AN2242

2 Circuit Description

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Circuit Description

The topology used in this schematic is a classical flyback, working in continuous and

discontinuous conduction mode with fixed frequency, achieves the best tradeoff between peak/

rms current ratio and the output capacitors' size. The nominal switching frequency, 65kHz, has

been chosen to get a compromise between the transformer size and the harmonics of the

switching frequency, optimizing the input filter size and its cost. 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 MOSFET is a standard and cheap 600V-2

Ω

max, TO-220FP, needing a small heat sink. The transformer is a layer type, using a standard

ferrite type EER28L. The transformer, designed according to the EN60065, is manufactured by

TDK. The reflected voltage is 70V, providing enough room for the leakage inductance voltage

spike with still margin for reliability of the MOSFET. The network D7, R7, C21 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 current source used at start-up that draws current from the DC bus and charges the

capacitor C19. After the voltage on C19 has reached the L6668 turn-on threshold and the

circuit starts to operate, the controller is powered by the transformer via the diode D5. After the

start-up, the HV current source is deactivated, saving power during normal operation and

allowing very good circuit efficiency during standby. The control system is Current Mode, so the

current flowing in the primary is sensed by R18 and R19 and is then fed into pin #12 (Isen). R5

and R6 are also connected to pin #12 (Isen). Their purpose is to compensate the power

capability change vs. the input voltage. The resistor R27 connected between pin #12 (Isen) and

pin #15 (S_Comp) provides the correct slope compensation to the current signal, necessary for

the correct loop stability. The circuit connected to pin #7 (DIS) provides over voltage 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. To definitely latch this state an internal circuitry of the L6668 monitors the Vcc

periodically reactivates the HV current source to supply the IC. After OVP detection and

Disable intervention the controller operation can be resumed only after disconnecting the mains

plug. The switching frequency is programmed by the RC connected to pin #16 (RCT) and in

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

#13 (ST-BY) and resistor R15, proportionally to the load consumption. The resistor dividers

R13 and R14 connected to pin #9 (SKIPADJ) allow to set the initial L6668 threshold to burst

mode functionality when the power supply is lightly loaded. Additional functions embedded in

the L6668 are the programmable soft-start and a 5V reference available externally. The output

rectifiers have been selected according to the calculated maximum reverse voltage, forward

voltage drop and power dissipation. The rectifiers for 5V and 12V outputs are Schottky, low

forward voltage drop type, hence they dissipate less power with respect to standard types. A

small heat sink for both devices is required, as indicated on the BOM. The other two output

rectifiers are Schottky too but with a smaller package. The snubber made up of R2 and C7

damps the oscillation produced by the diode D2 at MOSFET turn-on. The output voltage

regulation is performed by secondary feedback on the 12V, 5V and 3.3V output, while for the

1.8V output the regulation is achieved by the transformer coupling. The feedback network uses

a TS2431 driving an optocoupler, in this case a SFH617A-4, ensuring the required insulation

between primary and secondary. The opto-transistor drives directly the COMP pin of the

L6668. A small LC filter has been added on all outputs in order to filter the high frequency ripple

without increasing the output capacitors and a 100nF capacitor has been placed on each

output, very close to the output connector solder points, to limit the spike amplitude.