Live Lecture
Series #2
Designing ESD Safe Circuits
What Exactly Is ESD?
• ElectroStatic Discharge (ESD): is the release of static electricity when
two objects come into contact.
• Device level ESD: An event that occurs to an unmounted
semiconductor in an ESD controlled environment.
• System level ESD: An event that occurs to a finished electronic
device/widget.
Basics of ESD Protection (TVS) Diodes - Toshiba
Device Level Tests
Device Level Tests (cont’d)
Device Level Tests (cont’d)
System Level Tests
System Level Tests (cont’d)
What Is the Result of a System Level ESD Event?
• No damage at all, system continues as normal
• Soft Failure: No physical damage, but the system has a “lockup” or
“freeze.” May require physical intervention (i.e., power cycle.)
• Hard Failure: Physical damage to the system, may or may not show it
immediately (latent defect/failure are the worst).
What is Our Goal With Hardware ESD
Protection?
• It is not to try and eliminate all events.
• We want to reduce the event both in voltage and current so the
internal IC protection can handle it.
System-Level ESD Protection Guide – Texas Instruments
PIC16F616 - Microchip
LTSpice Simulation IEC61000-4-2
Based off: https://www.youspice.com/simple-spice-esd-generator-circuit-based-on-iec61000-4-2-standard/
LTSpice Simulation: Calibration
Using a 2R calibration resistor
LTSpice Simulation: No Protection
LTSpice Simulation: No Protection
Strategy 1: Series Resistors
• Limits the peak current.
• Damp ringing.
• Help protect diodes downstream.
• Immensely helpful for general EMC and SI.
LTSpice Simulation: 220R Series Resistor
LTSpice Simulation: No Protection
Strategy 2: Capacitors
• “Soaks up” the voltage spike.
• Smooths out the event.
• Higher the capacitor, the better it will perform*.
• Can’t use them on high-speed signals.
• *If not damped they can introduce ringing
LTSpice Simulation: 1nF Ceramic Capacitor
LTSpice Simulation: 220R Series Resistor
LTSpice Simulation: 10nF Ceramic Capacitor
Strategy 3: TVS Diodes
• Essentially act like a Zener diode.
• TVS diodes tend to have a faster response and a higher surge current rating.
• Reverse Working Maximum Voltage (VRWM): The maximum reverse voltage that
should be applied in normal operating conditions.
• Breakdown Voltage (VBR): The voltage where the diode just starts to conduct.
• Clamping Voltage (VCLAMP): The maximum voltage that the system will
experience during a surge.
• Dynamic resistance (RDYN): The estimated resistance of the diode when fully
conducting.
Basics of ESD Protection (TVS) Diodes - Toshiba
Strategy 3: TVS Diodes (Cont’d)
• Unidirectional TVS diodes protect from both positive and negative spikes.
• Bidirectional diodes only are needed for signals that go above and below 0V
(RS485)
• TVS capacitance must be lower than the limit of whatever is being protected.
LTSpice Simulation: 8.2V Zener (TVS)
LTSpice Simulation: 10nF Ceramic Capacitor
Strategy 4: Dual Schottky Diodes
• The goal with dual Schottky’s is to have them conduct before the internal diodes.
• This approach is highly dependent on the power rail impedance.
• This approach + series resistors can help limit current and overvoltage to input
pins.
LTSpice Simulation: Dual Schottky Diodes (100R PSU Impedance)
LTSpice Simulation: 8.2V Zener (TVS)
LTSpice Simulation: Dual Schottky Diodes (10R PSU Impedance)
What Do I Normally use?
• High-Speed Interface (USB/Ethernet/HDMI/Etc.): A single TVS diode
rated for that interface.
• Slow Interface, General Protection: Same as above, but will add a
series resistor before the TVS.
• Slow Interface, Higher Protection: Same as above, but will add a
capacitor in parallel with the TVS.
LTSpice Simulation: 8.2V Zener (TVS)
LTSpice Simulation: 8.2V Zener (TVS) + 220R
LTSpice Simulation: 8.2V Zener (TVS) + 220R + 1uF
PCB Layout considerations
• Protection needs to be as close to the connector/whatever as
possible.
• Low inductance traces.
• Don’t run protected traces near unprotected ones.
• Larger the ground plane, the better (enclosure included!)
Basics of ESD Protection (TVS) Diodes - Toshiba
Soft Failure Protection
• Primarily relates to firmware/software solutions.
• External watchdog monitor.
• CRC on all communication busses.
• Bit flip detection to force reset (RAM only).
Conclusion
• There is nearly an unlimited ways to protect a circuit.
• I showed 4 of the common ways that I do it
• It’s all about balancing cost/performance.
• When in doubt, use a TVS diode. If the signal is slow and you need
additional protection, add in a series resistor or parallel capacitor.
• The software/firmware side can’t be ignored.
