ATX/ATX12V Power Supply Design Guide
Version 1.1
Page 21
3.4.2. Short-circuit Protection
An output short circuit is defined as any output impedance of less than 0.1 ohms. The
power supply shall shut down and latch off for shorting the +3.3 VDC, +5 VDC, or
+12 VDC rails to return or any other rail. Shorts between main output rails and +5 VSB
shall not cause any damage to the power supply. The power supply shall either shut down
and latch off or fold back for shorting the negative rails. +5 VSB must be capable of being
shorted indefinitely, but when the short is removed, the power supply shall recover
automatically or by cycling PS_ON#. The power supply shall be capable of withstanding a
continuous short-circuit to the output without damage or overstress to the unit (for
example, to components, PCB traces, connectors) under the input conditions specified in
Section 3.1. The maximum short-circuit energy in any output shall not exceed 240 VA, per
IEC 60950 requirements.
3.4.3. No-load Operation
No damage or hazardous condition should occur with all the DC output connectors
disconnected from the load. The power supply may latch into the shutdown state.
3.4.4. Overcurrent Protection
Overload currents applied to each tested output rail will cause the output to trip before
reaching or exceeding 240 VA. For testing purposes, the overload currents should be
ramped at a minimum rate of 10 A/s starting from full load.
3.4.5. Overtemperature Protection
The power supply may include an overtemperature protection sensor, which can trip and
shut down the power supply at a preset temperature point. Such an overheated condition is
typically the result of internal current overloading or a cooling fan failure. If the protection
circuit is nonlatching, then it should have hysteresis built in to avoid intermittent tripping.
3.4.6. Output Bypass
The output return may be connected to the power supply chassis. The return will be
connected to the system chassis by the system components.
