Problem

The solar-array power system lockups in the battery-discharge mode when there is sufficient energy to charge the battery.

The problem occurs in a Direct Energy Transfer (DET) system consisting of a solar array directly powering the system voltage bus. System voltage regulation is provided by a shunt regulator working against the solar-array impedance. A battery provides power when there is insufficient energy from the solar array and a battery charger charges the battery when the solar-array output exceeds that required by the load.

On the Web

I have yet to find other information on the web about this problem.

References

For design, measurement, and analysis, two papers are recommended. The first, "Start-Up Transient of a DC-to-DC Converter Powered by a Current-Limited Source" discusses the problem, analysis and measurement techniques, along with solutions. The second "An Adaptive Start-Up Control Law for DC-to-DC Converters Powered From a Current- Limited Source" revisits the problem, including the shortcomings of previous solutions, and proposes an adaptive start-up approach. Other papers can be found in the timeline of key papers and in the bibliography.

Do not use this information for design without independent verification of the information.

Editor: Jerrold Foutz

Personal Anecdote

When trouble-shooting a system or circuit in the lab, it is natural to set the current limit of lab supply to just over the expected steady state current. It is not always observed that latchup has occurred, especially if the latched voltage is near the desired voltage. Circuits and systems often exhibit strange behavior in the latched mode. When asked into the lab to observe a problem with a switching-mode power supply, the voltage source and current limit setting is one of the first things I check. It often is the problem. As project lead, manager, and consultant, this has given me many opportunities explain the problem to those unfamiliar with it.

Do not use this information for design without independent verification of the information.

Editor: Jerrold Foutz

Solution

• Start-up into no load or a reduced load. The dotted orange lines in Figure 1 show how reducing the load (less input power) moves the power hyperbola in so that it clears the source V-I characteristics. Load shedding is often used in space-craft solar-array powered systems to shift from the undesired stable point to the desired stable point if latchup occurs.
• Prevent operation at low input voltages. Inhibiting switching-action until a preset minimum voltage is reached shifts the "resistive" part of the turn-on trajectory in Figure 1 up to help clear the source V-I characteristic. There is also another compelling reason (future topic) to do this. Hysteresis should be added so the supply turns on at a higher voltage than it turns off. This prevents on/off oscillations for slow turn-on or when the input voltage remains near the turn-on point.
• Extend or remove the current limit during start-up. This solution is not available for inherently current-limited sources such as solar arrays but is a common solution for systems whose source characteristics are controlled by feedback loops. In Figure 1 this is illustrated by starting with current limit set at the solid blue line, letting the desired operating point be reached (green circle), and then pulling the current limit into the desired steady-state current limit(dotted blue line). The disadvantage is that the system is unprotected at start-up and a timing race is possible.
• Limit the maximum duty cycle. This has the effect of increasing the slope of the "resistive" portion of the start-up trajectory. If done dynamically a timing race is possible and design tolerances may be a problem. This can be mitigated by adaptively modifying maximum duty cycle as a function of input voltage.

Do not use this information for design without independent verification of the information.

Editor: Jerrold Foutz