Talking about APFC

MrCU204

Talking about APFC [Copy link]

 

 

APFC can be seen as a switching version of parallel regulation.

In linear voltage regulation, the function of the transistor is a dummy load, and the load voltage drop is directly controlled by the transistor (the Zener diode can directly act as a dummy load. If it is used as a dummy load, it is not a benchmark).
The load voltage drop of APFC will also be affected by the working condition of Q1, but DC_output is higher than the power peak and is isolated by D5. Therefore, when Q1 is turned on, the load will not load the grid. The instantaneous load is only determined by Q1. The power factor of the grid depends only on the working condition of Q1 and is not affected by the load.
Optimizing and locking the power factor is the main job of APFC. But why does APFC have to be used as a substitute? Because the load is the only freewheeling channel of the inductor. If the operating mode of this circuit is CCM, the current of the grid can be continuous through the freewheeling of the inductor.

However, unlike LC smoothing, APFC rectification must be full-wave (unless two sets are used). Bridge rectification can kick out the power frequency transformer, but if you want to "filter" out the mains frequency, the capacity of the smoothing capacitor must be similar to that of ordinary capacitor smoothing!

MrCU204

Power supply, there are constant voltage source, constant current source, but what is APFC, it is a constant power factor source!

Why can the power factor be constant? First, DC_output is higher than the power peak, and the load branch is blocked by D5. Second, the AC power is converted by the boost circuit and is equivalent to a current source, which can decouple the load from the power supply voltage.

qwqwqw2088

Active power factor correction corrects the input current waveform by precisely controlling the on and off of switching elements.

I don't know if this is how you understand it. Let's learn it.

不亦心

The bus filter capacitor of PFC is much smaller than that of ordinary rectifier bus filter capacitor.

MrCU204

Most electronic circuits require pure DC power supply.
The boost topology also uses DC. However, this DC can be directly taken from the mains without any "filtering".
If you want to control the power factor, you cannot let any "filtering" link get involved. The boost topology has a boosting effect, but the selling point of APFC is not the boosting, but the direct connection between the circuit and the grid without the intervention of a filtering network.
However, the boosting effect is also necessary for power factor control, allowing the load to draw power from the grid while being decoupled from the grid, which is exactly the boosting effect of self-inductance.

MrCU204

The switching action of Q1 locks the timing of the grid load (phase shift power factor) and also sets the tone for the supply and demand relationship (waveform power factor). The rest is to use the load channel to make the grid load continuous.

Boost is a switching circuit, but the power supply is not pure DC. Although the supply to the smoothing capacitor is almost continuous, it is not a constant amplitude wave but an industrial frequency envelope. Therefore, the capacity of the smoothing capacitor must be larger than that of a general switching power supply.

不亦心

MrCU204 published on 2024-8-11 10:15 The switching action of Q1 locks the timing of the grid load (phase shift power factor) and also sets the tone for the supply and demand relationship (waveform power factor). The rest is to use the load...

Please let the data speak

MrCU204

Bu Yixin published on 2024-8-12 09:00 Please speak with data

I am a layman, and what I publish is not technology or academic, but ideas and understanding.

① Can't do half-wave rectification, why? !

②The circuit topology is always boost, why? !

③It is a substitute for electricity and must carry a load, why? !

④ Smoothing requires capacitors, and the capacitance is larger than that of ordinary switching power supplies. Why? !

qwqwqw2088

What? Do you want to complain about the half-wave rectifier?

The half-wave rectifier circuit only uses one half cycle of the AC power, resulting in large pulsation and low efficiency.
The key is that, for example, in a switching power supply, half-wave rectification may not provide sufficient DC voltage or current. In comparison, full-wave rectification or bridge rectification has obvious advantages.

qwqwqw2088

The circuit topology is always boost, why? !

Who said that? The Boost circuit is just a commonly used boost converter topology. Depending on the specific application requirements, other types of circuit topologies can also be selected, such as Buck, Buck-Boost, etc.

MrCU204

qwqwqw2088 posted on 2024-8-15 08:59 What, do you want to complain about the half-wave rectifier? The half-wave rectifier circuit only uses one and a half cycles of the alternating current, the pulsation is large, and the efficiency is low...

At first, we only tinkered with the phase shift factor, but later, the waveform factor gradually attracted attention. Compared with the entire sinusoidal AC electromotive force, isn’t half-cycle mounting also a kind of "distortion"? !

Therefore, half-cycle mounting is also a solution for low power factor application. Moreover, the AC power is generated by electromagnetic effect. Half-cycle mounting will be detrimental to the generator. Therefore, if you want to use half-wave rectification, you need to use two sets of APFC.

qwqwqw2088

MrCU204 posted on 2024-8-24 08:09 At first, we only tinkered with the phase shift power factor. Later, the waveform power factor gradually attracted attention. Compared with the entire sinusoidal AC electromotive force, half-cycle mounting is also...

