DC-DC converter generating negative current surges


#1

When designing with modular power conversion devices, be advised that not all such products support pre-biased startup. Stated differently, some devices may be damaged if input power is connected while there is voltage present at the device’s output. This can happen, for example, if a momentary loss of input power occurs during a period of light load and an output filter capacitor does not sufficiently discharge before input power is re-applied.

A Digi-Key customer was bitten by this issue recently, and in this case a primary symptom was that the converter was generating negative current surges nearly equal to the full-rated output of the converter during startup. In effect, the output of the supply was briefly switching polarity, which isn’t something desirable from a DC-output supply.

In this instance the device datasheet made no mention of the possibility of damaging the device by connecting input power while an output capacitor still had a charge; the manufacturer’s explanation was that support for pre-biased startup was considered a “feature” and that the datasheet doesn’t list features that are not included in the product…
The moral of this story is that it is that one should not assume that a modular power conversion device will survive a pre-biased startup. Even though it’s an extremely common function and more or less a prerequisite for survival in general applications, there ARE some parts out there that won’t tolerate it, and they don’t all make the fact clear.

In particular, pay close attention to devices described as using “synchronous rectification,” which is common among high-efficiency, high-output current, and/or low output voltage converters. These devices rely on an actively-controlled transistor instead of a diode to carry current during a portion of their switching cycle, and are much more susceptible to problems of this sort.