Design Of Primary Side Regulated Flyback Controller For LED Driver

White LEDs have the characteristics as follows: small, rugged, high luminous efficiency,energy saving, environmental and so on. Due to its advantages, it has broad applicationprospects. However, it’s necessary to use appropriate drive circuits to make full use of LEDs.Usually constant current driver is a good choice for LEDs.The task of this paper is to design a Primary Side Regulated flyback converter controllerthat can be used as LED driver. The solution proposed in this paper senses the output voltageand output current via the auxiliary winding, eliminating TL431and optical coupler on thesecondary side, decreasing the PCB size, reducing the cost and complexity and thus raisingthe overall efficiency. With optimized operating mode, both Constant Current and ConstantVoltage functions work in Pulse Frequency Modulation Mode (PFM), saving a specialfrequency hopping circuit to reduce EMI; At light loads conditions switching frequencydecreases in order to diminish the loss of power transistor, increasing overall efficiency.The entire control circuit consists of six sub-circuits: Power Management Unit_Over-Voltage Protection Circuit (PMU_OVP), Constant Current Function Circuit, DimmingFunction Circuit, Constant Voltage Functional Circuit, Logic Control Circuit, and Totem PoleDriver circuit. PMU_OVP provides power management for the entire control circuit,generating analog and digital power supply, reference voltages and reference currents andreset signal; more over, the over-voltage informations in VDD and Vo are latched byPMU_OVP, encouraging the Over-Voltage-Protection Operation. Constant Current FunctionCircuit makes the peak current Ipk in primary side of the transformer fixed per switchingcycle, detects the demagnetization time-tdem, and forces the switching cycle ts equal to twotimes the demagnetization time to achieve constant current output. It includesOver-Current-Protection circuit, DEM_detector and Constant_Current_Control circuit.Over-Current-Protection circuit realizes fixed Ipk function and provides soft-start and LeadEdge Blanking (LEB); the demagnetization time is detected by DEM_detector;Constant_Current_Control circuit makes ts=2tdem. A new dimming method that includesAnalog Dimming and PWM Dimming for PSR is provided by Dimming Function Circuit;User can choose flexibly either of these two ways to meet the corresponding requirements.More over, non-uniformity of Analog dimming function is studied and improved in this paper.Consant Voltage Function Circuit is compact and effective, clamping the outpout voltage,preventing the output voltage to jump up to damage the system. Logic Control Circuitdetermines the system to operate in CC, CV or Dimming state. Also, the maximum and minimum off-time is determined by Logic Control Circuit. Totem Pole Driver circuit convertslow voltage to high voltage to drive the external Power MOSFET.All sub-circuits and modules are designed and verified in Cadence. At last, the overallcircuit is built and tested by the CSMC0.5um40V thick oxide double poly BCD ST3100process spice model. The simulation result shows that the overall system built with the controlcircuit is able to work in the Whole Voltage Range (90V-270VAC); Moreover, high constantcurrent precision,2%is obtained; Analog Dimming and PWM Dimming can both workcorrectly, achieving large dimming ratio.