Electrical Fast Transients

Electrical Fast Transients

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EFT • It is possible for the transients to be coupled into a nearby electronics device by inductive coupling but, generally, they enter the product via the cable connections. • On signal ports, the spikes are almost invariably in common mode, i.e. on all wires at the same amplitude with respect to external earth. • On the mains port, they may appear either in common mode or differentially between phases. Common mode coupling includes the protective earth wire.

E Field and M Field Coupling

EFT • Poor filtering or inadequate screen termination on each interface then lets these transients pass into the electronic circuits where they appear as interfering signals at sensitive nodes. • Digital circuits tend to be more susceptible, since each short pulse can appear as a valid digital signal. • Occurring in bursts, there is a higher probability that one or more pulses will coincide with a critical timing edge. • Analogue circuits can also be affected, typically by saturation of sensitive amplifiers.

Electrical Fast Transient Effect • Internal circuit design is bandwidth limited wherever possible, and the PCB layout prevents large interference voltages from appearing within the circuit . • Interfaces must be filtered or screened to a structural low impedance earth so that common mode pulses are prevented from entering the circuit.

Test Standards • • • •

First Standard IEC 801-4 published in 1984. Changed later to 1000-4-4. IEC 61000-4-4 since 1995. Currently working on two new standards (IEC 61967 and IEC 62132) for the characterization of the electromagnetic emission and the immunity of ICs. • IEC 62132 describes several measurement methods. The susceptibility against transient disturbances, such as fast transients (bursts) is currently not included in this standard.

IEC 61000-4-4 Standard • • •

Basic standard for testing fast transient immunity . Accredited labs run world wide. The IEC 61000-4-4 defines immunity requirements and test methods for electrical and electronic equipment to repetitive electrical fast transients such as those originating from switching transients (interruption of inductive loads, relay contact bounce…). • The standard also defines the test voltage waveform, the range of the test levels as well as the test equipment and setup for coupling transients into power supply, control and signal ports of electrical and electronic equipment. • It applies a specified burst waveform via a defined coupling network to the mains connection and via a defined clamp device to any signal connection. Only conducted coupling is used.

EFT Test • A specified burst waveform is applied via a defined coupling network to the mains connection and via a defined clamp device to any signal connection. Only conducted coupling is used. • To standardise the test, the waveshape, number of pulses, their frequency and the burst length and repetition frequency are all specified. • The source impedance of the generator is required to be 50 Ohms and the waveform is calibrated into a 50 Ohms load . Actual load impedance may vary.

EFT TESTS

Test Levels

Pulse Generator

Measurement Setup

Coupling Capacitance = 800 nF Internal Resistance = 10 Ohms Burst Amplitude = 3 – 37 V

Failures

Failures •

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