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Link to the application note that I'm referring to below.

I was going through an application note by On Semi, System Level Surge Suppression Solutions for the CAN Bus -AND8253/D, where it says that the TVS diode's resonant frequency can be a design issue for high frequency EMI tests like BCI and that adding 5pF capacitors in parallel with the TVS diodes is the solution. I'm not sure I understand this very well, could someone please elaborate. My general doubt is that the resonant frequency of ESD diodes is usually not specified in the datasheets and also the impedance versus frequency curve is not available, so how do I determine the resonant frequency. ALso how is the resonant frequency a design issue for high frequency BCI , is it because at the resonant frequency the impedance is minimum ?

One other doubt is that in the screenshot that I have attached below, it says that the total capacitance of choke filter and TVS devices has to be less than or equal to 30pF for 1Mhz signal, but in the next paragraph it says that adding 5pF capacitors help in the design issue caused by resonant frequency of TVS diode for BCI, but wouldn't this 5pF add up to the existing capacitance of choke filter and capacitance of TVS diode and further increase the capacitance and maybe go above 30pF ?

enter image description here

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  • \$\begingroup\$ Are you aware of the resonant spiking issue associated with using parallel decoupling caps of different sizes? Same thing. Each component has parasitics that form LC networks. When multiple LC networks resonate together, there are resonant peaks and valleys. Valleys are good because low impedance shorts noise frequencies. Peaks are bad because they are high impedance and amplify noise frequencies worse than if the decoupling caps weren't there at all. As a result, I question the recommendation of blindly adding even more LC networks (capacitors) since that's like trying to dig out of a hole. \$\endgroup\$
    – DKNguyen
    Commented Apr 8, 2019 at 18:12
  • \$\begingroup\$ BTW, the same problem occurs when you add chokes as well since those are also LC networks. So they can make problems worse if the prominent noise frequencies in your circuit fall onto those resonant peaks. There is a limit to how deep the desirable resonant valleys can get but there is no limit on how high the undesirable resonant peaks are can be. Look Up Henry Ott's website for more info. I just got his book too and it goes into way more depths about why things are the way they are. \$\endgroup\$
    – DKNguyen
    Commented Apr 8, 2019 at 18:18
  • \$\begingroup\$ So what you are essentially saying is that you don't agree with the On Semiconductor application note, right ? I have also attached the application note that I'm referring to above. \$\endgroup\$
    – NIDHI
    Commented Apr 9, 2019 at 18:16
  • \$\begingroup\$ Yes. I think it's a mistake to add them in blindly (leaving room for them on your PCB is fine though in case you need them), but it has the potential to make something that was not a problem become a problem if you add them when you don't need them. Of course, if you have the means you can test it with and without to see if there is any improvement. \$\endgroup\$
    – DKNguyen
    Commented Apr 9, 2019 at 18:36

1 Answer 1

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Yes, you can add more capacitance at the expense of phase shifting data jitter (ISI) 1 to 2 decades down from the breakpoint of line impedance. But there are also subtle anti-resonant characteristics, beyond the scope here.

Adding caps to power dioes is also done in other applications.

  • It is common to see good Japanese Audio Amp designs with bigger caps (e.g. ~ 1 nf) across each power diode in a bridge at line f to suppression RF resonance, when the diodes turn off after very high dI/dt pulse currents. This is due to parasitic inductance and diode capacitance and high Rs of the diode as it turns off towards 0 bias.

    • This results in an RF resonance often in the AM band that can generation 0.6V of RF noise across the diode loop area and generate radiated EMI. Here reducing rise time with dV/dt=Ic/C does not affect the power diodes from switching current to the bulk caps.

I cannot verify the performance of adding 5pF to this TVS diode. Layout will affect their results and outght to look like this. enter image description here

This is how I graphically compute Q of parallel and series resonant parts.

enter image description here

TVS diodes are good up to 10Mb/s and maybe higher for this part perhaps 100Mb/s BUT NO HIGHER.

For ESD transient protection on VHF,UHF data rate , it is preferred to use Voltage Variable Material (VVM) ESD suppressors have unique properties of 0.1pF and low ESR.

https://www.littelfuse.com/products/polymer-esd.../xtremeguard-esd-suppressors.aspx

www.sunlordinc.com/UploadFiles/2011/12/01100729ED707227.pdf

enter image description here https://www.onsemi.com/pub/Collateral/TND412-D.PDF

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  • \$\begingroup\$ lol. The god of EMC. \$\endgroup\$
    – DKNguyen
    Commented Apr 8, 2019 at 18:57
  • \$\begingroup\$ @Toor yah I read his 1st edition book in late 70's? early 80's to train me as an EMC Engineer part of my career as RF Eng , Test Eng and ISDN Network Debug Eng. etc \$\endgroup\$
    – D.A.S.
    Commented Apr 8, 2019 at 19:01
  • \$\begingroup\$ Thank you for the answer, but could you please explain why and how the resonant frequency of the diode is a design issue for high frequency EMI. \$\endgroup\$
    – NIDHI
    Commented Apr 9, 2019 at 18:15
  • \$\begingroup\$ Cables are most often the biggest problems in a product EMC certification. Twisted pairs or ribbon cable with alternate grounds or even coax are never perfectly balanced so unintended radiation must be tested. If there is a signal on the data line that is clamped by the diodes to each rail then the cable "might" radiate that signal like an antenna. Suppressing the EMI internally is often done with ferrite beads , cap feedthru connectors or in this case the OEM suggests a small RF cap. If one has a sensitive Rx near this band this may be relevant. But it not for every design. So it depends. \$\endgroup\$
    – D.A.S.
    Commented Apr 9, 2019 at 18:25
  • \$\begingroup\$ So it depends. on the level of stray AC or SMPS noise that may cause the diodes to conduct and radiate back UHF noise. \$\endgroup\$
    – D.A.S.
    Commented Apr 9, 2019 at 18:29

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