Incorrect power supply filter design leads to unreliable hardware. This is distressingly common. Correct design of the power supply filter helps eliminate a whole class of mysterious circuit problems and improves power supply bypassing. To create a better design, follow these steps:
- Understand the power supply filter requirements.
- Use simple rules of thumb to find the component values.
- Iterate the design using a circuit simulator.
High frequency ripple passes right through a linear regulator. The ripple comes from switching power supplies, digital circuits, and radio interference. At frequencies above about 10kHz, most linear regulators begin to lose effectiveness. The small bypass capacitors distributed among ICs become effective at about 1MHz. A simple power supply decoupling filter made from an inductor and capacitor covers the gap between about 10kHz and 1MHz. Correct design of the decoupling filter ensures that it won’t cause more problems than it solves.
A good power supply filter can be built from a single inductor and damped capacitor. This is called an LC filter. Other designs are possible, with more or fewer components. The design process is to first generate the requirements for the inductor LB, choose a candidate for the inductor, and then design the filter around it. If an acceptable filter can’t be designed, figure out what was wrong with the inductor, choose a better inductor, and try again.
In the example design, the power supply regulator is assumed to be off-board, and a regulated voltage comes in through a connector. When there is a local regulator, the design is simpler and sometimes the power supply filter can be reduced.
The power supply filter comes after the regulator, so it needs to have a low DC voltage drop. The inductor datasheet has a value for the DC resistance. The voltage drop is about 20% more than this resistance times the current. The extra 20% accounts for the increase in the inductor’s copper wire resistance at higher temperatures.
The inductance value needed for the filter is not too hard to calculate. It should be about ten times larger than all the other inductances in series with the power supply. If there are no other inductors or ferrite beads in the supply, this inductance is due to cables and printed circuit board traces. 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. The inductance of power planes is much lower, and for this calculation, the length of power plane paths can be ignored.
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. A generous overall length is 500mm, which means that my power distribution inductance is something like 500nH. To make the power supply filter inductor about 10 times larger than this, I chose a 10uH +/- 30% inductor. The extra inductance accounts for the -30% tolerance. In addition to the initial tolerance, the inductor value drops with increasing current. For this part, at 2.4 Amps the inductance drops by another 35%.
I chose the Bourns SRU1028 series inductor. It has low height, is self-shielding, and readily available. I found it by searching Digi-Key for a low-cost 10uH inductor with a current rating of at least 2 Amps. I also like the Bourns datasheet because it has the specifications needed to make a good simulation model of the inductor.
These extra components cause the inductor to have the behavior in the impedance plot above. The solid curve is the dB magnitude of the impedance and the dashed curve is the phase angle of the impedance. Below about 1 kHz, the inductor acts like a small resistor RDC. Above 1 kHz it acts like an inductor, up to near the Self Resonant Frequency (SRF). For a narrow range of frequencies near the SRF, the inductor acts as a large value resistor with the value RQ. Above the SRF, the inductor acts as a capacitor CSRF.
The voltage source V1 is a 1 Volt AC source. The impedance can be graphed with the expression -1/(i(V1)). To learn LTspice, see my Simulation Series tutorials on YouTube. LTspice AC analysis is in part one and part two, and transient analysis is in part three. The total video time is about 12 minutes.
It is easy to transform the inductor model schematic into a low-pass filter by adding a capacitor to the schematic. I chose the Kemet capacitor T491A106010A, which is a 10uF polarized tantalum capacitor with a maximum ESR of 3.8 Ω and a voltage rating of 10 V.
Model the Load Network
- An inductor value that is much larger than the stray inductances
- A capacitor value much larger than the sum of the bypass capacitors
- Damping resistance to eliminate high-Q resonances
- Bypass capacitors sufficient to supply pulsed loads