Low-voltage high-current loads requiring precision voltage regulation are served by power systems that sense voltage remotely at the load. This creates the risk of an overvoltage (OV) condition at the load if the remote sense contacts are incorrectly connected. A new technique to supplement standard OV protection addresses this risk.
Previous supplemental protection does not protect low-voltage high-current loads. One must program a supplemental OV threshold high enough at the output terminals to accommodate the expected Ohms-law (IR) voltage drop in the cable, which results in a threshold value that exceeds the maximum voltage the load can safely tolerate. Load voltage can rise to damaging levels before protection acts. As one example, a load is not protected if remote sense contacts are shorted together.
A new supplemental method to protect the load acts by monitoring voltage drop in the output cable. A measurement circuit subtracts the remote sense voltage from the power system's output to determine a value for cable voltage drop. The power system is shut down if the measured drop violates a safe-operating area (SOA). The SOA is determined adaptively by measuring output current, and scaling it to represent the allowed IR voltage drop in the cable. For scaling to be effective, the output cable provided with the power system must have impedance characteristics that are defined in the manufacturer's specification.
When shorting the sense contacts together to produce a fault, the supplemental circuit sees a remote sense voltage measurement of zero volts. The circuit's measurement value for cable voltage drop now equals the output terminal voltage. The power supply shuts down when this value exceeds the cable SOA by more than the allowed design margin, which is well within a safe limit for the low-voltage load. When open-circuiting either one or both sense leads, the condition presents itself to the circuit in the same way as shorting the sense leads together. This is because a large-value resistor shunts these leads at the circuit interface. Protection is identical to the case with shorted sense contacts. Within the specified common-mode voltage range, this protection is effective against conditions of open, shorted, or polarity-reversed remote sense leads. The protection threshold is always below levels that would damage the load, since the maximum allowed cable IR drop is below the load-damaging level by design.
To avoid generating fault signals due to inductive voltage drops on the cable, the supplemental circuit activates an output shutdown only if an SOA fault persists for more than one millisecond. This is simpler than adjusting the SOA to include an inductance characteristic. It also has the benefit of improving noise immunity. One disadvantage is that it creates a dependency on the performance of the output's slew rate. System performance must limit the voltage increase that can occur within the one-millisecond delay. Fortunately, for loads that typify the system's output filter capacitance in parallel with load resistance, the power system slews in current mode as it powers up, and the slew rate decreases as load voltage increases. Slewing near SOA limits is slow enough not to present a problem during the delay time.
Early testing was done on several variations of the supplemental protection circuit to study the effectiveness in two system configurations. The configuration employed short cables with low impedance, terminated using load resistance in parallel with large filter capacitance. The second configuration substituted long cables, in conformance with system design specifications. Resistors were sized to accept full-rated supply current at program voltage. The power supply was programmed to regulate program voltage at the load; then output was enabled. Testing was repeated for all nine anticipated cases of misconnected sense lead termination, and maximum transient voltages seen at the load were recorded. In-regulation testing was performed only on the second configuration by removing either sense lead from the energized load. The second configuration was also tested using higher-value load resistances.
Results showed that the supplemental protection circuit worked within its expected parameters in every case tested. Voltage at the load during any startup test never exceeded 110 percent of the cable SOA maximum limit with improperly terminated sense contacts, and the power system shut down as expected. During the second test, when either sense contact was opened, load voltage overshot only minimally, then decreased to zero. All tests with improperly terminated or opened sense leads resulted in protection of the load from damaging transient voltages, and a prompt shutdown of the power supply output with the appropriate fault flag being indicated.
This solution is a good fit for low-voltage high-current load applications. It is also a good candidate to use in place of the earlier protection method for applications where maximum cable impedance can be controlled. Since the new method does not depend on programmed overvoltage thresholds, there is less opportunity for user error to defeat the supplemental protection. Effective OV load protection is delivered against what is perceived as an important fault condition: misconnection of the remote sense leads.
This supplemental overvoltage protection scheme is used on selected Sorensen programmable DC power supplies.
For more information, contact Don Novotny, marketing manager for Elgar/Sorensen Co., 9250 Brown Deer Road, San Diego, CA 92121; (619) 450-0085; (800) 525-2024; fax: (619) 458-0237. The author of this brief is Ray Maroon of Elgar/Sorensen.