Long cable ringing occurs when there is a high di/dt event such as plugging in a device to an on-board diagnostic connector. The surge current charging the device’s on-board capacitors or backup battery will resonate with the cable’s inductance, causing high voltage ringing. Longer cables, with higher parasitic inductance, will exhibit more severe voltage ringing. The new OBD-II standard dictates that the diagnostic connector be within two feet (0.61m) of the steering wheel while the main battery is far away under the hood or on a side of a truck. This new requirement makes the cable from the battery to the OBD-II connector longer and more prone to high voltage ringing.
Cable Ringing Causes High Voltage Faults
Figure 3 shows a lab setup to demonstrate cable ringing. A 24VDC power source is used to emulate a 24VDC battery from a truck. A 10-foot cable connects the power source to a ceramic capacitor (either 1uF or 10uF) to emulate an input capacitance of a fleet tracking device.
Figure 4 shows our first test, emulating cable ringing at initial plug-in when the in-rush current charging the capacitor (previously discharged) built up through the cable parasitic inductance resonates with the board input capacitance. With 10uF input capacitance, the peak ringing voltage is 32V with a voltage spike at 42.6V. With 1uF input capacitance, the peak ringing voltage is at 40V.
Figure 5 shows our second test, where we emulate a brief short-circuit condition across the cable. Once the short is removed, the short-circuit current built up through the cable parasitic inductance resonates with the board input capacitance. With 10uF input capacitance, the peak ringing voltage is 40V. With 1uF input capacitance, the peak ringing voltage is at 50.4V, more than doubling the source voltage of 24V.
We use 10 feet of cable in this experiment, which is a reasonable estimation of the truck cable length from its battery to an OBD-II connector just to demonstrate that the peak ringing voltage can easily double the input voltage source. The high peak ringing voltage can occur at different cable lengths and at different device input capacitance. In fact, peak ringing voltage can be calculated as:
Vpk=Ipk√(L/C)
Where Ipk is the peak short circuit current and √(L/C) is the characteristic impedance of the system. L in this case is the cable parasitic inductance and C is the device input capacitance.
Other Faults
Electronic components can encounter short-circuit faults. Short-circuit and/or overcurrent protection circuitry is essential for preventing fire hazards as well as isolating the power cable from a failed short device.
When the ambient temperature become excessive or if there is some other fault (overcurrent, etc.), the over-temperature protection prevents permanent damage by either scaling down the power dissipation or shutting down the device completely. Over-temperature protection prevents system overheating and fire hazards, and ensures that the system operates within its defined temperature limits.
Reverse voltage fault occurs when the battery is connected in reverse or the power cable is installed backwards. While unlikely to happen, reverse voltage fault usually produces expensive damage to the power cables and electronic devices connected to the cable without proper reverse voltage protection.
We’ve discussed the need to protect the device from many possible faults. Implementing fault protection circuits with discrete components can be quite tedious, expensive, and not fool-proof. The solution is large due to the high number of components. The system designer faces the challenge to verify and guarantee the circuit performance over time. Total cost of ownership is high due to system inflexibility when responding to a fault (open a switch, shut down the system, which requires a technician to restart).
Figure 6 illustrates a modern protection IC from Maxim’s Olympus family of devices, MAX17523 – 36V/1A. This highly integrated IC packs all needed protections into a single, tiny 16-pin TQFN 3x3mm package. This device is very simple to use while providing a rugged solution to the 12V transportation electronics. Some of the MAX17523 features are:
- High input voltage tolerance (+4.5V to +36V operating range)
- Reverse voltage protection (tolerates -36V negative input voltage)
- Reverse current protection
- Short circuit, over current protection
- Over temperature protection
Fig. 6: MAX17523 typical application schematic
For 24V transportation systems, a higher voltage rating protection IC is needed. The MAX17525 – Maxim’s Olympus Protection IC +5.5V to +60V, 0.6A to 6A is perfect for the job. The device is in a space saving 20-TQFN 5x5mm. Some of the MAX17525 features are:
- High input voltage tolerance (+5.5V to +60V operating range)
- Reverse voltage/current protection (tolerates -60V negative input voltage)
- Short-circuit, thermal foldback current-limit protection
- Over temperature protection
- Adjustable OVLO, UVLO, startup current, and forward current limit
