EVL6564-50WFLB Datasheet, PDF (2/6 Pages) STMicroelectronics – 50 W wide-range high power factor flyback converter demonstration board based on the L6564

General information and electrical schematic
EVL6564-50WFLB
1
General information and electrical schematic
The topology of this power supply is a typical flyback converter using a transformer to
provide the required insulation between the primary and secondary side. The converter is
connected after the mains rectifier and the capacitor filter that, in this case, is very small so
as not to damage the shape of the input current. The flyback switch is represented by the
Power MOSFET Q1, driven by the L6564. The board is equipped with an input EMI filter
designed for a 2-wire input mains plug. It is composed of one common mode line-filter stage
connected after the input connector. A varistor is also connected at the input of the board,
improving the immunity against input voltage fast transients.
At startup the L6564 is powered by the V_CC capacitor (C8) that is charged via the startup
network composed of R3, R4, R7, D2, DZ2, Q2 and PTC1. When the device begins to
switch, this network is opened by the diodes DZ6, R2, R43 and Q3, decreasing the power
dissipation during normal operation. After startup operation, the L6564 is supplied by the T1
auxiliary winding (pins 5-6) generating the V_CC voltage rectified by D5 and R17.
R33 is also connected to the auxiliary winding, providing to the L6564 ZCD pin the
transformer demagnetization signal, turning on the MOSFET at any switching cycle.
The voltage on the auxiliary is even used to detect an abrupt rise in the output voltage or a
feedback disconnection via the diodes D5 and D6 and the divider composed of R32 and
R12. When the voltage on the INV pin (#1) is lower than 1.66 V and the PFC_OK pin (#5) is
greater than 2.5 V, the OVP is active and the L6564 is latched.
The MOSFET is the STF11NM80, a standard and inexpensive 800 V device housed in a
TO-220FP package. The rectifier D3, the Transil™ DZ1 and capacitor C12 clamp the peak
voltage spike at MOSFET turn-off.
The R10 resistors sense the current flowing into the transformer primary side. Once the
signal at the current sense pin via resistor R11 has reached the level programmed by the
internal multiplier of the L6564, the MOSFET is turned off.
The dividers R1, R5, R8 and R13 provide, to the L6564 multiplier pin (#3), the information of
the instantaneous voltage used to modulate the current flowing into the transformer primary
side.
The capacitor C16 and the parallel resistor R14 complete an internal peak-holding circuit
that derives the information on the RMS mains voltage. The voltage signal at this pin, a DC
level equal to the peak voltage on pin #3 (MULT), is provided to a second input to the
multiplier for the 1/V2 function necessary to compensate the control loop gain dependence
on the mains voltage.
Additionally, the pin VFF (#5) is internally connected to a comparator providing the brownout
(AC mains undervoltage) protection. A voltage below 0.8 V shuts down (not latched) the IC
and brings its consumption to a considerably lower level. The L6564 restarts as the voltage
at the pin rises above 0.88 V.
The transformer is layer type, using a standard ferrite size EER-28L, manufactured by
Magnetica.
The flyback reflected voltage is ~140 V, providing enough room for the leakage inductance
voltage spike with still enough margin for reliability of the MOSFET. Employing the L6564
ensures a cleaner current sinewave than the old PFC devices (with L6562D/L6562A the
startup resistance was necessary on pin INV)) and it implements the VFF function,
especially useful in wide-range input voltage, to have a more constant overload protection
2/6
Doc ID 023237 Rev 1