EVL6566B-65W-QR STMicroelectronics, EVL6566B-65W-QR Datasheet - Page 7

• Manufacturer Part Number: EVL6566B-65W-QR
• Description: BOARD EVAL SMPS FOR L6566B
• Manufacturer: STMicroelectronics
• Type: Power Factor Correctionr

Specifications of EVL6566B-65W-QR

• Main Purpose: AC/DC, Primary Side
• Outputs And Type: 1, Isolated
• Power - Output: 65W
• Voltage - Output: 700V
• Voltage - Input: 8 ~ 23 V
• Regulator Topology: Flyback
• Frequency - Switching: 100kHz
• Board Type: Fully Populated
• Utilized Ic / Part: L6566B
• Input Voltage: 90 V to 265 V
• Output Voltage: 1.8 V to 12 V
• Board Size: 150 mm x 75 mm
• Product: Power Management Modules
• Dimensions: 150 mm x 75 mm
• Lead Free Status / RoHS Status: Lead free by exemption / RoHS compliant by exemption
• Current - Output: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• For Use With/related Products: L6566B
• Other names: 497-10165

AN2749
Circuit description
Brownout protection
Brownout protection prevents the circuit from working with abnormal mains levels. It can be
easily achieved using pin #16 (AC_OK) of the L6566B. The divider R5, R6, R46, R12 and
R13 is connected to the input DC bus and it is dedicated to sense the rectified input voltage
peak value and feed it into the L6566B AC_OK pin. An internal comparator then enables the
IC operations if the mains level is correct, within the nominal limits. If the input voltage is
below 85Vac, the startup of the circuit is inhibited, while the turnoff voltage has been set at
the voltage reached by the bulk capacitor after the hold-up time. The internal comparator
has in fact a current hysteresis allowing to set independently the L6566B turn-on and turnoff
voltage.
L6566B current mode control and voltage feed-forward function
R18 and R19 sense the MOSFET current and the signal is fed into pin #7 (CS), connected
to the PWM comparator. This signal is thus compared with the COMP (pin #9) signal, which
is coming from the optocoupler.
The maximum power that the converter can deliver is set by a comparator limiting the peak
of the primary current, comparing the CS and an internal threshold (VCSX). If the current
signal exceeds the threshold, the comparator limits the MOSFET duty cycle, hence the
output power is also limited.
Because the maximum transferable power depends on both the primary peak current and
the input voltage, in order to keep almost constant the overload set point that would change
according to flyback input voltage, the L6566B implements a voltage feed-forward function
via a dedicated pin. Hence, VCSX is modulated by the voltage on pin #15 (VFF) sensing the
bulk voltage by a resistor divider. A higher voltage causes a smaller VCS, MAX so that the
maximum power can be kept almost constant at any input voltage.
The resistor R27 connected between pin #7 (CS) and pin #12 (MODE/SC) provides the
correct slope compensation to the current signal, required for the correct loop stability.
L6566B short-circuit protection
In case of short-circuit, an internal comparator senses the COMP pin after the soft-start
time. In that condition the COMP pin goes high, activating an internal current source that
restarts charging the soft-start capacitor from the initial 2 V level. If the voltage on this pin
reaches 5 V, the L6566B stops the operation and enters in "hiccup mode". The L6566B
restarts with a startup sequence when the Vcc voltage drops below the Vcc restart level (5
V). Because of the long time needed by the Vcc capacitor to drop to 5 V, this results in an
increase of the duration of the no-load operation, thus decreasing the power dissipation and
the stress of the power components. This sequence is repeated until the short is removed,
after that the normal operation of the converter is automatically resumed.
Another comparator having its threshold at 1.5 V dedicated to protecting the circuit in case
of transformer saturation or secondary diodes short is provided too. If the voltage on the CS
pin (#7) exceeds this threshold two consecutive times, the IC immediately shuts down and
latches off. This is intended to prevent spurious activation of the protection in case of
temporary disturbances, for example during the immunity tests. Even in this case the IC
operation is resumed as soon as the Vcc voltage drops below 5 V. In this way a hiccup mode
operation is still obtained, avoiding consequent failures due to overheating of the power
components . The sequence described here effectively protects the circuit if during the short
operation the power supply delivers a significant high power at the output. Sometimes, in
multi-output SMPS in case of short on a low voltage output and especially when the power
supply is not delivering the full load, the power delivered to the shorted output could be
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