Engineers have tried to battle Electrostatic Discharge (ESD) for a long time now, as it can cause actual damage to your electronics. Nobody wants their equipment to stop functioning properly because somebody carelessly walked across a room in the middle of winter, on a carpeted floor, wearing a polyester jacket, brushing their hair with a PVC pipe and then decided to touch some doomed electronics! How dare they!? Haven't they heard of wearing a ground strap 24x7?
Jokes apart, the electronics industry has come up with tests and standards that components and products are recommended to meet in order to ensure ESD will cause no harm to the brave electronics that go forth into the world. Now, you also don't want your product's launch held up because your ESD protection circuit design was a little sloppy.
I learnt a whole lot to do with designing for ESD immunity when working on a product and bringing it to the hands of costumers.
As a EE system designer, are you wondering...
- How is ESD characterized? There are many different notations, but which is the most important?
- So many different levels of ESD immunity! What do they actually mean? What level of immunity should I try to achieve?
- How to design my ESD protection circuit? How to avoid a last minute redesign?
If you are interested in any of these questions... READ ON!
Section 1: Understanding Different ESD Standards
ESD as it relates to discharges into electronic components, are denoted with many different models. This is because, the ESD energy profile and waveform varies largely based on the particular discharge environment.
1.1 ESD standards for component level immunity in the manufacturing environment:
External charge is delivered by a human body to the device. Voltages are in the range of 1 kV – 10 kV.
Charged Device Model (CDM):
Electronic device itself gets charged by electrostatic induction and later discharges to an outside object. Voltages are in the range of 200 V – 2000 V
Machine Model (MM):
1.2 ESD standards for system level immunity in the real world:
IEC61000-4-2 ESD standard:
1.3 Are the HBM, CDM, MM models relevant? Not for a system level designer!
Section 2: Diving deeper into IEC61000-4-2 standard
Contact discharge involves discharging an ESD pulse directly from the ESD test gun that is touching the device under test. In the air discharge test, the ESD test gun is brought close to the device under test until a discharge occurs. The standards are defined so that each level's air or contact discharge is considered equivalent.
Section 3: Designing ESD protection
3.1 Prevent entry of ESD strike
3.2 ESD protection circuit
For many instances having the series resistor or the decoupling capacitor will not be appropriate since that might ruin the functionality of that trace, e.g. high speed data trace with a controlled impedance and low stray capacitance requirement.
- TVS diode clamping voltage: Once the TVS diode conducts, it will only bring the voltage down to the rated clamping voltage, which can be in the range of 10-50V. Make sure the downstream IC can handle this clamping voltage transient. If the IC gets into a latch up state, or is harmed in any other way by the transient voltage, one needs to find either a different ESD diode or add in extra protection such as a series resistor.
- How do I know the clamping the voltage for my TVS diode - at different levels of IEC waveform ESD strikes? Either read a TLP graph or look at the datasheet. More on this in another blog post.
- TVS diode working voltage: The diode should not conduct under normal operation of the trace it sits on, with added margin. This also includes choosing the right polarity and bi-directionality if needed for the diode.
- TVS diode stray capacitance: Adding a TVS diode will add a little extra capacitance on the PCB trace. This will be a problem with high speed signal with fast requirements on rise and fall times. Watch out and choose an appropriate TVS diode with low stray capacitance.
Determine the value of the series resistor based on clamping voltage, max. current the IC can handle and max. resistance the trace can handle under normal operation.
3.3 Layout Guidelines
- Place the TVS diode as close as possible to the point of entry of the ESD strike.
- Route with straight traces between the ESD entry point and the TVS if possible. Else use rounded curves or 45 degree curves at the most.
- Place the protected IC further away from the TVS diode.
- Do not use stubs between the TVS and the protected trace, instead route directly to the TVS and then to the rest of the system from there. Ensures the TVS can first suppress the ESD strike before it can reach the protected IC.
- Ensure there are enough ground vias near the TVS to enable minimum impedance. Adding any trace inductance on the ground side of the TVS will result in a significant voltage being seen across that inductance during an ESD event.
- Avoid vias between the ESD entry point and TVS if possible. If a via is required, route directly from the ESD entry point to the TVS before using the via to the protected IC. This ensures the ESD current will not experience a via stub between the protected trace and TVS.
- Ensure all vias on the ESD path are of the largest diameter, depending on the design constraints.
- In order to minimize EMI / ESD jump into other unprotected traces on the board. Do not route unprotected circuits in the area between the ESD entry point and the TVS diode.
- Understand the standards and always aim to achieve IEC61000-4-2 standard immunity. Do not be confused with the component level standards.
- Decide which class of ESD immunity is required for your product early on in the design phase.
- Contact discharge immunity usually also means air discharge immunity at the same voltage level.
- Carefully note all the different system parameters and design an ESD protection circuit by choosing a TVS and using other components like series resistor, decoupling capacitor, etc. if needed. Note things like TVS turn-on time, stray capacitance, TVS clamping voltage. Every detail matters.
- Optimize PCB layout to help ESD find the shortest path to ground.