This article describes the proposed solution for an European input 33.6W LED ballast using the FAN7527BM PFC controller, FAN4274IMU8 Dual RRIO CMOS Amplifier and the FQPF6N70 QFET. The input voltage range is 185VRMS – 265VRMS and there is one DC output with a constant current of 700mA @ 48Vmax.

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Schematic Diagram

The PSU is a constant current output flyback converter with high power factor using the FAN7527B PFC Controller (IC1) and the FAN4274IMU8 Dual RRIO CMOS Amplifier (IC202). The input voltage is rectified by the bridge rectifier consisting of D100…D103. Filtering is done by C102 to generate a DC voltage for the input of the flyback converter. The X2-Cap C101, common mode choke LF101 together with X1/Y1-Cap C108 act as EMI filter. D105, R114 and C107 form a clamping network that limits the voltage spike due to the energy trapped in the leakage inductance of the transformer. The FAN7527B is a PFC Controller which works in boundary conduction mode. R101 and R102 are for start-up. D107, R106 and C105 with winding W5 (pins 5/6) of the transformer form the Vcc voltage supply network. R104, R105, R116 and the parallel circuit of R103 and C103 form a voltage divider network for the multiplier input of the controller (pin3). R107, R108 and C104 form a compensation network for the output of the E/A amplifier.

Feedback signal from the optocoupler (IC101) output is fed by a voltage divider R111/112 to the inverting input of the E/A amplifier (pin1). R110 is for Zero Current Detection (ZCD) and is fed into the IDET pin5. Current sense signal is generated by R113 and fed into CS pin4. R109 is the gate resistor for the QFET (Q101). The R101/R102 and R104/R105/R116 resistors were implemented as pairs to ensure that the maximum voltage rating is not exceeded.

Constant voltage and constant current control are achieved by two circuits grouped around two opamps which are combined in one IC (IC202). One opamp (IC202A) is for constant current control and IC202B is for constant voltage control. Their outputs are ‘OR’ed together by D202 and D203. The programmable shunt regulator KA431 (IC201) delivers a reference voltage (Vref) of 2.5V. IC201 and IC202 are supplied by a simple linear regulator consisting of Q201, R211 and D204. This regulator gets its input voltage from winding W2 (pins 8/10) of T1 in combination with D200 and C200. For constant voltage control IC202B is used to compare Vref with a fraction of Vout.

This fraction is created by dividing Vout down to 2.5V with R206/210. The inverting input of IC202B is always at Vref. If Vout is below 48V the non-inverting input is below 2.5V and therefore the output is zero. If Vout is above 48V the non-inverting input is above 2.5V and therefore the output increases. This positive output then drives a current thru D203, R208 and the internal LED of IC101. R207 and R209 compensate the voltage control loop.

PCB Layout

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Bill of Materials

Source: http://www.fairchildsemi.com