When an AC-DC power supply’s input voltage is interrupted the DC output will only remain within regulation for a short period of time. This is specified on the power supply datasheet as the hold-up time. During this hold-up time the power supply relies on energy stored in its capacitors to maintain operation.

Hold-up time is important in certain vertical markets. The medical industry’s concern regarding hold-up time has increased since the release of the EN 60601-1-2; 2015 (Ed4) immunity standard. Primarily created to address the growing number of products used in home healthcare, this standard specifies multiple AC voltage dips ranging from 20 msec to 5 seconds. The longer outages are addressed by batteries or by designs that ensure no harm will occur to the patient or operator if the power supply output voltage drops out of the regulation band.

Airborne equipment is covered by the DO-160 standard. Section 16 of the standard refers to power input, simulating conditions of aircraft power from before engine start (using auxiliary ground based power) to after landing, including emergencies. The requirement is for a hold-up time of at least 200 msec.

In a recent article on their Power Supply Blog, TDK Lambda explores power supply hold-up time: What it is, where it’s important and, most importantly, several technical approaches for extending hold-up time. You can read that article here.