Power Supply and Emissions

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Questions and Answers

What is a common issue with mains rectifiers?

They are too inefficient

They are too large

They emit switching noise at harmonics of the fundamental frequency (correct)

They are too expensive

Up to what frequency can a 5-kVA power supply fail conducted emissions limits?

Several gigahertz

Several terahertz

Several megahertz (correct)

Several hundred kilohertz

What type of devices are known to have similar emissions to mains rectifiers?

Radar systems

Linear power supplies

Televisions

Thyristor-based DC motor drives and phase-angle AC power control (correct)

What is a common frequency range for switch-mode power converters?

2-500 kHz Signup and view all the answers

How many times the switching frequency can a switch-mode convertor have significant levels of emissions?

What is the primary function of conductors in terms of electromagnetic fields?

To convert conducted electricity into electromagnetic fields Signup and view all the answers

What can be interfered with by emissions from a 70-kHz switching power supply of a personal computer?

Long- and medium-wave broadcasting and part of the short-wave band Signup and view all the answers

At what distance does the transition to full electromagnetic fields occur?

One-sixth of the wavelength Signup and view all the answers

What is the frequency of clocks used in some personal computers?

1 GHz Signup and view all the answers

What happens when the wavelength is comparable with conductor length?

Resonances occur Signup and view all the answers

Why are all conductors considered antennas?

Because they can interfere with and be interfered with by digital technology Signup and view all the answers

What is the effect of increasing frequencies on treating conductors as electric or magnetic field emitters and receivers?

It becomes less adequate Signup and view all the answers

What is the purpose of providing a means to safely transfer lightning currents to the airframe in antenna mounting?

To prevent hazardous surges from being conducted into the airframe via antenna cables or wire harnesses Signup and view all the answers

What happens when the length of a whip antenna equals one fourth of its wavelength?

It becomes a perfect converter of signals to fields Signup and view all the answers

What type of devices are banned outright on commercial airline flights to protect avionics systems from interference?

Intentional radio frequency (RF) emitters like CB radios and walkie-talkies Signup and view all the answers

What is the result of conductors behaving as resonant antennas?

Signals are converted to fields with nearly 100% efficiency Signup and view all the answers

Why are conductors important in terms of signal contamination?

They are the principal means by which external fields contaminate signals Signup and view all the answers

What is the typical frequency range of signals produced by Personal Electronic Devices (PED)?

1-MHz range Signup and view all the answers

What is the purpose of bonding between airframe and flight control surfaces?

To provide a means to safely transfer lightning currents Signup and view all the answers

What is the relationship between frequency and wavelength in terms of the transition to full electromagnetic fields?

As frequency increases, wavelength decreases Signup and view all the answers

Which of the following is NOT a source of Electromagnetic Interference (EMI)?

Airframe riveted or bolted joints Signup and view all the answers

Why are most airlines extending the ban on portable radios and television receivers?

To prevent Electromagnetic Interference (EMI) with avionics systems Signup and view all the answers

What is the primary means of achieving bonding between airframe and flight control surfaces?

Riveting Signup and view all the answers

What type of devices are considered intentional emitters of Electromagnetic Interference (EMI)?

CB radios Signup and view all the answers

What is a major concern for passengers sitting near critical wiring inside the aircraft?

Indiscriminate use of electronic devices Signup and view all the answers

What is the typical material used for the inside of the passenger compartment?

Fibreglass Signup and view all the answers

How many miles of electrical wiring can be found in a commercial passenger jet?

150 mi Signup and view all the answers

What makes personal electronic devices a source of EMI?

Their proximity to aircraft systems Signup and view all the answers

What is a characteristic of external RF sources?

High power and frequency Signup and view all the answers

How can the airframe affect EMI?

By concentrating transient signals and broadcasting interference Signup and view all the answers

What is a similarity between the airframe and a satellite dish?

