Datasheet

DataSheet
30W, Single Output
VER:00 Page 51 of 61 Issued Date：2009/03/02
EMC Considerations (Continued)
Common ChokeL1 is definedas follows:
■
L: 450μH±35% / DCR: 25mΩ, max
Aheight: 9.8 mm, Max
■ 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 ofthe power module 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 atapproximately150 percent ofrated current for PXF40-xxSxx series.
Hiccup-
mode is a method of operation in a converter whose purpose is to protect the 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 these devices may exceed their specified limits. A
protection mechanism has to be used to prevent these power devices from being damaged.
The operation of hiccup is as follows. When the current sense circuit sees an over-curr
ent event, the controller
shuts off the converter for a given time and then tries to start up the converter again. If the over-
load condition has
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 operation 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 h
eat due to overload lasts for only a short duration in the hiccup cycle, hence the junction temperature of the
power devices is much lower.