What are the consequences if the power supply does not have PFC? On the importance of PFC to the power supply

This article is mainly about the related introduction of pfc, and focuses on the detailed description of the function characteristics and advantages and disadvantages of pfc.

The full English name of PFC is "Power Factor Correction", which means "power factor correction". Power factor refers to the relationship between effective power and total power consumption (apparent power), that is, effective power divided by total power consumption The ratio of the amount (apparent power). Basically, the power factor can measure the extent to which electricity is effectively used. The larger the power factor value, the higher the power utilization rate. Power factor is a parameter used to measure the power efficiency of electrical equipment, and low power factor represents low power efficiency. In order to improve the power factor of electrical equipment, the technology is called power factor correction.

The computer switching power supply is a capacitive input circuit, and the phase difference between its current and voltage will cause the loss of exchange power. At this time, a PFC circuit is needed to improve the power factor. There are currently two types of PFC, one is passive PFC (also known as passive PFC) and active PFC (also known as active PFC).

Passive PFC

Passive PFC is generally divided into "inductance compensation" and "Valley Fill Circuit"

"Inductance compensation" is to reduce the phase difference between the fundamental current and voltage of the AC input to improve the power factor. "Inductance compensation" includes silent and non-silent. The power factor of "inductance compensation" can only reach 0.7-0.8, which is generally near the high-voltage filter capacitor.

"Valley-filling circuit type" is a new type of passive power factor correction circuit, which is characterized by using the valley-filling circuit behind the rectifier bridge to greatly increase the conduction angle of the rectifier tube. By filling the valley point, the input current is changed from the peak pulse It becomes a waveform close to a sine wave, and the power factor is increased to about 0.9, which significantly reduces the total harmonic distortion. Compared with the traditional inductive passive power factor correction circuit, its advantages are that the circuit is simple, the power factor compensation effect is significant, and the input circuit does not need to use a large inductor with a large volume and a heavy weight.

Active PFC

The active PFC is composed of inductors, capacitors and electronic components. It is small in size and uses a dedicated IC to adjust the current waveform to compensate for the phase difference between current and voltage. Active PFC can achieve a higher power factor ─ ─ usually up to 98% or more, but the cost is relatively high. In addition, active PFC can also be used as an auxiliary Power Supply, so in the use of active PFC circuits, standby transformers are often not needed, and the ripple of the active PFC output DC voltage is very small, this kind of power supply does not need to use large-capacity filtering capacitance.

PFC is used to improve the power factor. There are active power and reactive power in the power grid. After the power factor is increased, the reactive power decreases. In my country, the active watt-hour meter is used, which cannot measure reactive power, so even if the power supply does not have a PFC, there will be no additional deductions for electricity charges. However, in foreign countries such as Europe, their watt-hour meters also calculate reactive power, and the absence of PFC means more electricity bills. Therefore, it is best to buy a power supply with PFC. The power factor of an active pfc power supply can be higher. It is recommended to buy an active PFC power supply.

Regarding the voltage, this does not have much to do with PFC, but it should be noted that some countries such as the United States and Japan use 110V alternating current, so you must purchase products that support 100-240V full voltage to avoid going abroad. Out of service.

The input voltage of the power supply can be seen from the nameplate of the power supply. The nameplate of a formal full-voltage power supply will be marked with the words 100-240V voltage, which is easy to identify. But PFC is not easy to see from the appearance, unless you disassemble the power supply, the simple way is to check the product information on the official website.

Disadvantages of no PFC power supply

1. When active PFC is low power, its own loss is greater than that of passive PFC. After all, it is a complicated circuit that consumes power when working; and passive PFC is an inductor. However, this problem is not obvious when few people make high-end Power Supplies work under low loads.

2. Active PFC has one of the most troublesome shortcomings: large electromagnetic interference. In order to deal with electromagnetic interference, the EMI filter circuit must be strengthened, and the circuit is more complicated. Some power supplies emit high-frequency noise during standby, which is also due to active PFC.

3. The passive PFC circuit is at the expense of the efficiency and stability of the power supply. Many people who don't understand power supply blindly add passive PFC circuit to the power supply, thinking that the PF value of the circuit can be easily solved in this way. In fact, this is not worth the gain in many cases.

4. Increasing a little PF value, at the expense of more excellent performance of the power supply itself, there are about two ways of passive PFC:

One is to directly remove the 400V large electrolysis in the front stage of the flyback circuit and replace it with CBB, so the voltage waveform input into the circuit is a sine wave, and then adjust the inductance of the power Transformer to achieve DCM and CCM operation At the critical point of the mode, if the adjustment is accurate, the PF value can be adjusted to 0.9. The shortcomings of this can also be seen at a glance, because the input voltage is reduced, the efficiency is reduced, and because there is no pre-filter circuit ripple If it becomes larger, the stability of the circuit is greatly reduced.

