Capacitor step-down current-limited power supply

The conventional method of converting AC mains power into low-voltage DC is to use a transformer to step down the voltage and then rectify and filter it. When limited by factors such as size and cost, the simplest and most practical method is to use a capacitor step-down power supply.

The basic circuit of a capacitor step-down simple power supply is shown in Figure 1. C1 is a step-down capacitor, D2 is a half-wave rectifier diode, D1 provides a discharge circuit for C1 during the negative half cycle of the mains power, D3 is a Zener diode, and R1 is a shutdown power supply. After the charge discharge resistor C1. In practical applications, the circuit shown in Figure 2 is often used. When a larger current needs to be provided to the load, the bridge rectifier circuit shown in Figure 3 can be used. The unstabilized DC voltage after rectification will generally be higher than 30 volts, and will fluctuate greatly with changes in load current. This is due to the large internal resistance of this type of power supply, so it is not suitable for large current power supply. application occasions.

Device selection

1. When designing the circuit, you should first determine the accurate value of the load current, and then refer to the example to select the capacity of the step-down capacitor. Because the current Io provided to the load through the buck capacitor C1 is actually the charge and discharge current Ic flowing through C1. The greater the capacity of C1 and the smaller the capacitive reactance Xc, the greater the charge and discharge current flowing through C1. When the load current Io is less than the charge and discharge current of C1, excess current will flow through the voltage regulator tube. If the maximum allowable current Idmax of the voltage regulator tube is less than Ic-Io, it will easily cause the voltage regulator tube to burn out.

2. To ensure that C1 can work safely, its withstand voltage should be greater than twice the power supply voltage.

3. The selection of the discharge resistor R1 must ensure that the charge on C1 is discharged within the required time.

Design examples

In Figure 2, it is known that C1 is 0.33μF and the AC input is 220V/50Hz. Find the maximum current that the circuit can supply to the load.

The capacitive reactance Xc of C1 in the circuit is: Xc=1/(2 πf C)= 1/(2*3.14*50*0.33*10-6)= 9.65K

The charging current (Ic) flowing through capacitor C1 is: Ic = U / Xc = 220 / 9.65 = 22mA.

Usually the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as: C=14.5 I, where the capacity unit of C is μF and the unit of Io is A.

Capacitor step-down power supply is a non-isolated power supply. Special attention should be paid to isolation during application to prevent electric shock.