DER-36
42W, 4 Output Set Top Box Supply
April 27, 2004
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.
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