Never underestimate electrostatic discharge (ESD) while working with data networking equipment

Introduction to ESD

Let’s start with basics you probably already know, but nevertheless. It’s important to explain the basics first. Electrostatic Discharge (or ESD) refers to the transfer of electrostatic charge between bodies at varied voltages that is caused by direct contact or induced by an electrostatic field.

Never underestimate electrostatic discharge (ESD) while working with data networking equipment
Never underestimate electrostatic discharge (ESD) while working with data networking equipment

When you walk across a carpet and touch a metal door knob, you experience a slight shock on your fingers. If the same ESD occurs in data networking equipment, the equipment can be damaged or destroyed.

In summary, ESD damage occurs due to the following:

  1. Direct electrostatic discharge to the device
  2. Electrostatic discharge from the device
  3. Field-induced discharges

ESD damage can be unknowingly caused when inspecting, sorting, or installing the ESD-sensitive devices.

So what is the best practice?

The following best practices are recommended to prevent ESD damage:

#1 – Permanent / metallic wrist straps

Use permanent or metallic wrist straps instead of disposable ones as metallic wrist straps have better connectivity to the skin of the operator, and are less prone to failure when compared to disposable wrist straps, which are made of inexpensive and not very resistant material.

Because of this, the grounding cable or strip in disposable wrist straps can easily break, making the wrist strap totally ineffective.

Since permanent (metallic) wrist straps are very reasonably priced (usually a few US dollars), the usage of disposable wrist straps is not worth the risk.

Permanent (not disposable) wrist strap
Figure 1 – Permanent (not disposable) wrist strap

#2 – ESD shoes

In all the installation sites that have conductive floors (tested according to the ANSI 20.20 standard), encourage wearing ESD shoes that have metallic elements, so that the static electricity gets discharged when the shoes come in contact with the ESD flooring.

However, we recommend that with ESD shoes, users should also wear a wrist strap when handling sensitive components. This is essential for users working at a bench where they can rest their feet on a table bar and lose contact with the floor.

Facilities with ESD flooring should test the flooring periodically to ensure that it maintains conductivity. However, be cautious during floor maintenance as floor waxes can reduce conductivity and create a level of insulation that prevents the effective dissipation of electrostatic charge.

Anti-static ESD shoes
Figure 2 – Anti-static ESD shoes
ESD testing station

When implementing ESD flooring and ESD shoes, the facility should also have an ESD testing station to permit users to test their shoes to ensure that they maintain good conductivity throughout the lifetime of the shoes.

#3 – ESD mat

Use an ESD mat (see Figure 3) that is grounded to the rack when performing a card swap. In fact, if it is not possible to immediately store the replaced or damaged cards in an anti-static bag, the ESD mat offers a surface to temporarily keep the electronic equipment and prevent ESD damage.

Figure 4 outlines how to ground the mat to the same potential as the chassis and the rack.

Figure 3 – ESD Mat

Preventing ESD damage during card installation and replacement:

Preventing ESD damage during card installation and replacement
Figure 4 – Preventing ESD damage during card installation and replacement

#4 – Test wrist strap resistance

Each time before using the wrist strap (permanent or disposable), test it using a multimeter to ensure that the resistance is less than 1 Mohm. This should be performed in addition to the periodic test of the ESD protective devices in accordance with the maintenance program of the customer or partner.

#5 – ESD protected areas procedure

If the installation site has ESD-protected areas, perform the following:

  • Post appropriate signage indicating the ESD-protected area, so that it is clearly visible to the people entering the area.
  • Allow only those who have completed the appropriate ESD training into the protected areas. The content of the training material should follow the ANSI 20.20 certification standard.
  • Deionize or use other discharge-mitigating techniques in the workstations to neutralize eventual electrostatic charges.
Typical ESD protected area
Figure 5 – Typical ESD protected area

See below descriptions of each of the points shown in Figure 5:

  1. Groundable wheels
  2. Groundable surface
  3. Wrist band and footwear tester
  4. Footwear footplate
  5. Wrist band and grounding cord
  6. Grounding cord
  7. Ground
  8. Earth Bounding Point (EBP)
  9. Groundable point of trolley
  10. Toe and heel strap (footwear)
  11. Deionizer
  12. Dissipative surfaces
  13. Seating with groundable feet and pads
  14. Floor
  15. Garments
  16. Shelving with grounded surfaces
  17. Groundable racking
  18. ESD Protective Area (EPA) sign
  19. Machine

Frequently missed requirements in the field

The following are examples of requirements that are frequently missed in the field:

Example #1 Always use an ESD wrist strap to prevent the damage from ESD between your body and sensitive electronic components during the installation of electronic devices.

