The second method commonly used to develop several low voltages, shown in Figure 3, is to use post regulators. One isolated dc-dc converter is used, typically the highest voltage needed, 3.3 V, 5 V or 12 V. The lower voltages are then down regulated from the main voltage. The post regulator can be a linear regulator or a non-isolated buck converter. This technique is especially attractive when several different voltages are needed. The post regulators can be placed directly at the load for the best performance.

A linear regulator can be used to develop a lower voltage from a higher one. The power dissipated in the linear regulator is equal to the current multiplied by the voltage drop across it. For example, the linear regulator supplying 3.3 V at 1 A from 5 V is 66% efficient and will dissipate 1.7 W. The linear regulator supplying 1.2 V from 5 V is only 24% efficient. For this reason, linear regulators are generally only recommended for low currents, typically 1 A or less.

The non-isolated synchronous buck converter schematic is shown in Figure 4. This is a switching circuit that can be used to develop a low voltage from a higher one. This circuit can achieve much higher efficiency than the linear regulator. For the military or hi-rel designer, it is not a simple drop-in circuit like the linear regulator. The synchronous buck must be built from discrete components: PWM controller, MOSFETs, capacitors, power inductor, etc. These components are not usually rated for the military temperature range or widely available in hermetic packages. So a compromise must be made and reliability must be sacrificed for performance. The risk and design effort is increased, since the buck converter must be redesigned or at least the printed circuit implementation must be redesigned for every new digital card that requires it.

To eliminate the compromise and allow the most efficient and most reliable system possible, VPT has introduced the DVPL0505S “point-of-load” dc-dc converter. The DVPL is a high-efficiency non-isolated synchronous buck converter, built with hermetic hybrid technology and rated over the full military temperature range, -55 °C to +125 °C. Only 1” × 0.8” in size, the DVPL can be located as close as possible to the load for the best performance. The DVPL operates from a fixed 5 V or 3.3 V input and with one external resistor, outputs 5 A at any voltage from 3.3 V down to 0.8 V. The efficiency is typically 90% to 97%, and is optimized over a load range of 1 A to 5 A.

With the HERO power system's new set of high-reliability, high-efficiency dc-dc converters, the system in Figure 3 can be improved as shown in Figure 5 by dropping in the DVHE and the DVPL as the post regulator. The design task is simplified, with a few hybrid blocks replacing many discrete components. The reliability is increased, with the entire power circuitry being hermetically sealed and rated for the full military temperature range. The bill of material is reduced, with only two part types required to generate three voltages.

The next progression is to realize that only one isolated converter may be needed for several digital boards. Instead of wiring the unregulated 28 V to every board, and then adding the required EMI filter and dc-dc converter on each board, it could be simpler to isolate only once. One EMI filter, the DVMC28 or DVMD28, and one isolated converter, the DVHE2805S, could generate a 5 V regulated bus with 90% efficiency. The 5 V could be bused to the various digital boards, where point-of-load converters would convert to 3.3 V and below for FPGAs, etc. The final conversion would take place directly at the load for best performance. This type of optimized system is shown in Figure 6.

The distributed low-voltage bus is often chosen to be 5 V or 12 V. Both are frequently used, but in the past, 12 V has been chosen for two reasons: the isolated converter has been more efficient with 12 V output than with 5 V output, and distribution losses can be less with 12 V. However, the availability of the DVHE2800S dc-dc converter with 90% efficiency at 5 V output tips the scales back in favor of the 5 V bus. It is more efficient than any currently available 12 V output hybrid dc-dc converter. The distribution losses can still potentially be lower for a 12 V system, but the distances involved are usually small and so they tend to be a small portion of the total losses and not a driving factor. The synchronous buck converter will also operate more efficiently at 5 V input, since it will have a better conversion ratio than at 12 V input. With both ends of the power conversion being more efficient, 5 V is the recommended low-voltage bus.

The circuit in Figure 6 is significantly more efficient than that of Figure 5. This is counter-intuitive, since an additional regulator was added to the 3.3 V power path. The first reason is that the DVHE isolated converter is more efficient at 5 V than at 3.3 V. The second is that the DVPL non-isolated converter is also more efficient at 5 V input, due to a better internal gate drive voltage.

Complex FPGAs and processors often add additional strict requirements on the supply voltages. Often one voltage may not exceed another, or two voltages may not differ from each other by more than a certain amount. These requirements are often met by tying voltages together with Schottky diodes, or by controlling the turn on and turn off sequence of the various voltages, often referred to as power sequencing. Power sequencing is difficult with isolated converters, due to long turn on delay times and primary referenced enable signals.

To greatly simplify this task, the DVPL0505S converter includes a TRACK input. The TRACK input can be used to accurately sequence the turn on and turn off of several converters. The TRACK input can also be used to implement voltage tracking, whereby several voltages follow the same startup curve until they reach their respective regulation points.

The lack of high-reliability, high-efficiency dc-dc converters has long been a stumbling block for the military power designer, causing increased design effort and compromises between reliability and performance. Comprised of the DVHE series of high-efficiency isolated dc-dc converters and the DVPL high-efficiency point-of-load converter, the HERO power system eliminates this impediment. Overall system efficiency is greatly improved while maintaining the highest reliability and simplicity. The elements of the HERO power system can be arranged and configured to meet almost any low-voltage power need with optimum performance.

Steve Butler leads VPT's engineering team as the vice president of engineering. Previously, he held positions as a lead engineer for AT&T Bell Labs/Lucent Technologies and as a researcher for the Virginia Power Electronics Center (VPEC) at Virginia Tech, where he researched spacecraft power converters and power systems. Butler is a patent recipient, frequently authors technical papers, and serves as a principal investigator for Small Business Innovative Research (SBIR) grants. He holds BSEE and MSEE degrees from Virginia Tech.

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