Thermal Design and Forced Cooling of High Voltage Power Supply for Airborne Early Warning Radar Transmitter
Airborne early warning radar systems provide critical surveillance capabilities for military operations, detecting and tracking aircraft and other targets at extended ranges. The transmitter is a key component that generates the high-power radio frequency pulses for target detection. The high voltage power supply for the transmitter must operate reliably in the challenging environment of an airborne platform, where thermal management is complicated by limited cooling resources and stringent weight constraints.
The radar transmitter converts DC power from the high voltage supply into radio frequency power that is radiated by the antenna. The transmitter typically uses vacuum electron devices such as klystrons or traveling wave tubes, or solid-state power amplifiers, depending on the frequency and power requirements. The efficiency of the power amplification is less than one hundred percent, meaning that a significant fraction of the input power is dissipated as heat.
The high voltage power supply itself also generates heat through losses in the power conversion circuits. Switching losses in the power semiconductors, core losses in transformers and inductors, and conduction losses in all components contribute to the total heat generation. The power supply efficiency determines the fraction of input power that is converted to heat rather than delivered to the transmitter.
Airborne platforms present unique challenges for thermal management. The available cooling resources are limited by weight, volume, and power constraints. Liquid cooling systems using fuel or dedicated coolant loops can provide effective heat removal but add complexity and weight. Air cooling using ram air or forced air from fans is simpler but has limited heat removal capacity, particularly at high altitudes where the air density is low.
The thermal design process begins with accurate estimation of the heat generation in all components. The heat generation depends on the operating conditions including the output power level, the duty cycle, and the ambient temperature. The thermal model must account for the transient nature of radar operation, where the power dissipation varies with the transmit and receive cycles.
Thermal resistance networks model the heat flow from the heat-generating components to the cooling medium. Each component has a junction-to-case thermal resistance that characterizes the heat flow from the semiconductor junction to the package surface. The case-to-heatsink thermal resistance depends on the mounting interface and the thermal interface material. The heatsink-to-ambient thermal resistance depends on the heatsink design and the cooling conditions.
Forced air cooling uses fans to increase the airflow over heatsinks and through equipment enclosures. The increased airflow reduces the thermal resistance and enables higher heat dissipation. The fan selection must consider the airflow rate, static pressure capability, noise level, and reliability. The fan power consumption adds to the total heat load and must be included in the thermal budget.
Liquid cooling can provide higher heat removal capacity than air cooling, particularly for high-power-density applications. Cold plates with internal channels circulate coolant to remove heat from high-power components. The coolant is then cooled by a heat exchanger that rejects the heat to the ambient air or to another cooling medium. The liquid cooling system design must consider the coolant properties, flow rates, and pressure drops.
The thermal design must ensure that all components remain within their specified temperature limits under all operating conditions. The junction temperature of semiconductors is particularly critical, as excessive temperature can cause immediate failure or accelerated degradation. The insulation temperature in transformers and inductors affects the insulation lifetime. Electrolytic capacitors have temperature-dependent lifetime that decreases rapidly with increasing temperature.
Altitude effects must be considered in the thermal design for airborne applications. The air density decreases with altitude, reducing the heat removal capacity of air cooling systems. The reduced air density also affects the fan performance, as the fan delivers less mass flow at the same volumetric flow rate. The thermal design must ensure adequate cooling at the maximum operating altitude.
Thermal cycling can cause fatigue failures in solder joints, wire bonds, and other interconnections. The temperature variations during radar operation, combined with the ambient temperature changes during flight profiles, create thermal cycling stress. The mechanical design must accommodate the thermal expansion and contraction without causing excessive stress on the connections.
Thermal simulation and testing validate the thermal design before hardware fabrication. Computational fluid dynamics simulations model the airflow and heat transfer in the equipment enclosure and through the heatsinks. Finite element thermal analysis predicts the temperature distribution in the components and structures. Thermal testing in environmental chambers verifies that the actual temperatures match the predictions and remain within limits.
