Design Con 2015

Tutorial: Improving the transient immunity of your microcontroller-based embedded design - Part 2

Ross Carlton, Freescale Semiconductor, Inc.

November 17, 2006

Ross Carlton, Freescale Semiconductor, Inc.November 17, 2006

The hardware design techniques employed for an application will establish the baseline immunity performance. The purpose of hardware techniques is to reduce the level or frequency content of immunity signals below that needed to cause performance degradation or long-term microcontroller (MCU) reliability problems.

Hardware techniques should be maximized to ensure desired electromagnetic compatibility (EMC) performance before attempting any software techniques. This is important because software techniques do not reduce the level of immunity signals to which the MCU is exposed " they only reduce the impact of these signals on system operation. Even though the application performance may not be degraded, exposure to immunity signals can adversely affect long-term reliability.

In order to produce an application that meets both the mandated 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 unrepeatable. The design process must also be iterative to ensure the best possible system design and PCB layout at the lowest cost.

A design that minimizes cost cannot be completed properly in one pass " regardless of the quality of personnel or tools. An EMC-compliant, low-cost application is the result of close and consistent collaboration between the EMC engineer and all other engineering disciplines (i.e. " electrical engineers, mechanical engineers, PCB layout engineers, etc).

Building Blocks of Transient Suppression and Control
Components used to suppress or control transients can be grouped into two main categories: 1) components that shunt transient currents (voltage limiters), and 2) 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 application will depend on the radio frequency (RF) characteristics of the component and the physical geometry of the board layout.

Resistors. A series resistor between two nodes can provide inexpensive and effective transient protection. Resistance can be used to create low-pass filters and to decouple power domains.

In these applications, a series resistor is used to block or limit transients with frequency- independent resistance. 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. Capacitors are 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 transient currents created by 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 metalized polyester film capacitor.

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. In general, ferrite beads are preferred because of their lossy nature.

As a result, introducing damaging resonances into the power distribution network is less likely. 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. 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. Filters are used to achieve greater performance than single capacitive or inductive components. Filters utilize multiple capacitive and inductive components that are specifically selected to achieve the desired performance.

Transient Voltage Suppressors. 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 utilized 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 as application.

Varistors. 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 employing varistors will need to accommodate this characteristic. Currently, MOVs are the best of the available non-linear devices for the protection of electronics from transient voltages propagating on 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 5V while diodes with reverse breakdown voltages of more than 8V use the avalanche mechanism.

Implementing the Solution
The available building blocks for transient suppression and control must be selected, assembled and implemented properly to ensure an effective solution. The choice of building block(s) and implementations must be determined on a case-by-case basis to work under the applicable constraints.

The constraints that most often limit or challenge the EMC designer include the functionality of the application, low-cost printed circuit board technologies, packaging (industrial design), bill of materials limitations, and the manufacturing process.

Now that we have a good understanding of the nature of the problems facing developers with respect to transient immunity and some of the basic building blocks that can be used to enhance immunity, it is now time to apply this knowledge to the overall hardware and software aspects of your embedded design. That is the topic of Part 3 in this series.

<>To read Part 1 in this series, go to "Defining the problem."<>
Next in Part 3: System Power & Signal Entry considerations

Ross Carlton has specialized in all aspects of electromagnetic compatibility (EMC) since his graduation from Texas A&M University with a Bachelor of Science in Electrical Engineering in 1985. He has been with Freescale Semiconductor for the last eight years where he has led the EMC design, test and support of Freescale's 8, 16, and 32-bit microcontroller products. In addition, Ross represents the U.S. as a Technical Expert to IEC Subcommittee 47A on integrated circuits where he is the project leader for IEC 61967-2, IEC 61967-3 and IEC 62132-2. He is currently involved in developing transient immunity test methodologies for standardization.

The author would like to thank Greg Racino as well as  John Suchyta, 8-Bit Applications Engineer at Freescale Semiconductor  for their inputs and guidance. Their contributions were critical to ensuring consistent and correct guidance.

References:
1. Ross Carlton, Greg Racino, John Suchyta, Improving the Transient Immunity Performance of Microcontroller-based applications. Freescale Application Note (AN) 2764).

2. IEC 61000-4-2, Electromagnetic compatibility (EMC) - Part 4-2: Testing and measurement techniques - Electrostatic discharge immunity test, International Electrotechnical Commission, 2001. 

3. IEC 61000-4-4, Electromagnetic Compatibility (EMC) - Part 4-4: Testing and measurement techniques - Electrical fast transient/burst immunity test, International Electrotechnical Commission, 2001.

4. Ronald B. Standler, Protection of Electronic Circuits from Overvoltages, John Wiley & Sons, 1989, pp. 265-283.

5. Ken Kundert, "Power Supply Noise Reduction", The Designer's Guide , 2004.

6. Larry D. Smith, "Decoupling Capacitor Calculations for CMOS Circuits", Electrical Performance of Electrical Packages Conference, Monterey CA, November 1994, Pages 101-105.

7. Ronald B. Standler, Protection of Electronic Circuits from Overvoltages, John Wiley & Sons, 1989.

8. Clayton Paul, Introduction to Electromagnetic Compatibility, Wiley & Sons, 1992.

9. Bernard Keiser, Principles of Electromagnetic Compatibility, Artech House, 1987.

10. T.C. Lun, "Designing for Board Level Electromagnetic Compatibility", Motorola Application Note (AN) 2321.

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