EMC FLEX BLOG A site dedicated to Automotive EMC Testing for Electronic Modules

 The EV does not mean the end of 12V automotive battery. For various safety reasons, complex modules are powered using two 13.5V / 200A batteries such that the Backup battery comes into play the moment the Main battery's voltage is outside operating voltage range.

The Ground Offset Test involves a voltage variation of +/- 1V on Supply Return line that may affect DUT circuitry referenced to an absolute 0V via remote ground. The diagram below shows how to use a combination of three power supplies to simulate the +/- 1V Ground Offset condition.

2022-06-29

Christian Rosu

Differential-mode RF emissions in a CISPR 25 component level configuration occur due to the flow of current (IDM) via signal paths in which the forward and return conductors are not routed together, thereby forming a conductor loop. The resulting magnetic field from the conductor loop is proportional to the current IDM, the area of the loop and the square of the frequency of the RFI current.

Common-mode RF emissions occur due to undesired parasitic effects, e.g. due to inductances in the current return path or unsymmetries during signal transmission. If we connect a cable to a DUT of it may function like an antenna allowing a common-mode current ICM to flow. Both signal and power supply lines can function as efficient antennas. Here, our rule of thumb is that line lengths that do not exceed λ/10 are uncritical, whereas longer lines (e.g. λ/6) must be treated as potential sources of RF emissions.

The magnitude of the voltage drop on the ground plane and thus the magnitude of the common-mode current coupled into the connected line are determined by the parasitic inductance and the slope steepness of the signal.

We cannot assume that differential mode radiated emissions are not dominant nor an infinite ground plane. A ground plane with finite width has inductance.

Common-mode RF emissions can also occur due to differential mode signal transmission.
If the parasitic terminating impedances of a differential mode transmission path differ substantially, in addition to the desired differential-mode current IDM a common-mode current ICM will also flow via the ground plane that connects the transmitter and receiver modules. This unwanted ground current ICM can then also be coupled into lines connected to DUT and cause emissions in the far field.

The strength of the common mode current and the level of radiated emissions depend on the inductance of the ground plane. The value of this inductance depends on the structure of the transmission line.

The ground plane inductance in a symmetric structure is:
L21 = (µ0/2π) * ln((tπ/W)+1)
Where:
W is the width of the ground plane
t is the height of the harness

The ratio of the height of the harness and the width of the ground plane determines the GP inductance.

As the harness is closer to the edge of the ground plane, the measurement tolerances are higher since the ground plane inductance increases. The tolerances in RE measurments are acceptable when the distance of the harness to the ground plane edge is 10 cm.
Since common mode radiated emissions occur through the ground plane (or the whole setup), the length of the ground plane can impact the tolerances in RE measurments. Longer the ground plane, higher the radiated emissions level.

Christian Rosu, 2022-03-07

RF Boundary is the element of an EMC test setup that determines what part of the harness and/or peripherals is included in the RF environment and what is excluded. It may consist of, for example, ANs, BANs, filter feed-through pins, RF absorber coated wire and/or RF shielding.

RF Boundary is also an RF-test-system implementation within which circulating RF currents are confined

• to the intended path between the DUT port(s) under test and the RF-generator output port, in the case of immunity measurements (ISO 11452-2, ISO 11452-4, ISO 1145-9), and

• to the intended path between the DUT port(s) under test and the measuring apparatus input port, in the case of emissions measurement (CISPR 25),

and outside of which stray RF fields are minimized.

The boundary is maintained by insertion of BANs, shielded enclosures, and/or decoupling or filter circuits. The ideal RF boundary replicates the circuitry of the device connected to DUT in vehicle.

The standard test harness lenght for automotive EMC electronic components is (1700mm -0mm / +300mm). This 1.7m test harness runs between the DUT and the Load Simulator (Shielded Enclosure) that plays the role of RF Boundary.

If the Load Simulator enclosure does not include all DUT loads and activation/monitoring support equipment, additional support devices may be placed directly on the ground plane. The connection of additional devices to LS enclosure must be done via short wiring running on the ground plane.

Testing at subsystem level is preferable to any simulation. Whenever possible, use production intent representative loads.

Running long coax cables directly from DUT outside the chamber via SMA bulk filter panel would violate the 1.7m test harness length rule invalidating the test result. Ideally is to use Fiber Optic to exchange data with devices placed outside the test chamber.

Running long coax cables between Load Simulator and a support device placed outside the chamber is acceptable as long as the I/O line in question is not just an extension from DUT without proper RF boundary at the end of maximum 2-meter length of standard test harness.

It is critical to use the test harness length as defined by CISPR-25, ISO 11452-2, ISO 11452-4, and ISO 11452-9 to achieve valid compliance for your product. The length of the test harness as well as the grounding method (remote vs local) can result in different RF emissions level. Longer the test harness, higher RF emissions above 100 MHz due to its resonance pattern. The local grounding would show less magnitude variation across resonance peaks above 100MHz.

Christian Rosu

2022-02-20

Pulse “1b” is defined differently by various international standards and/or OEM EMC specs.

Daimler and Chrysler have quite a similar definition for Pulse 1b. Us = 30V.

Nissan requirement for Pulse 1b is quite different (-100V).

The old 2007 version of SAE J113-11 is also significantly different for multiple pulse parmeters.

Christian Rosu, 2022-01-24

FMC1278R3 specification mention that CI 280 (ESD Test Methods) must be carried out prior to any other test methods. If CI 280 fails, continuing the rest of EMC validation must be decided by FORD. The same 2 samples must theoretically withstand all FMC1278R3 test methods selected by EMC Test Plan.

The order of the test methods is critical, and the test results listed by laboratory report applies only to the two samples provided (Part Number, Serial Number, HW/SW revision). Some Europeans & Japanes automotive makers allow the use of multiple groups of samples to be used for simultaneously running test methods, probably to to speed up the completion of validation. This means that no sample is exposed to the full validation test list leaving room for insufficient EMC compliance evaluations.

Example of potentially destructive test methods:

1. ESD on the first group of samples.
2. Transients on supply lines on a second group of samples.
3. Reverse Polarity on a third grtoup of samples

To compensate somehow such selective test methods allocation, the EMC Test Plan authors would require 3 samples per group instead of 2 samples per full validation. 

A parametric test is required following each immunity test method, and this may reveal some tolerances being pushed to one extreme if not outside the acceptable range. In a real scenario, following ESD powered one unit out of three was measured with 12 KΩ impedance on B+ line versus 16 KΩ prior to test. Other than that everything was functional, the DUT current consumption was the same before and after ESD. Theoretically this unit survived ESD and based on EMC Test Plan was not supposed to be tested for Transients on Supply Lines. By mistake this unit was tested for JASO Pulse B-2 (-260V) and the outcome was "sample damged on the second pulse". This EMC test plan trick was used to hide a poor DUT design performance for Honda that otherwise would have never pass FMC1278R3 spec.

Christian Rosu, Nov 8, 2021.