$i_{L_{Q_1}}$,-*] ++ (0,-0.4) coordinate (t1)
% transformer
(t1 -| mosfet3.S) coordinate (hilf1) % Connect Q1 with Q3S which is electrically wrong, but easier to draw
node[] (T) [transformer core,below right=0mm and 17mm]{} % This is the only variable that needs to be changed to adjust the distance from Transformer T from node t1 (underneath Q1)
(T.base) node[label=above:$N_1\quad N_2$]{}
node[circ] (E) at ($(T.A1)+(0.35,-0.35)$) {} % Wicklungssinn T
node[circ] (F) at ($(T.B1)+(-0.35,-0.35)$) {} % Wicklungssinn T
% bottom part of switch legs
(mosfet3.S |- T.A2) coordinate[] (t2) % likewise counter intuitive
to[red] ++ (0,-0.4)
% Alt ===========================
% node (mosfet4) [nigfete,below,anchor=D,lbl=$Q_4$] {}
% (t1 |- mosfet4.D) node (mosfet2) [nigfete,below,anchor=D,lbl=$Q_2$] {}
% Neu ==========================
node[white] (mosfet4) [nigfete,below,anchor=D,lbl=$$] {}
(t1 |- mosfet4.D) node (mosfet2) [nigfete,below,anchor=D,lbl=$Q_2$] {}
% =============================
% connection lines origins at transformer
%(T.A1) to (t1) % old, replaced by short b/c current arrows, see below
(t1) to [short,current/distance = 1,i=$i_1$] (T.A1)
(T.A2) to [short,-*] (t2)
(T.B1) to [short] ++ (0.5,0) coordinate (t3)
(T.B2) to [short] ++ (0.5,0) coordinate (t4)
% connection lines orign at mosfet
(t1) to (mosfet2.D)
(mosfet2.S) to ++ (0,-0.4) coordinate (s2)
(mosfet3.D) to ++ (0, 0.4) coordinate (s3)
(mosfet3.S) to (mosfet4.D)
(mosfet4.S) to ++ (0,-0.4) coordinate (s4)
% supply lines
(s3) -- (s1) to [short,*-] ++(-1.5,0) coordinate (czk+) to [short,*-o] ++ (-0.75,0) coordinate (s+) % added *-o, * to get dot at S1
(s4) -- (s2) to [short,*-] ++(-1.5,0) coordinate (czk-) to [short,*-o] ++ (-0.75,0) coordinate (s-) % added *-o, * to get dot at S2
% (s+) to [open, v=60<\volt>, invert] (s-) % old, with SI unit
(s+) to [open, v=$U_{in}$, invert] (s-) % new, with variable U_in
% secondary side dc rectifier
(t3) to[short,current/distance = -1.5,i=$i_2$,-*] ++(0.35,0) coordinate (AA) % move transformer outlet a bit to the right
(t4) to[short,-*] ++(1.85,0) coordinate (BB) % move transformer outlet a bit to the right
(AA) to [Do,name=d1,l=$D1$] ++(0,1.5) to[short,-*] ++(1.5,0) coordinate (c1)
(AA) -- (AA |- BB) to [Do,invert,name=d2,l_=$D2$] ++(0,-1.5) to[short,-*] ++(1.5,0) coordinate (c2)
(BB) -- (BB |- AA) to [Do,name=d3,l=$D3$](c1)
(BB) to [Do,invert,name=d4,l_=$D4$](c2)
% missing C_ZK inserted
(czk+) to [C,l=$C_{ZK}$] (czk-)
% missing L_m insterted
(hilf1) ++ (1,0) coordinate (hilf2)
(t2) ++ (1,0) coordinate (hilf3)
(hilf2) to[L,l_=$L_m$,-*,current/distance =
0.45,i=$i_{L_m}$,*-*] (hilf3)
% secondary side, L, C, out
(c1) to[L,l_=$L$,-*,i_=$i_L$,v^=$u_L$]
++(3,0) coordinate(c3) to[short,i=$i_{out}$,-o] ++(0.75,0) coordinate (out1)
(c3) to[C,l_=$C$,i=$i_{C}$,-*] (c3 |- c2) coordinate (c4) -- (c2)
(c4) to[short,-o] ++(0.75,0) coordinate (out2)
% voltage arrows primary
($(T.A1)+(0.35,0)$) to [open,v=$u_1$] ($(T.A2)+(0.35,0)$)
% voltage arrows secondary
(out1) to[open, v^=$U_{out}$] (out2) % delete later, voltage arrow
%(c1) to[open,v^=$u_L$] (c3) % optional instead voltage arrow of L, from c1 to c3
($(c1)+(0.35,0)$) to[open,v^=$U_{GR}$] ($(c2)+(0.35,0)$)
($(T.B1)-(0.35,0)$) to [open,v^=$u_2$] ($(T.B2)-(0.35,0)$)
;
% Änderung ============================
\coordinate[label={[red]right:X}](X) at (mosfet3.D);
\coordinate[label={[red]right:Y}](Y) at (mosfet3.S);
\draw[red] (X) to[C=$C_\text{XY}$] (Y);
\coordinate[label={[red]right:P}](P) at (mosfet4.D);
\coordinate[label={[red]right:Q}](Q) at (mosfet4.S);
\draw[red] (P) to[C=$C_\text{PQ}$] (Q);
\end{circuitikz}
\end{document}
[1]: https://tex.stackexchange.com/a/755954/46023
[2]: https://texwelt.de/upfiles/555555_8.png