Characteristics of capacitor step-down circuit and selection of components

Characteristics of capacitor step-down circuit and selection of components

In electronic production, in order to reduce the volume and cost, the capacitor voltage reduction method is often used instead of the bulky power transformer. If the capacitor voltage reduction method is used and the components are not selected properly, not only can the voltage reduction requirements not be met, but the circuit may also be damaged. This article introduces how to correctly select capacitor voltage reduction components from the perspective of practical application.
The simplest capacitor voltage reduction DC power supply circuit and its equivalent circuit are shown in Figure 1. C1 is the voltage reduction capacitor, which is generally 0.33~3.3uF. Assuming C1=2uF, its capacitive reactance XCL=1/(2PI*fC1)=1592. Since the on-resistance of the rectifier tube is only a few ohms, the dynamic resistance of the voltage regulator tube VS is about 10 ohms, the current limiting resistor R1 and the load resistor RL are generally 100~200, and the filter capacitor is generally 100uF~1000uF, and its capacitive reactance is very small and can be ignored. If R represents the equivalent resistance of all components except C1, the AC equivalent circuit of Figure 2 can be drawn. At the same time, the condition of XC1>R is satisfied, so the voltage vector diagram can be drawn.
Since R is much smaller than XC1, the voltage drop VR on R is also much smaller than the voltage drop on C1, so VC1 is approximately equal to the power supply voltage V, that is, VC1=V. According to the principle of electrical engineering, the relationship between the average value of the rectified DC current Id and the average value of the AC current I is Id=V/XC1. If C1 is in uF, then Id is in milliamperes. For 22V, 50 Hz AC, Id=0.62C1 can be obtained.
From this, the following two conclusions can be drawn: (1) When using a power transformer as a rectifier power supply, after the various parameters in the circuit are determined, the output voltage is constant, while the output current Id changes with the increase or decrease of the load; (2) When using a capacitor step-down as a rectifier circuit, since Id=0.62C1, it can be seen that Id is proportional to C1, that is, after C1 is determined, the output current Id is constant, while the output DC voltage changes within a certain range with the size of the load resistance RL. The smaller RL is, the lower the output voltage is; the larger RL is, the higher the output voltage is.
The value of C1 should be selected according to the load current. For example, if the load circuit requires a 9V working voltage and the average load current is 75 mA, since Id=0.62C1, it can be calculated that C1=1.2uF. Considering the loss of the voltage regulator VD5, C1 can be 1.5uF, and the actual current provided by the power supply is Id=93 mA.
The voltage regulation value of the voltage regulator should be equal to the working voltage of the load circuit, and the selection of its stable current is also very important. Since the capacitor step-down power supply provides a constant current, which is approximately a constant current source, it is generally not afraid of load short circuit. However, when the load is completely open, the entire 93 mA current will pass through the R1 and VD5 circuits, so the maximum stable current of VD5 should be 100 mA. Since RL is connected in parallel with VD5, while ensuring that RL takes a 75 mA working current, there is still 18 mA current passing through VD5, so its minimum stable current must not be greater than 18 mA, otherwise it will lose the voltage stabilization effect.
The value of the current limiting resistor cannot be too large, otherwise it will increase the power loss and the withstand voltage requirement of C2. If R1=100 ohms, the voltage drop on R1 is 9.3V, then the loss is 0.86 watts, and a 100 ohm 1 watt resistor can be used.
The filter capacitor is generally 100 microfarads to 1000 microfarads, but attention should be paid to the choice of its withstand voltage. As mentioned above, the load voltage is 9V, the voltage drop on R1 is 9.3V, and the total voltage drop is 18.3V. Considering that there is a certain margin, it is better to take the withstand voltage of C2 as above 25V.