ProblemLatchup of a switching-mode power supply or other constant power device on turn-on from a current-limited or high impedance source. Switching-moderegulators and other constant-power loads operating from current-limited or high impedance sources can have two stable operating points -- one of which is usually undesirable. The result can be latchup at a low input voltage (in the current-limited region), rather than the desired operating point at a higher input voltage.
Figure 1: Turn-On Trajectory With Current Limited Source
The problem is illustrated in the above simplified figure. The turn-on trajectory for the switching-mode power supply is shown in solid orange with directional arrows. For this trajectory, V represents the input voltage and I represents input current of the switching-mode power supply. As shown, the initial part of the trajectory is resistive (a gross simplification which does not affect the qualitative discussion and can be modified to get quantitative results). When the input voltage is sufficient for the feedback loop to start normal switching action, the input trajectory then follows a constant power hyperbola consistent with the power supply output voltage, load current, and efficiency. The output characteristic of the source, in this case a lab supply with an adjustable current limit, is shown in blue with V being the source output voltage and I being the source output current. The solid line indicates the case where the current limit is set high enough so it does not interfere with the normal turn-on trajectory. The green circle shows the normal operating point of the combination source/load. The latchup problem occurs when the current limit is reduced closer to the expected maximum steady-state operating current - which is where you normally set it. This is indicated by the dashed blue line. The turn-on trajectory in this case is from the origin to the yellow circle, an unstable operating point. The trajectory then moves up the current limit curve to the red circle, which is a stable but undesired operating point. The system has "latched up" with the source providing less than the desired input voltage, and the load drawing more than the normal input current. The system never reaches the desired operating point shown in green. Do not use this information for design without independent verification of the information. Editor: Jerrold Foutz |
