The input circuit of off power supply is an important circuit component of switching power supply. How to calculate and select the resistance and capacitance? Related content will be shared below. Most high quality switching power capacitors and resistors are a good match and fit.
I. Discharge resistance
The discharge resistor R1 should be selected as small as possible so as to leave enough space for the capacity selection of X capacitor. The selection of R1 should also consider voltage resistance (usually metal oxide film resistors are chosen, with a voltage reduction of 0.75) and power consumption (a reduction of 0.6 power rating). Assuming that the rated power of the selected resistor is PR and the maximum effective value of the input voltage is VINmax, then R1 "(Vinmax) 2 / (0.6×PR)
1. For example, PR = 2W, vinmax = 300V, R1> 75K, R1 = 100K. Another limitation of R1 is that instantaneous power consumption cannot exceed four times the rated power. The maximum instantaneous power consumption of R1 is related to the remaining voltage after a surge or lightning strike through the protection circuit. When the residual voltage is 1200V, R1 should also meet the following requirements: R1 12002 / (4×Pr)
2. By substituting Pr = 2W into the above equation, R1> 180K is obtained. Therefore, R1 = 100k does not satisfy this condition. Therefore, R1 = 200K is reasonable. It should be noted here that the position of R1 is also important considering the instantaneous power consumption of the discharge resistor R1. Putting R1 in the front is obviously inappropriate, but it's best to put it in the middle or back.
To further reduce R1, two or more parallel resistors can be used, depending on the situation. When two resistors of a 50A battery are in parallel, the discharge resistance is R1 = 100K.
II. X and Y capacitors
1. X capacitance
1) Selection of X capacitor
The choice of X capacitor is limited by discharge time. As required by safety regulations, the time from discharge of the input voltage to peak safety voltage of 42.4 V is less than 1s, which can be estimated by the following empirical formula: CX is the sum of all X capacitors. Cx "1 / (2.2×R1)
2) R1 = 100K is substituted into the above equation to obtain: Cx < 4.5uF, take Cx = 4.4uF, there are two capacitors in total, and the capacity of each X capacitor is 2.2uF.
3) Frequency Characteristics of Type X capacitors (low ESR and ESL)
For capacitors of the same material, the smaller the capacity, the better the frequency characteristics. The typical frequency characteristic of a capacitor is that the total equivalent capacitance reactance decreases as the frequency increases, but the capacitance reactance increases at a certain frequency. If this frequency is defined as the turning frequency of the capacitance reactance, the smaller the capacitance, the higher the turning frequency. Therefore, in order to obtain the same capacitance, multiple small capacity capacitors can be connected in parallel, which can improve the high frequency characteristics of the capacitor.
4) Voltage resistance requirements for X capacitors
The choice of X capacitors should also consider voltage resistance (derated according to 0.6 of the rated voltage) : since the X capacitor is near the input of the power line, it must be able to withstand transient high voltage (up to 1200V).
In summary, 2.2uF capacitors can be selected for each X capacitor in the circuit. Its rated voltage is 275VAC and instantaneous voltage is 1500VAC / 1s and 2500VAC / 0.1s.
2. Y capacitor
1) The choice of Y capacitor
The choice of Y capacitor capacity is limited by the leakage current. According to safety regulations, leakage current from phase line or neutral line to ground shall not exceed 3.5 mA at rated input voltage. Assuming that the capacitance of the phase line or neutral line to the ground is Cy, then 220×2πfo× Cy <3.5mA
2) Fo = 50Hz is the power frequency. Substituting the above formula, we get CY = (cy1 + Cy3) = (Cy2 + CY4) < 0.056uF. Considering that the equipment itself has a certain leakage current, Cy = 0.02uF. So each Y capacitor is 0.01uF.
3) For frequency characteristic requirements for Y capacitors, see selection of X capacitors.
When selecting X and Y capacitors, it is important to obtain relatively small capacitance through parallel connections, which will greatly improve the high frequency characteristics of the capacitors. Another important feature of the frequency characteristics of capacitors is that when the frequency is lower than the steering frequency, the relationship between capacitance reactance and frequency is: ZC = 1 / (2? FC), that is, the larger the capacity of a single capacitor, the smaller the capacitance. Reactance However, when the frequency exceeds the rotation frequency of the different capacitors, the total capacitance reactance tends to be the same as the frequency increases. In other words, for UHF (frequencies greater than 50MHz), capacitors of different capacities (for microcontrollers) have the same effect, e.g. 0.1uf equals 0.001uf.
In summary, two 4700pF or three 3300pF capacitors can be used in parallel for y capacitors in the circuit. Rated voltage 275VAC, instantaneous voltage 2500VAC / 1s, 5000Vac / 0.1s. This article can only give you a preliminary understanding of switching power supply. This will help you get started. At the same time, it needs to be constantly summarized so that you can improve your own professional skills.
