Understanding Power Supplies in a VXIbus Mainframe Introduction This paper describes the topics necessary to understand the requirement of a VXIbus Mainframe to meet the power demand of the system it houses. In the past, engineers designed rack and stack systems with little concern for the total system power demand. After all, the power was either coming from an unlimited AC supply or from a power conditioner that was chosen to meet the total power demand after selection of all the instruments. In a VXIbus system the power supply is an integral part of the enclosure housing. One of the advantages of the VXIbus is to share resources such as power. Why build power supplies onto each instrument when, through careful design, the necessary DC voltages can be provided from one common source? This idea is not new. In fact, it is used very effectively in many card modular systems such as computers, and VME systems. The problems associated with instrumentation are, however, a little more complex. For this reason tight controls are kept on the specifications relating to VXIbus power supplies so as to minimize the effect from one module to another. Voltages When VME was chosen as the foundation for an Instrumentation Extension, a number of issues had to be resolved. Pins on the outer rows of P2 were undefined and VME voltages were not ideal for instrumentation applications. VME provides for +5V and 12V levels. To provide a suitable voltage for powering analog signal sources and to derive other voltage levels (e.g. +/-15V DC), a +/-24V supply is provided. The -5.2V level is used for ECL devices and the -2V is used for termination of ECL loads. The VXIbus provides the following voltages: +5V, +/-12V, +/-24V, -5.2V, -2V and +5V Standby. Some of the undefined pins from P2 are used for instrument specific enhancements while the remainder allow for extra voltage levels. Number of Connector Pins per Module In conformance with the VME specs, the connectors for the backplane are of the high-density DIN type. Each pin on this connector is specified to carry 1 amp. This determines the maximum power delivered to a single card (Table 1). This large amount of power is hardly needed. Most modules require less than 50 watts resulting in a total demand for a mainframe of 650 watts (Table 2). However the mixture of voltages for this 650 watts will vary between applications. A digital system may need much more +5V than any other voltage. The system designers must pay attention to this when selecting a VXIbus mainframe. A table of the power demands of each of the modules to be used in the system should be developed so as to verify power requirements before purchasing a mainframe. Static Current / Dynamic Current The VXIbus specification takes a lot of care in ensuring that modules from many different manufacturers work together. Modules must handle the large number of variables such as cooling, power and EMC, once it is in a system. The VXIbus specification has placed limits on power parameters so that users and module manufacturers know what to expect (Table 3). This does not restrict a mainframe manufacturer from providing better than minimal performance. The amount of ripple and noise generated from a power supply varies with the load drawn and with the type of power supply. For example, with no load, a switch mode supply is normally more noisy than when loaded. However, as soon as a load is applied, the noise improves dramatically until a peak current is reached, at which point the noise increases once again. (Table 1) Available Module Current and Power VOLTAGE +5V +12V -12V +24V -24V -5.2V -2V MAXIMUM CURRENT* 7 1 1 1 1 5 2 TOTAL POWER (137 Watts) 35 12 12 24 24 26 4 * based on number of 1 amp pins To provide this power to all 13 slots would need a power supply capable of delivering 1781 watts. (See Table 2) (Table 2) Mainframe Current and Power VOLTAGE +5 +12 -12 +24 -24 -5.2 -2 Imp 91 13 13 13 13 65 26 POWER (1781 watts) 455 156 156 312 312 338 52 (Table 3) Power Supply Voltage Specifications VOLTAGE +5V +12V -12V +24V -24V -5.2V -2V +5V Stdby ALLOWED VARIATION +0.25V/ -0.125V +0.60V/ -0.36V -0.60V/ +0.36V +1.20V /-.72 -1.20V /+.72 -0.260V /+.156V -0.10V/ +0.10V +0.25V/ -0.125V DC LOAD RIPPLE/NOISE 50mV 50mV 50mV 150mV 150mV 50mV 50mV 50mV INDUCED RIPPLE/NOISE 50mV 50mV 50mV 150mV 150mV 50mV 50mV 50mV The VXIbus specification for DC load ripple/noise ensures that each manufacturer specifies -- using the same terminology -- the upper limit for the amount of current for each voltage. The power supply might supply more current but at the cost of a degraded ripple/noise performance. This degradation may not cause a problem for low performance instruments. Total Power Versus Usable Power The total power for the mainframe equals the sum of all static power levels. However, this can be misleading. Power supplies can handle high current on one voltage but may not be able to handle high current from all voltages at the same time. Switched mode supplies often use a power sharing technique. For this reason manufacturers will specify usable power as well as total power. The total power specification really does not help a system integrator. Current The VXIbus specification defines static current as Mainframe Peak Current (Imp). This current is established by loading each voltage until the noise and ripple specifications of the VXIbus are reached. Tests include frequency up to 20Hz hence, the term Static. Dynamic Current Current capacity from 20Hz to 1GHz defines the dynamic current (Imd) of a chassis. This demonstrates the ability of the mainframe to handle time varying currents demanded by such modules as arbitrary waveform generators or digital modules. Peak demand on the power supplies can affect the ripple and noise performance making these specifications important. To test a mainframe for dynamic current capability load the power supply to one-half the rated Imp. Monitor power supply voltages while varying the load. System Needs Both the Imp and Imd specifications are necessary to ensure that a module manufacturer can account for all the possible noise and ripple when a module is installed in a system. The effect of not meeting the specifications may not be a problem in some systems. But demanding applications need the assurance that these specifications can be met. Linear Versus Switch Mode Efficiency, weight and cost drive mainframe design. However, if the ultimate in low noise performance is needed, then these are ignored. A linear supply provides ripple and noise specifications much lower than required by the VXIbus specification. RF applications often need this level of performance. Conclusion VXIbus mainframes consist of more than some sheet metal, a backplane, fans and power supplies. Knowledge of the issues affecting high performance instrumentation is critical. The power supplies, cabling and backplane all affect each other and must be engineered as a system. TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13