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TRAILER ELECTRICAL INTERFACES – Part 2

Part 2: Centralized Power Switching Regulators (DC-DC Converters)

As we mentioned in Part 1 of this series, North American commercial vehicles’ electrical systems are different from those used in Europe and by the NATO military. Variations in voltages and connectors require the use of electrical trailer interfaces. Part 1 focused on power resistor voltage dividers. In this section we’ll go over another electrical trailer interface — centralized power switching regulators, otherwise known as centralized DC-DC converters — including this interface’s advantages and disadvantages.

How the Centralized (Bulk Power) Method Works

Bulk power is imported from the tractor through two cables to a 24V-to-12V DC-DC converter mounted on the trailer. The cables that carry this bulk power must be of sufficient diameter to handle the trailer’s total electrical demand without overheating. The DC-DC converter should efficiently change 24V to 12V DC. Signals are applied from the tractor electrical output connector pins to a relay network which is housed adjacent to the DC-DC converter. These signals energize the relays in the network such that the tractor signals (24 VDC) are translated into corresponding signals of voltage taken from the DC-DC converter output (12 VDC). These signal voltages are applied to the appropriate pins of the trailer input connector.

This interface method has two significant advantages over the resistor divider method (discussed in Part 1 of this blog). First, the transformation of power from 24V DC to 12 DC is efficient — usually approximately 90%. This implies that dissipative losses in the order of 100 watts would occur in the DC-DC converter. This condition is manageable from a heat evacuation standpoint. Second, this configuration offers excellent regulation because the DC-DC converter’s 12V DC output fluctuates marginally as load is varied.

However, centralized power switching regulators have disadvantages relative to other electrical trailer interfaces. First, towing a military tractor would require the addition of connectors and wire harnessing to enable the delivery of the trailer’s entire electrical demands through the two wire conductors carrying the bulk input power. Second, should the centralized switching regulator fail to function, the entire trailer may lose power. Third, a heavy circuit breaker must be installed at the power source for short-circuit protection. Fourth, the physics of such a large switching regulator (DC-DC converter) requires that some of its magnetic components be relatively large. This makes them vulnerable in conditions of shock and vibration. Finally, this approach does not necessarily allow for the conveyance of multiplexed signals such as PLC4 between tractor and trailer along designated circuit lines — a significant disadvantage in the context of 21st century commercial vehicle design. When it comes to ABS brake operation, PLC4 tractor-trailer signal transmission is critical for regulatory compliance. This aspect will be discussed in a later blog post.

In the third part of this blog series we’ll go over a third type of electrical trailer interface. This type is called distributed power switching regulators. This type of electrical trailer interface is the kind used in our trailer interface models.