Failed component due to ESD damage
Figure 6 – Failed component due to ESD damage

Example #2 Ensure that the wrist strap is properly connected to the equipment. Most Cisco equipment has an appropriate hole to plug in the wrist strap and ensure good connectivity to the device and ultimately the ground (Figure 7).

In case of limitations such as the length of the wrist strap preventing the connection to the proper connecting point on the device, ensure that the wrist strap is connected to an unpainted surface or a ground wiring that mounts on the device or a rack.

Avoid connecting the wrist strap to painted surfaces (see Figure 9 and Figure 10).

Permanent (metallic) wrist strap connected to the bond point of cisco equipment
Figure 7 – Permanent (metallic) wrist strap connected to the bond point of cisco equipment

If wrist strap is not available, touch a conductive part of the data networking equipment:

If wrist strap is not available, touch a conductive part of the cisco equipment
Figure 8 – If wrist strap is not available, touch a conductive part of the cisco equipment

Temporary wrist strap connected to a non-conductive part of the rack:

Temporary wrist strap connected to a non-conductive part of the rack
Figure 9 – Temporary wrist strap connected to a non-conductive part of the rack

Or a Non-conductive paint on a rack:

Non-conductive paint on a rack
Figure 10 – Non-conductive paint on a rack

Example #3 During installation or removal, handle electronic equipment using the available handles or edges. Even if you are using an ESD wrist strap, it is important to avoid touching the electronic components to prevent mechanical damage or depositing oil that is present on your hands.

How to handle cards during installation
Figure 11 – How to handle cards during installation

Example #4 When replacing a failed card, place it component side up on an antistatic surface (ESD mat) or in a static-shielding bag.

Example #5 Insert the card into the chassis completely until you can tighten the captive screws and/or levers to ensure a good connection between the backplane of the chassis and card. This is necessary for proper grounding and ESD protection of the cards.

Card not properly inserted in the chassis
Figure 12 – Card not properly inserted in the chassis

Reference // Guidelines and best practices for the installation and maintenance of data networking equipment by CISCO

Yup, it’s the motor drive that makes systems in motion all around us

What makes them move? The motor drive.

On the outset, it may be due to wheels as in the case of an automobile. What actually drives these movements, though, are motors. Additionally, many household appliances such refrigerators, air-conditioners, ventilation fans, washers, driers and so many others all require electric motors.

Yup, it's the motor drive that makes systems in motion all around us
Yup, it’s the motor drive that makes systems in motion all around us (photo credit: Texas Instruments)

One can see that motors are part of our day-to-day life. The goal of this technical article is to discuss motor drives and their power electronics – the various components and requirements through applications that we use and encounter in household and industrial environments.

An electric motor is a device that converts electrical energy to mechanical energy. It also can be viewed as a device that transfers energy from an electrical source to a mechanical load. The system in which the motor is located and makes it spin is called the drive, also referred to as the electric drive or motor drive.

The function of the motor drive is to draw electrical energy from the electrical source and supply electrical energy to the motor, such that the desired mechanical output is achieved.

Typically, this is the speed of the motor, torque, and the position of the motor shaft.  Figure 1 shows the block diagram of a motor drive.

Block diagram of a motor drive system
Figure 1 – Block diagram of a motor drive system

The functions of the power converter circuit in the motor drive are:

  1. Transfer electrical energy from a source that could be of a given voltage, current at a certain frequency and phase as the input
  2. To an electrical output of desired voltage, current, frequency and phase to the motor such that the required mechanical output of the motor is achieved to drive the load
  3. Controller regulates energy ow through feedback coming from the sensor block
  4. Signals measured by sensors from the motor are low-power, which are then sent to the controller
  5. Controller tells the converter what it needs to be doing. A closed-loop feedback system is the method of comparing what is actually happening to what the motor should
    be outputting, then adjusting the output accordingly to maintain the target output

Motor drive efficiency

Electric motors represent 45 percent of all electrical energy consumption across all applications. Increasing the efficiency of motor-drive systems could potentially result in a significant reduction in global electricity consumption.

With increasing demand of electricity along with industrialization and urbanization across the globe, the ability to supply energy is becoming even more challenging. As part of a global effort to reduce energy consumption and carbon emissions on the environment, various regulations across many countries have put forth and are continually working on governmental mandates to improve motor drive efficiency.

