Fall time refers to the time required for the output voltage to decrease from 90% to 10% of its initial steady-state value after the output is turned off. This parameter indicates the discharge speed of the output capacitance and is a critical factor for ensuring operator safety and protecting connected loads.
where t90% is the time when the output voltage reaches 90% of its initial value, and t10% is the time it reaches 10%.
For example, if a 100 V rated power supply drops to 90 V (90%) at 1 ms and 10 V (10%) at 6 ms after shutdown, the fall time is 5 ms.
- Linear power supplies: 10 ms to 30 ms
- Switching power supplies: Typically less than 5 ms
- High-voltage (HV) power supplies: Ranges from milliseconds to seconds (dependent on internal discharge circuits and load)
An excessively slow fall time leaves a residual charge on the output, which can create safety hazards like electric shock or delay subsequent test cycles. Conversely, an uncontrolled, overly rapid fall time can cause voltage transients (undershoot) that may damage sensitive components in the connected load, such as ICs.
Therefore, selecting a power supply with fall time characteristics suitable for the application is essential. Many modern power supplies feature active discharge functions or programmable shutdown sequences to ensure a safe and controlled power-down profile.