Both can concentrate and broadcast electromagnetic interference Signup and view all the answers

Why is the airframe a significant contributor to EMI?

Due to its ability to concentrate and broadcast transient signals Signup and view all the answers

What is the purpose of shielding in EMI management?

To eliminate line-of-sight entry points Signup and view all the answers

What is the effective frequency range for absorbing low-frequency magnetic waves?

1 kHz to 30 kHz Signup and view all the answers

How does a shield act on EMI?

Both b and c Signup and view all the answers

What happens to the EMI after passing through a shield?

It is highly attenuated Signup and view all the answers

Why is effective termination of shields to the connector backshell important?

To prevent radiation entering the system Signup and view all the answers

What is the purpose of building up the density of shield material?

To increase the effectiveness of shielding Signup and view all the answers

What is the effective way to shield high-frequency signals?

By reducing entry windows and ensuring adequate surface conductivity to ground Signup and view all the answers

What is the primary purpose of plating the skins of cases and cable shields?

To manage electromagnetic interference Signup and view all the answers

Study Notes

Mains Rectifiers and Switching Noise

Mains rectifiers emit switching noise at harmonics of the fundamental frequency, generating considerable frequencies depending on their power.
A 5-kVA power supply can fail conducted emissions limits up to several megahertz due to the switching noise of its 50- or 60-Hz bridge rectifier.
Thyristor-based DC motor drives and phase-angle AC power control have similar emissions, which can interfere with long- and medium-wave broadcasting and part of the short-wave band.

Switch-Mode Power Converters

Switch-mode power converters operate at fundamental frequencies between 2 and 500 kHz.
They can have significant emissions at 1000 times their switching frequency, interfering with radio communications up to and including the FM broadcast band.

Conductors and Electromagnetic Fields

All conductors are antennas, converting conducted electricity into electromagnetic fields, which can then leak out into the wider environment.
Conductors can also convert electromagnetic fields in their locality to conducted electrical signals, with no exceptions to this rule in our universe.
At distances greater than one-sixth of the wavelength of the frequencies of concern, E and M fields develop into full electromagnetic (EM) fields with both electric and magnetic components.

Resonance and Bonding

When wavelength is comparable with conductor length, resonances occur, allowing signals to be converted to fields (and vice versa) at almost 100%.
Bonding is crucial to prevent hazardous surges from being conducted into the airframe via antenna cables or wire harnesses, especially for externally mounted parts such as control surfaces, engine nacelles, and antennas.

Man-Made Sources of EMI

Intentional radio frequency (RF) emitters like CB radios, remote-controlled toys, and walkie-talkies are banned on commercial airline flights to protect avionics systems from interference.
Personal Electronic Devices (PED) such as laptop computers, handheld scanners, and game players can produce signals in the 1-MHz range, affecting avionics equipment performance.
External sources like radio and radar transmitters on the ground or radar from a passing military plane can be even more disruptive due to their high power and frequency.

Effect of Airframe and Shielding

The aluminium airframe can act as a resonant cavity, compounding the effects of both internal and external EMI by concentrating transient signals and broadcasting interference into nearby equipment.
Shielding is critical to eliminate EMI, and the frequency of the interfering signal is a crucial concern when designing an effective shielding solution.
Low-frequency magnetic waves are most effectively shielded by absorbing signals in highly permeable material, while high-frequency signals are most effectively shielded by reducing entry windows and ensuring adequate surface conductivity to ground.

Cable Shielding

Cable shielding involves placing a conductive material between the cable conductor and its outer jacket, or covering individual conductors within a cable with shielding material.
The shield can reflect energy or pick up noise and conduct it to ground, preventing EMI from reaching the conductors.
Shields must be effectively terminated to the connector backshell to prevent radiation entering the system at the backshell/connector/shield interface.

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Description

This quiz covers the emissions of switching noise in power supplies, including linear and switch-mode types, and their impact on conducted emissions limits.