Current mode control confusion

Thread Starter

SiCEngineer

Joined May 22, 2019
135
I have a couple of questions regarding current mode control. In peak current mode control, there is need for slope current compensation to avoid subharmonic instability. There is a lot of confusing, complicated literatrure to show why this is needed, but how do we calculate the slope amplitude to add to the sensed current?

Furthermore, does average current mode still use a peak current mode controller, but just the current reference is replaced with the output voltage reference point? Or is the circuitry for average current mode completely different to that of the peak current mode controller?

Thanks!
 

WendellB

Joined Feb 14, 2020
12
Dear SiCEngineer,

Hopefully the my referenced technical video will help you to understand SMPS current mode operation and feel comfortable about using it.
Run YouTube and type "MODERN SWITCH MODE POWER SUPPLY DESIGN, CLOSING FEEDBACK LOOP"
It is over an hour long and demonstrates all know loop closures known in 2006. But to focus on your question, start at an elapsed time of 24 minutes and 5 seconds and you will see the Half Frequency Phenomena demonstrated when using Average Current Mode Feedback.
If you then advance to 1 hour and 6 minutes into the video you will see peak current mode demonstrated and how to adjust the slope of the sensed current value and compensating current value to obtain the best line susceptibility figure. The video is not on my YouTube channel for I originally sent it to a friend to display on his channel. My channel with 6 other technical videos can be located by typing on YouTube
the name of one of those videos which is: "LLC RESONANT POWER CONVERTER DESIGN, GETTING STARTED" Unfortunately my contact information in my videos is invalid for I moved last year. Also I have been informed I cannot list it here either, so we must use this forum, best we can.

Best regards, hope what you view will help answer your questions.

Wendell Boucher
 

Thread Starter

SiCEngineer

Joined May 22, 2019
135
What I don't understand is how to select the voltage which is used for the current reference.

Does this mean that the output voltage is not directly controlled.

If you use the output voltage as the current reference, then when the error in the output voltage is zero, then the inductor current will also be zero. The inductor current will be largest at the converter start up and will slowly fall to zero when the error is zero. Is this true?
 

WendellB

Joined Feb 14, 2020
12
I have a couple of questions regarding current mode control. In peak current mode control, there is need for slope current compensation to avoid subharmonic instability. There is a lot of confusing, complicated literatrure to show why this is needed, but how do we calculate the slope amplitude to add to the sensed current?

Furthermore, does average current mode still use a peak current mode controller, but just the current reference is replaced with the output voltage reference point? Or is the circuitry for average current mode completely different to that of the peak current mode controller?

Thanks!
1582645771465.png
 

WendellB

Joined Feb 14, 2020
12
Hello, Wendell here again,

Above is a Simplis circuit model that simulates a buck converter operating in peak current mode.
If you download and install the product called "SIMETRIX/SIMPLIS Elements" which is a free downloadable program and you have the circuit model above then it will run and you will see exactly how the circuit runs.

This model is the one I wanted you to see operating in the video I mentioned earlier. To try to answer your latest questions. The circuit block with the output called Ri converts current flowing through inductor L1 into a proportional voltage which is called Vcurr. That voltage is then mixed with a fixed sawtooth voltage called
Vsaw to finally produce a sawtooth voltage which we in turn compare to the output voltage of the compensation amplifier, called here E3. The current feedback is called an INNER LOOP and the voltage feedback is an OUTER LOOP. The outer loop regulates the load voltage. So we do not loose any voltage regulation properties when employing a current feedback loop. What we do though is convert the power stages gain and phase response from a complex pole pair response with a gain slope of -2 to a simple pole response with a gain slope of -1 and change the maximum phase swing before the crossover frequency is reached from 180 degrees maximum to 90 degrees maximumd making the voltage feedback loop much easier to close. There are also other advantages to use a current loop but I will not mention them here.

If you get Simplis up and running at your site, let me know and I will send this Simplis program to you. But you will have to let me know your email address to do it. Unless this site has the capability to accomplish that software transfer, I simply don't know now. In conclusion, note that the Simplis circuit above allow you to calculate feedback gain and phase while the circuit is in switching mode. And it does it in about 4 seconds of clock time which is why I have been using it since 2002.

