A typical 20 V, 14 W constant voltage (CV), constant current (CV) output power circuit. The light output of the LED array is proportional to the amount of current flowing through it. Therefore, the LED driver should have a constant current output instead of a constant voltage output. In this design, the DC output is not isolated from the AC input, so the LED array and enclosure should be safely isolated from the user.
The AC input is rectified and filtered by BR1, C1, and C2. Inductor L1, together with C1 and C2, forms a π filter and provides EMI filtering. Fuse F1 provides protection in the event of a severe fault. In order to make the power supply work normally under no load without damage, the Zener diode VR2 is used for constant voltage adjustment and the voltage is maintained at about 21 V.
The constant current characteristic is achieved by detecting the voltage drop across the current detection resistor R7. The shunt regulator IC U3, together with R9, R8, and R8A, generates an accurate voltage reference of 0.07 V at the inverting input of the operational amplifier U2. When the set current is reached, the voltage on R7 will exceed the reference voltage, which will increase the output of the operational amplifier. At this time, D4 ??will be forward biased to drive the base of Q1, and then draw the current from the EN / UV pin of U1. Capacitor C7 and resistor R11 provide loop compensation. The current limiting method using an operational amplifier minimizes the current sampling voltage, thereby reducing losses and maximizing efficiency.
As long as the current drawn by the EN / UV pin exceeds 115 μA, the MOSFET in U1 is disabled on a cycle-by-cycle basis (on / off control). By adjusting the ratio of enabled to disabled switching cycles, the feedback loop can adjust the output voltage or current. The on / off control mode optimizes the converter efficiency under different load conditions at the same time, making it meet the energy efficiency standards.
Due to the high ambient temperature, U1 will operate in reduced current limit mode. This can improve the overall efficiency of the power supply and improve its heat dissipation performance. Primary clamps (D1, VR1, C3, and R3) control the maximum peak drain voltage internally
The 700 V BVDSS breakdown voltage of the MOSFET. Resistor R23 reduces high-frequency leakage inductance oscillation, thereby reducing EMI. The secondary-side output is rectified and filtered by diodes D2, D3, and C6.