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What is the active PFC of a power supply

For a computer to work, it needs to be connected to the electrical current, and a transformer, which we know as the power supply, processes the electricity from the electrical network and adapts it so that the motherboard circuits are not damaged. , graphics card, PCU, or any internal components. Idealistically, the electricity that enters would be the same as the one that leaves, but this is not the case since in the transformation process, part of the electricity will be lost as heat, and to avoid it is the PFC circuit of the transformer, which can be passive or active.

Active or passive PFC to save electricity

The transformation of energy into heat is due to the fact that when electricity is processed in transformers, the process generates heat with its use. As the first law of thermodynamics says, “energy is neither created nor destroyed, it is transformed“. That is, what happens if the energy that enters is transformed into heat? Which cannot be tapped on the device. You are losing energy by the mere fact of passing it through the power supply and regulating its voltage and voltage.

The PFC circuit of a power supply, in simple words, try to lose the least amount of energy in the transformation. Try to correct the power factor through coils and capacitors to save energy. For a home user, power factor correction will not cost you much. but for a huge building, or an industrial warehouse, it is not uncommon to see that next to the many differentiators of the electrical network, there are large fuses and capacitors. On a larger scale, a

First of all, what is the PFC circuit

Active power factor correction of a power supply refers to the method of increasing the power factor by using active electronic circuitry with feedback that controls the shape of the current drawn. There are commercial PFC controllers that can perform this task.

This is necessary because in conventional non-PFC AC-DC power supplies, a large filter capacitor “Co” is placed directly after the bridge rectifier.

Diagram of a rectifier circuit

In linear power supplies, the rectifier is connected through a low frequency transformer. On the other hand, in offline switching power supplies, it is powered from the AC input. In both cases, once "Co" is charged to almost the peak of the rectified voltage, most of the time, the diodes will be reverse biased and will not conduct electricity. Therefore, such a power supply will take power from the line in short pulses when the instantaneous input voltage exceeds the voltage across the capacitor. This produces harmonics whose level can exceed an applicable standard and negatively affect other users.

To draw a continuous sinusoidal current throughout the AC cycle, we can put an inductance before “Co”. In passive PFC, the inductance is large and it is not controlled. It usually corrects the PF to 0,7-0,85, so a lot of energy is lost as heat. This is why the passive PFC is out of use for computers and large appliances and is reserved especially for mobile phone chargers and small electronic devices. It is used when the power demand is below 100 W, when a high PF is not required and DC link regulation is not necessary.

In most other applications an active method is used, which is what the active PFC is called. This is a conceptual schematic of the active PFC boost converter.

Diagram of an active PFC circuit

Inductor "L" is controlled by a solid-state switch labeled "Q." The control circuit activates and deactivates this switch at a frequency "F" much higher than that of the network.

Differences between active PFC and passive PFC

El passive PFC It is a correction system that uses coils and capacitors to improve the power factor received by the source. It is a cheap way to solve the problem of losing electricity in the form of heat, but it does not solve it completely. The maximum power factor correction achieved by a passive PFC is 0,85.

In the case of the active PFC, the correction is done by a series of integrated circuits using MOSFETs. The resulting power factor is close to 0,99, so almost all of the electricity is transformed into useful power and very little is wasted as heat or noise.

How Active PFC works

During the on time “ton”, the current in the inductor increases by ΔI+=Vin×ton/L. When the switch opens, the voltage across “L” is reversed and all or part of the stored energy is released through diode “D”. During the off time “toff”, the inductor current decreases by ΔI-=(Vo-Vin)×toff/L.

The net change during a period “T” is ΔI=ΔI+-ΔI-=(Vin-Vo+D×Vo)/LF, where D=ton/T-(Duty cycle), F=1/T. By varying the duty cycle “D” we can vary ΔI. If done correctly, we can synthesize a desired form of I(t).

Block diagram of an active control PFC

A simplified block diagram of the PFC control is shown above. It contains two error amplifiers: a slow one for the voltage (Vea) and a fast one for the current (Iea). A replica of the rectified input waveform "Vin" is input to the multiplier, which produces the programming signal Iref for Iea, which monitors the current through a sense resistor Rs and compares it to Iref.

By varying the control signal of the pulse width modulator (PWM), the "Iea" forces the average value of the current to follow the shape of the mains voltage. “Vea” controls “Vo” through a divider and compares it to the reference “Vref”. The “See” error signal scales the output of the multiplier without changing its sine wave pattern.

As a result, this complex circuit can perform two tasks simultaneously. The first is to create a sinusoidal current and regulate the bus voltage “Vo”. This allows designers to achieve an "FP" as high as 0,99, so as not to waste electricity. But this power factor correction technique addresses only line frequency harmonics.

An EMI filter is still necessary in order to reduce the high frequency components generated by the switching mode operation of the power converters. This filter can cause some negative effects. Your differential mode inductors and capacitors across the line can introduce some offset angle between “Vin” and “I”, which is not corrected by the subsequent PFC circuit. This effect can be a problem if your application also has to meet a certain minimum PF limit.

Why active PFC is used

Now it's time for a simple explanation: when a power supply can produce 900 W of power to a motherboard and graphics card, the electricity that reaches the PC will not be the same as that consumed. In the process of transforming from the power outlet, the power supply will inevitably lose some electricity in the form of heat. For this reason, the PFC tries to ensure that this electricity is lost as little as possible.

