Datasheet

6
Output over current protection
When excessive output currents occur in the system, circuit protection is required on all power supplies.
Normally, overload current is maintainedat approximately 150 percent of rated current forthe PXE20-S Series.
Hiccup-
mode is a method of operation in a power supply whose purpose is to protect the power supply from
being damaged during an over-current fault condition. It also enabl
es the power supply to restart when the fault is
removed. There are other ways of protecting the power supply when it is over-
loaded, such as the maximum current
limiting or currentfoldbackmethods.
One of the problems resulting from over current is that
excessive heat may be generated in power devices;
especially MOSFET and Schottky diodes and the temperature of those devices may exceed their specified limits. A
protection mechanism has to beused to prevent those powerdevices from beingdamaged.
The operation of hiccup is as follows. When the current sense circuit sees an over-
current event, the controller
shuts off the powersupply for a given time and then tries tostart up the power supply again. If the over-
load condition
has been removed, the power
supply will start up and operate normally; otherwise, the controller will see another
over-
current event and shut off the power supply again, repeating the previous cycle. Hiccup operation has none of
the drawbacks of the other two protection methods, alth
ough its circuit is more complicated because it requires a
timing circuit. The excess heat due to overload lasts for only a short duration in the hiccup cycle, hence the junction
temperature of thepower devices is much lower.
The hiccup operation can be d
one in various ways. For example, one can start hiccup operation any time
an over-current event is detected; or prohibit hiccup during a designated start-
up is usually larger than during
normal operation and it is easier for an over-current event is detect
ed; or prohibit hiccup during a designated
start-up interval (usually a few milliseconds). The reason for the latter operation is that during start-
up, the power
supply needs to provide extra current to charge up the output capacitor. Thus the current demand during start-
up
is usually larger than during normal operation and it is easier for an over-
current event to occur. If the power
supply starts to hiccup once there is an over-
current, it might never start up successfully. Hiccup mode protection
willgiv
e the best protectionfor a power supplyagainst over current situations, since it will limit the average current
to the load at a lowlevel, so reducing power dissipation and case temperature in the power devices.
Short Circuit Protection
Continuous, hiccup and auto-recovery mode.