How to Design ESD capacitors for Automotive Applications

Introduction 

This post will help you understand methodology to design ESD (Electro Static Discharge) protection capacitors for Automotive application PCB's.

Prerequisite : For People with understanding of ESD, Circuit design, Working in Electronics field/ pursuing Electronics degree.


What is ESD 

Electrostatic discharge (ESD) is the sudden flow of electricity between two electrically charged objects caused by contact, or Air, by an electrical short, or dielectric breakdown.

What will happen if ESD protection is not implemented in PCB ? 

Damage to the PCB due to ESD will be either Latent or Catastrophic. When a PCB or its components suffers latent damage due to ESD, it may be partly degraded, and continue to run. However, the device becomes unreliable as its lifetime reduces, and its operational behavior affected.



Component damaged due to ESD

Catastrophic damage occurs when the device is permanently damaged because of ESD. A performance test can usually detect such damage. As catastrophic damages are detected at early manufacturing stages, the effect is not as expensive as that from latent damages.




PCB Track damaged due to ESD

ESD in Automotive Field 

Automotive Suppliers & OEM's requires minimum level 4 ESD immunity according to the IEC 61000-4-2 standard (±8kV contact discharge, ±15kV air discharge). This is applicable for PCB's which are used in 2 wheeler and 4 wheeler application's for Ex: Engine Control Unit, ABS control unit, Cluster's etc.

Design Calculation :

Let's take the Max requirement from the above standard which is 15kV. This is Machine Model (MM) [200 pF @ 0 ohms].




Machine Model for representation only

VMM/V1 = 15kV
C1 = 200pF
C2 = ??
V2 = 50V ( Let's take Capacitor with 50V rating)
R= 0Ω

L = Stray inductance (0H for calculation simplicity)

C2 is the ESD Capacitor which protects the PCB/DUT (Device under Test). This capacitor should be close to the Pins in parallel inside the PCB, so that along with the components inside the PCB, it should also protect the PCB tracks.
We know that Electrical charge Q stored in the capacitor is directly proportional to the Capacitance (C) and Voltage (V) across it. Q = CV. In our case Q = C1V1 = ( C1 + C2 )V2 Since charge remains same in both the conditions which means with C2 or without C2.

Let's substitute the values:
200X10-12 * 15X103 = ( 200X10-12 + C2 ) 50V



we get C2 = 60nF (approx) , since 60nF is not available in the market you can choose 100nF standard capacitor.


Conclusion: 

So if you have 100nF, 50V rated capacitor mounted close to the Pin on the PCB/DUT you will be protecting the DUT from upto 15kV ESD Applications. This is how you can design ESD protection Circuits and save the PCB's.


Points to be noted: 

1. You can change the voltage rating of the capacitor based on the type of signal expected in such pins. Also for Air discharge ESD, actual ESD on the pins will be lesser so you can reduce the capacitance value to say 10nF.
2. It's important to keep degradation of Capacitance in mind over time etc. This is common for capacitors across different applications. I will share the details in separate blog. 
3. If PCB is having multiple pins each pin should have ESD protection capacitors . This is because all pins are equally susceptible for (Electro Static Discharge). 
4. Care must be taken if signals are Frequency/Analog since these capacitors will affect the quality of the signal (Ex: Rise time, Fall time). I will explain in the next blog how to design ESD Protection circuits for such sensitive signals. 

Please feel free to comment, ask questions and improvement if needed.