DC-DC converters and highly capacitive loads

January 20, 2014 // By Dave Berry, Vicor
Capacitance at the input of the DC-DC converter plays a vital role in keeping the converter stable and playing a role in input EMI filtering. Large amounts of capacitance at the output of the DC-DC converter can provide significant challenges in the power system. A few simple techniques can be implemented within the power system to maintain an efficient and reliable design when powering highly capacitive loads.

Many loads downstream of the DC-DC converter need capacitance for proper operation. These loads can be pulsed power amplifiers or other converters that need capacitance at their inputs. If the capacitance at the load exceeds the value that the DC power system is designed to handle, the power system can exceed its maximum current rating at startup and during normal operation. The capacitance can also cause power system stability issues and lead to improper system operation and premature power system failure.

A few simple techniques can be implemented within the power system to maintain an efficient and reliable design when powering highly capacitive loads. Reducing the voltage rise time across the load capacitor at start up will keep the power system within its current rating, controlling the charge current into the capacitor during operation will keep the power system within its power rating and adjusting the control loop of the system will keep the power system stable and within system voltage ratings.

Start up Considerations

At start up, the typical DC-DC converter has a standard rise time set by the rise of an internal error amplifier reference. A discharged capacitor placed at the output of the converter will appear as a low impedance load. With this low output impedance, a few switching cycles of the converter can cause a change in voltage across the capacitor high enough to force the converter to exceed its output current rating. The capacitor can be pre-charged through a higher impedance path at the output of the converter. This high impedance element will limit the charge current into the capacitor until the capacitor is charged to a pre-defined voltage level. Once the pre-defined voltage level is reached, the high impedance path can be removed or shorted by a low impedance device such as an FET.

The converter can deliver its full rated current through this lower impedance path. When the FET shorts the impedance path,