Datasheet

DataSheet
40W, Single Output
VER:00 Page 51 of 62 Issued Date：2009/03/02
EMC Considerations (Continued)
This Common ChokeL1 has been define as follows:
■
L1: 450μH±35% / DCR: 25mΩ, max
Aheight: 9.8 mm, Max
■
L1: 830μH±35% / DCR: 31mΩ, max
Aheight: 8.8 mm, Max
■ Test condition: 100KHz / 100mV
■ Recommended through hole: Φ0.8mm
■ All dimensions in millimeters
Input Source Impedance
The converter
should be connected to a low impedance input source. Highly inductive source impedance can
affect the stability of the converter. An input external L-C filter is recommended to minimize input reflected
ripple
current. The inductor has a simulated source impedance of 12μH and the capacitor is Nippon chemi-con KY
series
220μF/100V. The capacitor must be located as close as possible to the input terminals of the converter for lowest
impedance.
Output Over Current Protection
When excessive output currents occur in the system, circuit protection is required on all converters
. Normally,
overload current is maintained at approximately 150 percent of rated current for PXF40-xxSxx series.
Hiccup-mode is a method of operation in a converter whose purpose is to protect t
he power supply from being
damaged during an over-current fault condition. It also enables the converter
to restart when the fault is removed.
There are other ways of protecting the converter when it is over-loaded, such as the maximum current limiting or
current foldback methods.
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 the
se devices may exceed their specified limits. A
protectionmechanism has to be used to prevent these power devices from beingdamaged.
The operation of hiccup is as follows. When the current sense circuit sees an over-current event, the contro
ller
shuts off the converter for a given time and then tries to start up the converter again. If the over-load condition ha
s
been removed, the converter will start up and operate normally; otherwise, the controller will see another over-
current
event and will shut off the converter again, repeating the previous cycle. Hiccup o
peration has none of the drawbacks
of the other two protection methods, 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 t
emperature of the
power devices is much lower.