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ESD Diodes as a Countermeasure for Static Electricity Discharge

Author Cadence System Analysis

Key Takeaways

  • An ESD diode is a protection diode used as a countermeasure for static electricity discharge in ICs and USB lines.

  • The ESD diode suppresses abnormal voltages and protects downstream components in a circuit. 

  • Low on-resistance, low parasitic capacitance, and instantaneous turn-on are some of the electrical characteristics of ESD diodes.

Electro Static Discharge ❲ESD❳ is a major issue affecting the USB data transmission rate, as well as ICs. In IC technology, the maximum rating of the components specified in a design should be satisfied in the fabrication stage. When the current and voltage applied to the IC are above the maximum rating, the components are subjected to Electric Over Stress ❲EOS ❳ due to ESD. Design engineers are constantly investigating what causes ESD and how to prevent it, as it contributes to failure and reliability issues. 

Preventing ESD is key to protecting devices from damage and preventing malfunction. An ESD diode, also known as an ESD protection diode, is a protection diode used as a countermeasure for static electricity discharge in ICs and USB lines. It absorbs the abnormal voltages from interfaces or external terminals and suppresses the ESDs and transient voltage pulses. 

ESD Diodes Suppress ESD

When charged bodies come in contact, there is a sudden flow of charges which generate static electricity. The static electricity in ICs and USB lines induce voltage and current spikes that ruin the purpose of the devices. ESD protection prevents high voltages and currents generated from the ESD strike from reaching the IC or USB. ESD diodes are connected between the interface connectors and the IC in order to be protected. By placing the ESD in the signal line, the EOS is prevented from damaging the IC.

The ESD strike creates large current spikes that flow through other devices, creating a high voltage drop. If the voltage drop is above the maximum voltage rating of each component, it can cause circuit issues such as EOS, electromigration, and gate oxide stress. 

The ESD diode absorbs the short-duration voltage pulses during the ESD strike. It breaks down and forms a low impedance between the interface and the IC. The high peak currents from the ESD strike flow to the ground through the ESD diode and limit the ESD strike voltages from damaging the IC or USB. The ESD diode is connected in reverse biased condition, and is operated at the region before the breakdown region. 

We can compare ESD diodes and Zener diodes. In one way, ESD diodes are one kind of Zener diode, as they operate at the reverse biased condition. The ESD diode suppresses abnormal voltages and protects the downstream components in the circuit from abnormal voltages and surges, whereas the Zener diode maintains a constant voltage across components in a circuit.

Characteristics of ESD Diodes

ESD devices exhibit exceptional protection when they have the following features and specifications: 

  1. Working Voltage

The operating voltage of an ESD diode is called the working voltage. The recommended practice is to select an ESD diode with forward and reverse voltage rating greater than the interface’s signal voltage. Usually, the working voltage is kept at 110% of the supply voltage. 

  1. Clamping Voltage

During the ESD strike, ESD diodes clamp the voltage to protect downstream components in the circuit. Clamping voltage indicates the extent of the ESD diode-based protection scheme. Generally, the 8 kV IEC ESD strike is approximated to 16 A Transmission Line Pulse❲TLP❳.

  1. Low On-Resistance

The low on-resistance of an ESD diode provides a low impedance path for large currents to flow to the ground, protecting the downstream components.

  1. Low Parasitic Capacitance

Like any normal diode, the ESD diode is also associated with parasitic capacitance. The parasitic capacitance should be low in high-speed interfaces to ensure signal integrity in ICs. 

  1. Instantaneous Turn-On Time

According to  IEC 61000-4-2 ESD waveform (8 kV), the current hits 30 A value within 0.7 to 1 ns time duration. If an ESD diode fails to turn on within this time, the ESD protection scheme does not work.

  1. Space Consumption

ESD protection circuits should only consume small areas. Nowadays, multichannel devices are available, offering space-saving options.

  1. Recommended Practices and Standards

The IEC 61000-4-2 rating gives the contact rating and air gap rating of the ESD protection device. The contact rating gives the maximum voltage rating the device can withstand when the ESD source is in contact, whereas the airgap rating deals with ESD strike discharged over an air gap. The higher the contact and airgap rating of an ESD diode, the better the protection offered to downstream devices from ESD. NFPA -77 recommended practice on static electricity is the other major standard to be considered when designing an ESD protection scheme. 

ESD Diode Clamp Protection Circuit

The ESD clamp circuit can be used as an anti-static circuit. In this circuit, the abnormal voltage at the interface connector from the ESD strike is clamped to the power rail voltage or ground voltage using ESD diodes D1 and D2, respectively. Under normal operation of the circuit, ESD diodes D1 and D2  are reverse biased. When the input signal voltage applied at the interface connector is above the power rail voltage VDD , the diode D1 conducts. When the input voltage is less than the ground voltage, the diode D2 conducts.

High-speed communication networks, miniature devices, and integrated noise filters often get impacted by ESD. Preventing an ESD strike from damaging devices is critical, and an ESD diode is one of the best countermeasures for ESD issues. It is reported that the incorporation of ESD clamping circuits in ICs and other interfaces have drastically reduced the Failure In Time ❲FIT❳ rate of devices. If you are looking to protect numerous interfaces or ICs from static electricity discharge, include an ESD diode-based protection scheme in your circuit. 

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