In the LED driving power supply circuit based on the principle of capacitor voltage reduction, the voltage reduction principle is roughly as follows: when a sinusoidal AC power supply u is applied to the capacitor circuit, the charge on the two plates of the capacitor and the electric field between the plates are functions of time. That is to say: the effective value and amplitude of voltage and current on capacitor also follow Ohm’s law. That is, when the voltage amplitude and frequency on the capacitor are fixed, a stable sinusoidal AC current will flow. The smaller the capacitive reactance is, the greater the capacitance value is, and the greater the current flowing through the capacitor is. If a suitable load is connected in series on the capacitor, a reduced voltage source can be obtained, which can be output through rectification, filtering and voltage stabilization. A problem that needs to be noticed here is that in this circuit system, the capacitor only uses energy in the circuit, but does not consume energy, so the efficiency of the capacitor buck circuit is very high.

Usually, the main driving circuit of LED power supply based on the principle of capacitor buck will be composed of Buck capacitor, current limiting circuit, rectifying filter circuit and voltage stabilizing shunt circuit. Among them, the step-down capacitor is equivalent to the step-down transformer in the ordinary voltage stabilizing circuit, which is directly connected to the AC power supply circuit and bears almost all the AC power supply u, so the metal film capacitor without polarity should be selected. At the moment when the power is turned on, it may be the peak to peak value of the positive or negative half cycle of U. at this time, the instantaneous current will be very large. Therefore, a current limiting resistor needs to be connected in series in the circuit to ensure the safety of the circuit, which is the main reason why the current limiting circuit is indispensable. The design requirements of the rectifier and filter circuit are the same as that of the ordinary DC regulated power supply circuit. The reason why a voltage stabilizing shunt circuit is needed is that in the voltage reducing circuit, the effective value of current I is stable and not affected by the change of load current. Therefore, in the voltage stabilizing circuit, there should be a shunt circuit to respond to the change of load current.