LC FILTER DESIGNER SERIES

I chose the Bourns SRU1028 series inductor. For this part, at 2.4 Amps the inductance drops by another 35%. The extra inductance accounts for the -30% tolerance. In addition to the initial tolerance, the inductor value drops with increasing current. To make the power supply filter inductor about 10 times larger than this, I chose a 10uH +/- 30% inductor. A generous overall length is 500mm, which means that my power distribution inductance is something like 500nH. In this example, I want the board to work on an extender cable that is about 300mm long, and the board is about 100mm X 100mm.

The inductance of power planes is much lower, and for this calculation, the length of power plane paths can be ignored. The not-terribly-accurate approximation for calculating this inductance is to take the maximum length for the power to travel, and multiply by 1nH per millimeter.

If there are no other inductors or ferrite beads in the supply, this inductance is due to cables and printed circuit board traces. It should be about ten times larger than all the other inductances in series with the power supply. The inductance value needed for the filter is not too hard to calculate. The extra 20% accounts for the increase in the inductor’s copper wire resistance at higher temperatures. The voltage drop is about 20% more than this resistance times the current. The inductor datasheet has a value for the DC resistance. The power supply filter comes after the regulator, so it needs to have a low DC voltage drop. When there is a local regulator, the design is simpler and sometimes the power supply filter can be reduced. In the example design, the power supply regulator is assumed to be off-board, and a regulated voltage comes in through a connector. If an acceptable filter can’t be designed, figure out what was wrong with the inductor, choose a better inductor, and try again. The design process is to first generate the requirements for the inductor L B, choose a candidate for the inductor, and then design the filter around it. Other designs are possible, with more or fewer components. Careful design with high-performance components can extend these frequency ranges, and not all designs have the same requirements for ripple rejection.Ī good power supply filter can be built from a single inductor and damped capacitor. The chart above shows typical power supply filtering frequency ranges. Correct design of the decoupling filter ensures that it won’t cause more problems than it solves. A simple power supply decoupling filter made from an inductor and capacitor covers the gap between about 10kHz and 1MHz. The small bypass capacitors distributed among ICs become effective at about 1MHz. At frequencies above about 10kHz, most linear regulators begin to lose effectiveness.

The ripple comes from switching power supplies, digital circuits, and radio interference. High frequency ripple passes right through a linear regulator. Iterate the design using a circuit simulator.Use simple rules of thumb to find the component values.Understand the power supply filter requirements.To create a better design, follow these steps: Correct design of the power supply filter helps eliminate a whole class of mysterious circuit problems and improves power supply bypassing. Incorrect power supply filter design leads to unreliable hardware.

Lc filter designer