Overcurrent Protection (OCP)

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What is a fuse?

Rules overview

Important Fuse Parameters

Slow/Fast blow

Important types of Fuses

ATO (Blade) Fuses

Cartridge Fuses

PTC Fuses

High Voltage Fuses

Fusible Links

Breakers

Breakers are a resettable form of overcurrent protection.

Overvoltage Protection (OVP)

Another major category of stresses which circuits can experience are overvoltage events. While overvoltage protection is not mandated (with the notable exception of the accumulator and BMS), it is advisable to include protections in any boards which are expected to be handled extensively, or which are going into a commercial product.

What is ESD?

ESD stands for electrostatic discharge. When a person walks around, particularly on a carpeted floor, their body can build up a charge thanks to the triboelectric effect. Shockingly (pun intended), this can result in charges of up to 35,000 volts accumulating on your person with respect to your surroundings. When this voltage is high enough, touching an object of lower potential can cause a painful (and sometimes visible or audible) static shock. When the electrostatic charge on a person or object is discharged through a PCB, an ESD event is said to have occurred.

In automotive applications, the IEC 61000-4-2 standard requires automotive electronics to possess a minimum of level 4 ESD protection, which necessitates protection against ±8kV from a source directly contacting the board, or ±15kV when arcing across an air gap.

What is a TVS Diode?

A TVS Diode (Transient Voltage Suppression Diode) is diode which clamps a signal at a maximum voltage by shunting current safely to a ground or power rail. They are typically specialized Zener diodes, or otherwise incorporate a Zener diode into their design.

Once a board is installed into its housing, the only parts of the board that a person is likely to touch are the connectors which leave the housing. Thus, TVS diodes are typically included on boards in close proximity to connectors.

Important Parameters of a TVS Diode

Reverse Standoff Voltage (also called V_RWM or Working Voltage) - The maximum voltage at which the diode is guaranteed to not conduct

Breakdown Voltage - The voltage at which the diode will enter avalanche conduction, and non-trivial amounts of current will conduct. Typically given as a minimum value, and sometimes also a maximum.

Clamping Voltage - The voltage that the TVS diode will prevent the bus from exceeding during an ESD event, provided its rated current/power is not exceeded. Given as a maximum value.

Peak Pulse Current - the maximum current the TVS can sink

Peak Pulse Power - How much power the TVS can absorb without damage

Operating Temperature - self explanatory

Package / Case - self explanatory

Channel Count - number of parallel diodes in a single package

Directionality - Bidirectional or Unidirectional (see note below)

Speed - normal or high speed (see note below)

Steering vs. Zener - A Zener TVS diode is simply a single Zener diode (unidirectional) or two opposite facing Zener diodes in series (bidirectional). A Steering (Rail-to-Rail) TVS diode uses normal (non-Zener) diodes to allow current to flow from GND to the signal line when an excessively negative voltage is applied, or from the signal to VCC when an excessively positive voltage is applied. A third Zener diode helps protect VCC by sinking excess current and clamping VCC at a set maximum value. Check out this article for more info on steering diodes and their applications.

Bidirectional vs. Unidirectional - the “direction” referred to here is not the sign of the transient voltage in the ESD event! Rather, it refers to the nature of the protected signal voltage with respect to ground. If the signal is always positive or always negative with respect to ground (e.g. a 5V rail, an I2C bus, or a CAN bus), then a unidirectional diode should be used. If the signal is sometimes positive, and sometimes negative with respect to ground (e.g. the current sensor signal for the accumulator - negative when charging, positive when discharging), then a bidirectional TVS should be used. More information on the distinction between Bidirectional and unidirectional TVS diodes can be found here.

High Speed TVS diodes - “speed” does not refer to the response time of the diode, but rather the frequency of the signal being protected. A high speed TVS diode has a lower parasitic capacitance, meaning that it will produce less distortion of high frequency signals.

Picking the right TVS Diode

Oftentimes, you can find TVS diodes specifically designed for a particular application, which can eliminate much of the burden of design. For instance, many diode arrays exist with the proper parameters and diode counts to provide ESD protection for USB and ethernet.

For more generic applications, the following steps should be taken:

Determine the sign of the signal with respect to ground. If it’s always positive, or always negative, use a unidirectional TVS. If it varies, use a bidirectional TVS. However, if you’re considering reverse polarity protection (worried about someone plugging something in backwards and flipping VCC/GND), then a bidirectional diode would always be a better choice, since it would prevent significant currents in this case.
If your signal is high speed, make sure to consider the diode capacitance. TVS diode capacitances are typically on the order of picofarads.
Determine the maximum signal voltage under normal conditions. For instance, if you’re protecting the 24V rail of the car, make sure to consider the max output of the DC-DC converter when determining the max bus voltage. This value is the minimum value you can select for Reverse Standoff voltage.
Determine the lowest maximum voltage rating amongst all components on the rail being protected. Ensure that the clamping voltage of the TVS is below this maximum voltage by a reasonable margin. Note that ESD-protected components may provide TLP ESD ratings, which should be deferred to when available.
The final step is to make sure that the TVS diode can handle sufficient current and power. Since automotive devices require at least level 4 ESD protection per IEC 61000-4-2, they should be able to withstand a contact discharge of 8kV with a peak current of 30A, with the overall test waveform appearing as follows:

IEC 61000-4-2 Discharge Waveform

The easy way to do this is to make sure to filter for Automotive/AEC-Q101 TVS diodes, and to make sure it meets IEC 61000-4-2 specifications:

Example: SD24C-01FTG from Littelfuse

Routing TVS Diodes

Place the TVS diode as close as physically possible to the connector, with as short and wide a trace as possible to minimize parasitic inductance and resistance. Make sure to put the TVS diode before any filtering or ICs on the trace being protected.

References

Reverse Polarity Protection

Can use a low forward voltage drop Schottky diode at the input to prevent currents in the reverse direction.

NER

Circuit Protection Guide