First, the design features
1. Global universal AC input voltage range—a constant current output with a globally accurate and stable design. 2. Small size and light weight. 3. A low-cost, low-cost solution with a small number of components. The topology used is highly secure in the event of a fault – the load can be protected 5, high efficiency (efficiency >60% at 85 VAC input)
6. Meet the requirements of EN55022 B for EMI
Second, the circuit schematic
Figure 1 circuit schematic
Third, the circuit analysis
The circuit in Figure 1 is a non-isolated constant current (CC) power supply using a buck-boost topology to drive the LEDs . Typical applications are night lights, neon billboard replacements, emergency exit signs or any application that uses LEDs for illumination. The AC input voltage is rectified and filtered by D1, D2, C1, C2, RF1 and RF2. The use of two diodes improves input surge tolerance (2 kV) and conducted EMI. Resistor RF1 uses a fusible flame retardant resistor, while RF2 uses a flame retardant resistor.
LinkSwitch-TN uses an on/off control to stabilize the output current. When the current flowing into the feedback pin (FB) exceeds 49 μA, the MOSFET switch is disabled in the next cycle. The 49 μA is the threshold current at a pin voltage of 1.65 V (±7% tolerance), so the FB pin voltage can be used as a voltage reference.
The voltage drop established on R3 after averaging C4 represents the magnitude of the output current. When the voltage of R3 exceeds 2 V, the voltage at the FB pin exceeds 1.65 V through the resistor divider of R1 and R2, so that a current of >49 μA flows into the FB pin. The 2 V voltage across R3 sets the output current to 100 mA, which is 25 mA per string of LEDs. If the load is open or the output is shorted and there is no feedback signal to the LinkSwitch-TN, it will enter the auto-restart state (only 5% of the time is on). To prevent excessive output voltage during no-load, an optional feedback circuit consisting of VR1 and D4 can be used. The value of VR1 is higher than the normal output voltage.
The size of the inductor L1 can be selected according to the LinkSwitch-TN Design Guide (AN-37) or the PIXIs design spreadsheet. The voltage of the LED string is input as the output voltage, and the sum of the currents of all the LEDs is used as the output current.
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