Coupling-Decoupling Networks (CDNs) are the unglamorous workhorses that get “hitched up” to an impulse generator in order to perform impulse testing on powered equipment. As such, performance and calibration requirements for CDNs have not been specified to the same level of detail as the impulse generator. This is no longer the case with the Third Edition of IEC 61000-4-5, the international standard for surge immunity testing.

What's New (and What's Not)

Figure 1: Differential-mode coupling on power lines using 18 uF capacitor (decoupling circuit also shown).
The Third Edition specifies new calibration requirements for the CDN: section 6.4 for CDNs used with 1.2×50/8×20 uS impulse generators, and section A3 in Annex A for CDNs used with 10×700/5×320 uS generators. The first type of generator produces an impulse with a nominal 1.2 uS rise time, and a 50 uS duration into an open circuit (high impedance) load. This same generator produces an 8 uS rise, 20 uS duration current waveform into a short circuit. This generator is used for all impulse tests in IEC 61000-4-5 except for tests on outdoor symmetrical communication lines.

The second type of generator (for outdoor symmetrical communication lines) produces an impulse that has a 10 uS rise time and 700 uS duration into an open circuit, and a 5 uS rise, 320 uS duration short-circuit current. This longer-wavelength impulse is considered more representative of the type of waveform that would be capable of traveling long distances. For the CDN, the components used, and the method of injecting the impulse onto the signal or power lines, differ for the two types of generators.

Figure 2: Common-mode coupling on power lines using 9 uF + 10 Ohms (decoupling circuit also shown).
The basic components for power-line coupling and decoupling of the 1.2×50/8×20 waveform are unchanged between the Second and Third Edition of IEC 61000-4-5. For differential-mode testing (from Line to Line, or Line to Neutral), an 18 uF capacitor is used as the coupling element (Figure 1). For common-mode testing (Line to Ground) a 9 uF capacitor in series with a 10-Ohm resistor is used (Figure 2). The impact of the Third Edition changes is felt primarily in the decoupling elements of the CDN, which are also shown in Figures 1 and 2.

The goal of the decoupling network is twofold. First, allow AC or DC power to be delivered from the AE (Auxiliary Equipment) port to the EUT (Equipment Under Test) port, so the decoupling network must be a low impedance to DC and low-frequency AC. The second goal is for the AE port (as seen through the decoupling components) to appear as a high impedance to the high-frequency impulse. This can be expressed in simple mathematical terms, which generally apply for all decoupling circuits:

For power and signals: (decoupling circuit impedance) << (EUT impedance)

For the impulse waveform: (decoupling circuit impedance) >> (EUT impedance)

CDN Calibration

The Third Edition adds new constraints to the decoupling circuit in sections 6.4.2 (power lines) and 6.4.3 (interconnection lines, also known as data or signal lines). Waveform characteristics are specified for the EUT port and must be met with the CDN connected to the impulse generator. For AC/DC power ports, there are additional requirements for the residual impulse that are presented at the AE port (residual impulse on the AE port must be relatively small). These two requirements are simply a quantitative statement (with limits specified) of what was previously stated: To the impulse waveform, the decoupling circuit impedance must be much greater than the EUT impedance. If this is the case, then most of the impulse voltage and energy will be applied to the EUT, and very little impulse voltage/energy will travel back to the AE port.

Table A.3 from IEC 61000-4-5 Third Edition
A limit is placed on the size of the inductors used in the decoupling network: 1.5 mH for power lines (the inductors shown in Figures 1 and 2). On the other hand, the capacitors in the decoupling circuit may seem relatively benign, as there is no specification in the standard for these components. However, there are conflicting constraints on these capacitors that are imposed by the requirements in the standard: prevent excessive voltage from appearing at the AE port (capacitors must be LARGE), and at the same time, the voltage and energy must be delivered to the EUT port and not absorbed by the decoupling network (capacitors must be SMALL). The residual voltage at the AE port during an impulse test (with nothing connected to the AE port) is generally limited to 15% of the overall surge voltage. An excerpt from section 6.4.2 follows:

“The residual surge voltage measured between surged lines and ground on the a.c./d.c. power port of the decoupling network with EUT and mains supply not connected shall not exceed 15% of the maximum applied test voltage or twice the rated peak voltage of the CDN, whichever is higher... All performance characteristics... shall be met at the output of the CDN with the a.c./d.c. power port open-circuit.”