DER-46
6 W peak, Multiple output PSU for DVB-T
April 20, 2005
4 Circuit Description
The schematic in Figure 1 shows an off-line flyback converter using the TNY264P. The circuit
is designed to operate from 195 VAC to 265 VAC input and provides isolated multiple outputs
(i.e. 2.5V; 3.3V; 5V; 12V; 33V).
4.1 TinySwitch Primary
AC input power is rectified by a full bridge, consisting of D1 through D4. The rectified DC is
then filtered by the bulk storage capacitors C1 and C4. Inductors L1, L2, C1 and C4 form a pi
(π) filter, which attenuates conducted differential-mode EMI noise.
The rectified DC rail is applied to one end of the transformer primary, the other end being
connected to the drain pin of the integrated MOSFET of U1.
To keep the peak DRAIN voltage acceptably below the BVDSS (700V) of U1, diode D5, C2, R1,
and R7 form a primary clamp. This network clamps the voltage spike seen on the DRAIN due
to primary and secondary reflected leakage inductance.
Capacitor C21 is the decoupling cap for the TOPSwitch.
R19, R20, R21 and R22 enables the under voltage (UV) line sensing that inhibits U1 to start
switching below 70VAC during start-up.
4.2 Output Rectification
The secondary of T1 has four windings AC stacked together. Each rectified and filtered to
provide 3.3V, 5V, 12V and 33V DC outputs. While the 2.5V output was derived from 3.3V
through a voltage dropping diode D12.
A zener (D9) - resistor (R3) pre load was employed to clamp the 33V from going over voltage
only when it is lightly loaded while the rest of the outputs are heavily loaded.
4.3 Output Feedback and control
The main feedback loop is closed on 3.3V output. DC feedback to the output voltage
regulator error amplifier (U4) comes from a divider network R9 and R11. The center point is
tied to the reference input of U4. Capacitor C24 and resistor R8 roll off the high frequency
gain of U4 while R12 sets the overall gain.
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