Tutorial: Improving the transient immunity of your microcontroller-based embedded design - Part 4

Transient immunity problems in many advanced MCU-based electronics designs often come to resemble the complex and completely unravelable Gordian's Knot that faced Greece's Alexander the Great on his way to conquer Persia.

He dealt with his complexity issues by slicing through the knot with his sword. In transient immunity design, the PCB power supply is the sword that can cut through many of the complexity issues. Solve that knotty problem and many of a system's transient immunity problems can be reduced significantly.

The transient protection in the power supply can be standalone or be designed to work in conjunction with protection at the power entry point. In either case, protection is required to prevent damage to the power supply and logic components as well as to prevent any performance degradation of the application.

Power supply designs typically fall into one of two categories: linear and switching. A basic representation of each type of power supply is shown in Figure 1 below. Each design style has its own considerations for ensuring transient immunity in the application.

Figure 1. Power supply types

Advances in power supply design technology have allowed the development of new low-cost versions of traditional power supply designs. While low-cost designs are very attractive, it must be noted that the cost reductions are typically achieved at the expense of EMC. Therefore, the successful implementation of a low-cost design will require greater planning and expertise to meet the required immunity performance levels.

Traditional Linear Power Supply
The linear AC-to-DC power supply can be approximated as a series resistance between the input and the output. Feedback control can optionally be used to provide a specified output voltage by varying the value of the series resistance. Traditional linear power supplies have many positive performance characteristics, such as excellent EMI performance, but are limited in applications by efficiency, heat dissipation, and size. A block diagram of a generic linear power supply is shown in Figure 2 below.

Figure 2. Generic linear power supply

When employed in a design, regulated supply and ground should not be DC connected to the AC mains unless required for functionality. In addition, there are four areas of a traditional linear power supply that require consideration and, possibly, protection. These areas are described as follows:

1. The transformer or the rectifier diodes, if a transformer is not used, need protection from excessive primary common mode and differential mode voltages on the AC mains. Protection components include fuses or fusible resistors (RF) to limit current, varistors (MOV) to clamp transient voltages, line-to-line "X" capacitors (C_X) to shunt differential mode noise, line-to-ground "Y" capacitors (C_Y) to shunt common mode noise, and chokes to impede both common mode and differential mode noise.

These protection components also work together to form a series of low-pass filters. An example of an AC mains EMI filter with both differential and common mode filter elements is shown in Figure 3 and Figure 4,  below, for both 2-wire and 3-wire power, respectively.

Figure 3. AC mains EMI filter for 2-wire power

Figure 4. AC mains EMI filter for 3-wire power

2. If a transformer (T) is used between the AC mains and the rectifier diodes (BR), the rectifier diodes need protection from excessive current and excessive reverse voltage. Differential mode protection is achieved by using a high-voltage, line-to-line aluminum electrolytic capacitor (C_Bulk). The addition of line-to-line "X" capacitors (C_X) from the secondary coil back to the primary reduces common mode noise.

In addition, for three-wire power systems, line-to-ground aluminum electrolytic capacitors (C_{Bulk_CM}) provide additional common mode protection. Note that while the rectifiers are shown in a full-wave bridge configuration, a half-wave configuration is also possible. An example of a step-down and rectification circuit is shown in Figure 5 and Figure 6 below, for both 2-wire and 3-wire power, respectively.

Figure 5. Transformer, rectifier and filter capacitors for 2-wire power

Figure 6. Transformer, rectifier and filter capacitors for 3-wire power

3. The voltage regulator input (if used) and the filter capacitors need protection from excessive voltage. Protection can be achieved by specifying a higher working voltage for the filter capacitor and by using a transient voltage suppressor such as a zener diode (D_Zener) as shown in Figure 7, below.

Figure 7. Regulator and filter capacitors

4. The voltage regulator output (if used) and loads need protection from excessive voltage and require bypassing to reduce noise as shown in Figure 7. Over voltage protection should be achieved by connecting a rectifier diode (D_Rect) from the voltage regulator output to the input to discharge the regulated power rail during power-down. In addition, decoupling capacitors (C_Bulk, C_Bypass) can be used to control noise on the secondary DC output. Transient voltage suppressors (D_Zener) can be added in parallel with the bypass capacitors if additional protection is needed.

Low-Cost Linear Power Supply
A low-cost version of the linear power supply is called a passive (capacitive/resistive) dropper power supply. This power supply type is suitable for current requirements in up to about 120mA. Diagrams showing two possible embodiments of a passive dropper power supply are shown in Figure 8a and Figure 8b, below.

