AN2521

Main characteristics and circuit description

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achieve a compromise between the transformer size and the harmonics of the switching

frequency, thereby optimizing the input filter size and the total solution cost. The power

MOSFET is a standard, inexpensive 800 V component housed in a TO-220FP package,

requiring a small heat sink. The transformer is the layer type, using the standard ferrite core

EER35. The transformer is manufactured by TDK and designed in accordance with

EN60950. The reflected voltage is ~130 V, providing sufficient room for the leakage

inductance voltage spike while maintaining a margin for the reliability of the power MOSFET.

The rectifier D8 and the Transil D4 clamp the peak of the leakage inductance voltage spike

at turn-off of the power MOSFET.

The controller L6668 offers maximum flexibility by integrating all the functionality needed for

high performance SMPS control with a minimum component count. A new feature

embedded in the device is a high voltage current source used at start-up which draws

current directly from the dc bus and charges capacitor C33. After the voltage on C33 has

reached the L6668 turn-on threshold and the circuit starts to operate, the controller is

powered by the transformer via the auxiliary winding and diode D11. After start-up, the HV

current source is deactivated, saving power during normal operation and allowing very good

circuit efficiency during standby.

The L6668 utilizes a Current Mode control system, so the current flowing through the

primary winding is sensed by R52 and R53 and is then fed into pin #12 (ISEN). Resistor

R41 connected between pin #12 (ISEN) and pin #15 (S_COMP) provides the correct slope

compensation to the current signal, necessary for correct loop stability in CCM mode at duty

cycles greater than 50%. The circuit connected to pin #7 (DIS) provides over-voltage

protection in case of feedback network failure, while the thermistor R58 provides for a

thermal protection of the power MOSFET (Q5). This pin is also connected to the

PWM_LATCH pin of the L6563 which is dedicated to stopping activity of the flyback

converter in case of PFC loop failure that could be damaging to the circuit. To definitively

reactivates the HV current source to supply the IC. After OVP detection and L6668 Disable

intervention, circuit operation can be resumed only after disconnection of 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 pin #13

(STBY) and resistor R42, proportionate with the load consumption. The resistor divider R60

and R61 connected to pin #9 (SKIPADJ) allows setting of 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 5 V reference, available externally.

Circuit regulation is achieved by modulating the voltage on the COMP pin (#10), by means

of the optocoupler U3. Also connected to the COMP pin is the Q6, Q8, R44, R62, C42 and

D13 network, which is dedicated to driving ISEN over its hiccup mode threshold in case of

overload or short condition. In this case the device will be shut down and its consumption

will decrease almost to pre-start-up level. The device will resume operation as soon as the

invoked until the short is removed. A short on-time and long off-time of the hiccup mode are

obtained allowing the average current flowing in the secondary side components to be kept

at a safe level, avoiding consequent catastrophic failures due to their overheating.

Output regulation is done by means of two loops, a voltage and a current loop working

alternately. A dedicated control IC, the TSM1014, has been used. It integrates two

operational amplifiers and a precise voltage reference. The output signal of the error

amplifiers drives optocoupler SFH617A-4 to transfer the information to the primary side and

achieve the required insulation of the secondary side. The output rectifier D7 is a dual

common-cathode Schottky diode. The output rectifier has been selected according to the