DER-99 Datasheet, PDF (8/29 Pages) Power Integrations, Inc. – 17.4 W Power Supply using TOP244P

DER-99
Set Top Box 17.4 W Power Supply
September 12, 2005
4 Circuit Description
4.1 EMI Filtering
L6 and C15 form the main EMI filter. C23 and C14 reduce radiated EMI, and the RC
networks across the output diodes also reduce high frequency conducted and radiated
EMI.
4.2 TOPSwitch Primary
The TOPSwitch has a built-in bulk voltage OVLO (over-voltage lockout) protection. It
senses the bulk voltage through R24 and R25. If the bulk voltage exceeds the set
threshold, it will shutdown until the voltage falls back to safe levels. This will prevent
failure due to Drain over-voltage during an AC voltage swell.
4.3 Power Good signal
Q1, Q2, Q3 and associated circuitry form the power-good signal. D14 charges C24
during the on-time of the TOP244P, and form a “forward mode” output. C24 has a
negative voltage that is proportional to the bulk capacitor when the TP244P is running.
VR1 and Q1 sense when the bulk cap is below a certain threshold. When AC power is
removed, the bulk voltage drops and Q1 turns on, rapidly discharging C30, turning off Q2
and turning Q3 on, and the power-good signal output goes low. This bulk voltage
threshold, and zener VR1 voltage is chosen so that this happens >5mS before the
outputs drop out of regulation when at full load. R27 and C30 form a delay so that at
power-up, the power-good signal is delayed >500 mS before it comes up.
Page 8 of 29
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