LTM2810 About the capacitor between GND1 and GND2

The evaluation board DC2832A states that C5 should be attached between GND.

What is the EMI when there is no capacitor between GND1 and GND2?

Thank you.

Parents
  • +1
    •  Analog Employees 
    on Mar 22, 2021 12:35 PM

    Hi Yito,

    That discrete bridge capacitor between GND1 and GND2 provides a return path for common mode current and helps us attenuate the EMI at low frequencies. It's one of the ways to optimize that board for low RF emissions. Without it, it could exceed the CISPR 22 class B limit. CISPR 22 is a widely used standard for electromagnetic compatibility within Europe. CISPR 22 requires certification over certain frequencies (has specified limits below 1.0 GHz). You can find more about the limits stated by CISPR 22 here CISPR 22: EN 55022 EMC Standard » Electronics Notes (electronics-notes.com)

    Thank you for your interest.

    -Naveen

  • I know CISPR22

    I want to know how much EMI (db) if i do not use bridge capacitor 

  • 0
    •  Analog Employees 
    on Mar 23, 2021 4:20 PM in reply to yito114

    Hi Yito,

    Is removing that capacitor a curiosity-driven experiment or do you need to know what happens to EMI levels for a particular frequency of operation for an application? You can email us directly with details if it’s for an application. Also, do let us know if you’re using DC2832A-A or DC2832A-B.

    -Naveen

  • This device is a candidate for use with an operating voltage of 1500V.
    To support CISPR 22, I think it is necessary for C5 (GND1 and GND2) to reduce EMI.
    Looking at the layout diagram of DC2832A-A, I thought that it might not be possible to secure the spatial distance(creepage) because the distance between GND is short, so I asked a question.

    Is it my belief that this capacitor cannot be added when used at an operating voltage of 1500V?

  • 0
    •  Analog Employees 
    on May 17, 2021 8:34 AM in reply to yito114

    Hi Yito,

    I believe that capacitor is used to implement stitching capacitance. Of course, for that reason, we have to select the capacitor with guaranteed creepage, clearance, and withstand voltage. They usually come in several grades. The Y2 grade is usually the recommended safety capacitor type for a stitching capacitor in a safety rated application. The main point of a stitching capacitor is that it requires the currents to flow to it, which can create asymmetrical image charge paths and added noise etc. We select these safety caps based on the isolation voltage rating they provide too. (Some examples given in table below). These safety caps usually help with the EMI attenuation at frequencies below approximately 400MHz. At higher frequencies, this capacitance is built into the PCB.

    Regards,

    Naveen

  • Please see the layout of the evaluation board DC2832 for the LTM2810.
    I would like to use a Y2 capacitor as an EMI down, but in that case, the space distance between GND will be about 3 mm.

    In order to secure the distance between GND for use at 1500V, I think that this capacitor cannot be mounted.
    Is there a CIPR compatible method with 1500V insulation?

Reply Children
  • 0
    •  Analog Employees 
    on May 17, 2021 3:50 PM in reply to yito114

    Hi Yito,

    You're right. Unfortunately, You cannot use that capacitor to get that Maximum Working Insulation Voltage of 1600 Vdc that the ltm2810 provides.
    The package has a creepage of >16mm to get that reinforced isolation. so if you'd use that capacitor, it's not possible to get that rating as you correctly pointed out. For that Maximum Working Insulation Voltage, you need a creepage of at least 8mm. Reinforced is twice of that. You can also check with Jason Naso, the primary app for this part. He'd probably be able to guide you on EMI levels if you drop that cap or alternatively, give you an EMI-based solution allowing a VIORM of 1600 Vdc etc. I'll have to look into this.


    Regards,
    Naveen