Simple method of 220V AC conversion to low voltage DC capacitor Buck

Simple method of 220V alternating current to low voltage DC resistance buck

The conventional method of converting 220V alternating current to low-voltage DC is to use the transformer to buck and then rectify the filter, but when the product is limited by the factors such as volume and cost, the simplest and most practical method is to use capacitive buck mode.
First, the circuit principle
The basic circuit of the capacitor buck simple power supply is shown in Figure 1,c1 as step-down capacitor, VD2 is half-wave rectifier diode, VD1 in the negative half postmenstrual of the mains to provide the discharge circuit, C1 is a voltage regulator diode, VD3 for power off after R1 charge discharge resistor. In practical applications, the circuit shown in Figure 2 is often used. When it is necessary to provide a large current to the load, the bridge rectifier circuit shown in Figure 3 can be used.
The DC voltage after rectification is generally higher than 30 volts, and will fluctuate greatly with the change of load current, this is due to the large resistance of such power supply, it is not suitable for high current supply applications.
Second, the device selection
1. When designing the circuit, the exact value of the load current should be measured, and then refer to the example to select the capacity of the buck capacitor. Because the current IO that is provided to the load through the buck capacitor C1 is actually a charge-discharge current flow through the C1 IC. The larger the C1 capacity, the smaller the tolerant XC, the greater the charge and discharge current flowing through the C1. When the load current IO is less than C1 charge and discharge current, the excess current will flow through the regulator tube, if the maximum allowable current of the regulator tube Idmax less than ic-io easily caused by the regulator tube burned.
2. In order to ensure reliable operation of the C1, its pressure selection should be greater than twice times the supply voltage.
3. The choice of the discharge resistor R1 must be guaranteed to release the charge on the C1 within the required time.

Three, design examples

For example, calculate the load current
Figure 2, the known C1 is 0.33μf, the AC input is 220v/50hz, the circuit can supply the maximum current load. C1 in the circuit of the Tolerance XC is:
Xc=1/(2πf C) = 1/(2*3.14*50*0.33*10-6) = 9.65K
The charge current (Ic) flowing through the capacitor C1 is:
Ic = U/XC = 220/9.65 = 22mA.
The capacitance of the buck capacitor C1 is usually the same as the load current IO: C=14.5*i, where the capacity unit of C is Μf,io unit is a.

Example two, according to the buck voltage element selection

As shown in Figure 2: If the capacitor Buck circuit will be 220V AC Buck to 10V DC, the parameters of each component should be how to choose. (resistors, capacitors, diodes, how large)

The C1 provides a maximum of 68mA current per 1u capacitor depending on the desired current size.

R1 is a discharge resistor, to prevent power loss after electric shock injury, generally take 820k-1m around.

Zener diode VD1 with 10v/1w.

VD2 is a rectifier diode, which can be used with 1n4001~4007.

C2 is a filter capacitor, 100u enough, because this circuit will not have a large current four, the application note
The following points should be noted when using capacitive buck:
1 Select the appropriate capacitance based on the current size of the load and the operating frequency of the AC, rather than on the voltage and power of the load.
2 current-limiting capacitors must use an nonpolar capacitor, and absolutely no electrolytic capacitor. And the voltage of the capacitor must be above 400V. The most ideal capacitance is the oil immersion capacitance of the iron shell.
3 Capacitor buck cannot be used for high power conditions because it is unsafe.
4 capacitive buck is not suitable for dynamic load conditions.
5 Similarly, Capacitance Buck does not fit the capacitive and inductive loads.
6 when DC operation is required, use half-wave rectification as far as possible. Bridge-type rectification is not recommended. And to meet the conditions of a constant load.
7 Capacitive Buck-type power supply is a non-isolated power supply, in the application of special attention to isolation, to prevent electric shock.

Problem:

In Figure two, why the Zener diode is placed before the rectifier diode ...