The use of the PFC is widespread in business and domestic environments. Because it helps to reduce the electrical cost, especially the active PFC for home appliances and computers. Passive PFC is only used in low-voltage and mobile devices anymore. It is not viable or used in anything larger because the energy lost in heat is much less in quantity than if it were powering an entire PC. Also, at least in the European Union, since 2001 it is a minimum imposed to meet the 80 PLUS requirements for power supplies.

The use of active PFC can be summed up in that less energy is consumed than with passive PFC by making better use of the energy it receives.

Can we improve the power factor of a PC source?

The change from a passive PFC to an active PFC already supposes a considerable improvement in the power factor of a source. But you have to understand that at least for almost 20 years, all PC power supplies are already with active PFC due to the high amount of energy they move so as not to waste energy.

Most likely the power supply you have for your PC already has a power factor close to 0,90 or even higher. If not, it is possible to increase it with capacitors and MOSFETs that improve energy retention. But doing it in a home shop is risky and can damage the power supply if done wrong. It can also be inefficient, since the power factor that would be increased and the energy that would be saved may not be enough to offset buying better capacitors and the time spent.

If you think you can improve the power factor of your power supply, consult an electronic engineer first. You can check the details of the power factor of your power supply, and if you see that it is far from 0,99 and it can be increased, it is possible, but not sure.

For a home user, improving the power factor may not be so serious or important. But for a series of professional equipment, such as a rendering farm or high-end workstations that by the nature of their work require to be constantly active, an increase in power factor is a reduction in the energy consumed in the long term because it is is reducing large-scale consumption in percentage terms. Simply put, if you're not in a big company, you already have a sufficiently efficient active PFC power supply, at least in the last 20 years.

What is a UPS?

UPS (stands for Uninterruptible Power Supply) are a combination of a series of current filters to protect the equipment along with a battery that will power it in the event of a blackout. We can see them as an energy reserve in the event of occasional blackouts or a failure of the electrical network. With this, we will have time to turn off the PC safely, or even continue using it for a while depending on the size of the battery. They are essential in workstations, so that, once we see that there has been a blackout, we can save the work without losing it, since we do not trust the automatic saves of the programs.

Image of a Salicru SPS Net DC UPS

UPS for PCs can be of three types:

  • standby: The cheapest. The current passes through the filters and goes directly to the PC. When the current drops, a switch passes the current supplied by the battery.
  • Interactive: They have additional protection systems that compensate for voltage drops and line voltage problems.
  • Online: The current passes through the battery converted to direct current at all times and is converted back to alternating current but of high quality.

Can a UPS be used as active PFC?

UPS can cause problems if used as active PFC. This is due to the shape of the current wave that it provides to the power supply when it supplies us from the battery. The ideal wave, called sinusoidal or sinusoidal, has a perfectly smooth voltage path. When the UPS is running on battery, it uses a pseudosine wave, which is close to ideal, but not exactly. It is because the voltage wave is more “staggered and irregular” for lack of a better term to describe it, and that can be taken into account by the power supply as a blackout or an electrical problem and directly shut down.

A poor quality UPS does not offer the best voltage for a power supply with active PFC. The ideal is to use an interactive UPS or an online one so that the computer can be nourished and not damage the power supply.

The circuitry required to produce a perfect sine wave from battery DC current is expensive. For this reason, many UPSs that are sold at a low price are limited to getting as close as possible. Most standby and interactive type UPSs generate a pseudo sinusoidal wave while there are some interactive ones, and all the online ones that have a sinusoidal output.

Image of a Mini Router UPS with Active PFC from Eaton

If you have a power supply with passive PFC, such as the mobile, nothing happens. But if you have active PFC, the UPS may not fulfill its function if it is a low quality one. The moment it starts to draw current from the battery, the PSU runs a serious risk that the PSU will detect that pseudo-sinusoidal current as insufficient and cut off the supply. There is a small exception, in which if at the moment you switch to battery you are using a very low proportion of the possible supply of the UPS, probably the power supply will not turn off.

One solution, if you don't want to spend a lot of money on an interactive UPS or an online one, is use a protection circuit. Although ideally, it would be necessary to ensure that the UPS can reproduce the sinusoidal probe in the event that said protection circuit fails.

What have we learned today?

It is clear that the active PFC regulates the loss of energy in the form of heat, thereby reducing the electricity bill, especially in cases on a larger scale.

The power factor of a power supply can be increased. But it requires knowledge of electronic engineering to be able to do it easily. Furthermore, if we use it with a UPS, it must be a quality one because if we use a low-quality one, the power supply can take it as a low-quality current and stop working by not offering a correct voltage waveform.

The active PFC is what has made it possible for us to enjoy equipment with considerable electrical consumption today. Power supplies over 1000 W now available to enhance high-consumption graphics. In the case of having a power factor of 0,99, the maximum consumption of those 1000 W would be about 1010 W with an active PFC. That is, 10 W are being converted into heat, which is very little compared to the figures that would give us power factors closer to 0,85, which is the limit of the passive PFC, most used in mobile chargers.

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Benjamin Rosa

Madrileño whose publishing career began in 2009. I love investigating curiosities that I later bring to you, readers, in articles. I studied photography, a skill that I use to create humorous photomontages.

A comment

  1. Demsey says:

    This document is very good, thank you!

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