This article describes an efficient LED driver that provides 12.5V output voltage, 0.1 A output current drive within the input voltage range of 85V AC to 265V AC. It is a solution for replacing the power supply of passive blocking bucks and low components.
First, design features
1. Global Universal AC Input Voltage Range - a global general precise and stable constant current output
2, small size light
3, replace the cost of the source of passive blocking buck, low cost, small number of components
4, the use of topology is high in failure - protecting the load
5, high efficiency (85 VAC input, efficiency. 1882.60%)
6, reaching EN55022 B to EMI requirements
Second, the circuit schematic
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Figure 1 Circuit Schematic
Third, circuit analysis
The circuit in Fig. 1 is a non-isolated constant current (CC) power source using a buck-boost type topology for driving the LED. Typical applications for nightlights, neon billboards, emergency exit signs, or any applications that use LEDs to illuminate. The AC input voltage is filtered in D1, D2, C1, C2, RF1 and RF2. Use two diodes to improve the tolerance (2 kV) and conduction EMI of the input surge. The resistor RF1 is to use a melting resistance resistance, while RF2 uses a flame retardant resistance.
LinkSwitch-TN uses an on / off control mode to stabilize the output current. When the current flowing into the feedback pin (FB) exceeds 49 μA, the MOSFET switch is prohibited in the next cycle. 49 μA is a threshold current when the pin voltage is 1.65 V (± 7% tolerance), and therefore, the FB pin voltage can be used as a voltage reference.
The pressure drop established on R3 after C4 represents the size of the output current. When the voltage of R3 exceeds 2 V, the voltage of the FB pin exceeds 1.65 V via R1, R2, and thereby there is. 1882.49 μA current flow into the FB pin. The voltage of the R3 2 V sets the output current to 100 mA, that is, each string LED flows through 25 mA. If the load open or output is shorted, there is no feedback signal to the LinkSwitch-TN, then it will enter the automatic restart state (on only 5% of the time). In order to prevent the output voltage at the time of no, the optional feedback circuit composed of VR1 and D4 can be employed. The 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 PIXIS design form. Enter the voltage of the LED string as the output voltage, the sum of the currents of all LEDs as the output current.
Fourth, design points
1. The output and input are not secure isolated.
2, the output voltage variation of the circuit shown in Figure 2 ranges from ± 12% (including temperature variation of 50 ° C).
3. In order to reduce the noise coupling to reduce EMI, the input filter element is to be away from the source and L1 inductance of LinkSwitch-Tn; the DC input filter capacitor C1 and C2 can be placed in the middle of the AC input and U1 / L1 as shielded.
4. Select C4 ≥ 20 (15 ms / R3) to filter current detection voltages. If the C4 value is greater than 50 (15 ms / R3), the linearity of the output constant current can be slightly improved.
5. Select C5 based on the acceptable peak current on the LED string. A larger capacity capacitor reduces the current of the peak LED. The typical value is 100 NF to 100 μF, and the low ESR capacitance is selected. If this capacitor does not, the peak output current is the internal limit of U1.
6. The value of R1 and R2 in the corresponding diagram, R3 can be calculated by R3 = 2V / IO.
7, the total output current provided by the power supply is determined by the number of LED strings, and is limited by the value of L1 and the limit of the U1 limit. For the circuit shown in the figure, the load should be less than or equal to 100 mA and the total power is less than or equal to 1.25 W.
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Figure 2 circuit is connected to 3 string LEDs (4) (10 V, 66.6 mA)
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