Power Supply Standard Eases VPX System Design Efforts

By: David W. Lee, Curtiss-Wright Controls Electronic Systems

Ruggedized military electronic systems require reliable power supplies. Standardized VITA 62 VPX power supplies ease VITA 46 system designs while facilitating the overall maintenance and support concept of VPX.

The big push toward off-the-shelf subsystems due to budget and schedule constraints has led to the development of a new VITA standard. The VITA 62 Power Supply Standard defines form factor and pin out for VITA 46 VPX systems going forward. Figure 1 shows an implementation of VITA 62 with the optional 2-Level Maintenance ESD cover. The latest draft was released in January at the last VITA meeting, with the first formal release some time in the second quarter of 2011. Key technical contributors include suppliers in embedded electronics such as General Electric, North Atlantic Industries and Curtiss-Wright.

Historically, the development of military-grade power supplies required specialized knowledge. Development can be lengthy and expensive. Therefore, many system integrators today simply reuse existing designs or purchase them off the shelf to meet time-to-market requirements. Next generation smart power supplies are growing in popularity, thanks to their versatility and their ability to fulfill system level requirements. Smart power supplies may communicate via VITA 46.11 Intelligent Platform Management Bus (IPMB).

In addition, due to logistics concerns, the VPX community has longed for standardized power supplies that are interchangeable from different vendors to expand the supplier base. With these in mind, the VITA 62 working group was established in September 2009 to create a new power supply standard from the ground up that will directly support these market trends.

Importance of VITA 62

The primary focus of VITA 62 is to provide an electrical and mechanical specification that is compatible with VITA 46 and other specifications in the VPX suite. In the past, a power supply’s form factor and pin assignment varied from vendor to vendor because of the lack of standardization. Furthermore, due to the nature of the power supply, many were custom designed for the chassis that they reside in. PICMG 2.11 Positronics connectors and DIN 41612 connectors are commonly used for power supplies in the embedded environment; however, there is no standard pin out for DIN 41612 and thus the modules from different vendor are not interchangeable. PICMG 2.11 defines a pin out but is more geared toward CompactPCI systems and lacks many of the advanced features of a VPX system (Figure 2).

The lack of a pin out standard, as most electronic system integrators already realized, would practically lock the system design to a particular power supply vendor, which is highly undesirable from a logistics and costs standpoint. Instead, a nonproprietary mechanical and electrical standard opens up the opportunity for a wider supplier base, mitigates long-term supplier risks (e.g. product lifecycle issues and sole-source supplies), and reduces the overall program costs.

According to Bruce Thomas, a member of the VITA 62 committee, the number one reason to use items designed to this standard is logistics. VITA 62 is an open standard, meaning that nothing at the interface is proprietary. “There are many technical reasons for VITA 62. But the main driver is that system designers can purchase these modules from multiple COTS vendors to control costs,” says Thomas.

VITA 62 Overview

In order for VITA 62-compliant power supplies to work in a VPX environment, the standard needs to be compatible to both VITA 46 and VITA 48, which most of the existing COTS VPX modules are designed to. VITA 46.11, System Management, is also directly supported. Figure 3 shows a simplified interconnect diagram of a VITA 62 3U power supply.

The 3U and 6U connectors are designed to support high mate/de-mate cycles, which is important to support field maintenance. The VITA 62 connectors also feature high current carrying pins (40A and 20A) for the primary input voltage (DC/AC) and secondary voltages VS1, VS2 and VS3. A number of signal pins fulfill all other digital and analog interfacing needs. There are also plenty of User-Defined (UDx) pins for application-specific purposes.

VS1 is typically used for higher voltages, like 12V and 48V. VS3 is 5V. The definition of VS2 is slightly different between 3U and 6U. In a 3U system, VS2 provides 3.3V. However, on the 6U, the VS can either be used as a 12V output or a 48V return. The 3.3V auxiliary voltage is mandatory to support VITA 46.11 operations. +12/-12V auxiliary voltages are optional in case they are needed to support low-power legacy hardware, like analog I/O PMCs.

ENABLE and INHIBIT lines provide on/off controls on the primary and secondary side, respectively. SYS_RESET, Current Sharing (VSx_SHARE), Remote Sense (VSx_SENSE), System Management (SMx) and Geographical Addressing (GAx) are also supported but not required. The FAIL discrete reports any failure. On the 6U, a Filtered Output is also provided for external holdup circuitry if needed.

In a large system where more than one power supply is required, current sharing pins can be used to balance the power outputs between two or more similar modules. To support a system-wide nuclear event circumvention capability, an optional NED (Nuclear Event Detector) output provides a means to notify other modules in the system that a nuclear event is occurring and circumvention mechanisms should be activated.

Support for Onboard Intelligence

Smart power supplies go a step further beyond simply providing system voltages. They provide system integrators with tools that are tightly coupled with the functionality of the power supplies, all within the modules themselves. Built-in features such as self tests, current/voltage/temperature monitoring, failure reporting, controlled power-up and power-down sequencing and nuclear event circumvention can be modularized to reduce the system development effort. The VITA 62 standard specifically supports future growth in this area.

Conventional power supplies, on the other hand, only perform basic power conversion functions and would require external circuitry to support any control and monitoring functions, which take up valuable real estate elsewhere in the system as well as increase system integration time. With a programmable microprocessor, a smart power supply enables flexible and rapid development that scales to the functional needs of many electronic systems.

To support this trend, the draft VITA 62 standard defines a modular packaging standard for 6U and 3U VPX power supplies, making true COTS solutions possible. While the basic functions of power supplies remain the same, a modular design that conforms to standard pin-out and form factor allows system designers to quickly connect the dots and build the system with confidence.

Self-Contained Smart Power Supply

A self-contained smart power supply alleviates the need to put the power monitoring circuits, which require custom I/O and board area, elsewhere in the system. This allows the entire system to be designed using true embedded off-the-shelf building blocks—such as SBCs, network switches, I/O modules and so on.

To communicate with the host processor in a VITA 46.11-compliant systems, I2C-based IPMB can be used. For example, the microcontroller (IPMC) on board a power supply can continuously monitor the functionality of the module, and, in the event of a power failure, the host computer will be notified of the fault and allow it to react appropriately depending on the application. This is particularly useful in a large system. 

Internal to the module itself, Power Management Bus (PMBus) may also be used to communicate with now increasingly popular PMBus-enabled components, like power monitors, DC-to-DC converters, trims and power distribution modules. PMBus is an open standard drafted in 2004 by a coalition of industrial partners, including Texas Instruments. The PMBus forum became part of System Management Interface Forum, Inc. and is endorsed by the Point-of-Load Alliance (POLA) and the Distributed-power Open Standards Alliance (DOSA). To ensure interoperability and simplicity, PMBus is based on System Management Bus (SMB) protocol over an I2C physical layer, similar to IPMB in concept. The adoption of PMBus would further simplify power supply designs and free up board area for other features.

Power: Not an Afterthought

A power supply is critical to the integrity of any ruggedized electronic system. In its simplest form, a power supply provides functions such as voltage regulation, surge protection and EMI filtering, all of which are to ensure that the electronics that it supports function correctly and to protect them from disturbances from the input power.

VITA 62 includes provisions for advanced features such as system management, current sharing, NED output and on/off controls. These new power supplies will allow system designers to concentrate on other aspects of their electronic systems, achieving quicker time-to-market. A VITA 62-compliant power supply not only simplifies system level designs, it also significantly reduces the lifecycle costs of the entire system. With custom and standard engineered system solutions, a modular approach means that you can optimize and scale your power solutions for your specific needs.  


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