EVL6566B-65W-QR STMicroelectronics, EVL6566B-65W-QR Datasheet - Page 8

• 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

Circuit description
AN2749
lower than the total maximum deliverable output power of the power supply. In that case the
situation would be critical because the power supply is delivering a high output current that
could overheat the secondary side components, but the reduced power at primary side
would not be able to trigger the overload protection at primary side. Therefore, an output
voltage sensing on the lower voltage is implemented. Thus, in case a low voltage output is
shorted, the abnormal lower value of that output is detected and the COMP pin is tied to
GND by Q2 and Q3, pushing the circuit to work in hiccup mode.
Overvoltage protection
The pin #11 (ZCD) is connected to the auxiliary winding by a resistor divider. It implements
the OVP against feedback network failures. When the ZCD pin voltage exceeds 5 V, the IC
is shut down. This protection can be set as latch or auto-restart by the user with no
additional components. On the board it is set as latched. Therefore the operations can be
resumed after a mains recycling.
Standby power saving and light-load frequency foldback function
The L6566B implements a current mode control, thus it monitors the output power by pin
COMP, which level is proportional to the load. Thus, when the voltage on pin COMP falls
below an internal threshold, the controller is disabled and its consumption reduced; Normal
operation restarts as soon as the COMP voltage rises again. In this way a low consumption
burst mode operation is obtained.
The burst mode feature is dedicated to reduce the no load consumption but another target
of this application is achieving a high converter efficiency even at light load, when normally it
is quite low. To improve the efficiency at this load condition a reduction of the switching
frequency is needed, in order to save AC and transition losses of the power section. A
simple circuit realizing a frequency foldback, connected to the L6566B oscillator (pin #13 -
OSC) is implemented on the board. This solution allows improving light-load efficiency
requiring just three external components.
Frequency jittering
To improve the EMI behavior of the application and make it quieter thus minimizing the EMI
filter, the L6566B is provided with a dedicated pin, FMOD (#6), allowing the user the
possibility to modulate the oscillator frequency in case the device is set for working in fixed
frequency mode. The FMOD pin (#6) gives the possibility to select both the modulation
frequency and the modulation depth. On the board, the capacitor C36 connected from this
pin to GND is alternately charged and discharged by internal current sources. As a result,
the voltage on the pin is a symmetrical triangular waveform with the frequency related to its
value. By connecting a resistor (R31) from this pin to pin OSC (#13) it is possible to
modulate the current sourced by the OSC pin and then the switching frequency. In this
board a 400 Hz modulating frequency and ±5 kHz of frequency deviation have been set.
The modulation reduces the peak value of EMI emissions by means of a spread-spectrum
action. Please refer to
Chapter 4: Conducted noise measurements on page 23
for the
relevant measurements and comparison of the circuit working with or without the frequency
jittering function.
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Doc ID 14630 Rev 2