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Is the power filter capacitor's capacity the better?

April 15, 2019

When most readers learn the concept of capacitors, the first application that comes in contact with may be filtering. The most common and simplest single-capacitor filter circuit is shown in the following figure:

The relevant input and output waveforms are shown in the following figure:

The principle of the capacitor filter circuit is very simple: When the input ripple voltage ui is higher than the voltage across the filter capacitor, the capacitor is charged, and when the input ripple voltage ui is lower than the voltage across the filter capacitor, the filter capacitor starts to discharge to bear the power to the load. Responsibility compensates for the decreasing trend of the input ripple voltage ui so as to reduce the ripple level (ripple coefficient) of the ripple voltage. We have also been educated: The larger the filter capacitance, the smaller the output voltage ripple after filtering.

The greater the capacity of the power filter capacitor, the better?

First of all, there is no doubt that the higher the capacity, the higher the cost, but more importantly, the filter capacity is large to a certain extent, and the benefit brought by the capacitance capacity will be less.

As in the previous bridge rectifier filtering, the filter capacitor capacity is from 10uF to 100uF, the ripple voltage improvement is 64V-22V=42V, the ripple improvement from 100uF to 1000uF is 22V-4.24V=19.6V, and from 1000uF to 4700uF The ripple improvement value is only 4.24-1.35=2.89V, as shown in the following figure:

Obviously, it can be seen that the larger the capacity of the filter capacitor, the corresponding ripple voltage is reduced, but the greater the filter capacitance, the less benefits that can be obtained. From an economic point of view, the marginal benefit is smaller. (Low cost), not worth doing;

Second, the necessity of excessive filtering capacity. If a thing is not necessary for implementation, then we do not need to implement it. It seems to be nonsense. However, this is also a rule of suitability followed in circuit design (just enough).

When the ripple voltage of the input ripple DC voltage is controlled within the allowable range by the filter capacitor (circuit), although the output DC voltage is still somewhat fluctuating (not very stable), we believe that the historical mission of the filter capacitor has been Upon successful completion, there will be a voltage regulator circuit behind the filter circuit for more precise regulation, as shown in the following figure:

Every part of the circuit system has its main responsibility. We don't need to spend more effort on the filter circuit to perform tasks that it is not good at. This is the same reason everyone should do what he does best. At the beginning of the article we have already described the purpose of the filter capacitor: reduce the AC ripple voltage (ripple coefficient), rather than output a stable voltage;

Third, the feasibility of excessive filter capacitors. If the capacity of the filter capacitor is too large, the charging current (ripple current) will also increase. Excessive ripple current will be a fatal injury to the circuit system.

If the above two points do not become your reason for using a larger capacity filter capacitor (for example, if you say you are lavish, I would like to do the best product thanksgiving society, to serve the motherland, spend more money do not care), but in With ripple current limitations, you don't want to use capacitors with too much capacitance. (Filter capacitor will say: I don't care if you want to make a good product, but you're going to make me too big and I'm going to damage the circuit. I do not back the pot).

Most readers may have knowledge of the ripple voltage, but in fact there is also Ripple current, which is defined as the maximum AC ripple current the capacitor can withstand at the maximum operating temperature. The RMS value (effective value), and the specified ripple is a sine wave in the frequency range (100Hz to 120Hz).

The performance of ripple current on voltage is ripple voltage (ripple). The maximum allowable ripple current that a capacitor can withstand is limited by parameters such as temperature, loss angle, and AC frequency. In the data sheet, it is usually expressed by IR. The ripple current shown in the figure (the figure below is from the VISHAY Aluminum Electrolytic Capacitor 038 RSU data sheet):

The ripple current of this series Electrolytic capacitor is shown in the figure below:

The figure above is part of the data of the filter capacitor with a voltage resistance of 25V. Under the same process and capacity, the higher the withstand voltage is, the higher the corresponding ripple allowable current is, and the larger the capacity of the filter capacitor is, the bigger the result is. Ripple current?

For the same bridge rectifier filter circuit, when the capacity of the filter capacitor is too large, the relevant waveform is as follows:

When the circuit system is just powered on, the voltage across the filter capacitor is zero, the input ripple voltage ui will gradually increase, and the filter capacitor is charged at the same time. If the capacity of the filter capacitor is too large, the charging speed of the capacitor will be compared. Slow (the voltage rises slowly), when the input ripple voltage ui peaks, the input peak voltage at this time and the voltage difference across the filter capacitor are the highest, and there is no impedance between the two, as shown in the following figure:

The high-voltage and low-resistance state will cause an instantaneous large current. The larger the capacity of the filter capacitor is, the larger the instantaneous charge (ripple) current will be. At this time, the state of the circuit is equivalent to the following figure:

This instantaneous current (also called variable surge current) is likely to exceed the maximum ripple current of the filter capacitor, which will damage the filter capacitor. If the filter capacitor is short-circuited, other related components (such as rectifier diodes, fuses, etc.) Switching tube) may also be reimbursed in an instant.

Of course, in many cases, the power supply filter capacitor must be very large, so we must add a corresponding protection circuit, for example, we can string a current limiting resistor in the circuit, and then use additional relays to switch control, as shown below:

When the power supply is just powered on, the relay switch is turned off, and the current limiting resistor R1 is connected in series in the circuit to prevent excessive ripple current. After the filter capacitor has entered the normal operating state, we close the relay switch. The current limiting resistor R1 is shorted so that the current limiting resistor R1 can avoid unnecessary power consumption.

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