Datasheet

Data Sheet

Output Over Current Protection

Whenexcessive output currents occur in the system, circuit protection is required on all power supplies. Normally,

overload current is maintained atapproximately 150 percent of rated current forPXD20-xxSxx 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 enables 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 curren

t 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 these power devices from being damaged.

The operation of hiccup is as follows. When the current sense circuit sees an over-

current event, the controller

shuts off thepowersupplyfor a giventime 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 met

hods, although 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 operatio

n can be done 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 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 usuallylargerthan 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 will give the best protection for a power supply against over

current situations, since it will limit the average current to the load at a low level,

so reducing power dissipation and

case temperature inthepower devices.

Short Circuitry Protection

Continuous, hiccup and auto-recovery mode.

During short circuit, the converter shuts down.Theaverage current during this condition will be very low and the

device is protected.