As for a traditional linear AC-to-DC power supply, this supply type can be approximated as a series resistance between the input and the output with a zener diode (D_Zener) to establish the output voltage. This low-cost linear power supply design eliminates the conversion efficiency, heat dissipation, and parts cost of the traditional design style; however, at the cost of increasing the complexity of achieving EMC.

Figure 8a. Passive dropper power supply (buck regulator)

Figure 8b. Passive dropper power supply (inverting regulator)

EMC complexity is increased in these designs because one of the AC mains lines actually becomes one terminal of the regulated DC power supply. This is to say that either the VDD or VSS pin(s) of a microcontroller are directly connected to the AC mains. As a result, the microcontroller will be subjected to all disturbances on the AC mains. This situation can easily cause microcontroller susceptibility problems unless the proper measures are taken.

It is highly recommended that point of entry power filtering, as shown for a traditional linear power supply in Figure 3 and Figure 4, be used. If the power entry point is not filtered, using a passive dropper power supply will require the designer to expend a maximum of time and effort implementing the necessary immunity controls. For the protection of any attached DC-DC regulators, utilize the protection recommended for traditional linear power supplies shown in Figure 7.

An additional problem with this power supply type is that application self-compatibility becomes a real issue " particularly in applications with relays that switch AC mains power to inductive loads such as motors and compressors. Unless the transients generated by these switched loads are properly suppressed, the microcontroller will also be subjected to them as well.

Traditional Switching Power Supply
The switching AC-to-DC power supply varies the duty cycle of a series switch according to feedback from the output. Traditional switching power supplies deliver higher efficiency at the expense of higher noise on the DC output. A block diagram of a generic linear power supply is shown in Figure 9, below. For switching power supplies, it is important to optically isolate the feedback loop to ensure the regulated supply and ground are isolated from the mains to maximize immunity performance.

Figure 9. Generic switching power supply

When employed in a design, regulated supply and ground should not be DC connected to the AC mains unless required for functionality. In addition, there are four areas of a traditional switching power supply that require consideration and, possibly, protection. These areas are described as follows:

1. The rectifier diodes need protection from excessive primary common mode and differential mode voltages on the AC mains. Protection components and EMI filter designs are the same as for linear power supplies as shown in Figure 3 and Figure 4.

2. While not required specifically for protection, the rectified voltage must be filtered and smoothed. Differential mode filtering is achieved by using a high-voltage, line-to-line aluminum electrolytic capacitor (C_Bulk) as shown in Figure 10, below.

In addition, for three-wire power systems, line-to-ground aluminum electrolytic capacitors (C_{Bulk_CM}) provide additional common mode filtering as shown in Figure 11, below.

Figure 10. Rectifier and filter capacitors for 2-wire power

Figure 11. Rectifier and filter capacitors for 3-wire power

3. The switch, controller, and feedback circuitry will need protection as specified or recommended by the manufacturer of the switching controller. Care should be taken to ensure that the regulated supply and ground are not DC connected to the AC mains unless required for functionality. Use optical isolation in the feedback circuit whenever possible, or as recommended by manufacturer of the switching controller.

4. The voltage regulator input (if used) and the filter capacitors need protection from excessive voltage. Protection can be achieved by specifying a higher working voltage for the filter capacitor and by using a transient voltage suppressor such as a zener diode (D_Zener) as shown in Figure 12, below.

Figure 12. Regulator and output protection

5. The voltage regulator output (if used) and loads need protection from excessive voltage and require bypassing to reduce noise as shown in Figure 12. Over voltage protection should be achieved by connecting a rectifier diode (D_Rect) from the voltage regulator output to the input to discharge the regulated power rail during power-down. In addition, decoupling capacitors (C_Bulk, C_Bypass) can be used control noise on the secondary DC output. Transient voltage suppressors (D_Zener) can be added in parallel with the bypass capacitors if additional protection is needed.

Low-Cost Switching Power Supply
A low-cost version of the traditional switching power supply called a non-isolated switching power supply, designed as an alternative to the passive (capacitive/resistive) dropper power supply, is also available.

This power supply is suitable for current requirements up to about 400mA but may increase as the switching technology improves. Diagrams showing two possible embodiments of a non-isolated switching power supply are shown in Figure 13 and Figure 14, below. This low-cost switching power supply design reduces the parts cost and layout complexity of the traditional design style; once again, at the cost of increasing the complexity of achieving EMC.

Figure 13. Non-isolated switching power supply (buck regulator)

Figure 14. Non-isolated switching power supply (inverting regulator)

As for the passive dropper power supply, EMC complexity is increased in these designs because one of the AC mains actually becomes one terminal of the regulated DC power supply.

