Meeting the Challenge of High-Voltage Power Distribution in New Ground Vehicles
On today’s battlefield, military vehicles have to perform in harsh environments with increasingly complex and sophisticated on-board electronics. To achieve the required level of complexity, greater demand is placed on the power electronics to deliver higher power in higher densities. To address the need for such high power density, a typically implemented 28-V system involved converting to a higher voltage distribution bus to decrease the current. The higher voltage system, with its smaller conductors, bus bars and switches, addressed the critical goal of reducing the weight of the system making room for more electronics while providing the vehicle the ability to travel further with the same amount of fuel. Most of the power hungry devices such as motors and generators are already designed for high-voltage systems so in new generations of ground vehicles, power system architecture has changed from a 28-V system to a 610-V power distribution system. The same overall power can be achieved in both systems. However, in the old 28-V system, a vehicle with a 10 kW power requirement would distribute 350 A with heavy wiring and switching components. In the new 610-V system, 10 kW of power requires just 16 A. The higher voltage results in lower current and reduces the size of the wiring but also significantly changes the requirements of the switches. While the weight reduction goal has been met and cost savings achieved by reducing the amount of copper required to conduct the extra current, new components are needed for the switching.
A New Approach
To implement a new power distribution system, it became necessary not only to re-size the switching components but to adopt a new technology. For the traditional 28-V system, solid state switches used Metal Oxide Field Effect Transistors (MOSFETS) which are efficient at lower voltages. Modules containing MOSFETS are common and readily available as multiple channel switching modules. However, to perform the same task with a 610-V bus, insulated gate bipolar transistors (IGBTs) were required since they are more efficient at the higher voltages. The challenge designers faced was that these devices had not been integrated in a multi channel device with the required intelligence to interface with and protect the equipment to which the power was being switched.
To address the challenge, a four-channel “smart switch” power module was developed to be compatible with the 610-V power distribution system. The switch is intended to be used with a higher level controller switch on/off, and monitor power to a load. When commanded by higher level control electronics, this feature-rich device is capable of transferring 25 A from input to output in each of three channels, and 35 A through the fourth channel simultaneously. Using a 10-kW load example, the current rating is perfectly sized with sufficient derating to provide the best reliability and guarantee long life. In addition to this steady state current rating, the device is rated for a 100-A pulse condition. Included in the module are load current sensing and fault protection as well as optically isolated on-off commands. Fault protection, specifically short circuit protection uses desaturation detection and latches off the effected IGBT. A reset command is required by the control module in order to turn on the IGBT. The current sense output is used by the control module to execute these functions.
The core of the smart switch is International Rectifier’s Gen 5 IBGT die and IR2214 driver IC. The IGBT is a 1200 V breakdown voltage utilizing non punch through (NPT) technology with the ability to withstand a short circuit condition. The driver IC is also part of the short circuit protection because it includes transistor desaturation protection. When the desaturation condition exists, the IC smoothly shuts down the IGBT to prevent over-voltages and reduce any electromagnetic emissions.
With size and weight being a design driver, a rugged lightweight plastic package capable of withstanding the environmental screening of Mil-PRF 38534 was developed. To reduce the overall footprint of the device, a two tiered component placement technique was implemented. The bare die power semiconductors and the current sensors are solder-attached and wire bonded to an insulated metal substrate (IMS) to achieve the lowest thermal resistance. The thermal resistance is 1.0 degree C per Watt for the IGBT and 2.0 degree C per watt for the diode. The IMS substrate makes up the module base. The remainder of the circuit elements, gate driver, opto isolator and charge pump, and mounted on a printed circuit board and interconnected to the IMS as a mezzanine level within the package. The package sidewall is plastic with copper pins molded into the plastic. The components are encapsulated and covered to complete the module. The devices are screened to military specifications to provide the highest reliability and will operate over a temperature range of -40 to +125 degrees celcius.
The result is a feature-rich device housed in an extremely dense, lightweight package that achieves the design goals of achieving further reductions in size and weight.