DER-55
TNY267 Multi Output Power Supply
April 20, 2005
4 Circuit Description
4.1 Input EMI Filtering
C10, and C13 are the bulk capacitors. Together with L4, they form the EMI filter.
TinySwitch-II’s built-in frequency jitter helps achieve low EMI in spite of this very simple
EMI filter.
4.2 Primary Clamp Snubber
D3, R1, R2, and C1 form the primary clamp snubber to clamp the voltage spike at the
Drain pin after turn-off. D3 is a 1N4007G, a glass-passivated version of the standard
1N4007, with controlled reverse recovery. Its use, along with R2, improves EMI and
efficiency. If a controlled recovery diode is not available, a fast diode must be used
instead.
4.3 Output filtering
C3, C5, and C7 are the main output capacitors. C4, C6, and C8, along with L1, L2, and
L3, form second-stage output filters.
4.4 Output Feedback
Sensing resistors R4 and R5 detect any variation in the amplitude of the 3.3V and 5V
outputs. These changes, due to changes in the output load or input voltage, are sampled
to the input pin of the TL431 shunt regulator. The shunt regulator compares these
variations with its internal voltage reference and drives a feedback current trough U1B
proportional to the to the variations detected by R4 and R5. Opto-coupler U1 closes the
control loop of the supply by transferring the feedback current to the output of the EN/UV
pin on the primary side of the circuit.
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