Modern digital electronics present a challenge to designers of power systems for critical reliability applications in military, avionics and space programs. The latest high-performance FPGAs, ASICs and processors require increased high performance from the power supply. Typical requirements include low voltages, high currents, tight regulation, fast transient response, and even supply voltage sequencing. These requirements present an even greater challenge to the military designer who does not have access to the latest new components but instead must choose from a limited set of high-reliability power converters and discrete components.

VPT's high-efficiency reliability optimized (HERO) power system includes a set of hybrid dc-dc converters that offer a new solution to the low-voltage problem, and allow the military designer to assemble the smallest, most reliable and most efficient system possible.

The typical 28 V military power bus is governed by MIL-STD-704 for aircraft, MIL-STD-1275 for vehicles, or by specific system or program requirements. The nominal steady-state voltage can vary significantly and the transient voltage can vary from the teens to the forties under disturbances, transients and surges. The simplest way to construct a power system is to convert the 28 V into regulated load voltages — 5 V, 12 V and 15 V for example — and bus the load voltages to different loads or subsystems. This centralized approach has several drawbacks including cumbersome distributed wiring for multiple voltages, poor regulation and transient response at the load, and lack of configurability or easy upgradability.

The natural progression was to distribute the 28 V power bus directly to the load cards or subsystems. Then each load card could have its own, much smaller, dc-dc power converters to convert the unregulated 28 V into specific regulated voltages. These dc-dc converters have since been somewhat standardized, and are readily available from various suppliers including VPT. The distributed 28 V system is smaller, lighter, has better performance, and is more easily configurable than a centralized power system.

The typical high-performance digital board may require several different voltages, 5 V and below. For example, each FPGA or DSP will require one voltage for the I/O circuitry and another to power the processor core. A typical FPGA might require 3.3 V for the I/O and 1.2 V for the core. Multiple FPGAs with different core voltages can result in several voltages ranging from 5 V down to 1.2 V or lower on a single board.

There are several ways to generate the required low voltages. The first, shown in Figure 1, is to use an isolated dc-dc converter for each voltage. The efficiency of isolated 28 V input hybrid dc-dc converters has in the past been mediocre. A typical hybrid dc-dc converter might be 84% efficient for a 12 V output model, 80% efficient for a 5 V output model, and 72% efficient for a 3.3 V output model. Below 3.3 V the efficiency drops rapidly. The overall efficiency of the system shown in Figure 1 is 63.3%. The total power dissipated is 14.6 W. This low efficiency creates a problem for the modern digital board. It results in wasted power, excess heat that must be dissipated, and higher overall operating temperatures. A better solution is needed.

VPT's HERO power system is comprised of two components: a high-efficiency isolated hybrid dc-dc power converter and a smaller hybrid point-of-load converter built to military specifications for hi-rel use.

The first step to an improved high-reliability, low-voltage system is a better isolated dc-dc converter. To meet this need, VPT has introduced the DVHE2800S series of high-efficiency isolated dc-dc converters as components of the HERO power system. A wide input range, 500 V of isolation, and compatibility with VPT's standard EMI filters allow this series to drop into almost any 28 V application. Hermetic hybrid packaging and rated operation over the full military temperature range, -55 °C to +125 °C, meet the strict environmental and reliability requirements dictated by most military and avionics systems.

The DVHE2800S maintains preferred features for easy incorporation into system designs. Patented magnetic feedback circuitry eliminates optocouplers and their accompanying problems. Low input and output noise simplifies board design and eliminates the need for additional capacitors in most applications. Full undervoltage lockout circuitry, overcurrent and short circuit protection are standard. The input voltage range is 16 V to 40 V continuous, with transient capability of 50 V for one second. The package footprint is one inch by two inches and is available with and without mounting flanges. Plus, it offers single low-voltage outputs, each trimmable, at up to 50 W of output power. Using advanced synchronous rectification techniques, the DVHE2800S achieves efficiencies of up to 90% for the 5 V output, 88% for the 3.3 V output, 86% for the 2.5 V output, and 84% for the 1.9 V output. Output transient response is optimized with fast response times for demanding digital loads.

Replacing a standard isolated dc-dc converter with a DVHE2800S series dc-dc converter can reduce power losses by three to four times. The application from Figure 1 can be reconfigured in this manner as shown in Figure 2. The overall efficiency is improved from 63.3% to 85.5%. The total power lost in the power converters is reduced from 14.6 W to 4.3 W. This reduction in power losses will be accompanied by reduced operating temperatures and increased reliability. The total current drawn by this application would be 1.1 A at 28 V input, down from 1.4 A, over a 20% reduction.

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