What is Hold-up Time?
Hold-up time is the length of time a power supply can operate in regulation after failure of the AC input. Linears have very short hold-up times due to the CV squared energy storage product of their low voltage secondary side output capacitors. Switchers have longer times due to their higher voltage primary side energy storage capacitor.
The holdup time, usually measured in milliseconds is an indication of the time the supply will maintain an output voltage within the specified range, after a loss of input supply power. It is also the time the powered circuit will continue its normal operation without resetting or rebooting during the power outage.
When selecting power supplies, UPS systems and other redundant systems, the switchover requirements must be considered. The redundant system must have a switch on time that is less than the hold-up time of the affected supply.
A typical hold-up time is the time a power supply takes to reduce from 100 percent to 90 percent of its rated output when a power outage or a supply fluctuation occurs. The general requirement is at least 16ms to allow sufficient time for UPS to take over. The hold-up time is usually specified by the manufacturer and ranges from 15 milliseconds to 50 milliseconds.
The supply during the holdup time is usually from the storage device such as the capacitor and must fall within the regulated limits. An example is a 5V dc supply with ±10% (±0.5V) output regulation; its hold-up time is the duration between the time the input is removed and the time the output falls to 4.5V.
Figure 1: Turn on and Hold-up time for 5V DC to DC converter
The hold-up time in linear supplies is very short and power goes off almost immediately after an input supply failure. In switched mode supplies, the holdup time is much longer because of the input electrolytic capacitor which provides some hold-up time to protect against momentarily drops and transient power outages. The time is a dependant on the energy storage capacity and the loading of the power supply.
Most circuits and in particular the high-reliability, and sensitive systems, have a requirement to ride through short power interruptions. During the holdup time, the power supply continues to deliver power to the circuit after a power outage, shutdown or breakage. Depending on the nature of the power interruption, the supply may either cut off the power completely after the hold-up time in case of a complete power outage, or continue delivering power if the interruption was momentarily and power resumed back to normal.
The hold-up time is largely dependent on the size of the hold-up capacitor and larger capacitors provide longer times. The capacitors stores energy during the normal input power operation, enough to supply the load when the rectifier output falls below a certain level, or when there is a complete power failure. The capacitor discharges through the converter circuit for a time determined by the capacitance and the load. The capacitance required for the linear is usually higher than that of switching supply due to energy required to supply for power losses in the linear supply.
Figure 2: Basic rectifier and regulator with a hold capacitor.
Additional capacitance can be added externally to increase the holdup time for both the DC to Dc and AC to DC converters. However, the external method is expensive in terms of cost and space. Another disadvantage is that the external circuit interferes with internal DC bus of the power supply. The external hold up time circuitry can be connected at the input or at the output of the supply, each has its advantages and disadvantages.