The remote sense lines are a vital part of any good
system power supply. As shown in Figure 1, by using a second, separate pair of leads
for sensing, the output voltage is now regulated right at the DUT rather than
at the output terminals on the power supply. Any voltage drops in the force
leads are compensated for; assuring the highest possible voltage accuracy is
achieved right at the DUT.
Figure 1: Remotely sensing and regulating output voltage
at the DUT
Of course for this to work correctly the sense leads need
to have a good connection at the DUT. However, what if the sense leads become
disconnected, misconnected, or shorted?
One might think if one or both of the sense leads became
disconnected, the sensed voltage would then become zero, causing the output
voltage on the force leads to climb up out of control until the over voltage
protect (OVP) trips. This turns out not to be the case, as a co-contributor
here, Gary had pointed out in a previous posting “What happens if remote sense
leads open?” (Click here to review). Basically a passive protection mechanism
called sense protect maintains a backup connection between the sense line and
corresponding output terminal inside the power supply in the event of a sense
line becoming disconnected.
While sense protect is an indispensable feature to help
protect your DUT by preventing runaway over-voltage, if a sense lead is open
the voltage at your DUT is still not as accurate as it should be due to
uncompensated voltage drops in the force leads. This can lead to miscalibrated
DUTs and you would not even know that it is happening. To address this some
system power supplies include an active open sense lead fault detection system.
As one example our 663xx Mobile Communications DC Sources check the sense lead
connections during each output enable and will issue a fault protect and shut
down the output if one or both sense leads become disconnected. It will also
let you know which of the sense leads are disconnected. It can be enabled and
disabled as needed. I had written about this in a previous posting “Open sense
lead detection, additional protection for remote voltage sensing” (Click here to review).
Taking sense protection further, we have incorporated a
system we refer to as sense fault detect (SFD) in our N6900A and N7900A
Advanced Power System (APS). It can be enabled or disabled. When enabled it
continually monitors the sense lead connections at all times. If it detects a
sense fault it sets a corresponding bit in the questionable status group
register as well as turn on status annunciator on the front panel to alert the
user, but does not disable the output. Through the expression signal routing
system a “smart trigger” can be configured as shown in Figure 2 to provide a
protect shutdown on the event of a sense fault detection. In all, sense fault detect on APS provides a
higher level of protection and flexibility.
Figure 2: Configuring a custom opens sense fault protect
on the N6900/N7900 APS
What happens if the sense leads become shorted? Unlike
open sense leads, in this case the output voltage can rise uncontrolled. The
safeguard for this relies on the over voltage protect system. The same thing
happens if the sense leads are reversed. The power supply will think the output
voltage is too low and keep increasing the output voltage in an attempt to
correct it. Again the safeguard for this relies on the over voltage protect
system. The N6900/N7900 APS does actually distinguish the difference when the
sense leads are reversed by generating a negative OVP (OV-) fault, giving the
user more insight on what the fault is to better help in rectifying the
problem.
Remote voltage sensing provides a great benefit by being
able to accurately control the voltage right at the DUT. Along with the
appropriate safeguards against sense lead misconnections you get all the
benefit without any of the corresponding risks!