Datasheet

40W, Dual Output
VER:00 Page 26 of 35 Issued Date：2009/03/02
EMC Considerations (Continued)
Common Choke L1 is definedas follows:
■ PMT-048
L: 450μH±35% / DCR: 25mΩ, max
Aheight: 9.8 mm, Max
■ PMT-053
L-830μH±35% / DCR- 1mΩ, max
Aheight: 8.8 mm, Max
■ Test condition: 100KHz/ 100mV
■ Recommended through hole: Φ0.8mm
■ All dimensions in millimeters
Input Source Impedance
The converter shou
ld be connected to a low impedance input source. Highly inductive source impedance can
affect the stability of the converter. Input external L-
C filter is recommended to minimize input reflected ripple current.
The inductor is a simulated source impedance of 12μH and capacitor is Nippon chemi-con KY series 220
μF/100V.
The capacitor must be locatedas close as possible to the inputterminals of the converter for lowestimpedance.
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 approximately150 percent of ratedcurrent for PXF40-xxDxx series.
Hiccup-mode is a method of operation in a converter whose purpose is to protect th
e 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 preventthese powerdevices from being damaged.
The operation of hiccup is as follows. When the current sense circuit sees an over-
current 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 op
eration 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 te
mperature of the
power devices ismuch lower.