DER-36

42W, 4 Output Set Top Box Supply

April 27, 2004

Page 6 of 23

Power Integrations

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4

Circuit Description

4.1

Input EMI Filtering

Components L5, L8, L10, C4, and C10 provide EMI filtering, while L4, L5, L10, SG1, and

RV1 provide line surge protection.

4.2

TOPSwitch Primary

Components D2, R1-2, and C5 comprise an RCD clamp with damping to limit the primary

leakage spike. Use of a normal recovery diode in this clamp circuit allows some leakage

energy to be recycled. Capacitors C16 and C19 bypass the U2 control pin. Components

C19 and R9 also provide frequency compensation for U2. Resistor R8 sets the U2

current limit to 90% of its nominal value.

4.3

Output Rectification

Output rectification and filtering is provided by D4 and C8-9 for the 5V output, D1 and C7

for the 6.8V output, D3 and C2 for the 12V output, and D7 and C12 for the –10V output.

Components L1, 3, L6, L7, L9, C3, C6, C13 and C15 are used for additional high

frequency filtering. Ferrite bead L2 is used to center the 12V output closer to its nominal

value.

4.4

Output Feedback

Output feedback is provided from the 5V output. Resistors R7 and R11 program the

output voltage. The output error signal is coupled back to the supply primary via R5 and

U1. Capacitor C18 and 10 compensate U3, while C17 allows current through U1 during

startup to prevent output overshoot. Resistor R4 discharges C17 when the supply is off.