By Kelly Casey for Mouser Electronics
Automobiles produced today have a longer life expectancy than at any time in the past. This is due, in part, to improvements in materials and design. One critical change has been the increase in electronic equipment and systems that have replaced mechanical devices. Without proper electrical protection, however, these electronic systems can fail without warning, leaving the consumer to wish for the “good ol’ days”.
Designing electronics for the automotive environment is very challenging. Wide temperature swings must be anticipated. Most applications require the industrial temperature range of –40 to +85 °C. Under-hood applications can be even more extreme. Humidity can range from desert lows to swamp highs. Add in seaside salt mist, road salt, or just lots of time; and corrosion can affect the quality of electrical connections and electrical insulation. Shock and vibration must be taken into consideration, along with the possibility of pinch points occurring where wiring is run throughout the vehicle where there is relative motion of doors, seats, windows, mirrors, pedals, steering columns, etc.
ESD Protection
Electrostatic discharge has several sources. A charge can build up on passengers as they move about the vehicle interior. The simple act of plugging in a personal device to the car’s sound system can inject ESD into the system inputs. Dry air moving over a car antenna can cause ESD to build up on the radio inputs. Technicians working on electronic systems can inject ESD into ports not normally accessible by the car occupants.
Multi-Layer Varistor (MLV) devices are the classic ESD protection device as represented by the MLA series from Littelfuse. These have capacitance values in the hundreds of pF, so they may not be appropriate for high-speed data lines. Polymer ESD devices (e.g. TE Connectivity PESD series) offer capacitance below 1pF. The ultimate low capacitance ESD devices employ an encapsulated air gap. The CG0402MLU family of devices from Bourns is an example with a maximum capacitance of just 0.05pF. All three of these technologies, however, have rather high instantaneous “let through” voltages that often exceed 100V during ESD events. To limit these voltages, one must use silicon diode technology such as the Littelfuse SP3021 series, which offer much lower clamping voltages and low capacitance at under 1pF. One drawback is that the silicon technology devices can cost ten times as much as the other technologies.
Load Dump Protection
Load dump power surges are unique to automotive applications. Load dump pulses are momentary voltage spikes on the car’s 12V power bus caused by the rather slow regulation of the alternator output voltage. The sudden removal of a large load on the power bus (such as a disconnected battery, a blown power fuse, etc.) will cause the alternator output to suddenly jump to 60 volts or more. It can take as long as 400 milliseconds for the voltage to recover to normal tolerances.
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| Figure 1. Load Dump Protection |
Compared to ESD pulses, which are measured in picoseconds, or lightning pulses, which are measured in microseconds, these load dump pulses are very, very long. TVS diodes have surge ratings measured with a pulse with a half-life of just one millisecond. This means that a typical 600W TVS diode may not survive even a 10W pulse that lasts 400 milliseconds. For a 15-volt diode, that’s less than one amp! Fortunately, many alternators include surge suppression diodes that can reduce the load dump protection requirements of equipment installed in the car. Solutions for load dump requirements may include metal oxide varistor (MOV) or transient voltage suppressor (TVS) diode devices as overvoltage protection coupled with fixed resistors or positive temperature coefficient (PTC) resistors used to limit the surge current (see fig 1). One must be very careful in selecting components as the datasheets are geared toward much shorter surge pulses and extrapolating the surge withstand ratings to the load dump pulse range is tricky. Silicon overvoltage devices offer precise clamping voltages that assure the protected circuit will survive.
Short Circuits (Application Loads) and Power Faults
It is possible to design the equipment I/O ports to be “hardened” to withstand these events using active protection circuitry (current limiters, auto shutdown, etc.). Yet all too often the need for such protection is not realized until late in the design cycle. At that point the design flexibility is often limited to adding additional protection circuitry.
One-time fuses can be used to open under fault conditions – however, this only prevents further damage or a fire. Once the fuse is operated, the circuit will not be operational and the equipment must be repaired, or more likely, be replaced.
PTC devices act as automatically resetting fuses in many applications. At normal operating currents, a PTC has a low resistance value. However, when fault currents are present, the PTC device warms up, increasing its resistance and pinching off the fault current. When the power or the fault is removed so that the PTC is allowed to cool to its original temperature, it will reset to its normal low resistance. To avoid nuisance tripping, one should be careful to select PTC devices with an extended temperature range. The Bourns MF-RHT series PTC devices have a maximum operating temperature of up to 125 °C, well above the normal PTC maximum operating temperature of 85 °C. Designers should also note that at very low ambient temperatures, PTC devices might take a very long time to trip. Fortunately, the driving circuitry also has substantial thermal margin in these situations.
To protect against power cross events, TVS diodes are typically deployed to clamp the voltage on control lines and sensor inputs. For prolonged overvoltage events, current limiting must be included to prevent the TVS diode from overheating.
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Figure 2: Circuit Protection for I/O Signal or Data Lines |
Figure 2 shows a typical I/O line where the TVS diode provides the overvoltage protection and the PTC provides both shorted load protection and power cross current limiting.
Whatever your circuit protection requirements might be, make sure to think about it early on in the design process. Mouser.com is a good place to find what you need, and they break cks if you are manufacturing a low-volume prototype run.
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