Capacitor buck power supply principle and precautions

In the common small low-voltage power supply, in addition to the switching power supply module, there is also a power supply that utilizes a capacitor buck (actually a capacitor current limit). The capacitor buck power supply has the characteristics of small size, economy, reliability, high efficiency, etc., but the disadvantage is not as good as the power supply with transformer transformer.

The capacitor step-down power supply introduces alternating current into the load through the capacitor, and has 220V voltage to the ground, which is easy to cause electric shock. If it is applied to an internal circuit that does not require human contact, the disadvantage is not a disadvantage. Capacitor buck power supplies are low-cost and non-isolated low-current power supplies. For example, LED bulbs used in plastic housings can output voltages ranging from a few volts to tens of volts, and the output current is also in the tens of milliamps. There are also electronic power conditioners for refrigerators or switches for remote control power supplies, which are made by depressurizing capacitors.

The working principle of the capacitor step-down power supply is to limit the maximum operating current by utilizing the capacitive reactance generated by the capacitor at a certain AC signal frequency. Compared with the resistor step-down, for the lower frequency 50Hz AC, the thermal energy loss generated on the capacitor is small, and the capacitor step-down is better than the resistor step-down.

Note on the use of capacitor step-down circuit:

1. After the capacitor is stepped down, it needs to be sorted, filtered and stabilized by the voltage regulator to obtain a voltage stable power supply (the rectifier circuit can be half-wave rectified).

2. According to the current of the load and the working frequency of the AC, select the appropriate capacitor. The capacitor withstand voltage is preferably above 630V. Capacitors with polarity cannot be used.

3, the current limiting capacitor must be connected to the live line, and add string anti-surge impact and fuse resistors and parallel discharge resistors.

4. If the power switch is added, it is necessary to connect the load in parallel to prevent the inrush current.

5. Pay attention to the power consumption of the Zener tube. It is forbidden to disconnect the Zener tube.

6. The capacitor step-down circuit is not suitable for capacitive loads, inductive loads, dynamic loads and high-power loads. Generally, the current is small and unsafe.

7, does not apply to the microcontroller, the general interference is serious.

8. During the assembly and debugging process, the 1:1 isolation transformer must be connected to the AC220V circuit to prevent electric shock.

The output DC voltage of the capacitor step-down circuit is not isolated from the input AC power source and is commonly used in electronic devices that do not need to be isolated. For example, in the control, detection, and analysis of electronic devices, in electronic devices such as household appliances, especially in the field of small household appliances has a wide range of applications.