Switching Power Supply Current-Limit Latchup(4 of 6)

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