Electrical Fast Transient-EFT Failure? Steps to consider in design
Electrical and electronic products designed for the market might have to be operated in not so conducive environments. Design engineers therefore require the products to be tested before releasing to the market. There are several mandatory compliance tests defined in respective industry to certify a product based on the application and functionality. Electrical Fast Transient (EFT) Immunity test is one basic test method defined by immunity standards listed under Electromagnetic Compatibility (EMC) directive. It is important, for the safe operation, the system is immune to EFT. IEC 61000-4-4 standard defines the methods and requirements for the EFT immunity testing. Under EMC directive 2014/30/EU, equipment manufactured specifically for use at a named ‘fixed installation’ may not have to comply with EMC requirements when it is supplied. But testing to EN/IEC 61000-4-4 at specified levels will generally be a requirement by the customer to ensure that the fixed installation complies with EMC Directives Protection Requirements.
This standard helps designers to understand the test procedure to be followed to qualify a product. One has to understand that passing EFT test is more of an internal design aspect of the system rather than the whole system. Many designs which didn’t have a proper protection fail in the compliance tests and must go back to the lab taking out lot of time and design efforts, design modifications leading to long product development cycle. This can many times increase the development cost of the product.
EFT in real world
Electrical Fast Transients (EFT) are caused by transient currents during a make or break of contact. In a real time, scenario, when there are loads like motors, other inductive loads connected to the circuit, there can be a chance that a short time-period transient with high frequency may be generated. This is also called arcing. These transients can damage the system. Therefore, an appropriate protection mechanism to counter these transients need to be employed at the design stage.
Common Causes of EFT
- Inductive loads, such as relays, switch contactors, or heavy-duty motors when de-energized, produce bursts of narrow high-frequency transients on the power distribution system
- Fast transients produced when the utility provider switches in or out the power factor correction equipment.
- Sparking that occurs whenever an AC power cord is plugged in, equipment is switched on/off, or when circuit breakers are opened or closed
- Lightning strikes also produce EFT events. EFT transients are coupled to end equipment typically via power lines.
- Subway trains and electric buses impose large EFT surges onto the power grid and subsequently AC mains with their constant arcing
- A common mistake in switching power supply layout is to have a big loop on the SMPS high switching current paths. This path must be heavy and as short as possible.
Design Practice to maximize immunity to Electrical Fast Transient Burst
In order to maximize the immunity of electronic device in the event of electrical fast transient bursts, the following steps should be adhered strictly to:
- Internal circuit design shall be bandwidth limited wherever possible
- PCB layout shall prevent 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.
Note: The sensitive output resistor divider feedback net of an SMPS should not be routed parallel to a vibrant EFT source like the inductor, which should always be a shielded type and mounted such that a sufficient air gap (at least 2 mm) must be reserved around to evacuate the heat.
Hardware Design Measures
The hardware design techniques used for an application will establish the baseline immunity performance. The purpose of hardware techniques is to protect the Microcontroller Units (MCUs) from performance degradation or long-term MCU reliability problems. Hardware techniques should be maximized to ensure desired EMC performance before attempting any software methods.
This is important because software techniques do not reduce the level of transients to which the MCU is exposed — this can only reduce the impact of EFT signals during system operation. Even though the functional performance may not be degraded, exposure to transients can adversely affect long-term reliability. In order to design a product that meets both the regulatory EMC requirements and minimizes cost, the design process must be both methodical and iterative.
Rigorous system and PCB design methodologies are required to ensure quality and consistency in the design process. Without such methodologies, achieving EMC compliance will be accidental and unpredictable. The design process must also be iterative to ensure the best possible system design and PCB layout. An EMC-compliant, low-cost design is the result of close and consistent collaboration between all engineering disciplines (i.e., electrical engineers, mechanical engineers, PCB layout engineers, etc.) including EMC.The hardware design techniques used for an application will establish the baseline immunity performance. The purpose of hardware techniques is to protect the Microcontroller Units (MCUs) from performance degradation or long-term MCU reliability problems. Hardware techniques should be maximized to ensure desired EMC performance before attempting any software methods.
Transient Suppression and Control Components
Components used to suppress or control transients, as well as their implementation details and RF characteristics, are described in technical documentation from the component manufacturers as well as in many books, papers, and articles. Therefore, this application note will not go into detail on component selection and specific usage. The following paragraphs provide a basic description of how the most typically used components are employed in low-cost designs for achieving the desired level of transient immunity.
Components used to suppress or control transients can be grouped into two main categories:
- Components that shunt transient currents (voltage limiters)
- Components that block transient currents (current limiters)
Note that depending on the rise time (frequency bandwidth) of the transient, a component may function as either a shunt or a block. For instance, at a slow rise time (low frequency bandwidth) an inductor will have little impedance (a shunt). At faster rise times (higher frequency bandwidth), an inductor will have greater impedance (a block). As a result, transient suppression components must be carefully selected for the optimal operating conditions. The actual performance of the component in the design will depend on the frequency-based characteristics of the component and the PCB layout.
Series resistance between two nodes can provide inexpensive and effective transient protection blocking or limiting transients with frequency-independent resistance. Resistance can be used to create low-pass filters and to decouple power domains. Series resistance is primarily suited to protecting digital or analog signals that carry low currents and can accept a modest voltage drop (across the series resistance). Typically, wire-wound or carbon-composition resistors are used due to their ability to survive large transient currents. Important characteristics to consider when selecting resistors are the steady-state maximum power rating, maximum working voltage, and dielectric withstand voltage. The parasitic shunt capacitance and series inductance of a resistor do not require special consideration in transient protection applications.
