Summary:
Figure 1 shows a typical 20 V, 14 W constant voltage (CV), constant current (CV) output power supply circuit. The amount of light output from 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 housing 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-shaped filter and provides EMI filtering. Fuse F1 provides protection in the event of a serious fault.
In order to operate the power supply under no load without damage, the Zener diode VR2 is used for constant voltage adjustment and the voltage is maintained at approximately 21 V.
The constant current characteristic is achieved by detecting the voltage drop across the current detecting resistor R7. The shunt regulator IC U3 works with R9, R8, and R8A to generate an accurate voltage reference of 0.07 V at the inverting input of op amp U2. When the set current is reached, the voltage on R7 will exceed the reference voltage, which will increase the output of the op amp. At this point, D4 is forward biased, driving the base of Q1, which pulls the current out of the EN/UV pin of U1. Capacitor C7 and resistor R11 provide loop compensation. The current-sampling voltage is minimized by the op amp's current-limit method, which reduces losses and maximizes efficiency.
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