Military electronics continue to push the performance envelope in all directions. Each new system design faces the same challenges: the need for more processing power, tighter specs, and shorter development time. Continual advances in system performance often require similar advances in the power system. VME architecture is common in many military applications, as systems can readily be built around standard or custom circuit cards. Off-the-shelf VME power supplies are available, but often don’t meet the necessary requirements or haven’t kept up with recent performance advances. Usually, neither schedule nor budget allow for a full custom power supply development effort.

Figure 1. MIL-STD compliant 28V input power supply.

Fortunately, an optimized VME power supply solution can be built from standard off-the-shelf high-reliability or COTS DC-DC converter modules. This solution can be rapidly developed at minimal cost since most of the design effort is internal to the modules. Input power bus requirements such as MILSTD-704, MIL-STD-1275, RTCA DO-160 Section 16, and DEF STAN 61-5 can be met by combining standard DC-DC converter, EMI filter, and transient protection modules. Control and telemetry functions, secondary filtering, and other special requirements can be implemented with discrete circuitry. This modular approach can be made to fit almost any application, achieving the same end performance as a custom power supply with much lower risk.

For military, avionics and other high reliability applications it is best to choose DC-DC converters from a manufacturer who focuses on these applications. Look for high quality standards such as J-STD-001 and IPCA- 610 class 3, or for the most critical applications, MIL-PRF-38534 Class H or Class K. Products should have a wide temperature range, -55°C to 100°C or more, a wide input voltage range, and rugged mechanical construction. Additionally, the environmental qualification should be to MIL-STD levels.

Figure 2. Input Voltage Conditioning Circuit.

28 VDC Input Requirements

The power requirements for most VME applications are defined by a government or commercial standard. For example, equipment for an aircraft application might have to meet MILSTD-704 or RTCA DO-160, while a vehicle application might need to meet MIL-STD-1275. These documents specify steady state voltage ranges, but also voltage ripple, abnormal conditions, and undervoltage and overvoltage transients. An EMI requirement such as MIL-STD-461 governing the conducted emissions and conducted susceptibility will also need to be met. Many DC-DC converters are available for the 28 VDC power bus, but assembling a robust system design which meets every requirement can still be complex. A typical compliant design includes several DC-DC converters, an EMI filter, and possibly a preconditioning or transient protection module as shown in Figure 1.

In the various standards governing input power bus conditions, there are several areas which cause headaches for power designers. MIL-STD-704 revision A includes an 80V transient, a minimum steady state voltage of 15V, and up to 2V peak of ripple on the input; while in later revisions, the transient is 50V, the minimum steady state voltage is 16V, and the ripple is 1.5V peak. DO- 160 includes transients up to 80V, transients down to 17V, and ripple up to 2V peak. Some systems may be required to operate through an engine starting condition where the input can drop as low as 10V or 12V, and others might be further required to operate through a momentary loss of input.

Figure 3. MIL-STD-1275 transients.

Compliance starts with the DC-DC converter. Its input voltage range should be as wide as possible to cover the input power bus conditions. Typical DC-DC converter input voltage ranges are 16V to 40V, 15V to 50V, or 9V to 60V with transient capability of 50V, 80V or 100V. If the DC-DC converter does not meet the full input requirement standalone, additional circuitry can be used to achieve compliance.

Input Voltage Conditioning Techniques

The two-stage input voltage conditioning circuit shown in Figure 2 accommodates input transients that extend both above and below the operating range of the DC-DC converter. The first stage performs the overvoltage protection function while the second stage boosts low input voltages into range.

The overvoltage transient protection activates when the input voltage exceeds the input range of the DC-DC converter. Short duration, high voltage spikes are first clamped with a transient voltage suppressor device. Longer duration transients, such as the 80V surge in MIL-STD-704A or in DO-160 or the 100V surge in MIL-STD-1275, must be limited with a series device. As illustrated in the figure, this is accomplished with a series pass MOSFET, operated in its linear mode. An N-channel MOSFET is chosen for low on-resistance and high power handling capability. Care should be taken to ensure the MOSFET stays within its safe operating area, as it dissipates high instantaneous power, dropping 50V while passing several amps. A charge pump drives the gate voltage above the input, turning the MOSFET “on” fully for low power loss during normal operation. A Zener clamp on the gate forces the MOSFET to act as a source-follower, or effectively a series pass linear regulator, during an input voltage transient. The output is then safely limited to less than 50V.

Figure 4. AC input power supply constructed with 28V input DC-DC converters.

The “boost” portion of the input voltage conditioning circuit is necessary when operation is required for input voltages below the range of the DC-DC converter. Typical such scenarios include operating through the 6V “initial engagement surge” of MILSTD- 1275 or during engine starting in MIL-STD-704 and DO-160. If continuous operation is not required, the DCDC converter can be allowed to naturally turn off and back on. A synchronous boost topology is utilized, with the output diode replaced by a MOSFET, for lower losses and increased efficiency. During normal operation, the boost remains off, and the high-side MOSFET fully on. This requires a high side drive with 100% duty cycle capability, typically implemented with a charge pump and level shifter for the gate signal. As the input voltage drops, the boost must respond, turning on very quickly, otherwise a dip on its output could cause the downstream DC-DC converter to glitch or turn off momentarily. This requires a fast control loop with a fast mode transition.

Figure 3 gives a worst case envelope of the input voltage transients from MIL-STD-1275D along with the output envelope of the preconditioning circuit. Voltages above the range of the DC-DC converter are limited by the series pass MOSFET, while inputs below the input range are boosted. The resulting output is controlled within the operating range of the DC-DC converter.

Figure 5. VME power supply built from off-the-shelf DC-DC converter modules.

Accommodating Higher Voltage and AC Inputs

High voltage power buses, such as 270 VDC or 115 VAC can be accommodated in several ways. At the card or box level, it’s advantageous to first perform a bulk conversion to 28 VDC, then use individual converters to regulate the different low power outputs. This approach works with both DC and AC inputs, and can often result in a simpler overall design, as it not only minimizes the amount of high voltage wiring, but also takes advantage of the variety of 28V input products available.

A typical aircraft power system design is shown in Figure 4. The input shown is a 3-phase wye connected grounded-neutral system typical of MIL-STD-704 with a nominal voltage of 115V AC and a nominal frequency of 400 Hz. A 3-phase six-diode rectifier and bulk capacitor is used to convert the 3-phase AC to a nominal 270 VDC. A bus converter is used to convert the 270 VDC into a 28V power bus which can then be converted to various lower voltages with standard 28V input DC-DC converters.

The Final Assembly

With proper system design, standard 28V input DC-DC converters can satisfy a variety of input power bus requirements. The DC-DC converters, EMI filters and accessory modules are readily mounted to a PCB and packaged in a conduction or convection cooled housing. Power modules with a temperature rating of -55°C to +100°C can easily meet a final power supply specification of -55°C to +85°C rail temperature. Additional discrete circuitry can be added for enable signals, input and output monitoring, over temperature protection, status LEDs, and even output sequencing and timing. The input and outputs can also be tweaked with additional filtering for ultra-low ripple or with post linear regulators.

The VPTVME-28, shown in Figure 5, is a highly configurable VME power supply built from VPT Series COTS modules. The output voltages and power levels, as well as the number of outputs and I/O signals, are configurable for almost any application.

This article was written by Steve Butler, Director of Advanced Product Development, VPT, Inc., (Blacksburg, VA). For more information, Click Here .