Land systems - November 02nd, 2016

Active Surge Protection - Understanding 'Load Dump' Def-Stan 61-5 Part 6, Issue 6

On 6 February 2009, the United Kingdom Ministry of Defence published Issue 6 of Defence Standard 61-5 part 6. Issue 6 was released with the purpose to address all the power quality issues identified at both platform and component level. The Issue 6 version of this specification was a substantial change over the issue 5 version these included:

• Specifies mandatory design requirements such as circuit protection, earthing, capacity and spare capacity, environmental de-rating requirements, wiring configurations, schematics

• Mandatory requirement for a compliant PCDR (Power Critical Design Review). The PCDR documents compliance to the design requirements, test methods and limits
• Enhanced platform test regime (Annex A) measurements including earth paths, voltage drop, over/under voltage, circuit protection, imported ripple and load dump
• Enhanced component test regime (Annex B) measurements including inrush current at temperature extremes, isolation tests, conducted short transients and load dump

However one of the key changes included the survival of electrical equipment when subjected to a DIT08.B Load Dump.

System design engineers concerned themselves with the power surges and transients associated with these types of high energy surges, recognising that these were the most difficult to achieve compliance to and had the largest effect on system integration and reliability.

What is the 'Load Dump'

Load dump occurs when a heavily loaded electrical system exhibits a loose battery terminal,  the resulting voltage transient presented onto the electrical system by the poor connection is defined in the commercial vehicle standard ISO 7637-2.
Previously, the largest surge requirement on the 28V system was when the system ran without a battery connected, and was fed from only the main alternator. The surge was 80V for a period of 80mS and was  the result of an instantaneous change in power demand, or power feed-back from a regenerative effect  on the system. The figure below shows both the new load dump and 80V surge from Issue 5:

Calculating the energy the 28V DC system load dump surge is around a factor of 4.5-5 times that of the 80V surge. The peak voltage is over 2.5 times the magnitude, meaning that existing Active surge protection designed for issue 5 in the equipment would be inadequate to cope with the Issue 6 energy levels.
The 80V surge is seen as a common occurrence on 28V vehicles and it is necessary that the equipment continues to operate continuously throughout the duration of the surge. Most Electronic equipment fitted to 28V vehicles typically utilise front end Galvanically Isolated DC-DC converter technology to reduce the input voltage to lower voltages for the downstream electronic circuitry. The Power conversion efficiency is normally optimised to a typical input range of 36 to 40VDC. 

Knowing the input voltage range of the front end DC-DC converter, a very common methodology of protection to protect the converter is a simple clamp circuit using a series pass element that prevents the voltage increasing above the maximum allowed input voltage of the front end converter. This circuit could be as simple as a MOSFET with a large reliable SOA (safe operating area) and charge pump circuitry with additional passive components and TVS protection. In normal operation the charge pump provides full enhancement gate bias to the MOSFET so that the device looks like a very low resistance connection from input to output, however during the surge it switches on harder and becomes more resistive. The increasing surge voltage is dropped across this series element (MOSFET) and is dissipated as heat within the device ensuring that the output voltage is clamped below the maximum allowed level. An additional timer circuit can be added to protect the MOSFET from Thermal damage by monitoring the clamp time.

During the 202V load dump surge it is recognised that it is actually a rare event and allowing compliance to the specification can include operator intervention, to replace a fuse for instance,and/or temporary interruption of operation of the equipment powered from this rail, Therefore a simple solution would be to detect this large surge voltage and then safely turn the system off. This method of protection is only suitable if the downstream electronics are able to safely shut down in a mode that allows the system to re-boot, once the surge has passed.
The above means to gaining compliance with the specification can be met by the following criteria

• ‘the equipment shall continue to operate as intended during and after each test. No degradation of performance or loss of function or unwanted operation is allowed, when the equipment is used as intended’
  

However some electronic equipment could lose valuable data and be subject to an extended reboot period that might not be acceptable in some environments. Safety Critical and Mission System would not be allowed to fall in to this category, for instance weapon control systems that control many safety interlocks and weapon functionality would need to work through the 'Load Dump' to ensure safety of the Operator and other Personnel.

Careful consideration is needed when defining the platform power architecture for the equipment. A combination of allowing some parts of the equipment to shut down whilst maintaining full power to Mission and safety critical equipment often results in the most cost effective and safest solution.

EES have designed and developed a comprehensive range of standard COTS high efficiency protection Electronic Payload Modules (EPM) ensuring that the equipment continues to operate during this ‘Load Dump’ surge without any interruption to its output voltage. Our Electronic Payload modules Provide active surge protection, with the output of the EPM being clamped at < 36 VDC.

Careful consideration is needed when defining the platform power architecture for the equipment. A combination of allowing some parts of the equipment to shut down whilst maintaining full power to Mission and safety critical equipment often results in the most cost effective and safest solution.

EES have designed and developed a comprehensive range of standard COTS high efficiency protection Electronic Payload Modules (EPM) ensuring that the equipment continues to operate during this ‘Load Dump’ surge without any interruption to its output voltage. Our Electronic Payload modules Provide active surge protection, with the output of the EPM being clamped at < 36 VDC.