Page 51 - 2021 High-Reliability, Harsh Environment Connectivity eBook

Thermal Challenges
7. Managing thermal issues requires strategies to dissipate heat in composite structures to create a stable electrical/
electronic environment.
Dissipating heat in composite structures is difficult. Pick-up voltage (VPI) and coil resistance (RC) change as the temperature of wires and relays change. To ensure electrical stability, designers must determine the steady-state characteristics for the temperature and voltage combination of a DC relay’s operating conditions. The same is true for AC applications, although their VPI varies less with temperature than DC relays. This evaluation ensures proper product selection.
8. Battery charge cycles need to enable higher energy transfer rates by optimizing heat distribution and balance.
High-power charging (HPC) for direct current (DC) presents a challenging electric load profile in eVTOL and UAM applications. Individual components are subjected to temperature extremes at resistance points along the HV path. For HPC system safety, a simulation can apply dynamic load profiles along the complete HV path to identify potential thermal bottlenecks. Every microohm (μΩ) of resistance counts. Areas where resistance should be reduced include cable attachment (termination technology), contact interfaces (crimps and contact types), and contact materials, as well as applying optimized high-voltage contactors and relays. Thermal sensing, thermal system protection, and thermal modeling can also be used to avoid hot spots and design an HV path that can carry short-time dynamic loads.
Power Management Challenges
9. Handling power management calls for the need to use high-frequency switching to enable rapid bus transfer in
the event of power loss.
Variable-frequency AC power is used for typical aircraft loads. But fixed-frequency 400Hz AC power enables smaller and lighter transformers and motors, as well as faster transfer of bus power if power is lost, all of which is ideal for eVTOL applications. Used throughout the aviation and aerospace industry, HARTMAN power switching technology from TE offers 400Hz AC contactors rated up to 500A and lightweight high-performance DC contactors rated for up to 1,000A. Hermetically sealed enclosures provide protection in severe environments, and multiple main contact configurations and auxiliary contact configurations are available. Additionally, modular power distribution units or backplane-type panels can be customized for fixed-wing aircraft and rotorcraft applications.
10. Hybrid electromechanical and solid-state power switching technologies call for the need to evaluate the advantages and disadvantages of both.
Solid-state relays (SSRs) offer silent operation, low electrical interference, functionality over a wide range of input voltages, low power consumption, and no electrical arcing. Additionally, zero-voltage crossover minimizes surge currents. SSRs are essentially electronic circuits; they employ no moving parts and resist the effects of physical hazards including shock, vibration, and changes in altitude. But as with any electronic circuit, SSRs are sensitive to ambient heat and require a heat sink. They are also vulnerable to power surges.
Electromechanical relays (EMRs) are far more tolerant of surges and overloads and can switch any AC or DC load up to their maximum rating. EMRs will operate at full load over a wide temperature range without requiring a heat sink.
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