Qspice #5: Closed-Loop Frequency Response of DC-DC Converters
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- Опубліковано 7 вер 2024
- This video shows how to measure the closed-loop frequency response of a DC-DC converter using Qspice. Two procedures are shown: i) using the .bode statement, and ii) using the .fra statement.
Several Qspice simulations are presented to illustrate the methodology.
#powerelectronics #qspice #marcosalonsoelectronics
Excellent video Sir. It is of immense help to us.
Best Regards,
Sanjay
Thanks!
This is an interesting approach which, in my opinion, cannot beat piece-wise linear (PWL) simulators like SIMPLIS or PSIM for speed and granularity. But Qspice and LTspice are free and available to anyone so it is good to watch this. Just a comment, what is displayed is the open-loop gain, not the closed-loop gain. The open-loop gain is the product of the power stage response by the compensator response. You have a closed loop for keeping the dc point at the right value but you extract the open-loop gain in the end.
Thanks for your comments. It is interesting your comment about the name of the response. This will allow me to clarify some points.
If you watch the video carefully you will see that I never said that what we are obtaining is the closed-loop gain. I say that we are obtaining the closed-loop response of the converter, as shown in the title. By this I mean the open-loop gain of the closed-loop-operated dc-dc converter. Only that this is too long for me, so I just say the loop gain of the converter operating in closed loop. For me saying open-loop gain is confusing because it can also refer to the loop gain of the converter operating without feedback loop, just pwm modulator + converter.
The name open-loop gain in closed-loop, which as I said is a rather complicated name for me, comes from the fact that to obtain this gain we have to open the closed loop and multiply the gains of all the blocks we find in the loop. This is shown in all control books. But this is physically impossible because if we break the loop the closed-loop operation would immediately fail to work. This is why we need to inject the perturbation in the loop while keeping the closed loop in operation. So, this is another reason why I prefer just saying "loop gain".
Also, the name closed-loop response could also refer to the response of output voltage over reference voltage, as shown in video PE #45: ua-cam.com/video/29jOpPIEeBw/v-deo.html. So, we see that language can be very imprecise, this is why we need images to support it. I think the video shows pretty clearly what is being done beyond the chosen language.
@@MarcosAlonsoElectronicsThank you for this comprehensive reply. Inserting an ac source in series, as highlighted by Dr. Middlebrook years ago, is actually a perturbation the system operated in a closed dc and ac loops tries to fight. You could very well physically open the loop and separately bias the control input on the dc-dc converter and modulate the bias in ac to extract the power stage response. If it works ok for low-gain systems, you would have difficulty to bias the operational amplifier and it would certainly saturate, hence making the compensated open-loop gain extraction impossible. This is why you close the loop in dc to keep the correct operating point and, by virtue of a mathematical treatment, you extract the open-loop gain. In my terminology, the closed-loop gain is when you ac-excite Vref and check the response on Vout while the loop is physically closed. In this mode, you would have extracted the closed-loop transfer function of the system and this is what you refer to in your second video I believe. But I agree, it is difficult to combine all terms in one sentence as I would otherwise designate this operation as "ac-open-loop gain characterization from a dc-closed-loop-operated dc-dc converter" : ) Thank you.
Excellent Series ....Sir could you please make a video on average model of converters based on Vorperian pwm switch model
Thanks. I will think about it. On this channel there are several videos on average modelling of dc-dc converters. I like the procedure shown in these videos better.
@@MarcosAlonsoElectronics Thanks sir for response..is there any model which can predict sub harmonic oscillation and CCM- DCM transition?
All files are now available here: github.com/marcosalonsoelectronics/Qspice-5
Those files are useless without the component library
Fatal error: Could not open file "C:\Users\Alonso\Documents\QSPICE\MyLibrary\MyLibrary.lib"
@@pahom2 Sorry, I forgot to say that you can download the library from my website: sites.google.com/view/j-marcos-alonso/home
This library is introduced in the first videos of this series
The simulation time of the Qspice scan loop is much longer than that of the software simplis. What is the reason? thank you professor
I think it depends on the semiconductor models used for the simulation. I have seen that by using an ideal switch instead of the actual transistor the simulation time is only 5 seconds and the results are pretty much the same. I have posted this file on the channel repository: github.com/marcosalonsoelectronics/Qspice-5
Thanks!
thank you professor@@MarcosAlonsoElectronics
0:30 Sorry, I should have said here the .meas FRA analysis
Hi Professor, thank you very much for the videos.They are very helpful as you explain the physics behind the circuits also. I am long time power system engineer and recently developed interest in electronics / circuit designs so i am learning on my own and trying to develop understanding of the hardware design and practical considerations while designing the circuits. I just started learning LTspice and now wordering if it would be better to start with Qspice and since it has the ability to intergate the c++ code. What would you recommend ? I already did some analysis in LTspice , or in other words, if i do Qspice, can i stil jump to LTspice. From my limited experience, i do not see any big difference since the engine is the same but maybe Qspice is the future. I would apprecaite your input.
Hello, thanks for your nice comment and question.
In Qspice you have everything you have in LTspice plus some interesting and useful new things like C++ and Verilog modules. To me this is a huge asset of Qspice. I like also the way Qspice does frequency response analysis (FRA). If you learn Qspice you will be able to use LTspice too.