EPR-29 – 4 Output, 11 W, Universal Input, DVD Supply
07-July-03
4 Circuit Description
The circuit shown is an AC to DC multi-output flyback power supply using the highly
integrated TNY268 power IC from the TinySwitch-II family. The circuit is designed for
90 VAC to 265 VAC input with 3.3 V, 5 V, 12 V, and -12 V outputs.
4.1 Input EMI Filtering
Capacitor C3, inductor L1 and capacitors C1 and C4 provide differential mode EMI
filtering. Capacitor C8 (Y-capacitor) helps to reduce common-mode and radiated EMI. In
conjunction with these components, the frequency jitter feature of the TNY268 and
shields in the transformer, allow the unit to pass CISPR22B/EN55022B specifications for
conducted emissions without the need for a common-mode choke.
4.2 TinySwitch-II Primary
Diodes D1, D2, D3, and D4 and capacitors C1 and C4 provide a high-voltage DC bus for
the primary circuitry. The DC bus is applied to the high side of the transformer T1
primary winding. The other side of the transformer primary is driven by the integrated
switching MOSFET of the TinySwitch-II U1. Components D5, R5, C2, R7, and VR1 form
the clamp network to limit the voltage rise at MOSFET turn-off due to the transformer
leakage inductance. This circuit clamps the MOSFET voltage to a safe level during
normal power supply operation. The Zener VR1 actively clamps at peak output load, but
does not contribute to the clamping at normal load conditions.
Diode D5 is a slow-recovery diode and allows some of the clamp energy to be recovered
and transferred to the output. After clamping the leakage inductance energy, diode D5
does not turn off immediately. This allows some current to flow in the reverse direction,
out of C2 and couple through the transformer to the secondary side. Resistors R5 and
R7 damp ringing while R5 also limits the reverse current through D5 when U1 turns on.
The energy recovery of this clamp scheme contributes to slightly higher efficiency and
also better cross-regulation.
Diode D9 and capacitor C10 rectify and filter the voltage from the transformer bias
winding. Current is fed from C10 via R6 to the BYPASS pin of U1. Supplying U1 from
the bias winding reduces power consumption by inhibiting the internal DRAIN current
source normally used to power the IC from the high voltage DC bus during the internal
MOSFET off time. This helps improve the no-load power consumption and light load
efficiency of the power supply. Capacitor C19 is the decoupling capacitor for the
TinySwitch-II. Opto-transistor U2A provides feedback from the secondary side to control
TinySwitch-II regulation.
4.3 Output Rectification
Diodes D7, D11, D10, and D8 together with capacitors C15, C11, C12, and C9 provide
rectification and filtering of the secondary transformer voltages. Capacitors C20, C17,
and C18 with inductors L2, L3, and L4 provide filtering to reduce high switching frequency
Power Integrations, Inc.
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