17-May-04
EPR-16 – LinkSwitch 2.75 W Charger/Adapter
R1 was selected to program the peak power point to be 500 mA when a transformer with
a nominal LP value was used. Initial values are selected using the expression (from
Power Integrations Application note, AN-35):
R1 ≅ (VFB - VC (IDCT)) / IDCT
≅ (54.1 – 5.75) / 2.3 mA
≅ 21 kΩ
The closest standard 1% series value of 20.5 kΩ was selected. See AN-35 for a more
detailed explanation of clamp and feedback component selection.
C3 sets the auto-restart period and also the time the output has to reach regulation
before entering auto-restart from start-up. If a battery load is used then a value of
0.22 µF is typical. However, if the supply is required to start into a resistive load then this
should be increased to 1 µF to ensure enough time during start-up to bring the output into
regulation. The type of capacitor is not critical; either a small ceramic or electrolytic may
be used with a voltage rating of 10 V or more.
4.5 Output Stage
Diode D6 should be rated for 80% of applied reverse voltage and thermally for average
current multiplied by forward voltage at maximum ambient. Here a 1 A, 60 V Schottky
diode was used to reduce the losses and improve efficiency, although fast or ultra-fast
PN diodes are acceptable.
A snubber formed by C6 and R4 may be fitted across D6 to improve radiated EMI
performance.
Capacitor C5 should be rated for output voltage and ripple current. Depending on the
application, the designer may choose not to derate for ripple current. If the application is
battery charging of equipment such as PDAs or cell phones, the duty cycle of operation
at high ripple current is likely to be low, perhaps only 1 hour per day. In this case the
capacitor temperature can be allowed to rise significantly during charging without concern
for the overall capacitor lifetime.
Power Integrations
Page 11 of 36
Tel: +1 408 414 9660 Fax: +1 408 414 9760
www.powerint.com
POWERINT [ Power Integrations ]