The second is, the valley filling circuit, adding three diodes, 400V electrolysis replaced with two 250V, this circuit is better than the previous circuit, because the ripple of the input stage is not that big. But the valley value of the input voltage is half of the original electrolytic direct filtering, and the peak value remains unchanged. It will also reduce the efficiency and stability of the power supply.

Power factor (PF) refers to the ratio of actual power (effective power) to apparent power (apparent power) (kW/kVA), and we all know that power P is equal to the product of voltage and current (P=V×I ). In addition, there are two basic circuit loads in the circuit, one is "resistance (circuit load composed of various resistances in the power supply)", and the other is "reactance (composed of inductance coils and capacitors in the power supply). Circuit load)”.

If the entire circuit is a linear load (a load with a constant circuit impedance), then the power supply voltage and current will be sinusoidal and have the same phase. And if in this pure resistance circuit, then the voltage and current will reverse polarity at the same time, then that is to say, at each moment, the product of voltage and current is "positive". That is to say, in the circuit, there is no "reverse direction (negative direction)" energy movement, and the load power generated at this time is called "real power".

In a pure reactance load circuit, there will be a certain time difference between voltage and current, and there will also be a phase difference (the maximum theoretical value is 90 degrees, and the general situation is 45 degrees), then the product of voltage and current, It may not be "positive" every moment. In the first half cycle, the energy is "positive", and the energy in the other half cycle is "negative". That is to say, the power source obtains energy from the grid in the first half cycle, and in the second half cycle, the energy will be Return to the national grid. So if it is calculated according to one cycle, the energy obtained by the power supply will be "zero" and there will be no energy.

The above two descriptions are purely theoretical ideals. But in practical applications, there will be a lot of resistance, inductance and capacitance in the circuit, there will be a load at the same time, it will also produce "energy" in different directions. Therefore, all forward energy, we call it "real power", and the energy that returns to the grid in the reverse direction is called "useless power", then the combination of "real power" and "useless power" is what we had before. Said "apparent power".

But as we mentioned before, "power factor" is actually the ratio of "actual power" to "apparent power". The most ideal ratio is "1". Of course, this cannot be done, so it can only be infinitely close to "1". This value is generally called "power factor".

What we need to point out here is that residential users only need to pay for the electricity consumed by the actual power (wattage), but will not pay for the useless power that is returned to the grid. For commercial factories, the use of unnecessary power will be added because the base of their power consumption is too large.

Although for residential users, we don’t need to pay for the useless power, according to the "EU EN61000-3-2 Standard" (of course, China also has relevant regulations), any switching power supply with a power of more than 75W needs at least Install the passive PFC module. In addition, in the 80Plus power supply certification, the power factor is required to exceed 0.9 or even more.

However, a few years ago, many power supply manufacturers mostly used passive PFC modules in their power products. The PFC module is a filter that reduces harmonic currents and converts non-linear loads into linear loads. The power factor generated by capacitors and inductances will be closer to the unit value.

Therefore, what we are going to talk about next is the active PFC and passive PFC circuits. Compared with active PFC, passive PFC has a lower power factor, and passive PFC is only suitable for 230V high-voltage power grids. For 115V low-voltage power grids, passive PFC also requires a voltage doubler to adapt to grid specifications. However, passive PFC is more efficient than active PFC!

In addition, we will also see a thermistor in the active PFC circuit, which is also used to limit the sudden inrush of current, especially when the power supply is energized and started.

Active PFC circuits usually have two different modes, DCM (Discontinuous Conduction Mode) and CCM (Continuous Conduction Mode). Among them, DCM refers to the working state where the PFC MOSFET is turned on when the inductor current is zero; CCM refers to the working state where the inductor current is always above zero and the PFC MOSFET is turned on. Therefore, in the MOSFET, all The energy of reverse recovery will be wasted.

The second mode (CCM) in the power supply PFC circuit is mainly used for power supplies with a power output of more than 200W, because it can provide relatively low current noise peaks, which means that high-power power supplies can effectively suppress current ripple and output more It is a smooth current. However, the disadvantage of CCM is that it consumes more energy and generates additional EMI when the boost diode is turned off. Therefore, we often see that an X capacitor is usually added after the power rectifier bridge.

That's it for the related introduction of pfc. If you have any deficiencies, please correct me.