Makes sense,

Half-cycle mounting will cause serious distortion of the current waveform because it only uses half a cycle of the AC power. This distortion will reduce the power factor and generate a large amount of harmonics, which is detrimental to the power grid and connected devices.

qwqwqw2088

If the generator is mounted half-way, there will be many problems.

The main issue is the existence of harmonics. The internal losses of the generator are large and may lead to overheating.
Harmonics also affect the life of the generator and
pollute the power grid.

In actual field applications, full-wave rectification is usually used instead of half-wave rectification.

qwqwqw2088

How could the host be a layman?

Your paragraph involves several terms as follows

Phase shift power factor: It is to adjust the phase difference between current and voltage through phase control to improve the power factor.

Waveform power factor: refers to the effect of power waveform distortion on power factor. For example, non-sinusoidal current or voltage will reduce power factor.

Half-cycle mounting: is a rectification technique in which only one half-cycle (positive or negative) of the alternating current is used, while the other half-cycle is ignored.

APFC: It is a power electronics technology used to improve the power factor of the power supply, reduce current harmonics, and keep the input current and input voltage of the power supply in phase.

MrCU204

qwqwqw2088 posted on 2024-8-15 09:02 Why is the circuit topology always boost? ! Who said that? The Boost circuit is just a commonly used boost converter topology. According to the specific...

SCR can only carry DC power, but when it is used as a controlled rectifier, it is still in the AC area.

The power supply used in the switching circuit is mostly pure DC (the switching tube is placed after the rectifier and will not withstand reverse voltage like the SCR in the controlled rectifier). Can you use pure DC from the mains without smoothing the wave? !

However, once passive filtering is used, the power factor is useless, right? Passive filtering is large and heavy, and has poor load adaptability. If you want to get the highest and fixed power factor, you should kick out the passive filtering and let the switch tube and the rectifier be directly coupled. The only way to output pure DC without filtering is probably boost, right? !

qwqwqw2088

MrCU204 posted on 2024-8-24 08:32 SCR can only carry DC power, but when it is used as a controlled rectifier, it is still in the AC area. The power supply used in the switching circuit is absolutely...

I don't know what you mean by
SCR, which stands for Silicon Controlled Rectifier, can only carry DC power.
But when used in a controlled rectifier, it handles the rectification of AC power, but we are concerned with its rectified DC output.

qwqwqw2088

Most of the power supplies used in switching circuits are pure DC, which is true! No objection.
This is because the switch tubes in the switching circuit are usually placed after the rectifier. They will not withstand the so-called reverse voltage like the SCR in the controlled rectifier. When obtaining pure DC power from the mains, don’t they need to be rectified and filtered?

qwqwqw2088

As for
"passive filtering is large and bulky, and has poor load adaptability",
you can consider using active filtering or other more advanced filtering technologies instead of just relying on passive filtering.

Boost is just a switch mode power supply or a topology form that converts low DC to high DC. The output DC voltage still contains a certain amount of ripple.
To achieve a completely unfiltered pure DC output, more complex circuit design and control methods may be required.

MrCU204

qwqwqw2088 posted on 2024-8-24 11:49 As for "passive filtering is both large and cumbersome, and the load adaptability is also poor", you can consider using active filtering or other more advanced filtering technologies...

Thank you for your advice!

It is big and heavy. In addition to the industrial frequency filter components, there is another one, which is the center-tapped transformer. A few years ago, I bought a 300-watt one with an output of 12V×2 and a size of 15cm×15cm×15cm. An APFC of the same wattage is probably not half the size of this one, and its weight is not even 10% of it. So, for full-wave rectification used in switching circuits, the dual half-wave solution is absolutely inappropriate. Using a bridge rectifier is the way to go!

The thyristor has no other function except to act as a switch, but it seems that the controlled rectifier is not classified as a switching circuit? !

It seems that the use of triodes in switching circuits has become a default practice. However, if you want to control the power factor of the power grid, there is nothing simpler and more direct than directly coupling the APFC with the rectifier.

The common point between switching circuits and controlled rectifiers is the output (both are ripple DC), but the rectifier (regardless of whether it is controllable or not) must be powered by a sinusoidal pulse, and except for the boost topology, the switching circuits seem unable to operate normally under sinusoidal pulse power supply, so direct coupling with the rectifier is not allowed!

SVC can be regarded as the predecessor of APFC, but SVC can release reactive power like a capacitor, while the purpose of APFC is only to shield and prevent the load from affecting the power factor, and it has no monitoring and correction function on the power grid. In addition, if a filter is inserted between the rectifier and the boost circuit, do you think this boost circuit can still be APFC? Just be a DC booster!

qwqwqw2088

I still don't know what to say

Why, compare the volume of the center-tapped transformer with that of the APFC

They all work differently and are designed differently