EPR-000013 Datasheet, PDF (9/56 Pages) Power Integrations, Inc. – Engineering Prototype Report (EP13) 43 W / 57 W pk, 5 Output TOPSwitch팜-GX (TOP246Y) Power Supply

08-May-2001
EPR-000013 – 43 W Multiple Output TOP246 Power Supply
A voltage divider consisting of resistors R10, 11 and 13 monitors the voltage on the 5 V
and 3.3 V outputs. The resistor values are weighted so that the voltage feedback loop is
controlled mostly by the 5 V output, with some contribution from the 3.3 V output.
Sharing the voltage regulation control between the two outputs in this manner improves
the cross regulation for the 3.3 V output at the expense of a slight change in the
regulation of the 5 V output.
The voltage from R10, 11 and 13 is applied to the reference pin of shunt regulator U3.
These resistor values and the reference voltage of U3 are used to set the output voltages
of the supply. Resistor R7 is used to set the overall gain of the supply control loop, while
R8 provides bias current for U3. R9 and C19 provide frequency compensation for U3 to
help stabilize the power supply control loop. Capacitor C20 is used to provide open loop
feedback through optocoupler U2 during start-up, which in conjunction with the built-in
soft start-up feature of TOPSwitch-GX, completely controls the start-up drain current
profile, preventing transformer saturation and output overshoot.
Optocoupler U2 applies the feedback signal from U3 to the CONTROL pin of U1.
Resistor R15 and capacitor C24 form a snubber across D10 that reduces the reverse
recovery transient from this diode, improving EMI performance. Inductor L7 is a ferrite
bead placed in series with the 12 V, 18 V and 30 V output windings of T1. This bead acts
as a small saturable reactor to improve the centering and cross regulation of these
outputs. R6 provides a small amount of pre-filtering for the 30 V output, and is used to
help prevent peak charging of this output due to leakage spikes.
C1, L1 and C6 provide common-mode and differential mode EMI filtering for the power
supply. Fuse F1 protects against gross circuit faults. Varistor RV1 is used to clamp
differential mode line transients. Thermistor RT1 reduces the initial current surge when
AC power is first applied to the circuit.
Page 9 of 56
Power Integrations, Inc.
Tel: [phone] Fax: [phone]
www.powerint.com