I naturally gravitate towards endurance exercise rather than high-intensity as I grow older. There’s something about keeping a steady pace for an extended period that I now find much more physically rewarding and mentally stimulating. Whether riding my bike or taking a backcountry trip, incorporating this constant and measured movement has become second nature.
In many ways, circuits also enjoy steady operations instead of constant source spikes. While plenty of design goes into filtering and conditioning signals, it is partially unavoidable. Disturbances up the line or even weather discharges can lead to systems experiencing excitations and responses that are distinct from standard conditions. To check overall reliability and relative susceptibility to transient signals, PCBs utilize electromagnetic compliance (EMC) testing – specifically, in this case, EMC burst testing – to enact any proactive changes for best performance.
The Standards and Limitations of EMC Burst Testing
Transient response is a cornerstone of circuit analysis. As much as designers would like to average the behavior of their signals and power systems, the more appropriate treatment is to consider the time-variant changes in performance. An EMC burst test is part of a wider battery of electromagnetic compliance testing. It checks how well a device can withstand short, repeated durations of voltage in the kV range. Despite being a stress test, it’s important to note that EMC burst testing (also known as a fast transient burst or FTB) does not guarantee that a passing device will withstand similar or greater circuit excitations in the field. This may initially seem like a testing oversight because what’s the use in the minimum values of a test that don’t accurately subject a device to real-world conditions? In truth, EMC burst testing can be further compounded by variability in testing. Essentially, a valid test signal may change noticeably if procedures vary even slightly, leading to inconsistencies that are difficult to interpret.
| IEC/EN 61000-4-4 Testing Level Specifications | ||
| Level | Power Supply Line†
Voltage (kV) / Current (A) |
Signal frequency (kHz) |
| 1 | ±.25 / 10 | 5 |
| 2 | ±.5 / 20 | |
| 3 | ±1 / 40 | |
| 4 | ±2 / 80 | |
| x | * | * |
*: Values specified by product.
†: For all other lines (signal, control, I/O, etc.), half both voltage and current, e.g. ±1 kV / 40 A for Level 4.
While EMC burst testing may not capture these excitations’ complete and nuanced behavior, it provides an important baseline for tests that engineers can modify to the requisite parameter level for better accuracy. Consider also that these tests may be conducted by design team sizes ranging from startups to established companies. To prevent unnecessary market restrictions, the International Electrotechnical Commission (IEC) provides a more general testing ruleset that can be independently scaled up by parties. Budget is also a concern as tests, and associated equipment can rapidly escalate in cost. However, ROI from cost savings covering warranties, recalls, etc., can be significant.
Simulating Flyback Voltage Discharge
The burst test is a fairly straightforward result of a physical phenomenon. Switches and relays in systems of significantly high power are liable to experience some arcing when their position changes from closed to open. The driving electromagnetic force comes from the circuit inductance. At a steady state, the inductor (assuming a lumped element model) possesses a magnetic flux that wishes to maintain static. The open circuit immediately disrupts the current flow, and the inductor will use its stored magnetic energy to create a large potential and resume current flow.
The energy required to facilitate current across an air gap is far greater than a standard conductive pathway. This discharge of stored energy can easily damage components or the terminal contacts it flows through. Assuming the battery is still connected and the conduction path to the inductor has not been damaged, this process can repeat in an oscillatory manner. The flyback voltage will continue building up to a discharge event, more rapidly when the distance is short at the beginning of the open circuit and less so as the gap increases until the energy built up cannot arc across the gap.
The act of a closed switch opening can result in damaging and unsafe arcing.
EMC burst testing does not simulate the increasing air gap/decreasing discharge frequency. Instead, it provides a near-instantaneous transient pulse equivalent to the amplitude of the test level, which slowly decays as energy leaves the system, and the process is repeated. These signals are pulsed at 5 kHz with delays of no circuit excitation between these rapid burst periods. This is a significant difference compared to arcing events induced outside of testing. However, boards still greatly benefit from a standard that covers some of the reliability concerns of high-amplitude transient signals.
How Your Contract Manufacturer Gets The Most Out of Testing
With some of the shortcomings of EMC burst testing in mind, what adjustments can an engineering firm make to maximize performance outcomes? Test layout is of extreme importance. With the large voltages being applied, capacitive coupling to items near the test equipment must be acknowledged.
Some foremost thoughts to keep in mind when EMC burst testing:
- The ground plane is the single most important component of an accurate test. The burst test equipment will be tethered directly to the ground, while the evaluation boards must maintain a minimum 10 cm distance from the ground plane on all sides.
- Distance between any conductive surfaces and the evaluation boards must be a minimum of 50 cm.
- Cable lengths should remain <1 m and follow the ground rule spacing of the device under test. The pulsed bursts will be sent along lines using a compliance clamp bonded to the ground. It should be the only cable that can violate the length/ground clearance condition.
The transient bursts can be applied in any combination to the transmission lines, including all at once. The best practice for exhaustive testing would be to check the reaction to each combination to gain a comprehensive understanding of the board’s response. This knowledge can then be applied piecewise to optimize the system.
EMC bursts tests form an important fraction of general EMC. As transmission speeds climb, the need for immunity to EMI issues becomes increasingly prevalent. For system designers, the best solution is pairing with contract manufacturers who have experience preparing boards for all EMC testing and know how to correct designs to avoid wasting time and budget on retesting.
At VSE, we’re a team of engineers who build electronics with our customers collaboratively. Alongside our talented manufacturing partners, we’re engaged in producing high-quality PCBAs that change and save lives — every day.
