# Calculation method of electrolytic capacitor on input side and output side

We generally calculate according to how many percentage points the ripple voltage on the electrolytic capacitor is required to be below the minimum input voltage and maximum output. Of course, if there is a hold time requirement, it needs to be recalculated according to the hold time requirement, and the larger value of the two is used.

Calculation of electrolytic capacitor on the input side:

We generally calculate according to how many percentage points the ripple voltage on the electrolytic capacitor is required to be below the minimum input voltage and maximum output. Of course, if there is a hold time requirement, it needs to be recalculated according to the hold time requirement, and the larger value of the two is used.

If under the lowest input voltage, the input power of the power supply is Pin and the lowest input AC voltage effective value is Vinacmin, then we generally think that the rectified DC voltage at this time is Vinmin=1.2×Vinacmin, because during the two AC charging cycles, The power supply to the following converters is guaranteed by capacitor energy storage, so the voltage drop can be calculated: C×ΔV=I×Δt, ΔV is the voltage ripple, generally 10%-20% of Vinmin, I It is the discharge current of the capacitor to the following circuit=Pin/Vinmin and Δt is the time interval between two charges (that is, the discharge time of the capacitor in a power frequency cycle), which can be considered as 0.8×1/(2×fac), to put it plainly That is, in the half-sine cycle after AC rectification, 80% of the time is supplied to the subsequent converter by the energy storage of the electrolytic capacitor.

Then we can calculate the filter electrolytic capacitor capacity after AC rectification at the input.

Calculation of electrolytic capacitor on the output side:

Electrolytic capacitor on the output side. The electrolytic capacitor on the output side works at high frequency, and the ripple current has a great influence on it. We generally calculate the electrolytic capacitor on the output side according to the limiting conditions of the ripple current. The relationship between the effective value of the ripple current on the electrolytic capacitor and the effective value of the current of the secondary rectifier diode and the output current is:

Electrolytic capacitor manufacturers usually give the rated ripple current IRCrms of the electrolytic capacitor at a certain frequency and a certain temperature. But in actual use, we need to consider temperature effect and frequency effect. The ripple current that the actual capacitor can use is IRCrms×temperature coefficient×frequency coefficient. Different manufacturers may provide different reference points for temperature coefficient and frequency coefficient, so please pay attention to the conversion. If the manufacturer does not provide, then the following values ​​can be used for reference:

Temperature Coefficient:

105°C: 1

85°C: 1.7

65°C: 2.1

Frequency factor:

100KHz: 1

10KHz: 0.9

1KHz: 0.8

120Hz: 0.5

50Hz: 0.32

If the ripple current of a single electrolysis is not enough, multiple ones can be used in parallel. In addition, the use of multiple in parallel also helps to reduce the output voltage ripple.

Whether the choice of the actual final electrolytic capacitor is appropriate, in addition to ensuring sufficient voltage margin. The more important thing is the temperature and temperature rise of the electrolytic capacitor. Every time the temperature of the electrolytic capacitor increases by 10 degrees, the life span is halved. Therefore, the operating temperature of the electrolytic capacitor will be limited by the design life of the power supply.

On the other hand, because the temperature rise of the capacitor may be caused by external heat, it may also be caused by its own loss. Therefore, we have another restriction here, that is, the self-temperature rise <5℃.