Latchup 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.
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.
Any switching-mode power supply or other constant-power device operating from a current-limited or high impedance source.
The source can be any current-limited source such as a voltage regulator with current limiting, a solar array (which has inherent current limiting), or a high impedance source, such as a telephone loop carrier system.
The load can be any constant-power load such as a switching mode power supply, etc. that draw more current from the source at lower voltages than at higher voltages in order to maintain constant power. This covers a wide variety of circuits and devices.
The problem can occur in any system with a current-limited source driving a constant-power load. This combination is becoming increasingly common with the trend towards distributed power systems.
Reduction of the load at start-up, or static, dynamic, or adaptive modification of the start-up trajectory or current limit. The following can be used keep the source/load lines from crossing.
- Start-up into no load or a reduced load.
- Prevent operation at low input voltages (hysteresis is usually desirable).
- Extend or remove the current limit during start-up (not applicable to inherently current-limited sources).
- Limit the maximum duty cycle of the switching-mode regulator during start-up.
- Use adaptive control to limit the duty-cycle as a function of input voltage.
- 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.
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.
On the Web
I have yet to find other information on the web about this problem.
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.