PE #47: Effect of EMI Filter on the Dynamic Response of DC-DC Converters

In this video we study the effect of EMI filter on the dynamic response of a DC-DC converter. This is an important topic that is usually neglected in many basic courses on power electronics. The video shows how to obtain the control-to-output transfer function of a DC-DC converter including the effect of the EMI filter. Also, the effect on the stability of the converter in closed loop is studied. It is shown how the inclusion of the EMI filter can make unstable an originally stable closed-loop operated converter. Examples and simulations results are shown.

[@bbug705076]

Hi Alonso I have bought you some coffee through paypal. Thank you for these online courses. I look forward to more content of your power electronic course : )

[@MarcosAlonsoElectronics]

Hello, I got it. Thank you very much. All contributions will be used to make this channel better

[@bbug705076]

@@MarcosAlonsoElectronics Could you provide more information for the Gd? I still do not understand where D squared Zf come from even though I watch the PE# 3 modeling of DCDC converter.

[@MarcosAlonsoElectronics]

@@bbug705076 You have to solve the system on the left shown at 6:23. You have to subtitute i_L=v_o/Z_p in the expression of v_i and then v_i in the expression of v_o Then solve for v_o over d

[@MarcosAlonsoElectronics]

All files are available at: github.com/marcosalonsoelectronics/PE-47

[@slopezc-BS]

Hi Marcos! Thank you very much for all your videos, they are awesome. I have doubts regarding the resistance (ESR) of the capacitors (defined as rC). If at the output of my converter I have several ceramic capacitors the total ESR becomes smaller and smaller (around 1 mOhm?) and that makes my system very close to being unstable (~170º) before reaching 0 dB. One solution would be to add an electrolytic capacitor but I don't think this is the best solution due to the ripple current at the output of the converter... What ESR should we take for the simulation? Because the ESR of a ceramic capacitor is different depending on the frequency. Thank you very much for all you do! Edit: I'm using your LTspice file from minute 15:00 without the EMI filter.

[@MarcosAlonsoElectronics]

Thanks for your comment. Yes, the zero created by the capacitor ESR helps with the converter stability. If rC is very small, the zero will be at very high frequency and its effect will be negligible. I think that in this case you need to redesign the compensator accordingly to meet your requirements. Regarding the second question, we model the real cap as an ideal cap with a series resistor (the ESR). For this we take the cap to an analyzer and measure its impedance versus frequency. From it you can get the values of the model, the ideal cap and the ESR. The ESR shoud not change with the frequency once you get the model in this way. If you measure the cap with a analyzer point by point at different frequencies you will get different values of C and ESR. This is not the best way to obtain the model. The value of the ESR is at the frequency at which the impedance is minimum. The impedance versus frequency of a cap is as follows: Impedance * * * * * * * * ** --------------> freq Before the minimum, the cap behaves as a capacitor, at the minimum is the ESR and beyond the minimum is an inductor (due to the parasitic series inductance. Usually we neglect this inductance in the model). The cap has to operate below the minimum to behave as a capacitor of course.

[@bbug705076]

Thanks for the reply. Below is the equation I come to after substitution. V_o = (d(s)*((V_in - D*I_L*Z_f) - (D^2*v_o*Z_f/Z_p))*(Z_p/(Z_l+Z_p)) How do you get rid of v_o?

[@MarcosAlonsoElectronics]

Your equation is not correct. After substitution you get: v_o= ( d*(V_i - D*I_L*Z_f) - D^2*v_o*Z_f/Z_p ) *Z_p/(Z_L + Z_p)

[@bbug705076]

@@MarcosAlonsoElectronicsoh yes, I mistakenly added one more left parenthesis to the left of the V_i. Anyways, how do you get rid of v_o in the equation?

[@bbug705076]

And how do you turn the denominator Z_l+Z_p into Z_l + Z_p + D^2 *Z_f

[@Aleksei_MOI]

Where can I download your library for ltspice?

[@MarcosAlonsoElectronics]

The library control.lib and other LTspice components that I use in the videos are available from my website: sites.google.com/view/j-marcos-alonso/home There is a playlist explaining the components of the library. The first video is: kzbin.info/www/bejne/jZi9hH6adqZlbpI Thanks!

[@skyrangerbob]

Very nice explanation. Intuatively I would have guessed the instability, but it was good to see the analysis. Thank you.

[@MarcosAlonsoElectronics]

Thanks!

[@조현엽-d7e]

Can you give a lecture on the closed loop analysis of current mode buck?

[@MarcosAlonsoElectronics]

Thanks for the suggestion. There is a video about current-mode control here: kzbin.info/www/bejne/aJe5mmaCfZiXbZI

[@Aleksei_MOI]

Very interesting, thank you

[@MarcosAlonsoElectronics]

Thanks!