All these requirements make it compelling to have an efficient power converter system using switched-mode power supplies (SMPS). The SMPS uses semiconductor power switches (also called power electronic switches) in a switch mode and on and off states only, that yields 100 percent efficiency in an ideal situation.

Power electronics systems are primarily designed using silicon-based power management with power semiconductor switches. These switches are power MOSFETs, bipolar junction transistors (BJTs), and isolated gate bipolar transistors (IGBTs) that have made significant improvements in their performances. Examples include lower on-state resistance, increased blocking voltage, and higher drive currents.

Furthermore, a lot of development is taking place using wide-band-gap semiconductors such as silicon carbide (SiC). SiC is of particular interest to motor drives that transfer very high power at high- voltage levels.

Motor drive classifications

Before we delve into motor drive applications and the role of power electronics in these systems, here is a quick overview on how motor drives are classified (Figure 2).

Figure 2 - Classification of motors (*PMSM = permanent magnet synchronous motors)
Figure 2 – Classification of motors (*PMSM = permanent magnet synchronous motors)

Table 1 summarizes where AC (induction) and DC (brushed and brushless) motors are used in terms of voltage and power levels, along with the pros and cons of each.

Comparative analysis of motors
Table 1 – Comparative analysis of motors

Power converter in motor drives

The drive configuration for motors summarized in Table 1 above are generally the same. However, what differs is the power converter topology in the power converter circuit. Since the bulk of these applications are moving towards brushless DC (BLDC) or induction motors, our focus will be on applications that use these two types of motors.

In general, selecting a motor drive may require looking at the power and voltage levels while addressing questions that depend on the application.

Examples could be the starting torque, load inertia, pattern of operation, environmental conditions, or the motor’s ability to regenerate.

AC motor drives

The AC motor drive, as the name suggests, requires an AC input to the induction motor used to drive large industrial loads such as HVAC for commercial buildings – pumps and compressors, factory automation, industrial equipment that requires provisions for speed adjustments such as conveyor belts, tunnel boring, mining, paper mills, and many others.

An AC motor drive takes an AC energy source, rectifies it to a DC bus voltage and, implementing complex control algorithms, inverts the DC back to AC into the motor using complex control algorithms based on load demand.

Figure 3 below shows a block diagram of an AC motor drive. The power stage and power supplies are marked in teal.

Power stage

The power converter topology used in the power stage is that of a three-phase inverter which transfers power in the range of kW to MW. Inverters convert DC to AC power. Typical DC bus voltage levels are 600-1200V. Considering the high power and voltage levels, the three-phase inverter uses six isolated gate drivers (Figure 3).

Each phase uses a high-side and low-side insulated gate bipolar transistor (IGBT) switch.

Operating usually in the 20-30 kHz range, each phase applies positive and negative high-voltage DC pulses to the motor windings in an alternating mode.

High-power IGBT requires isolated gate drivers to control their operations. Each IGBT is driven by a single isolated gate driver. The isolation is galvanic between the high-voltage output of the gate driver and the low-voltage control inputs that come from the controller.

Block diagram of an AC motor drive
Figure 3 – Block diagram of an AC motor drive

The emitter of the top IGBT floats, which necessitates using an isolated gate-driver. In order to isolate a high-voltage circuit with that of a low- voltage control circuit, isolated gate-drivers are used to control the bottom IGBTs.

Gate drivers convert the pulse-width modulation (PWM) signals from the controller into gate pulses for the FETs or IGBTs. Moreover, these gate drivers need to have integrated protection features such as desaturation, active Miller clamping, soft turn-off and so on.

These isolated gate drivers usually suffer from low drive strength, especially when the drive current capability is below the 2A range. Traditionally, these drive applications use discrete circuits to boost the drive current. Recently, there have been several gate driver ICs developed to replace the discrete solution.

Figure 4 illustrates this trend.

Three-phase inverter topology with boost gate driver power supplies for IGBT gate drivers in the power stage
Figure 4 – Three-phase inverter topology with boost gate driver power supplies for IGBT gate drivers in the power stage

In order to take advantage of the low conduction losses in IGBTs, gate drivers need to operate at voltages much higher than their threshold voltage in the range of 15-18V. Furthermore, an IGBT is a minority-carrier device with high input impedance and large bipolar current-carrying capability. The switching characteristics of an IGBT are similar to that of a power MOSFET.