This is to say that either the VDD or VSS pin(s) of a microcontroller are directly connected to the AC mains. As a result, the microcontroller will be subjected to all disturbances on the AC mains. This situation can easily cause microcontroller susceptibility problems unless the proper measures are taken.

It is highly recommended that power point of entry filtering, as shown for a traditional linear power supply in Figure 3 and Figure 4, be used. If the power entry point is not filtered, using a non-isolated switching power supply will either require strict compliance with the schematic and layout recommendations of the switching controller manufacturer or require the designer to expend significant time and effort implementing the necessary immunity controls. For the protection of any attached DC-DC regulators, utilize the protection recommended for traditional linear power supplies shown in Figure 7.

An additional problem with this power supply type is that application self-compatibility becomes a real issue " particularly in applications with relays that switch AC mains power to inductive loads such as motors and compressors. Unless the transients generated by these switched loads are properly suppressed, the microcontroller will also be subjected to them as well.

PCB Floorplan
Before a PCB layout begins, care must be taken to properly place components. Low-level analog, high-speed digital, and noisy circuits (relays, high-current switchers, etc.) must be separated from each other to limit coupling between the PCB subsystems to a minimum. Begin the PCB design by partitioning the available board space into separate functional areas as shown in Figure 15, below.

Figure 15. PCB Segmentation

Each regulated DC power domain is isolated by its own decoupling filter (DF). The decoupling filter is typically a low-pass filter with both series and parallel elements as shown in Figure 16, below. The series elements, or blocks, are chosen based on the functional and EMC requirements and are typically resistors, inductors, or ferrite beads. The parallel components, or shunts, are capacitors.

Figure 16. Generic Decoupling Filter

Each digital logic component, such as the MCU, or other sensitive circuit block should be provided with a high-frequency bypass capacitor (BP) as shown in Figure 15.

The bypass capacitor, in addition to providing a local source of charge to reduce emissions, serves to limit transients at the protected device's power pins. In addition, low-pass filters (LPF) should be provided for each input and output to prevent noise that is coupled to connected cables from disturbing circuitry on the PCB.

When placing components, consider the potential routing of traces between the different functional areas, particularly clocks and other high-speed signals. The layout should be iteratively reviewed and corrected until all EMI risks have been addressed.

PCB Power Distribution
After the initial PCB segmentation and component placement is complete, the power distribution system should be defined. The design of the power distribution system is the most important part of ensuring PCB EMC since it is the basis for all EMC controls. The ground and supply nets should be implemented as planes or short, wide traces. The ground (VSS) system should be defined first and the supply (VDD) system second.

To design a successful grounding scheme, the designer must be aware of the paths that ground currents will take to identify possible common-mode impedance problems, reduce loop areas, and prevent noisy return currents from interfering with low-level circuits.

A good methodology is to start with a ground plane and selectively remove copper for power and signals. Avoid the use of vias and wire jumpers to connect different areas of ground. Vias and wire jumpers add inductance that can create common impedance noise between circuits that could cause functional degradation.

Ensure that all MCU pins tied to VSS are connected using a plane or short, wide trace to provide a common reference with a minimal voltage differential between any two connections. Such voltage differentials generate noise currents in the ground system of the PCB and the MCU.

After the ground system has been routed on the PCB, the supply system should be designed. Supply lines should run parallel to the ground lines on the same or adjacent layers if physically possible. If not, do not compromise the ground layout for the sake of the supply layout. Supply system noise can be decoupled with filters, but the ground system cannot. If discrete inductance is required, wire jumpers can potentially be used.

Some additional design guidelines include:

* Isolate digital, analog, high current, and PCB I/O grounds from each other.
* Connect different grounds at single point, typically at the power supply.
* Consider adding impedance in the ground path only when necessary.

When routing the ground and supply distribution systems, it is important to consider the location and connection of any filtering or decoupling components. Creating a good power distribution system is an iterative process that will require several passes.

Figure 17. Routing Regulated Power off the PCB

In the case where regulated power (VDD and/or VSS) is routed off the PCB using a connector, it should be isolated from filtered DC power as shown in Figure 17, above. Capacitors should be connected between the connector supply pins and unfiltered DC power. The typical value of the capacitors (C) is 1-100nF while the typical value of inductance (L) is 100 microH-100 milliH.

Next in Part 5:  Defensive software design
To read Part 1, go to: Defining the Problem
To read Part 2, go to: Hardware Techniques - The basic circuit building blocks
To read Part 3, go to: System power and  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 and 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