First, the principle of capacitor step-down
The working principle of the capacitor step-down is to limit the maximum operating current by using the capacitive reactance generated by the capacitor at a certain AC signal frequency. For example, at a power frequency of 50 Hz, a 1 uF capacitor produces a capacitive reactance of approximately 3180 ohms.
When an AC voltage of 220V is applied across the capacitor, the maximum current flowing through the capacitor is approximately 70 mA. Although the current flowing through the capacitor is 70 mA, there is no power consumption on the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary current, and the work done is reactive power.
According to this feature, if we connect a resistive component in series with a 1uF capacitor, the voltage obtained across the resistive component and the power dissipation it generates depends entirely on the characteristics of the resistive component.
For example, we connect a 110V/8W bulb in series with a 1uF capacitor. When connected to an AC voltage of 220V/50Hz, the bulb is illuminated and emits normal brightness without being burned. Because the 110V/8W bulb requires 8W/110V=72mA, it matches the current limiting characteristics of the 1uF capacitor.
Similarly, we can also connect a 5W/65V bulb and a 1uF capacitor in series to 220V/50Hz AC. The bulb will also be lit without being burned. Because the 5W/65V bulb also has an operating current of about 70mA.
Therefore, the capacitor buck is actually using the capacitive reactance current limit. The capacitor actually acts as a limiting current and dynamically distributing the voltage across the capacitor and the load.
The following figure shows the typical application of RC capacitor. C1 is the step-down capacitor. R1 is the bleeder resistance of C1 when the power is off. D1 is the half-wave rectifier diode. D2 provides the discharge circuit for C1 in the negative half of the mains. Otherwise When capacitor C1 is fully charged, it will not work. Z1 is a Zener diode and C2 is a filter capacitor. The output is a stable voltage value of Zener diode Z1.
In practical applications, the following figure can be used instead of the above figure. Here, the Z1 forward characteristic and the reverse characteristic are used, and the reverse characteristic (that is, its voltage regulation characteristic) is used to stabilize the voltage, and the forward characteristic is used in the commercial power. A negative half cycle provides a discharge loop for C1.
In larger current applications, full-wave rectification can be used, as shown below:
At small voltage full-wave rectified outputs, the maximum output current is:
Capacitance: Xc=1/(2Ï€fC)
Current: Ic = U/Xc=2Ï€fCU
Pay attention to the following points when using capacitor step-down:
1. Select the appropriate capacitor according to the current of the load and the operating frequency of the AC, not the voltage and power of the load.
2, the current limiting capacitor must use non-polar capacitors, absolutely can not use electrolytic capacitors. Moreover, the withstand voltage of the capacitor must be above 400V. The most ideal capacitor is an iron-shell oil-immersed capacitor.
3. Capacitor buck cannot be used for high power conditions because it is not safe.
4. Capacitor step-down is not suitable for dynamic load conditions.
5. Similarly, the capacitor step-down is not suitable for capacitive and inductive loads.
6. When DC operation is required, half-wave rectification should be used as much as possible. Bridge rectification is not recommended. And to meet the conditions of a constant load.
Second, the device selection
1. When designing the circuit, first determine the exact value of the load current, then refer to the example to select the capacity of the step-down capacitor. Since the current Io supplied to the load through the step-down capacitor C1 is actually the charge and discharge current Ic flowing through C1. The larger the capacity of C1 is, the smaller the capacitive reactance Xc is, and the larger the charging and discharging current flowing through C1 is. When the load current Io is less than the charge and discharge current of C1, the excess current will flow through the Zener diode. If the maximum allowable current Idmax of the Zener diode is less than Ic-Io, the regulator tube will burn out.
2. To ensure reliable operation of C1, its withstand voltage should be greater than twice the supply voltage.
3. The selection of the bleeder resistor R1 must ensure that the charge on C1 is released during the required time.
Third, the design example
It is known that C1 is 0.33μF and the AC input is 220V/50Hz. The maximum current that the circuit can supply to the load is obtained. 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
Generally, the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as follows: C = 14.5 I, wherein the capacity unit of C is μF, and the unit of Io is A. Capacitor buck power supply is a non-isolated power supply. In application, special attention should be paid to isolation to prevent electric shock.
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