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High voltage charge pumps minimise EMI

01 Apr 2016  | Tony Armstrong

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Switching regulators are a hit for many power conversion supplies due to their size, output flexibility and efficiency advantages. Conversion efficacy of these supplies can now attain 98% levels, depending on operating conditions. Nevertheless, despite these advantages, they necessitate compromising on other parameters, one of the most difficult of which is noise.

But what is switching regulator "noise?" To have a better understanding of this term, let's start with the fact that switched mode power supplies create wideband harmonic energy. This undesirable energy manifests itself in two components, radiated and conducted, which are routinely labelled in the industry as "noise." However, this is really a misnomer, since a switching regulator's output "noise" is not noise at all, but coherent high frequency residue directly related to the regulator's switching. The correct term for this phenomenon is electromagnetic radiation, or more commonly, EMI. And yes, it comes in both radiated and conducted forms.

Since quiet, well-regulated power supplies are important for optimum performance in many circuit applications, it is important to be able to mitigate any noise generated as part of this conversion process. An obvious way to attain this is to simply use a linear regulator. However, although they supply quiet power supply rails, their conversion efficiencies are poor at high step-down ratios and this can lead to thermal issues with the design in high output current applications.

Correspondingly, switching regulators usually have higher conversion efficiencies than linear regulators, leading to simpler thermal design when high output currents are required by the end application. It is well understood that component selection and circuit board layout can play a significant role in determining the success or failure of virtually all power supplies. These aspects set their functional EMI and thermal behaviour. For the un-initiated, switching power supply layout may seem like a "black" art, but it is in fact a basic aspect of a design often overlooked in the early stages of the process.

Since functional EMI requirements always have to be met, what is good for functional stability of the power supply is also usually good for its EMI emissions, too. Furthermore, good layout from the beginning does not add any cost to the design and can actually provide cost savings by eliminating the need for EMI filters, mechanical shielding, EMI test time and numerous board revisions.

Moreover, the potential problems of interference due to noise can be exacerbated when multiple DC/DC switchmode regulators are used in a design to generate multiple rails of if they are paralleled for current sharing and higher output power. If all are operating (switching) at a similar frequency, the combined energy generated by multiple regulators in a circuit is then concentrated at one frequency. Presence of this energy can become a concern, especially if the rest of the ICs on the printed circuit boards (PCBs), as well as other system boards are close to each other and susceptible to this radiated energy.

This can be particularly troubling in industrial and automotive systems that are densely populated and are often in close proximity to electric noise generating sources, such as mechanically switched inductive loads, PWM drive power outputs, microprocessor clocks and contact switching. Furthermore, if switching at different frequencies, intermodulation products can alias into sensitive frequency bands.


Switching regulator emissions
In industrial, medical and automotive environments, switching regulators usually replace linear regulators in areas where low heat dissipation and efficiency are valued. Moreover, the switching regulator is typically the first active component on the input power bus line, and therefore has a significant impact on the EMI performance of the entire product design.

Conducted emissions ride on the wires and traces that connect up to a product. Since the noise is localized to a specific terminal or connector in the design, compliance with conducted emission requirements can often be assured relatively early in the development process with a good layout or filter design as already stated.

Radiated emissions are a different matter altogether. Everything on the board that carries current radiates an electromagnetic field. Every trace on the board is an antenna and every copper plane is a resonator. Anything, other than a pure sine wave or DC voltage, generates noise all over the signal spectrum. Even with careful design, a power supply designer never really knows how bad the radiated emissions are going to be until the system is tested. And radiated emissions testing cannot be formally performed until the design is essentially complete.

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