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In my schematic, I have a DC-DC buck converter that takes in 5V and outputs 3.3V.

a. When I connect the 5V the DC-DC converter outputs 0.7V

b. When I remove the FL1 and replace it with a diode, the DC-DC outputs 3.3V

c. When the 5V source is removed, the input to DC-DC reads 0.7V

I am guessing there is a short circuit between the 3.3V and 5V ground plane. How do I narrow down the short circuit?

enter image description here

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  • \$\begingroup\$ Are you sure the diode is in the right direction? A short on 3.3V to GND wouldn't read 0.7V. It would be a hard 0. To me it sounds more like a mistake in a circuit. Check the diode orientation, check the DCDC symbol. If you see nothing, use divide and conquer methodology to find what's wrong. \$\endgroup\$
    – TQQQ
    Dec 13, 2021 at 7:46
  • \$\begingroup\$ Measure the resistance of the FB with a meter. \$\endgroup\$
    – Andy aka
    Dec 13, 2021 at 9:07
  • \$\begingroup\$ @Andyaka It measures 0.7ohm \$\endgroup\$
    – Adnan
    Dec 13, 2021 at 12:19

1 Answer 1

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Ferrite beads are not always good when placed on power rails.

The one you selected has an impedance of 1.5 kOhms at 100 MHz. This indicates that the inductance might be a bit higher and thus cause some problems such as ringing: Since the input is applied to the inductor L1 as a pulse train, the bead may ring during this switching.

If you have to use a ferrite bead before applying the switcher IC then use one with lower impedance and along with a suitable capacitor so that they form a low-pass filter.

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  • \$\begingroup\$ Thank you very much for the insight. I replaced the Ferrite Bead with a 0ohm resistor and it's working now. I have seen Ferrite at the power input for filter purposes. Could you please tell me when are the cases when Ferrites cause issues such as this one? Do I need to choose one with low impedance? \$\endgroup\$
    – Adnan
    Dec 14, 2021 at 2:28
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    \$\begingroup\$ @Adnan A ferrite bead is a "special form" of an inductor. So they may make trouble if used in applications where switching currents involved such as switching regulators or hi-speed digital transceivers (ethernet, USB etc). Think about the inductor in a boost converter, for example. If you are to use a bead in an application, always follow the manufacturer's guidelines and recommendations. For you application, it appears that you should use one with a lower impedance. Or, you can use an ordinary inductor along with capacitors to form a PI filter. But this depends on the EMC requirements. \$\endgroup\$ Dec 14, 2021 at 6:53

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