This September marks one year since revisions were released regarding the MIL-STD-1275 standard, and there were notable implications for system design engineers when considering overvoltage-protection solutions for electronic systems in military land vehicles. The standard is far-reaching across many military tactical vehicles and systems. So what are the key changes in the revised MIL-STD-1275F for overvoltage-protection requirements in 28-VDC electrical systems?
The changes impact many vehicles and systems. They include armored personnel carriers, infantry fighting vehicles, ambulances, reconnaissance, command posts, CBRN trailers, cabins and gun carriages. For systems, the standard covers electronic warfare, communications and information systems and remote weapon systems.
Most of these applications have common 28-VDC voltage characteristics at the input power terminal of the electrical and electronic equipment directly connected to the power distribution network. There are legacy solutions that can be partially used for overvoltage protection here. Newer semiconductor solutions also exist that are optimal to meet new standard requirements.
The release of MIL-STD-1275 Rev F
In September 2022, MIL-STD-1275 Rev F (2022) for 28-VDC voltage characteristics for military vehicles was released. Table 1 summarizes the differences between the new standard and the previous MIL-STD-1275E (2013) standard.
Table 1: Comparison of the MIL-STD-1275F and MIL-STD-1275E standards (Source: ProTek Devices)
One of the main changes to the new standard is the addition of a new 500-ms load-dump pulse requirement. The new load-dump pulse specifies 5 × 500-ms pulses over 25 seconds that is more severe than the 50-ms load-dump specification in the previous standard. It is noted that the new 500-ms load-dump pulse requirement in MIL-STD-1275F standard is also more severe than the 350-ms load-dump specification (10 pulses over 10 minutes) in the ISO1675-2 Road Vehicles commercial automotive standard.
For the military land vehicle electrical power or equipment design engineer, the most common occurrence of a positive voltage surge is an “alternator load dump.” This occurs when the alternator is working to charge a partially or fully discharged/flat set of batteries and the connection to the battery positive terminal is suddenly disconnected. The alternator cannot immediately decrease its output to compensate for the sudden loss of load, so the excess surge energy delivered during this settling period is distributed to the military vehicle’s electrical system.
Due to this inrush of transient surge current, electronic systems can be damaged or destroyed after the load-dump pulse has been applied. Without overvoltage protection, the high-energy 5 × 500-ms pulse train over 25 seconds can severely damage or catastrophically destroy critical electronic systems in military land vehicles.
Electrical transients covered by MIL-STD-1275
There are several types of transient waveforms associated with the vehicle’s power supply system that are referenced in MIL-STD-1275. Some are critical events during operation when connected to the 28-VDC main power supply.
One is a voltage spike, as shown in Figure 1. This is an energy-limited transient waveform that typically results from the interaction of the power delivery system wiring and switching of reactive loads. It can also be a mismatch in impedance between the wiring harness and equipment. Equipment must be protected against these repetitive 250-V, 50-ns voltage spikes.
Figure 1: Voltage spike waveform for 28-VDC systems (Source: ProTek Devices)
Then there is the voltage surge. It is a transient waveform resulting from switching reactive loads containing a significant level of stored energy or sudden disconnection of a constant load. Surges like 100-V, 500-ms pulses may also occur due to the application of high-demand loads. For military land vehicles, equipment must operate without degradation or damage when subjected to injected and emitted voltage surges within specified lower and upper limits.
Next, we come to reverse polarity, which is the inverted connection of the EUT’s power terminal(s) to the military vehicle’s power system. Damage to electronic systems can be caused by reverse voltage being applied and excess reverse current flowing into the downstream circuitry.
In addition, military vehicle equipment must be compatible with applicable performance specification requirements for electrostatic-discharge (ESD) transients. ESD is not covered by the MIL-STD-1275 standard. ESD standard requirements are instead covered in MIL-STD-461 CS118. It specifies the requirements for the control of emission and susceptibility characteristics of electronic, electrical and electromechanical equipment and subsystems.
The need for load-dump protection
System design engineers can simulate a common occurrence of a load-dump transient. This can happen if a discharged battery is disconnected while the alternator is generating charging current with other loads remaining on the alternator circuit.
Figure 2: Alternator-load–dump pulse waveform (Source: ProTek Devices)
In Table 2, we can see the applied transient can be as high as 110 V and may take up to 505 ms to decay. Considering that series resistance is 500 mΩ, the surge pulse described will require a maximum energy of about 1.05 kJ for the longer 500-ms pulse duration.
Table 2: MIL-STD-1275F positive voltage surge test parameters (Source: ProTek Devices)
The average power for the quasi-periodic power function for the five pulses every five seconds is about 210 W, which is equal to 5.1 kJ for the total 25-second pulse train. The repetitive test of five pulses with five-second intervals requires any load-dump protection solution to sustain the high energy that is dissipated through the chassis or PCB.
Load-dump protection solutions
There are several opportunities for military land vehicle and equipment designers to address overvoltage circuit protection. One is a switching solution. With this, it might have a power supply designed to use a wide-input–range non-synchronous buck controller. This would offer protection for load dumps in an automotive military vehicle environment.
It provides protection to load-dump transients by monitoring excess surge-current flow and switching/disconnecting the input supply to the controller or electronic subsystem for the specified duration of the pulse. It then reconnects with a fixed delay when conditions return to normal.
But such solutions have some limits. The input operating ranges are known to function up to 75 V. This is acceptable for typical load-dump transients. But for 100-V transients above the controller upper operating range, the input supply needs to be disconnected to protect the chipset.
With a loss of power supply to the military vehicle systems, during the switching mode, the disconnection may impact operation of the vehicle and its systems. For example, communication systems, for critical command and control of military forces, that may go offline might require system recalibration or reloading. Also, these chipset solutions require many external discrete components and PCB real estate during assembly.
Newer semiconductor overvoltage-protection solutions use a shunting technique. This is a simpler implementation method of protecting a military land vehicle electronic subsystem. It’s done by shunting the applied surge energy to the 28-VDC power input with a semiconductor transient-voltage suppressor (TVS) device.
Semiconductor silicon TVS diodes contain a p–n junction like a Zener diode but with a larger cross-section, which is proportional to its surge power rating. These TVS diodes are clamping devices that limit voltage spikes by the low-impedance avalanche breakdown of the p–n junction, making them a more suitable semiconductor solution for load-dump protection.
Figure 3: Unidirectional TVS semiconductor diode in shunt protection mode (Source: ProTek Devices)
System design engineers should be aware of the recent changes to the MIL-STD-1275 standard for military land vehicles. System designers must ensure their circuit-protection design is fully compliant with new load-dump protection requirements. The solution must also offer extended operating life performance. A designer should understand that standardized surge simulations are there because they occur in real life.
Newer, more reliable semiconductor solutions employ a shunt capability for surge energy pulses. With a proper circuit-protection semiconductor solution, an electrical transient can be clamped instantaneously. This diverts the applied surge current away from the protected device and then can immediately return to protection mode. Compared with older switching solutions, a semiconductor solution can also provide long life design and increased performance benefits. Finally, they improve upon manufacturing cost and design-in time for system design engineers.
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