No-Load Operation; Power-Good Signal - IBM 5170 Technical Reference

No-Load Operation
No damage or hazardous conditions occur when primary power is
applied with no load on any output level. In such cases, the
power supply may switch off, and a power/on cycle will be
required. The power supply requires a minimum load for proper
operation.
Power-Good Signal
The power supply provides a 'power-good' signal to indicate
proper operation of the power supply.
When the supply is switched off for a minimum of 1 second and
then switched on, the 'power-good' signal is generated, assuming
there are no problems. This signal is a logical AND of the dc
output-voltage sense signal and the ac input-voltage sense signal.
The power-good signal is also a TTL-compatible high level for
normal operation, or a low level for fault conditions. The ac fail
signal causes power-good to go to a low level at least 1
millisecond before any output voltage falls below the regulation
limits. The operating point used as a reference for measuring the
1 millisecond is normal operation at minimum line voltage and
maximum load.
The dc output-voltage sense signal holds the 'power-good signal'
at a low level when power is switched on until all output voltages
have reached their minimum sense levels. The 'power-good
signal' has a turn-on delay of at least 100 milliseconds but not
longer than 500 milliseconds. The following figure shows the
minimum sense levels for the output voltages.
Power Supply 3-5