AN1376 Datasheet, PDF (4/35 Pages) STMicroelectronics – 25W QUASI-RESONANT FLYBACK CONVERTER FOR

AN1376 APPLICATION NOTE
The switching frequency (minimum is ~30 kHz @V_in = 80 V_DC) has been chosen to get a com-
promise between the transformer size and the harmonics of the switching frequency, in order
to optimise the input filter size and its cost. The MOSFET is a standard and cheap 600V-1.76Ω
typ., TO-220FP. It needs a small heat sink. The transformer reflected voltage is 90V, providing
enough room for the leakage inductance voltage spike with still margin for reliability. The net-
work D9+D3 clamps the peak of the leakage inductance voltage spike. These two components
are SMT, allowing cost saving of the manual labour with respect to a passive solution, needing
manual insertion on the PCB. A 220pF HV capacitor has been added across the drain to opti-
mise MOSFET losses by a small snubbing effect on the drain voltage rate of rise.
The controller L6565 is activated by a couple of dropping resistors (R1+R14, for voltage and
power rating reasons) that draws current from the DC bus and charges the capacitor C11. This
circuit dissipates only about 240mW @ 264 Vac, thanks to the extremely low start-up current.
During the normal operation the controller is powered by the transformer via the diode D4. The
network Q101, C102, R104 acts as a spike killer, improving the auxiliary voltage fluctuations
and the performance in short circuit. R12+R15 and R11 compensate for the power capability
change vs. the input voltage (Voltage Feed-forward). A 1nF ceramic capacitor bypasses any
noise on pin #3 to ground (C23). The current flowing in the transformer primary is sensed by
the resistor R6. The circuit connected to pin1 (FB) provides for the over voltage protection in
case of feedback network failures and open loop operation.
The output rectifiers have been chosen in accordance with the maximum reverse voltage and
power dissipation. The rectifiers for 3.3V and 7V outputs are Schottky, type STPS10L60FP.
These diodes are low forward voltage drop, hence dissipating less power with respect to stan-
dard types. Both are the same to decrease the component diversity, as well as for the capac-
itors C1 to C3 and C19. The diode D8 needs a small heat sink, as indicated on the BOM. The
other two output rectifiers are SMT, fast recovery. The snubber R102 and C101 damps the os-
cillation produced by the diode D1 at MOSFET turn-on.
The output voltage regulation is performed by secondary feedback on the 3.3V output, while
for other voltages the regulation is achieved by the transformer coupling. The feedback net-
work is the classical TL431 driving an optocoupler, in this case an SFH617A-4, insuring the
required insulation between primary and secondary. The opto-transistor drives directly the
COMP pin of the controller. The 5V output is linearly post-regulated from the 7V output to get
a very stable voltage. A zener regulator assures the 30V stability at low cost. The 5V regulator
needs to be dissipated.
A small LC filter has been added on the +12V, +7V, +3.3V in order to filter the high frequency
ripple without increasing the output capacitors.
A 100nF capacitor has been connected on each output, very close to the output connector sol-
dering points to limit the spike amplitude.
The input EMI filter is a classical Pi-filter, 1-cell for differential and common mode noise. A NTC
limits the inrush current produced by the capacitor charging at plug-in.
The transformer is slot type, manufactured by Eldor Corporation, in accordance with the
EN60950.
Here following some waveforms during the normal operation at full load:
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