TOP250YN 75W Flyback LED Driver

(Note that the two circuit diagrams need to be combined into a complete circuit diagram)

This article introduces a flyback constant voltage/constant current 75W LED driver with single-stage (single switch) PFC. The circuit is shown in the attached figure. The input voltage range of Ac is 208~277V. The output is 24V, 3.125A, which is used to drive a 75W LED array lamp.

TOP250YN is a TOPSwitch-Gx offline switch launched by PI. It adopts 7-pin TOP-220-7C package. It has built-in PWM control and protection circuit and 700V power MOSFET.

The method of achieving high power factor using a single-stage flyback converter is to keep the switching duty factor of the MOSFET in U1 constant within a single AC cycle, which requires that the current flowing into the control terminal (pin C) of U1 remain unchanged. The method is to increase the capacitance of capacitor C5. Add a first-stage emitter follower Q1 between U1 pin C and U2B, and connect capacitor C10 to its base. From the emitter of Q1, if the current gain of Q1 is hfe, C10 increases to C10×hfe. This capacitor, together with C5, is sufficient to keep the current on the control terminal of U1 constant, achieving a high power factor of 0.978 (@277Vac) to 0.992 (@208Vac), and the input current harmonic content is lower than the allowable limit of IEC61000-3-2.

Voltage feedback is provided by VR2, R16 and U2A, which provide constant voltage output by controlling the duty cycle of U1 operation. Under no-load condition, the output voltage is limited to 28V. R11, R12, R13, Q2, Q3, Q4 and their related circuits and U2A form a constant current circuit, setting the output current at 3.1A (±10%).

D12, C15, C16, R18, R19, R20 and Q5 form a soft start circuit. It can pre-charge capacitor C10 before the output reaches the regulated voltage to prevent output overshoot. D12 and C15 generate an independent auxiliary power supply. The charging voltage rise rate on C15 is much faster than the voltage rise rate on the main output of C11 and C12. After Q5 is turned on, the current enters U2A through R21, and U2B pre-charges C10, thereby eliminating the output overshoot during startup.

The output rectifier uses two diodes D10 and D11. They are connected to separate secondary windings, which can reduce power dissipation and improve power distribution between the two diodes. Improve rectification efficiency. Post-output filter L5 and C17 are used to reduce switching noise on the output. The efficiency of the entire circuit is between 85.32% (@277VAC) and 86.01% (@208VAC).

The transformer uses an EER28 PC40 core and an EER28 10-pin bobbin. The primary winding has 26 turns. The secondary windings are 6 turns each, and the optocoupler bias winding has 3 turns. It also has two shields consisting of 1 turn of copper tape.