Preview:
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The Pulse Withstanding Chip (PWC) resistor is designed to operate in a high energy ESD application.
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The PWC resistor along with Transients Surge Suppression Diodes (TVS) may be used to protect the input of sensitive logic circuits.
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The Analog Devices (Maxim) MAX22199ATJ+ is used as a case study. This IC is used to increase the survivability of a PLC in harsh industrial environments.
Introduction
As a resident of Northern Minnesota, I’ve learned to deal with the pain and shock of static electricity. It’s especially bad in the middle of winter, when the cold pulls the moisture out of the atmosphere. In fact, I once had a pickup truck with seats perfectly designed to generate static. SNAP! The cab lit up with that unmistakable blue light as an inch-long discharge left my finger. I learned to use a metal key to lessen the pain.
Getting technical, we can explore the Electrostatic Discharge (ESD) and the survivability of electronic components. Specifically, let’s look at an industrial application featuring the Analog Devices (Maxim) MAX22199ATJ+. This is a digital input IC designed to accept eight 24 VDC inputs and interface with a microcontroller via a SPI interface. ESD protection is a core design element for this high-impedance input circuitry. The reliability and survivability of the associated Programmable Logic Controller (PLC) is dependent on the IC’s design and PCB implementation. It’s like a candy with a hard, impenetrable ESD shell and a yummy sensitive logic inside.
It’s at this interface between the real world and input circuitry that we find the Pulse Withstanding Chip (PWC) resistors. They are often accompanied by Transient Voltage Suppression (TVS) diodes. These resistors are purpose-built to absorb ESD energy without self-destruction or flashover, providing a cushion, thereby extending the capability of the TVS diodes.
Brief ESD introduction
Human body electrostatic discharge is a common cause ESD damage. We can model the body as a charged capacitor and a series resistor. The capacitor’s energy is discharged into a circuit via a direct contact or via an air gap.
The energy will take all available paths, causing destruction to sensitive electronics. For example, ESD will easily push through MOS gate structures instantly destroying or significantly shorting the life of electronics. It can easily jump across a closely spaced PCB traces, jump between closely spaced IC pins, and even jump across small SMD resistors and capacitors.
Tech Tip: ESD is more prevalent than many of us realize. My favorite demo is to use a small neon lamp. Connect one leg to ground and leave the other exposed. The ground clip of an oscilloscope provides a convenient connection. Scuff your feet and touch the lamp. If you see an orange flash, your body was charged to at least 90 volts. Depending on your footwear and the floor type, you may be able to activate the neon by simply dragging your feet.
Brief introduction to ESD protection at the PCB level
At a basic level, ESD damage is all about containment, clamping, and current limiting to prevent destruction of the miniscule on-chip silicon structures.
Containment
Electricity will take all available paths not just the fabled “path of least resistance.” The high voltage discharge will easily bridge circuit board traces and components. The implication is that we should use physically large devices with large gaps or even PCB trenches between any section subject to ESD. This will limit the discharge to a single controlled path.
Clamping to limit voltage
Clamping using fast acting diodes is the last line of defense to protect the IC. We can use gas discharge tubes, closely spaced (raked) PCB structures, and fast diodes and TVS diodes.
Note that clamping is reliant on current limiting. After all, the clamp can’t do their job if the impulse current is exceeded, or the pulsed energy is exceeded. Things tend to explode without current limiting.
Current limiting
Current limiting is associated with resistors. Specifically, our pulse withstanding resistors.
What are the attributes of a pulse withstanding resistor?
In this context, pulse withstanding resistors are designed to operate with high energy (high voltage) microsecond width pulses. They must reliably do so without breakdown (sudden lowering of resistance leading to destruction of the clamps), flashover, or explosion due to the high (E^2)/R. They should also avoid parasitic effects such as inductance which would worsen the impulse situation.
The PWC resistor is a surface mount resistor optimized for pulsed application. Years ago, we would have specified a carbon composition resistor. This older technology was good at absorbing an impulse in its bulk (large internal mass) resistance material. Today, the PWC resistors are usually constructed using thick film on a ceramic surface mount chip with sizes between 0402 up to 2512, such as this Stackpole example.
When you consider PWC resistors, think in terms of survivability under violent pulsed operation. The resistor must stay together without changing properties.
Application example using the MAX22199ATJ+
The MAX22199ATJ+ is an interface chip. When installed in a PLC, the inputs are exposed to the world and subject to ESD. The block diagram for the octal 24 VDC input to SPI converter is shown in Figure 1. Observe that the input channel features a bidirectional TVS diode. We can expect this diode to be fast but limited due to the small size of the die in the 32-TQFN package.
The effectiveness of the TVS diode may be increased if a current limiting resistor is used. This is shown in Figure 2 with the inclusion of a 1.5 kΩ 1 W CMB0207, RPC2512, CRCW2512-IF or similar PWC resistor. The impulse energy is now dissipated in the series resistor as well as the internal PWC resistor. Provided the resistors are spaced far apart, our three criteria are satisfied including single path, current limited, and clamped.
As a side note, Figure 3 suggests that additional levels of isolation are desirable. For example, the MAX14483AAP+ performs galvanic isolation between the MAX22199ATJ+ IC and the microcontroller. Back to our candy, this is the demarcation point between our robust field device side and the yummy sensitive logic side.
Tech Tip: Many PLC engineers have encountered this galvanic input isolation. It is readily apparent when the input block of the PLC is designed with its own supply and return connection (GNA vs GNB in Figure 2).
Figure 1: Functional block diagram of the Analog Devices (Maxim) MAX22199ATJ+.
Figure 2: Typical implementation of the Analog Devices (Maxim) MAX22199ATJ+ with 1.5 kΩ pulse withstanding resistors on each input.
Alternatives
While we are exploring the MAX22199ATJ+ we should mention that there are alternatives to the PWC resistor. Figure 3 shows an implementation using a bidirectional Littelfuse SMAJ33CA TVS. This is a relatively large TVS capable of dissipating 400 W peak for a 10 uS impulse. It has a clamp voltage in the 50 volt range. Observe that the Figure 3 schematic still includes a series resistor for each of the MAX22199ATJ+ input channels. This allows additional protection by current limiting the energy to the internal TVS diode.
Figure 3: Typical implementation of the Analog Devices (Maxim) MAX22199ATJ+ with large TVS diodes for each input.
Breakout board
The MIKROE-6072 as shown in Figure 4 is low-cost breakout board featuring the MAX22199ATJ+. This design utilizes the TVS diode alternative as shown in Figure 3. This MIKROE product provides a low-cost way to experiment with the MAX driver chip.
Figure 4: Image of the MIKROE-6072 featuring the MAX22199ATJ+ with Littelfuse TVS diodes.
Development kit
Before departing, we should mention that Analog Devices offers a development kit for the MAX22199ATJ+ as shown in Figure 5. With this kit we can explore both the PWC resistor and the TVS diode solutions. Resistors are used for the upper four inputs, while TVS diodes are used in the lower four inputs.
Wouldn’t it be fun to try an ESD gun on the inputs!?
Figure 5: Image of the development kit for the MAX22199ATJ+.
Parting thoughts
The Pulse Withstanding Chip (PWC) resistor is a critical component for ESD protection. In a properly designed system, the resistor limits current (dissipates impulse energy) allowing the IC’s diode clamps to limit the voltage presented to the sensitive electronics.
Do not substitute a conventional resistor for a PWC resistor!
The lesser resistor may come apart (explosively), break down, or flash over when subject to an ESD event. It’s like the circuit burning to protect the fuse.
Please like this article if you learned something new.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.