● Learn about the effects of noise on power supplies.
● Gain a greater understanding of how to mitigate noise in power supplies.
● Learn more about the types of noise source mechanisms.
Sound--that virtually ever-present invisible force that moves at 1,125.33 ft/s--can be both a constructive force or a destructive force depending on the form and application. However, waves that propagate in the form of noise tend to be on the destructive side of this equation. With regard to applications such as power supplies, noise affects performance, accuracy, and even consistency.
The Effects of Noise on a Power Supply
In nearly every metric in use today, designers view noise as one of the biggest concerns and detriments to their designs. The adverse effects of noise on electronic circuits, including power supplies, are well documented and thus, worthy of this lofty concern. So, why does noise warrant agonizing over a design? The initial cause for concern is the unpredictability of the noise itself. In general, noise can arise from either an external source or an internal source. Even the origin of the noise can bring about added concerns.
In the case of an external source of noise, it is technically an uncontrollable source. This means the designer will need to be diligent and creative to offset these effects. In the case of an internal source, while it may be easier to control, the results are still the same as an external source. These adverse effects include detrimental impacts on functionality, performance, accuracy, and even consistency. In some cases, it can prevent devices from meeting desired design objectives or even complete failures in anywhere from the prototype stage to the finished product.
The Importance of Mitigating Noise in a Power Supply
Though there is a vast range of sources from which noise can arise, the overall effect of this parameter is adverse. The consensus here is to address the noise issue at the circuit level rather than at the power supply. However, that will only address half of the problem and quite possibly leave out the more critical aspect of the need for noise mitigation. The reason for this assessment is because we build a circuit, and even a system's foundation, upon a power supply's ability to function correctly.
Furthermore, if noise exists on a power supply's output rails then the entire system's ability to function as designed is hindered. In most cases, issues created by power-supply noise are both inconsistent and intermittent. As you might imagine, that can exacerbate the ability to troubleshoot, accurately diagnose, and eliminate the problem.
Another reason why it is critical to address noise at the power supply level is that virtually every device incorporates a power supply. Therefore, regardless of your design, it will most likely utilize a power supply and is, thus, susceptible to noise issues.
EMI Mechanisms in Electronic Design
Noise, or interference, is the bane of electronic functionality, and EMI (electromagnetic interference) is at the top of that list. The following are the primary mechanisms that generate EMI:
Conductive coupling is a direct contact created by a physical-coupling path such as a cable or wire. This can occur on PCBs as well through its traces or metal enclosure.
Radiative coupling is produced through the act of radiating through the air or within a vacuum. Keep in mind that each trace on a PCB is a prospective antenna and, therefore, a possible coupling path.
Capacitive coupling arises when a variable electrical field occurs between two nearby conductors, like, for example, in a capacitor (i.e., two plates with a potential difference divided by allotted space).
Inductive coupling arises when there is a variable magnetic field between two equivalent conductors. The result of this occurrence produces a parasitic induced voltage.
How to Reduce Power Supply Noise
In a simplified view, we define power-supply noise as a combination of undesired periodic ripples and spikes merged with random noise from internal or external sources. Also, the design of the line regulation will control the amount of input-referred ripple. This concept is comparable to the power-supply rejection ratio (PSSR), which is the amount of the input signal a linear regulator allows to pass to the output. Overall, it is not exclusively a function of the control chip but rather the workings of the circuit as a whole.
In general, there are three methods of addressing power-supply noise and mitigating ripple.
We can utilize a filter to remove noise from a power supply similar to how a filter can remove noise from a signal. You can also consider output capacitors as part of the filtration process since they react counter to the output impedance of the power supply circuit. In summary, an increase in the output capacitance results in noise mitigation.
Note: Remember, a capacitor has both an ESR (equivalent series resistance) and an ESL (equivalent series inductance). Using a capacitor with a lower ESR and ESL will result in lower noise. But keep in mind that some power supplies utilize the ESR to furnish the error signal for feedback. Therefore, if you reduce it drastically, i.e., by exchanging electrolytic capacitors with ceramic capacitors, the result is instability in your power supply.
Bypassing the control chips in your power-supply design will facilitate a reduction in noise. Although, circumventing the chips that are fed by the power supply will not result in a reduction of noise at the power supply. However, it will afford a decrease at the power pins of the chips.
Note: If bypassing the chips in your power-supply circuit, utilize the standard guidelines by placing the capacitor near the power pins and use ceramic capacitors. If possible, use surface-mount capacitors since they have low ESL and ESR. Keep in mind that physical size matters, and it, along with value, will determine effectiveness.
Adding a second low-noise regulator to the power-supply output will mitigate noise. This method is less cost-effective, which may be a deterrent to its use. In most cases, the implementation of this method incorporates the use of a low-dropout (LDO) linear regulator. In summary, it can reduce any output ripple by at least an order of magnitude or more. Using an LC or RC filter in conjunction with an LDO can reduce noise even more.
Power supply noise is an undeniable concern of all designers in the field of electronics. Its ability to affect every aspect of design only makes the need to mitigate it all the more vital to the success of the overall design. The unpredictability of external noise sources coupled with internal threats makes reducing noise a paramount objective of every designer or engineer.
Computer power supply.
Reducing noise in your power supplies and having successful circuit designs is reliant on having the right set of PCB design tools. Whether you are utilizing a single-sided board or a multi-layer design, you will need the right set of PCB layout and design software. Allegro PCB Designer and Cadence's full suite of design tools can help you create designs from verified component models and analyze all aspects of functionality. You'll also have access to a set of tools for MCAD design and preparing for manufacturing.
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