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!
