This paper introduces a flyback constant voltage, constant current 75W LED driver with single stage (single switch) PFC. Its circuit is shown in the figure. The AC input voltage range is 208~277V, and the output is 24V, 3.125A, which is used to drive 75W. LED array lights.
TOP250YN is a TOPSwitch-GX off-line switch from PI. It is packaged in a 7-pin T0-220-7C package with built-in PWM control and protection circuitry and a 700V power MOSFET.
The method of achieving high power factor with a single-stage flyback converter is to keep the switching duty cycle factor of the MOSFET in the UI constant during a single AC cycle, which requires the current flowing into the U1 control terminal (pin C) to be constant. It is to increase the capacitance of the capacitor c5. A first-stage emitter follower Q1 is added between the UI pins C and U2B, and the well is connected to the capacitor CIO at its base. From the Q1 emitter, if the Q1 current gain is hFE, the CIO is increased to ClOxhFE. The capacitor and c5 - From. Sufficient to keep the current on the U1 control terminal constant, achieving a high power factor of o-978 (@277VAC)-0.992 (@208VhAC), the input current harmonic content is lower than the IEC61000-3-2 allowable limit. Voltage feedback is provided by VR2, R16, and U2A to provide a constant voltage output by controlling the duty cycle of U1 operation. Under no load, the output voltage is limited to 28V. R11, R12, R13, Q2, Q3, Q4 and their associated circuits and U2A form a constant current circuit, and the output current is set at 3.lA (plus or minus 10%).
D12, C15, C16, R18, R19, R20, and 05 form a soft-start circuit to pre-charge capacitor C10 before the output reaches regulation, preventing output overshoot. D12 and C15 produce an independent auxiliary power supply. The rate of rise of the charging voltage on C15 is much faster than the rate of voltage rise on the main outputs of C11 and C12. After 05 is turned on, current enters U2A via R21, and U2B precharges C10, thereby eliminating output overshoot during startup.
The output rectifier uses two diodes, D10 and D11, and is connected to separate secondary windings to reduce power dissipation, improve power distribution between the two diodes, and improve rectification efficiency. The post output filters L5 and C17 are used to reduce the switching noise on the output. The efficiency of the entire circuit is between 85.32% (@277VAC) - 86.01% (@208VAC).
The transformer adopts EER28PC40 core and EER2810 lead frame, the primary winding is 26 turns, the secondary winding is 6 turns, the optocoupler bias winding is 3 turns, and the well contains two shield layers consisting of copper strips.
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