For a given condition when turned on, the IGBT behaves much like to a power MOSFET, showing similar current rise and voltage fall times. However, the switching current during turn-off is different.

At the end of the switching event, the IGBT has a “tail current” that does not exist for the MOSFET. This tail is caused by minority carriers trapped in the “base” of the bipolar output section of the IGBT. This causes the IGBT to remain turned on.

Unlike a bipolar transistor, it is not possible to extract these carriers to speed up switching, as there is no external connection to the base section. Therefore, the device remains turned on until the carriers recombine. This tail current increases the turn-off loss which requires an increase in the dead time between the conduction of two devices for a given phase of a half-bridge circuit.

Having a negative voltage (–5V to –10V) at the gate helps to reduce the turn-off time by helping to recombine the trapped carriers. When the IGBT is turned on the high dv/dt and parasitic capacitance between gate and emitter generates voltage spikes across the gate terminal. These spikes can cause a false turn-on of the bottom IGBT. Having a negative voltage at the gate helps to avoid this false turn-on trigger.

Usually 15V to 18V is applied to the gate to turn-on the device and a negative voltage of –5V to –8V is applied to turn off the IGBT. This requirement is key to determine the power supply rating to the IGBT driver.

Typically, such a power supply is a PWM controller with a topology that has the ability to scale the output power while driving these high-power IGBTs. Typical inputs for these power supplies are regulated to 24V (to be explained shortly).

One example of a classic topology used for this power supply is the push-pull isolated converter. This topology is similar to a forward converter with two primary winding. The advantage that push-pull converters have over y-back and forward converters is that they can be scaled up to higher powers, in addition to higher efficiency.

Other power supplies

Figure 3 shows an offline power supply that draws power from the three-phase universal AC line to a regulated 24V DC output. Because of the low-power level (below 75W), power factor correction (PFC) is not needed.

These offline power supplies are typically y-back topology converter ICs that could be a controller with external MOSFET, or an integrated MOSFET controller or switcher.

The choice of the power supply IC is flexible and is influenced by the power level, number of outputs, and accuracy of the regulation. This offline power supply is usually a separate module.

The 24V DC output is the system power bus in the AC motor drive system that is input into the bias power supply for the power stage and non- isolated DC/DC converter. This non-isolated DC/DC regulator from the 24V system provides power to the controller, communications and safety microcontrollers, interface transceivers, and data converters.

BLDC motor drives

The brushless DC (BLDC) is on trend for becoming the most popular choice, replacing brushed DC and AC motors in markets such as HVAC, especially for its higher efficiency and high reliability. Of particular interest are power tools and household appliances such as refrigerators, air-conditioners, vacuum cleaners and other such white goods.

Using BLDC in these market spaces lowers the system’s overall weight.

Figure 5 shows a block diagram of the BLDC motor drive in a cordless (battery-powered) power tool such as an electric drill. Power blocks are shown in blue.

Figure 5 - Block diagram of a cordless BLDC motor drive
Figure 5 – Block diagram of a cordless BLDC motor drive

Power stage

A BLDC power stage is also an inverter similar to an AC motor drive, except that the input can be single- or three-phases. DC-rail voltages are typically 48-600V, depending on the power levels. The switch is usually a power MOSFET switching at around 100 kHz. Gate drivers are high-side, low- side or half-bridge drivers per inverter phase with no isolation requirement.

Protection features are not as critical as those needed for the AC motor drive, except for dead-time control to avoid shoot- through since the high-side and low-side drivers are operating from one IC.

Power supplies

Bias power to the controller and gate drivers comes off a regulated power supply from the battery. A typical battery used in this space is the 18V nominal Lithium-Ion (Li-Ion) five-cell battery. Since these are cordless tools, a wall charger is required to charge the drill periodically.

Typically, charging in the range of 50–1000W is done using an isolated controller that is topology-specific, depending on the power level.

Also, PFC is generally not needed unless the power level is in the few hundred W. Typical charging controllers are based off of a y-back, interleaved y-back, or push-pull topologies.

To summarize…

Motor drives are becoming more efficient as power electronic devices such as power switches (IGBTs and MOSFETs), gate drivers and bias supplies are being incorporated. We discussed two key and popular motor drive systems: AC and BLDC, and covered the functionalities and role of gate drive circuits and associated bias supplies.

Key areas such as isolation, voltage levels and protection features were highlighted.

Reference // Power electronics in motor drives: Where is it? by Nagarajan Sridhar (Product Marketing Manager High Performance Isolated Power Solutions Power Management Texas Instruments)