Output Rating; Output Noise - HP 654 A Series Operating Manual

Output Rating
The rated output voltage and current specification in "Performance Specifications" applies at
the output terminals of the power supply. With remote sensing, any voltage dropped in the
load leads causes the supply to increase the voltage at the output terminals so it can maintain
the proper voltage at the load. When you attempt to operate at the full-rated output at
the load, this forces the supply voltage at the output terminals to exceed the supply's rated
output. This will not damage the supply, but may trip the OVP (overvoltage protection)
circuit, which senses the voltage at the output. When operated beyond its rated output, the
supply's performance specifications are not guaranteed, although typical performance may be
good. If the excessive demand on the supply forces it to lose regulation, the Unr annunciator
will indicate that the output is unregulated.
Output Noise
Any noise picked up on the sense leads also appears at the output of the power supply
and may adversely affect the load voltage regulation. Be sure to twist the sense leads to
minimize external noise pickup and route them parallel and close to the load leads. In noisy
environments, it may be necessary to shield the sense leads. Ground the shield only at the
power supply. Do not use the shield as one of the sense conductors.
Note
HP 657xA Series - The signal ground binding post on the rear panel is a
convenient place to ground the sense shield.
Stability
Using remote sensing under unusual combinations of load-lead lengths and large load
capacitances may cause your application to form a low-pass filter that becomes part of the
voltage feedback loop. The extra phase shift created by this filter can degrade the supply's
stability and result in poor transient response. In severe cases, this may cause output
oscillations. To minimize this possibility, keep the load leads as short as possible and tie wrap
them together.
In most cases, following the above guidelines will prevent problems associated with load lead
inductance. This leaves load load-lead resistance and load capacitance as the major source
of reduced stability. Further improvement to the stability of the supply may be obtained by
keeping the load capacitance as small as possible and by decreasing the load-lead resistance by
using larger diameter wires. However, if heavy guage wire (>AWG 10) is used, conditions may
arise where the load-lead inductance and load capacitance can form an undamped filter. This
can actually reduce the damping in the system and create a destabilizing phase response.
Note
If you need help in solving a stability problem with any Series 654xA, 655xA,
or 657xA supply, contact an HP Service Engineer through your local HP Sales
and Support Office.
Series 657xA Network, If a large bypass capacitor is required at the load and the load-lead
length cannot be reduced, then a sense-lead bypass network may be needed to ensure stability
(see Figure 4-5). The voltage rating of the 33 /iF capacitors should be about 50% greater than
the anticipated load-lead drop. Additon of the 20-11 resistors will cause a slight voltage rise at
the remote sensing points. For utmost voltage programming accuracy, the supply should be
recalibrated with the DVM at the remote sensing points (see Appendix A).
4-8 User Connections and Considerations