The problem of capacitor placement in the filter circuit behind the rectifier bridge

Aguilera

The problem of capacitor placement in the filter circuit behind the rectifier bridge [Copy link]

 

For the filter circuit behind the rectifier bridge, we usually choose different capacitor values C, one large and one small, for better filtering.
But there is a question: if the two capacitors are placed in front and behind each other as shown in the figure below, which one will have better filtering effect?
Why? Can you explain the reason?

maychang

I don't know what the frequency of the AC voltage input to the rectifier bridge is. If it is industrial frequency (50Hz or 60Hz), then there is actually no difference between these two connection methods, because the inductance of 3.3mH is too small to have much effect.

Fred_1977

Don't worry too much about this, the difference may not be that big, or it may be related to the actual board, which varies depending on the load.

Aguilera

maychang posted on 2022-3-29 11:57 I don't know what the frequency of the AC voltage input to the rectifier bridge is. If it is industrial frequency (50Hz or 60Hz), then there is actually no difference between these two connection methods, because 3. ...

How to determine the inductance value?

Also, after the inductance value is determined, let's go back to the question on the first floor. What is the difference between the large and small capacitors? Which one has a better filtering effect?

Aguilera

Fred_1977 posted on 2022-3-29 13:38 Don't worry too much about this, the difference may not be that big, or it may be related to the actual board, which varies depending on the load.

Thanks for the answer

maychang

Aguilera posted on 2022-3-29 13:40 How to determine the inductance value? Also, after the inductance value is determined, it goes back to the question on the first floor. What is the difference between the large and small capacitors before and after, and which one is better for filtering...

I said on the 2nd floor: "I don't know what the frequency of the AC voltage input to the rectifier bridge is."

You haven't mentioned the frequency of the AC voltage to be rectified yet. How can there be any "more"?

chunyang

For power frequency filtering, the original poster's circuit is useless, a capacitor is enough.

Aguilera

maychang posted on 2022-3-29 13:46 I said on the second floor: "I don't know what the frequency of the AC voltage input to the rectifier bridge is." Until now, you haven't said the AC voltage to be rectified...

Does this CLC filter circuit have a lot to do with the frequency of the AC mains in front of the rectifier bridge? How to determine the inductance value?

As shown in the figure below, the capacitors are placed in front and back positions. Which one has better filtering effect?

The current mains power is 50Hz

maychang

Aguilera posted on 2022-3-30 17:20 Does this CLC filter circuit have a lot to do with the frequency of the AC mains in front of the rectifier bridge? How to determine the inductance value? The capacitor is placed before and after the following figure...

If the power frequency is 50Hz, chunyang on the 7th floor has given a clear answer: it is of no use, a capacitor will do.

maychang

Aguilera posted on 2022-3-30 17:20 Does this CLC filter circuit have a lot to do with the frequency of the AC mains in front of the rectifier bridge? How to determine the inductance value? The capacitor is placed before and after the following figure...

If the AC frequency is above 20kHz, it must be a switching power supply. However, a switching power supply cannot use this type of bridge rectification plus CLC filtering.

Aguilera

chunyang posted on 2022-3-30 15:41 For power frequency filtering, the original poster's circuit is useless, and a capacitor is enough.

Why is one capacitor enough for power frequency filtering?

I have seen many rectifier bridges with such circuits behind them.

Aguilera

maychang posted on 2022-3-30 17:32 If the AC frequency is above 20kHz, it must be a switching power supply. And a switching power supply cannot use this kind of bridge rectifier plus CLC filtering.

Now I want to ask about the size of these two capacitors and whether there are any EMI issues before and after.

Aguilera

maychang

Aguilera posted on 2022-3-30 17:32 Why can we see such a circuit behind many rectifier bridges with only one capacitor for power frequency filtering?

"I have seen many rectifier bridges with such circuits behind them."

I have already said on the second floor: 3.3mH inductance is too small at industrial frequency and has no effect.

The power frequency rectifier uses CLC filtering, and the inductance should be measured in H, at least a few tenths of H (low voltage and high current). Your capacitor has a withstand voltage of 400V and a capacity of only 22uF, which is obviously high voltage and low current.

maychang

Aguilera 发表于 2022-3-30 17:39

Calculate it yourself: L1 is 1mH, under power frequency conditions, is the inductive reactance of this inductor larger, or is the 10 ohm resistor of RF1 larger?

chunyang

Aguilera posted on 2022-3-30 17:32 Why can we see such a circuit behind many rectifier bridges with only one capacitor for power frequency filtering?

In the past, large-capacity capacitors were more expensive than inductors, especially in high-voltage applications where the grid was directly rectified, so this was necessary. The situation is different now, so don't apply the old calendar. Take apart the power adapter of a digital device, a PC power supply, or even the power supply of a lamp, and look at the filter circuit after AC rectification. Only in special cases should two capacitors be connected in parallel instead of one capacitor, that is, the height of the equipment is strictly limited and a single large-capacity capacitor cannot be installed. In this case, two capacitors are connected in parallel instead, and generally two capacitors of the same capacity are connected in parallel. Another situation is filter capacitor + high-frequency decoupling capacitor. In this case, the decoupling capacitor with a small capacity cannot be treated as a filter capacitor.

Aguilera

chunyang posted on 2022-3-30 19:11 In the past, large-capacity capacitors were more expensive than inductors, especially in high-voltage applications where the grid is directly rectified, so this was the only way to go. Now the situation has long been...

The teacher mentioned these situations and then carefully check the information.

I have been busy these days and have not been able to reply to your posts.

Thank you teacher for your patience

Aguilera

maychang posted on 2022-3-30 17:52 Calculate it yourself: L1 is 1mH, under power frequency conditions, is the inductance of this inductor larger, or is the 10 ohm resistor of RF1 larger?

OK

PowerAnts

I remember that more than ten years ago, Lou Gong (awey) said something somewhere: What does the filter filter depend on, whether it benefits others or yourself?

Filtering twice the industrial frequency is certainly not enough, but if it is to filter high-frequency noise, it may not be enough.

Putting large capacitors on the load side: benefits yourself! Putting them on the input side: benefits others!

Aguilera

PowerAnts posted on 2022-4-7 19:22 I remember that more than ten years ago, Mr. Lou (awey) said something somewhere: It depends on what the filter is filtering, whether it is beneficial to others or to oneself. Filtering twice the power frequency is of course not enough. If...

Sorry, I'm stupid.

What is the function of the capacitor on the right? I think the capacitor on the right is to filter out the noise from the power grid and affect the power supply.

For the picture on the first floor, it means that there is still a difference in the size and placement of these two capacitors, such as front and back.