Friday, February 15, 2013

Addressing the challenge of sequencing multiple bias supplies on and off


A challenge test engineers are perennially faced with is how to best sequence the bias voltages powering their DUT, when their DUT requires several bias voltages. Many DUTs are sensitive to sequencing and an improper sequence may lead to the DUT hanging up, or worse, suffer damage as a consequence. Not only is sequencing an issue when powering the DUT on, but it can also be an issue when powering the DUT down as well. In addition to sequencing, the slew rates of the various bias voltages can likewise be important to the DUT correctly powering on.

Simply relying on the timing of output-on and output-off commands sent from the test system controller to all the system DC power supplies individually tends to be far too imprecise, especially for critical sequencing timing requirements. The actual turn-on time of a typical system DC power supply can be many tens of milliseconds, and will vary considerably between different models of power supplies. The turn-on and turn-off times of each will need to be carefully characterized in order to know when a command for a specific bias voltage needs to be sent in relation to the other bias voltages. It is very likely the sequence of commands sent for outputs to turn on or off may be in a different sequence to the outputs actually changing, due to delay differences between different DC power supplies! An even bigger problem however is most system controllers are PCs which may randomly experience a large delay in sending out a command, if a higher level service request interrupts and pre-empts execution of the test program.

An alternative approach often taken is adding some custom hardware to control output sequencing. This can assure correct sequencing, but adds a lot of complexity, is usually inflexible, and may introduce other issues and compromises.

At Agilent we added system features to our N6700 series multiple output modular DC power system that support correct power-on and power-off sequencing. The output-on and output-off controls for the individual outputs get grouped together. The N6700 platform knows and compensates for the actual delays of all the various DC power output modules so that the desired delay value entered will be what is accurately achieved. Figure 1 shows setting up an N6705A to achieve a desired turn-on sequence of DC outputs for powering up a PC mother board. Figure 2 shows the actual result. A more detailed description of this PC motherboard example is given in our application note: “Biasing Multiple Input Voltage Devices in R&D”. While the N6705B DC Power Analyzer mainframe is regarded as being primarily for R&D, which this app note is referencing, the low profile rack-mountable N6700 series mainframes have these very same features and suit automated test systems in manufacturing and other environments.



Figure 1: Setting Output Delays



Figure 2: Output Turn-on Sequence Results


Just like setting up the power-on sequence, separate delays for power-off can also be entered, as seen in the set up screen shown in Figure 1, for the expected shut down of the DUT. However, what if there is an emergency shut down due to an abnormal condition and you still want to assure a certain power-off sequence? A colleague worked out the procedure for setting up the N6700 series DC power system to provide an orderly shutdown of the outputs, in the event of a problem on one of the outputs. In this example it happens to be an overvoltage condition on one of the outputs, but any of a number of fault conditions can be acted on to initiate an orderly shutdown. Details of this procedure are provided in another application note; “Avoid DUT Damage by Sequencing Multiple Power Inputs Off Upon a Fault Event”.

So when faced with the challenge of having to properly sequence multiple DC bias voltages powering your DUT, reconsider trying to engineer a solution to accomplish this. Instead, look for features that provide this kind of capability in the system DC power supplies you are looking to use, already built-in. It makes a lot of sense having sequencing built into the power supplies and it will make your life a lot easier!

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