I have a question about pulse transformer ,regarding to isolation, EMI reduction and heating.

  • My PCB has 6 layers (4 signal and 2 power planes) and I am using pulse transformer to isolate and convert 3.3 from/to 5V. Maximal output current is 200mA.
  • Switching frequency for this transformer is 600kHz.
  • Is it better to use ceramic or tantalum or electrolytic capacitor at output?

I am wondering, should I keep area below transformer without copper ( all layers and planes) or should I place GND or isolated GND polygons, maybe on top layer?

My first thought was to remove copper on every layer and plane, below, but then make some fills (outward) around isolated GND pin and connect it with couple of vias to internal plane.

For capacitors I placed ceramic at input because of lower ESR and tantalum at output because better voltage stability.

What are your suggestions?

  • \$\begingroup\$ If your 3.3V and 5V systems are electrically isolated, then it is best to have an isolation region with no copper, and your isolation devices (optos, transformers etc) being the only things bridging the isolation region. \$\endgroup\$ Feb 12, 2016 at 12:17
  • \$\begingroup\$ What does the pulse transformer data sheet say or recommend? \$\endgroup\$
    – Andy aka
    Feb 12, 2016 at 12:18
  • \$\begingroup\$ Nothing about that in datasheet. \$\endgroup\$
    – Haris778
    Feb 12, 2016 at 13:05

1 Answer 1


Regarding heat: I would consider 200 mA at 5V as low power. You probably don't need to worry about substantial heat through the coil, or need a ground plane underneath it. The only thing that might get hot would be the switching element (IC or output FET).

Regarding EMI: You're circuit probably has a large input cap (33 uF) and a large output cap (10uF) after the coil to stabilize the rails. If you are concerned with switching noise, it would most likely come from ringing. Use a few filter components after the output cap, but as close as possible to the coil. I generally use a 1 uF cap, 0.1 uF cap, and a ferrite bead matched to 100x impedance ratio of my switcher's frequency vs, the DC resistance, below is a link to one. If you are concerned about radiated signals from your coil, consider using a shielded inductor. For any signal traces crossing pathes with your power signals, try to make them cross at perpendicular angles, instead of running parallel, the minimize chances of inductance.

Ferrite Bead: http://www.digikey.com/product-detail/en/742792114/732-1621-1-ND/1639565


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