A It is the small unwanted residual periodic variation of the direct current (DC) output of a power supply which has been derived from an alternating current (AC) source. The wave form is shown as figure below.



There are two AC contents, also known as Ripple and Noise (R&N), on the DC output. The first one, coming from sine wave rectification, is at a low frequency which is 2 times of the input frequency; the second one is at high frequency which is from the switching frequency. For measuring high frequency noise, configurations of an oscilloscope with a bandwidth of 20MHz, a scope probe with shortest ground wire possible, and add 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are requires to be made.

A When current drawn exceeds the rating of the PSU, the protection circuit will be triggered to protect the unit against overload/overcurrent. 
Protections of overload/overcurrent can be divided into several forms:
(1)FOLDBACK CURRENT LIMITING
Output current decreases about 20% of rated current, shown as curve (a) in the figure below. 
(2)CONSTANT CURRENT LIMITING
Output current remains at a constant level and within the specified range while the output voltage drops to a lower level, shown as curve (b) in the figure below. 
(3)OVER POWER LIMITING
Output power remains constant. As output load increases, output voltage decreases in proportion, shown as curve (c) in the figure below.  
(4)HICCUP CURRENT LIMITING
Output voltage and current keep pulsing ON and OFF repeatedly when protection is activated. The unit automatically recovers when faulty condition is removed.
(5)SHUT OFF
 Output voltage and current are cut off when output load reaches protection range. 
NOTE: Protection mode of some of the products combines with different types of the forms mentioned, such as constant current limiting + shut down.


Recover method:
(1)Auto Recovery: PSU recovers automatically after faulty condition is removed.
(2)Re-power on: PSU restarts by manual AC re-power on after faulty condition is removed.
Note：Please do not operate PSU in overcurrent or short-circuit condition for a long period of time to prevent a shorten lifespan or damaging the PSU.

A Some power supplies provide a "Power Good" signal when they are turned on, and send out a " Power Fail" signal when they are turned off. This is usually used for monitoring and controlling purpose.
Power Good: after the output of a power supply reaches 90% rated voltage, an TTL signal (about 5V) will be sent out within the next 10-500ms.
Power Fail: before the output of a power supply is less than 90% rated voltage, the power-good signal will be turned off at least 1ms in advance.