Monday, September 20, 2010

Sequencing Multiple Power Supply Outputs

Today’s high performance power supplies continue to add more and more capabilities to make the test engineer’s job easier. The capability that I am going to talk about here is sequencing on or off multiple power outputs. This capability allows you to set the order and timing that each power supply output powers on or off. This is useful for designing and testing embedded system designs. Embedded systems can be made up of any combination of microcontrollers, FPGAs, ASICs and memory chips. These individual integrated circuits often have multiple power input requirements that must be properly sequenced on and off to prevent latch-up. Latch-up may cause a wasteful initial surge of current at turn on, or it may be severe enough to inflict permanent damage to the semiconductor device. Ultimately these devices will have a power distribution system with regulators that will ensure the proper sequencing and timing for each power supply turn on and turn off, but during initial design and testing the power distribution system is often not in place yet so test and measurement equipment is used in its place to simulate the proper turn on and off conditions of the design.
In the past power sequencing was typically done in one of two ways: using programmable power supplies with software or using supplies with custom switches. The first way uses programmable supplies and then in software the supplies are properly sequenced on or off. The drawback to this method was unless you were running a real time operating system (windows is not a real time operating system) there was no way to accurately guarantee the timing from one supply output to the next with better than 30 ms of precision. The other method required additional hardware in the form of switch cards and control circuits. A hardware timed control circuit would provide the precision timing that a computer operating system could not. The control circuit would then control the sequencing of the supply output on or off using switches. If you wanted to avoid switch bounce you had to use solid state or mercury switches versus traditional mechanical switches. The problem with this method is complexity it adds to the testing process.
With the capability built into the supply you avoid the complexity of dealing with multiple pieces of hardware and since the timing is done inside the hardware of the supply it is highly accurate. Agilent’s N6700B series of supplies and the N6705B DC Power Analyzer are examples of supplies with built-in power output on / off sequencing. Each one has up to four power supply outputs per mainframe that can be sequenced on or off. Sequencing from one mainframe to another is also possible for applications that require sequencing of more than four power supply outputs. The figure shows a picture of the N6705B DC Power Analyzer being used to test an embedded design that requires turn on power sequencing.


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