07-July-03
EPR-29 – 4 Output, 11 W, Universal Input, DVD Supply
ripple from the outputs. This is implemented with resistor R12 and capacitor C6 on the
lower current –12 VDC output.
4.4 Output Feedback
Resistors R15 and R16 sense the 5 V and 3.3 V outputs and with resistor R13 form a
voltage divider network at the input of the TL431 (U3). Capacitor C14 provides
compensation for the TL431 (to provide high DC gain but low high frequency gain). The
TL431 drives current through optocopler LED U2B, which in turn is fed from the 5 V rail
(before the post filter) by resistor R9. The value of R9 programs the high frequency gain
of the control loop. The ON/OFF control scheme used by TinySwitch-II (U1) ensures low
output ripple, fast transient response, high low-load efficiency, and low no-load
consumption while minimizing audible noise by ensuring the transformer flux density is
low when the switching frequency enters the audible range.
Capacitor C7 provides a soft-finish characteristic to “close” the feedback loop early during
power up, thus eliminating startup output overshoot. Resistor R8 provides a discharge
path for C7 after power down.
Power Integrations, Inc.
Tel: +1 408 414 9200 Fax: +1 408 414 9201
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