DER-95

30 W Adapter for Portable DVD

September 12, 2005

Page 7 of 22

Power Integrations

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4

Circuit Description

The schematic in Figure 2 shows an off-line flyback converter using the TOP245Y. The

circuit is designed for 90 VAC to 265 VAC input and provides an isolated 9.7 V, 3.0 A

continuous output.

4.1

Input EMI Filtering

Conducted EMI filtering is provided by pi formed filter C7, C4, L1, The switching

frequency jitter feature of the TOPSwitch-GX family allows the use of a small, low cost

common mode choke for L1.

C5, R1 and R3 form a primary clamp. This network clamps the voltage spike seen on the

DRAIN due to primary leakage inductance. Diode D3 and capacitor C8 provide rectified

and filtered bias supply for U1.

4.2

Reduced current limit

Resistor R4, R6, R9 and R13 are used to set the maximum current limit for both high line

and low line.

R2 is used for input undervoltage lockout (UVLO).

4.3

Output Rectification

The secondary of T1 is rectified and filtered by D4, C1 & C3. Post filter choke L2 and C15

provide additional high frequency filtering.

4.4

Output Feedback

DC feedback to the output voltage regulator error amplifier (U3) comes from a voltage

divider network R8 and R16. The center point of this network is tied to the 2.5 V REF pin

of U3. Capacitor C11 and resistor R12 roll off the high frequency gain of U3 while R5 sets

the overall gain.

C9 is a “soft finish” capacitor to eliminate output over shoot during start up.