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Fig. 1
This screen shot shows measurement of an AC/DC switching power
supply turn-on after the front-panel switch was pressed.
Fig. 2
This screen shot shows measurement of an AC/DC switching power
supply turn-on after the front-panel switch was pressed.
Dealing with synchronization and
time uncertainty requires care
Aggregating data to develop a
system timing diagram is possible,
but time-consuming
Complexity increases with the
number of power rails to be
observed
Setups must be perfectly consistent
One measurement channel must be
used to provide synchronization
Using an MSO to extend
channel count
A mixed signal oscilloscope can
provide additional channels for
power supply sequencing. For
this to work, the MSO must have
suitable voltage range on the digital
inputs and independently adjustable
thresholds. For example, a Tektronix
MDO4000C with the MSO option
offers 16 digital inputs with
independent thresholds for each
channel and a ± 30 Vp-p dynamic
range up to 200 MHz, making it
suitable for most of the voltage
levels you’d find in a typical design.
Note that this approach works well
if the objective is strictly to measure
timing relationships, but it does not
allow for measurement of rise/fall
times or the shape (monotonicity) of
the power-on/off.
8-channel scopes speed
the process
Using an oscilloscope with 8 analog
channels cuts the time and hassle
significantly versus any of themethods
covered so far. With an 8-channel
oscilloscope, power supplies with
up to eight power rails can be
characterized using analog probes. To
measure turn-on and turn-off timing
relationships on power supplies with
more than eight power rails, a mixed
signal oscilloscope with digital signal
inputs and independently adjustable
thresholds can also be used.
Now, let’s look at some typical
power sequencing measurement
applications.
Turn-on delay with
remote on/off
The switching power supply under
test in the screen shot below produces
a high-current, regulated 12 VDC
output. This power supply is remotely
controlled with a switch on the front
panel of the instrument. Shortly
after the switch is pressed, the +5 V
standby voltage is turned on, enabling
the switching converter to start. After
the +12 V output is in regulation, the
Power Good (PW OK) signal goes high
to indicate to the load that the supply
is reliable.
The +5 V standby voltage signal
provides a simple rising edge trigger
for the acquisition of the relevant
signals. Automatic measurements
verify that the delay to the output
voltage turn-on is <100 ms, and the
delay from output voltage turn-on to
PW OK is in the specification range of
100 - 500 ms.
New-Tech Magazine Europe l 33