Fall time is the time it takes for the output voltage to drop from 90% to 10% of its initial steady-state value after the power supply's output is turned off. This parameter indicates the discharge speed during shutdown and is a critical factor for ensuring safety and protecting the connected load.
where t90% is the time at which the output voltage drops to 90% of its initial value, and t10% is the time it reaches 10%.
For example, if a 100 V-rated power supply reaches 90 V (90 %) at 1ms and 10 V (10 %) at 6ms after output shutdown, the fall time equals 5 ms.
- Linear power supplies: 10-30 ms
- Switching power supplies (synchronous rectification): Less than 5 ms
- High-voltage (HV) power supplies: Adjustable via variable discharge resistors (several ms to several hundred ms)
An excessively slow fall time can leave a residual charge on the output for an extended period, creating safety hazards like electric shock or causing equipment malfunctions. Conversely, an overly rapid fall time can cause abrupt voltage changes and transients (spikes or dips) that may damage sensitive components in the connected load, such as capacitors and integrated circuits. Therefore, designing the fall time characteristics to suit the specific application is essential. To achieve this, engineers may implement snubber circuits, discharge resistors, or soft-shutdown functions to ensure a controlled power-down sequence.