Best regards

Wendell
 

Thread Starter

SiCEngineer

Joined May 22, 2019
135
Hello, Wendell here again,

Above is a Simplis circuit model that simulates a buck converter operating in peak current mode.
If you download and install the product called "SIMETRIX/SIMPLIS Elements" which is a free downloadable program and you have the circuit model above then it will run and you will see exactly how the circuit runs.

This model is the one I wanted you to see operating in the video I mentioned earlier. To try to answer your latest questions. The circuit block with the output called Ri converts current flowing through inductor L1 into a proportional voltage which is called Vcurr. That voltage is then mixed with a fixed sawtooth voltage called
Vsaw to finally produce a sawtooth voltage which we in turn compare to the output voltage of the compensation amplifier, called here E3. The current feedback is called an INNER LOOP and the voltage feedback is an OUTER LOOP. The outer loop regulates the load voltage. So we do not loose any voltage regulation properties when employing a current feedback loop. What we do though is convert the power stages gain and phase response from a complex pole pair response with a gain slope of -2 to a simple pole response with a gain slope of -1 and change the maximum phase swing before the crossover frequency is reached from 180 degrees maximum to 90 degrees maximumd making the voltage feedback loop much easier to close. There are also other advantages to use a current loop but I will not mention them here.

If you get Simplis up and running at your site, let me know and I will send this Simplis program to you. But you will have to let me know your email address to do it. Unless this site has the capability to accomplish that software transfer, I simply don't know now. In conclusion, note that the Simplis circuit above allow you to calculate feedback gain and phase while the circuit is in switching mode. And it does it in about 4 seconds of clock time which is why I have been using it since 2002.

Best regards

Wendell
Hello, Wendell here again,

Above is a Simplis circuit model that simulates a buck converter operating in peak current mode.
If you download and install the product called "SIMETRIX/SIMPLIS Elements" which is a free downloadable program and you have the circuit model above then it will run and you will see exactly how the circuit runs.

This model is the one I wanted you to see operating in the video I mentioned earlier. To try to answer your latest questions. The circuit block with the output called Ri converts current flowing through inductor L1 into a proportional voltage which is called Vcurr. That voltage is then mixed with a fixed sawtooth voltage called
Vsaw to finally produce a sawtooth voltage which we in turn compare to the output voltage of the compensation amplifier, called here E3. The current feedback is called an INNER LOOP and the voltage feedback is an OUTER LOOP. The outer loop regulates the load voltage. So we do not loose any voltage regulation properties when employing a current feedback loop. What we do though is convert the power stages gain and phase response from a complex pole pair response with a gain slope of -2 to a simple pole response with a gain slope of -1 and change the maximum phase swing before the crossover frequency is reached from 180 degrees maximum to 90 degrees maximumd making the voltage feedback loop much easier to close. There are also other advantages to use a current loop but I will not mention them here.

If you get Simplis up and running at your site, let me know and I will send this Simplis program to you. But you will have to let me know your email address to do it. Unless this site has the capability to accomplish that software transfer, I simply don't know now. In conclusion, note that the Simplis circuit above allow you to calculate feedback gain and phase while the circuit is in switching mode. And it does it in about 4 seconds of clock time which is why I have been using it since 2002.

Best regards

Wendell
Hello Wendell,

Thank you for being so helpful, the reply helps a lot. Could you please let me know what your email is, and I will send you an email, so that we can exchange SIMPLIS files and continue our discussion in more detail.

Thanks,
SiC.
 

WendellB

Joined Feb 14, 2020
12
Hello Wendell,

Thank you for being so helpful, the reply helps a lot. Could you please let me know what your email is, and I will send you an email, so that we can exchange SIMPLIS files and continue our discussion in more detail.

Thanks,
SiC.
Hi Sic,

Last time I posted an email it was removed by a moderator who said I broke the rules he sent me. I read them as advised and there was no mention of them. I will try again here is my email: wendellboucher@gmail.com Glad you got some help so far.

Best regards

Wendell
 

djsfantasi

Joined Apr 11, 2010
6,127
Emails should not be exchanged in the public forum. It increases the chances of being spammed and possibly creates a chance of you being hacked.

Sending emails via a private message is preferred. If you are a new member, you need ten posts before you can private message.
 
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