Power Supply Design Guide

SSI
EPS1U Power Supply Design Guide, V1.1
7 Protection Circuits
STATUS
Required
Protection circuits inside the power supply shall cause only the power supply’s main outputs to shutdown. If the
power supply latches off due to a protection circuit tripping, an AC cycle OFF for 15 seconds and a PSON
#
cycle
HIGH for 1 second shall be able to reset the power supply.
7.1 Current Limit
STATUS
Required
The power supply shall have current limit to prevent the +3.3 V, +5 V, and +12 V outputs from exceeding the
values shown in Table 17. If the current limits are exceeded, the power supply shall shutdown and latch off. The
latch will be cleared by toggling the PSON
#
signal or by an AC power interruption. The power supply shall not be
damaged from repeated power cycling in this condition. –12 V and 5 VSB shall be protected under over current
or shorted conditions so that no damage can occur to the power supply. All outputs shall be protected so that no
damage occurs to the power supply under a shorted output condition.
Table 17: Over Current Protection
Voltage Over Current Limit (Iout limit)
+3.3 V 110% minimum; 150% maximum
+5 V 110% minimum; 150% maximum
+12 V 110% minimum; 150% maximum
7.2 240VA Protection
STATUS
Recommended
System designs may require user access to energized areas of the system. In these cases the power supply may
be required to meet regulatory 240VA limits for any power rail. Since the +12V rail combined power exceeds
240VA it must be divided into separate channels to meet this requirement. Each separate rail needs to be limited
to less than 20A for each +12V rail. The separate +12V rails do not necessarily need to be independently
regulated outputs. They can share a common power conversion stage. The +12V rail is divided into two rails for
the 250W and 350W power levels. +12V1 is dedicated for providing power to the input of the processor voltage
regulator(s). The +12V2 rail is used to power the rest of the main board +12V power needs and peripherals
devices.
Table 18: Over Current Protection
Voltage Over Current Limit (Iout limit)
+3.3 V 110% minimum; 150% maximum
+5 V 110% minimum; 150% maximum
+12V1 18A minimum; 20A maximum
+12V2 18A minimum; 20A maximum