Question

1.Briefly describe the operation of the common mode LC filter of a power supply.

2. Describe briefly with the aid of a diagram, the thermal resistances involved in heat dissipation by a heat sink.

3. Describe briefly the EMI emission and susceptibility issues with Power supplies of medical electronic equipment.

Answer 1

1.

An LC filter combines inductors (L) and capacitors (C) to form low-pass, high-pass, multiplexer, band-pass, or band-reject filtering in radio frequency (RF) and many other applications. Passive electronic LC filters block, or reduce, noise (EMI) from circuits and systems, and separate, or condition, desired signals.

While ideal filters would pass desired signal frequencies with no insertion loss or distortion, and completely block all signals in the stop-band, real filters have DC and AC resistances that contribute to insertion loss, requiring careful component selection. Selecting the exact values of the parts for a particular application requires high quality components as well as complete specifications and performance models. The simplest to design and implement are the low-pass and high-pass types.

Coilcraft high-Q, tight-tolerance, surface-mount RF chip inductors and air-core inductors help you achieve top performance in all of these LC filter categories.

How do you design LC filters?

The alignment (type) of the filter determines the flatness of frequency behavior and the sharpness of the cut-off. There are many types of alignments, including those with the most commonly desired characteristics such as Butterworth, Bessel, Chebyshev, and elliptic.

The simplest LC filter consists of one inductor and one capacitor. Higher-order filter alignments use more components to give a sharper, more defined roll-off in attenuation of unwanted noise. For example, Elliptic (Cauer) filters give the sharpest roll-off and are the least sensitive to component variation. As a trade-off, there is more pass-band ripple and stop-band ripple in Elliptic LC filters.

2.

Electrical or electronic components housed in sealed enclosures are especially susceptible to over heating due to the formation of hot spots. Heat sinks may be used on components to provide localized cooling to mitigate the effect of hot spots and/or to provide extra cooling for critical components.

The performance of a heat sink in a sealed enclosure is influenced by the size of the enclosure, proximity of other heat generating components, the ability of air to flow freely through the heat sink and the internal/external surface emissivity of the enclosure. In order to be able to estimate heat sink performance let’s make some assumptions regarding the enclosure layout and heat sink.

1. The structures in the enclosure do not significantly obstruct the movement of air flow throughout the enclosure
2. Air flow through the heat sink is not restricted due to the lack of ventilation in the enclosure
3. Radiation exchange between other heat generating components and the heat sink is negligible.

If you strongly believe that these assumptions are not valid for your situation then a computational fluid dynamic/finite element  analysis of your design will be your most accurate alternative analysis method.

Figure 1. Heat sink housed in a sealed enclosure

Figure 1 shows a heat sink in a sealed enclosure with the heat transfer modes identified. The heat sink is oriented vertically in the enclosure as shown in figure 1. The heat generating components are assumed to be dispersed evenly through out the enclosure so that there are no areas where the air temperature varies widely from the average.

3.

Electromagnetic Interference

Electromagnetic interference (EMI) is a phenomenon that may occur when an electronic device is exposed to an electromagnetic (EM) field. Any device that has electronic circuitry can be susceptible to EMI. With the ever-increasing use of the electromagnetic spectrum and the more complex and sophisticated electronic devices, issues of EMI are attracting attention. When addressing EMI issues, consider a source, a path, and a receptor. The electromagnetic energy from the source propagates through the path and interferes with the operation of the receptor. All three must exist to have an EMI problem. The path can be conducted, radiated, inductive, or coupled with a capacitor or with electrostatic discharges, or a combination of any of the above. Therefore, to understand the effects of EMI, consider two factors: Emissions and immunity (also known as susceptibility). Emissions are a measure of electromagnetic energy from a radiofrequency source. Immunity concerns the degree of interference from an external electromagnetic energy source on the operation of the electronic device. The device will be immune below a certain level of EMI and become susceptible above that level. The three most common EMI problems are radio frequency interference, electrostatic discharge, and power disturbances. This chapter will focus on radiated interference from various radiofrequency sources.