The working principle of the LED driver is to limit the maximum operating current by using the capacitive reactance generated by the capacitor at a certain AC signal frequency. Therefore, the working principle of the capacitor step-down is not complicated.
At 50Hz, the capacitance generated by a 1uF capacitor is about 3180 Ω.
When the AC voltage of 220V is added to both ends of the capacitor, the maximum current flowing through the capacitor is approximately 70mA.Although the current flowing through the capacitor is 70mA, but does not produce power consumption on the capacitor, if the capacitor is an ideal capacitor, then the current flowing through the capacitor is a virtual current, and its work is reactive power.
The use of capacitive buck schematic diagram is a common small current power supply diagram , with small volume,low cost and current is relatively constant and other advantages, and is also commonly used in the driving diagram of the LED.
The following figure shows an actual LED driver diagram with capacitor step-down: Most of the application diagram do not have a varistor or transient voltage suppression transistor. It is recommended that the connection, due to varistor or transient voltage suppression transistor can effectively discharge the mutation current in the moment of voltage mutation (such as led lightning, large power equipment start, etc.), effectively bleed the abrupt current to protect the diode and other transistors, and their response time is generally in the order of micro milliseconds.
The pressure resistance of the filter capacitor C2,C3 depends on the load voltage, which is generally 1.2 times times the load voltage, and its capacitance capacity depends on the size of the load current.