Capacitors can be used in a variety of transient protection roles: bypassing or charge storage (as a limiter of voltage variations) and power decoupling (as a shunt element in a low-pass filter or a series element in a high-pass filter). In either role, the capacitor can be used to effectively shunt fast transients of limited energy, such as ESD or EFT. Capacitors are not practical for shunting larger (28.4 kA) Peak transient current due to lightning, surge, or switching large inductive loads. Important characteristics to consider when selecting capacitors are the maximum DC voltage rating, parasitic inductance, parasitic resistance, and over-voltage failure mechanism. When used in conditions where the maximum voltage rating may be exceeded, capacitors should be of the self-healing type, such as the metallized polyester film capacitor.
Filter Capacitor Selection
To brief word about capacitors as EFT/EMI filters, EFT testing frequencies tend to be in the 100-200 MHz range, (~5 ns rise time). When selecting capacitors as noise filters, users should always consider two important characteristics of the capacitor: maximum frequency limitation and self-resonance. The maximum frequency limitation of various types of capacitors is shown in below Table.
Self-resonance is the frequency at which a capacitor no longer behaves like a capacitor and instead becomes more like an inductor.
Capacitor Frequency Limits
Ensure that the type of capacitor you are using to filter out noise has a higher self-resonance frequency than that of the noise you are trying to filter out. Find below lists typical self-resonance frequencies of various values of capacitance.
|Capacitor Type||Frequency Limitation|
|Aluminum Electrolytic||100 kHz|
|Tantalum Electrolytic||1 MHz|
Capacitor Self-resonance Frequencies
Capacitor self-resonance frequency is the frequency at which resonance occur due to the capacitor’s own capacitance and residual inductance. It is the frequency at which the impedance of the capacitor becomes zero.
The insertion loss of capacitors increase until the frequency reaches the self-resonance frequency, and then decrease due to residual inductance of the lead wires and the capacitor’s electrode pattern existing in series with the capacitance. Since noise is prevented from going through the bypass capacitor to the GND due to the residual inductance becoming dominate, the capacitors insertion loss begins to decrease with increasing frequency. (The frequency at which the insertion loss begins to decrease is called self-resonance frequency)
|Capacitor Value||Leaded||Surface Mount|
|1 µF||2.5 MHz||5 MHz|
|0.1 µF||8 MHz||16 MHz|
|0.01 µF||25 MHz||50 MHz|
|1000 pF||80 MHz||160 MHz|
|100 pF||250 MHz||500 MHz|
|10 pF||800 MHz||1.6 GHz|
Ferrite Beads and Inductors
Ferrite beads and inductors are used to decouple power domains by creating low-pass filters. In these applications, a series ferrite bead or inductor is used to block or limit transients with frequency-dependent impedance. Series inductance is primarily suited to protecting power lines and digital or analog signals that carry high currents or cannot accept the voltage drop imposed by a series resistance. Important characteristics to consider when selecting ferrite beads or inductors are the maximum DC current rating, parasitic resistance, permeability of the ferrite material, DC resistance, and parasitic inter-winding capacitance in the case of wound inductors.
Common Mode Chokes
Common-mode chokes present a large inductance in series with common-mode sources and small or negligible inductance in series with differential-mode sources. These inductances suppress common-mode signals while having a negligible effect on power frequency differential-mode signals. As a result, the common-mode choke is one of the most effective transient protection components. When used with capacitors to form a low-pass filter, common-mode chokes can be even more effective at designated frequencies.
Important characteristics to consider when selecting a common-mode choke are the maximum differential-mode DC current rating, common-mode inductance, differential-mode inductance, and DC resistance.
Filters are used to achieve greater performance than single capacitive or inductive components. Filters use multiple capacitive and inductive components that are specifically selected to achieve the desired performance
Transient Voltage Suppressors (TVS)
The transient voltage suppressor (TVS) is used to control and limit the voltage developed across any two or more terminals. The TVS accomplishes this task by clamping the voltage level and diverting transient currents from sensitive circuitry when a trigger voltage is reached. TVS devices tend to have response times in inverse proportion to their current-handling capability. As a result, two devices (one with slow response and high current capability and one with fast response but low current capability) are often required to achieve the desired protection level. TVS devices can be used to suppress transients on the AC mains, DC mains, and other power supply systems. They can also be used to clamp transient voltages generated by the switching of inductive loads within an application.
The varistor (or voltage-variable resistor) is a non-linear, symmetrical, bipolar device that dissipates energy into a solid, bulk material such as a metal oxide in the case of the common metal oxide varistor (MOV). As a result, the varistor will effectively clamp both positive and negative high current transients. The one issue with varistors is that the actual trigger voltage can vary widely from the specified value. Transient protection designs using varistors must accommodate these characteristics. Currently, MOVs are the best of the available non-linear devices for the protection of electronics from transient voltages propagating through the AC mains.
Avalanche and Zener Diodes
The avalanche and Zener diodes are silicon diodes intended for operation in the reverse breakdown mode. The primary difference between these two diodes is the mechanism of reverse breakdown: avalanche or Zener. Typically, the Zener diodes have a reverse breakdown voltage of less than 5 V while diodes with reverse breakdown voltages of greater than 8 V use the avalanche mechanism.
Summary of Design Considerations
To Design productd compliant of Electrical Fast Transient bursts (EFT) tests:
- Use Ferrite bead in series and capacitor to ground to filter out the transients.
- Use TVS diodes at the input of the connector.
- Take appropriate measures in the layout like avoid any routing near the connector and the signal input at the connector should see the decoupling capacitor.
- Whatever energy that comes to the circuit through the connector should be routed back to the connector with a shortest and low impedance path.