I am working on a buck converter and I want to obtain the Bode diagrams of the open-loop circuit. The below circuit is what I made. However, the DC-gain is -96 dB while it should be very small around -6 dB according to the transfer function. So, am I missing something? Could you please help me out with this?
The AC analysis is a linearized analysis. That means it simulates the small-signal behavior of each element at its operating point.
In the case of the switch, that means it is held in either the on or off state, and behaves as a resistor with either the ROFF or RON value.
The -96 dB transfer gain is due to the leakage through the switch's 1 megohm off resistance.
If the reason you're simulating this circuit is enable designing the switching control loop, really you should just remove the switch and diode and drive the left end of the inductor with your voltage source. Because the control loop critical frequencies will be much lower than the switching frequency, the whole switching circuit (input source, switch and diode) just looks like a voltage source with a value depending on the switch duty cycle when you're designing the control loop.
The discussion is interesting an in particular, the (always) comprehensive reply given by a concerned citizen. However, it shows, in my opinion, that LTspice is not really suited for extracting the ac response of a switching circuit. The problem lies with the stimulus frequency, its amplitude (to avoid saturation) and the switching period. The ac plots that are provided start at 1 kHz but going down to 10 Hz (or even for a PFC circuit) would probably imply a tremendous amount of time. It is not to criticize LTspice that I find excellent, but my feeling is that it is not meant for making this measurement practical.
Instead, look at the below circuit captured with Elements, the free demo version of SIMPLIS:
This is a voltage-mode buck converter with current limit. It is a switching circuit and the program will deliver the cycle-by-cycle waveforms (to make sure the operating point is correct) but also the transfer functions of your choice. In this example, the total simulation time is 5 s on my computer:
This circuit is part of the newly-developed free templates that I posted on my page.
The other option, as underlined in my comments, it is to resort to an average model and LTspice will deliver the information we want very quickly:
The switching component has disappeared and you work on a circuit linearized by the simulation engine. A .AC statement is now valid and will do the job at any frequency.
The Photon's answer says why you can't perform an .AC analysis on this particular type of circuit, and Verbal Kint's comment deals with the possibility of making an .AC analysis possible by using an averaged model (which I'll leave up to him to answer).
I'll show you two other methods that can be done in .TRAN.
One of them is found in the LTspiceXVII/examples/Educational/FRA folder, in My Documents, as part of the default installation of LTspice(XVII). This method implies setting up a literal small-signal source, which is the equivalent of Middlebrook's LTspiceXVII/examples/Educational/LoopGain.asc and LoopGain2.asc, but for .TRAN.
Among the three shematics in there, Eg2.asc shows how to setup a frequency sweep in .TRAN and some .MEAS scripts in order to extract the frequency information. It's all done with an LTC3611, but you can implement it easily in your schematic like this:
This is a very basic voltage-mode buck converter, don't give it too much thought other than to see it's a buck converter and how it can be set up for this analysis. Note that I used *pi in the .MEAS scripts, because I have checked the option User radian measure in waveform expressions inside the Control Panel > Waveforms. That's why you see "only" a few degrees, because they're radians. In Eg2.asc, the commands make use of 360, so degrees, and they rely on that option being ticked off (which is the default). The result is this:
Note that I've made a few changes compared to the original schematic: freq sweep is from 50k to 500k, with 50 steps per decade, not octave. The option nomarch is also added.
The second method is similar, in that the previous one dealt with .STEPping a parameter and then collating the results of the .MEAS scripts, while this one does everything dunamically, trying to perform as a real-life FRA would, giving the results in (quasi-)real-time. The downside is that it is slow, but it delivers.
This schematic belongs to the user analog spiceman from the LTspice user group, so I'll only post the result of the simulation of FRA-CCM.asc (one schematic from the FRA folder), and if you want to give it a try, you'll have to go to the group's Files area and get the files from there (registration is needed, though).
Are you sure the switch is working properly in the AC simulation? I don't believe that the pulse source operates in the AC simulation.
If you take the voltage division of the switch off resistance (1MΩ), you get an output voltage of 15µV (for a 1V input), which is -96.48dB.
LTspice is quite capable of doing this work, sweeping the bode plot on the switching circuit. Much better results can be obtained than are shown here, but you have to know how to set up the frequency response analyzer properly. That's no different from the real world.
We have given several seminars on this, and the speed of LTspice is more than acceptable if your models are set up properly.
