DER-60
Adapter – TNY268
May 27, 2005
4 Circuit Description
A Flyback converter is used to obtain 3.5V 2.5A output from 90-265 VAC input. The bias
winding, which also serves as core cancellation winding to reduce EMI noise, provides
current to the BP pin of TNY268P, to reduce the No-Load Input Power consumption by
about 0.1 W.
4.1 Input Rectification and EMI Filtering
Fuse F1 protects the charger against any fault condition, and input current exceeds 1A.
Diodes D1, D2, D3, and D4 form Full-bridge rectifier, and rectify the AC voltage into DC
voltage and charge the capacitors C1 and C2. L1, L2, C1, and C2 form π – filter and
attenuate EMI noise. Here, C1 and C2 act as both storage capacitors and part of EMI
filter, which reduces the total cost.
4.2 TOPSwitch Primary
This design uses RCD (C3, D5, R4, and R3) clamping across primary winding to limit the
drain voltage below 700V, when the Mosfet inside U1 turns OFF. The capacitor C4
connected to BP (by-pass) pin of U1 stores energy and provide power for the internal
circuit of U1 and also to turn ON the U1’s Mosfet, during power-up and steady state
operation. C11, D11, and R19 form bias supply components, which provide power to the
BP pin during steady state operation. This will reduce the No-load input power
consumption. R45 limits the current flowing to BP pin, and it is tuned to reduce the No-
load input power to minimum. The opto-coupler transistor pulls down enable (EN) pin of
U1. TinySwitch-II keeps on switching as long as the pull down current < 250 µA. U1 will
stop switching if the pull down current exceeds 250 µA.
4.3 Output Rectification
When U1 Mosfet is turned ON, current flows through transformer primary and stores
energy. When U1 is ON, output diode D20 is OFF. When the U1 Mosfet is OFF, D20 is
forward biased, and the stored energy is transferred to the secondary and stores in C22,
C23 and C25. The snubber C21 and R20 across secondary winding will improve EMI.
4.4 Output Feedback
Resistors R30, R31 divide down the supply output voltage and apply it to the reference
pin of error amplifier U22. Shunt regulator U22 drives optocoupler U20 through resistor
R28 to provide feedback information to the U1 EN pin. Capacitor C27 drives the
optocoupler during supply startup to reduce output voltage overshoot. C26 plays a role in
compensating of the power supply feedback loop.
Power Integrations
Tel: +1 408 414 9200 Fax: +1 408 414 9201
Page 6 of 25
www.powerint.com
POWERINT [ Power Integrations ]
