Protection circuits: ESD, reverse voltage, overcurrent
Most of what destroys boards in the field comes in through the connectors. How to stop it with TVS diodes, fuses and reverse-polarity protection.
ESD (static electricity)
A momentary voltage of several kV enters a pin from a hand or a cable. Put a TVS diode or ESD protection array close to the connector on every signal that leaves the board. The path from the protection device to ground has to be short for it to work.
Reverse voltage (reverse connection)
Someone will plug the power in backwards.
| Method | Loss | Note |
|---|---|---|
| Series diode | Vf × I (large) | Simplest |
| Schottky diode | About half | Small currents |
| P-channel MOSFET | I² × Rds(on) (small) | Suited to large currents |
| Ideal diode controller | Very small | Automotive |
Overcurrent
- Fuse: replaced once it blows
- Polyfuse (PTC): resistance rises when hot and recovers when it cools. Common on USB ports
- Electronic cut-off: eFuses and hot-swap controllers that sense current and turn off a MOSFET
Overvoltage and surge
A vehicle supply has load dump, where the voltage jumps to tens of volts when a load is disconnected. Automotive boards use a large TVS and input parts with generous voltage ratings. For protection circuits, first write down "which accident are we assuming?" and choose to match.