Inductors are available in many different styles and can come shielded or unshielded. Initially the first power supply prototype included a bobbin core inductor, which is a less inexpensive wire wrapped, or unshielded, inductor. As stated in the LM2576 data sheet, "because the magnetic flux is not completely contained within the core, the bobbin core generates more electromagnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe." [1]
The first power supply prototype tests did experience some minor fluctuations with some of the scope readings that seemed to coincide with the type of EMI interference that a bobbin core inductor can cause. Since a bobbin core inductor was used with the first power supply prototype, the second prototype changed the unshielded inductors with shielded inductors to verify this theory of possible false reading.
Unshielded Inductors: [2]
The inductors used in the first PS prototype, shown below, look as though they have shields but they are just covered with a heat shrink type of material and do not help with reducing EMI.
Power Supply Prototype 1 with Unshielded Inductors :
106mV Peak to Peak Voltage Ripple, a likely cause from using unshielded inductors:
Shielded inductors: [3]
Applying shielded inductors in the second power supply prototype, shown below with clear shields, were an improvement over the unshielded inductors and drastically reduced the EMI, that could be seen from the oscilloscope below, during our tests from the first prototype.
Prototype 2 Power Supply with Shielded Inductors:
The tests conducted from each of the rails, with the shielded inductors, showed less than 20mV peak to peak voltage ripple, compared to the previous 106mVp-p with the unshielded inductors. The shielded inductors proved to reduce the ripple significantly during these tests and also a good lesson learned.
[1] Texas Instruments, “LM2576xx Series Simple Switcher® 3-A Step-Down Voltage Regulator”, LM12576 datasheet, June 1999 [Revised May 2021].
[2] SUMIDA, “RCH-895: Pin Power Inductors ”, RCH-895 datasheet, [Revised Jan 2017].
[3] Murata Power Solutions, “1200RS Series: Through Hole Radial Lead Shielded Inductors ”, 1200RS datasheet, 2021.