AN2600

Circuit description

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Circuit description

The topology used in this power supply is a standard flyback, working in continuous

conduction mode with fixed frequency. The switching frequency (65 kHz) represents a good

trade-off between the transformer size and the harmonics of the switching frequency, in

order to optimize the input filter size and cost. The Power MOSFET is a 600 V-1.6

Ω in

SuperMESH® technology, housed in a TO-220FP (insulated package). The reflected

voltage is 70 V, providing enough room for the leakage inductance voltage spike with still

margin for reliability. The network D53-R68- R83-R84-C84 clamps the peak of the leakage

inductance voltage spike.

The controller is the new L6668, integrating all the functionalities needed to control an

SMPS with high performance and minimum component count, offering the maximum

flexibility. A new functionality embedded in the device is a high voltage startup internal

circuit which draws current from the DC bus and charges the IC supply capacitor (C96). As

soon as the voltage on this capacitor reaches the L6668 turn-on threshold, the controller

starts to drive the MOSFET, and the IC is supplied only by the auxiliary winding of the

transformer via the diode D62 (After startup, the HV current source is deactivated, saving

power during normal operation).

The control system is Current Mode, so the current flowing in the primary is sensed by R71,

R80 and R81 then filtered by R65 and C100 and fed into pin 12 (Isen). No slope

compensation is needed as the duty cycle is kept well below 50%. The circuit connected to

pin 7 (DIS) provides overvoltage protection in case of feedback network failures and open

loop operation. An internal comparator senses this pin voltage and in case its threshold is

exceeded, the L6668 stops operating and reduces its consumption. After Disable

intervention the controller operation can be resumed only by disconnecting the mains plug.

The switching frequency is programmed by the RC network connected to pin 16 (RCT) and

in case of reduced load operation, the controller can decrease the operating frequency via

pin 13 (ST-BY) and resistor R69 proportionally to the load consumption.

The output rectifiers have been chosen according to the maximum reverse voltage and

power dissipation. The 3.3 V and 5 V outputs rectifiers are Schottky type, featuring lower

losses compared to standard PN diodes. The diodes D50, D54 and D55 need a small heat

sink, as indicated on the BOM. The other output rectifiers are fast recovery. The 12 V output

is obtained from the 16 V output by a high-current low-drop voltage regulator.

The output voltage control is performed by the secondary feedback on the 5 V and 3.3 V

outputs, while for the other outputs the control is achieved by the transformer coupling. The

feedback network uses a TL431 driving an opto-coupler, in this case a PC817, ensuring the

required insulation between primary and secondary. The opto-transistor drives directly the

COMP pin of the L6668. An LC filter has been added on all outputs in order to filter the high

frequency ripple without increasing the output capacitors.

The input EMI filter consists of two X2 capacitors with a common mode choke in between.

The NTC limits the inrush current charging the input capacitor at plug-in.

The transformer is layer type, manufactured by Pulse Eldor Corporation, in accordance with