I want to plot the open loop gain dB20 and phase curve of a DC/DC converter using PAC analysis, but how to break the circuit and add PAC source?
You cannot insert a L-C filter to isolate AC signal and pass DC signal, because the PSS analysis need to do transient analysis first, if you insert filter the transient analysis is not correct.
I'm not familiar with PAC analysis, but a general method is to insert a series voltage source in the feedback path and calculate the loop gain as ratio of voltages at both sides.
Not generally. If you have a circuit branch with Zsource << Zload, it's just one step. In most active circuits, this is feasible.
Also Middlebrook is describing the method initially as using either voltage or current injection, with respecitive prerequisites for the circuit topology. **broken link removed**