First, the design features
1, very high efficiency: 82%
2, the number of components is small: only 40 components
3. It does not require common mode inductance to meet the requirements of EN55022B for conducted EMI.
4, the valley filling circuit makes the power supply meet the IEC61000-3-2 THD limit
5, ON / OFF control to suppress the higher power frequency ripple voltage caused by the valley filling (THD correction) circuit
Second, the circuit schematic
Figure 1. Schematic of an LED lamp driver with passive PFC
Third, the way of working
The flyback converter shown in Figure 1 uses a device in the TinySwitch-III family (U2, TNY279PN) to deliver up to 1.8 A of load current to six high-brightness lumen LEDs (LXHL series).
The output voltage is slightly lower than the forward voltage drop of the LED string. Therefore, when the LED string is connected to the power source, the power source operates in constant current (CC) mode. If the LED string is not connected to the power supply, the voltage regulator VR1 provides voltage feedback to adjust the output voltage to approximately 13.5 VDC. A 100 mW resistor (R11) senses the output current and drives the optocoupler through an op amp (U1) to provide feedback to U2. TinySwitch-III devices are regulated by turning off or skipping the MOSFET switching cycle. When the load current reaches the current setting threshold, U1 drives U3 to turn on. The phototransistor in U3 pulls current from the EN/UV pin of U2, causing U2 to skip the cycle. Once the output current drops below the current setting threshold, U1 stops driving U3, U3 stops pulling current from the EN/UV pin of U2, and the switching cycle is re-enabled. The TL431 (U4) provides a reference voltage to U1 for comparison with the voltage drop across R11.
The output rectifier (D9) is located on the lower leg of the secondary winding of the transformer (T1) to reduce EMI noise. RCD clamps (R16, C4, and D13) protect the MOSFET drain from flyback voltage spikes.
The valley fill circuits (D5, D6, D7, C15, C16, and R15) limit the 3rd and 5th harmonic values ​​of the power frequency current so that the power supply meets the total harmonic distortion (THD) requirements specified in IEC61000-3-2.
U2's frequency jittering function, the shield winding in T1 and the Y capacitor across C1 (C8) reduce the generation of conducted EMI, so a simple pi-type filter (C13, L1, L2, and C14) can make the power supply satisfied. Limitation of EN55022B.
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