What is the difference between PFC power supply and switching power supply

This article is mainly about the related introduction of PFC power supply and switching power supply, and focuses on the detailed explanation of the difference between PFC power supply and switching power supply.

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 related professional terms: [1]

1.

corrector, power factor (PFC)

Power factor corrector

2.

power-factor corrector (PFC)

Power factor corrector

3.

power factor control (PFC)

Power factor control

Everyone who has paid the electricity bill knows that the same use of electricity in the grid, why the price of industrial electricity and residential electricity are not the same?

Most people may give this answer: "Industrial electricity is to create value and make commercial profits; secondly, industrial equipment pollutes the environment more; thirdly, the transmission cost of industrial electricity is high." This answer explains something. Question, but if you have professional knowledge or know what power factor is through the previous study, then you can definitely give a more professional answer: "The electrical equipment used in industry is mostly inductive or capacitive equipment. The power factor is lower than that of residential electrical equipment, resulting in higher reactive power in the grid. Power companies need to generate more power to maintain this reactive power, which wastes this part of electrical energy, so industrial power users need to do This part of the wasted electric energy pays for it."

So what method do we have to reduce reactive power, or how to increase the power factor to the best value? Great scientists have helped us work out a solution: power factor correction. The technique of increasing the power factor of electrical equipment to close to 1 is called power factor correction.

Let's take a look at what power supply companies and industrial users do.

l Power factor correction method of power supply company

For the power supplier, the easiest way is to increase the transmission voltage. You can also add power factor correction equipment to various central substations and transmission networks to increase the power factor of the entire grid itself and reduce transmission losses, as shown in Figure 2.

l Power factor correction method for industrial power users

For users of industrial electricity, power factor correction equipment can be added to the low power factor load circuit, or a high power factor load can be used, as shown in Figure 3.

Switching power supply is a kind of power supply that uses modern power electronic technology to control the time ratio of switching on and off to maintain a stable output voltage. Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs. With the development and innovation of power electronics technology, switching power supply technology is constantly innovating. At present, the switching power supply is widely used in almost all electronic equipment due to its small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of the electronic information industry.

The main purpose

Switching power supply products are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instrumentation, medical equipment, semiconductor refrigeration and heating, air purifiers, electronic refrigerators, liquid crystal displays, LEDs Lighting, communication equipment, audio-visual products, security monitoring, LED light strips, computer cases, digital products and instruments and other fields.

Main type

There are two modern switching power supplies: one is DC switching power supply; the other is AC switching power supply.

DC/DC converters can be divided into two categories according to whether there is electrical isolation between input and output: one is called isolated DC/DC converters with isolation; the other is called non-isolated converters without isolation DC/DC converter.

Isolated DC/DC converters can also be classified according to the number of active power devices. There are two types of single-tube DC/DC converters: Forward and Flyback. Two-tube DC/DC converters include Double Transistor Forward Converter, Double Transistr Flyback Converter, Push-Pull Converter and Half-Bridge Converter Four kinds. The four-tube DC/DC converter is a Full-Bridge Converter.

According to the number of active power devices, non-isolated DC/DC converters can be divided into three types: single tube, double tube and four tube.

There are six types of single-tube DC/DC converters, namely, Buck DC/DC converters, Boost DC/DC converters, and Buck Boost DC/DC converters. , Cuk DC/DC converter, Zeta DC/DC converter and SEPIC DC/DC converter. Among the six single-tube DC/DC converters, Buck and Boost DC/DC converters are basic, and Buck-Boost, Cuk, Zeta, and SEPIC DC/DC converters are derived from them. The double-tube DC/DC converter has a double-tube series-connected boost (Buck-Boost) DC/DC converter. Four-tube DC/DC converters are commonly used as Full-Bridge Converters.

When the isolated DC/DC converter realizes the electrical isolation between output and input, it is usually realized by a transformer. Because the transformer has the function of voltage transformation, it is beneficial to expand the output application range of the converter and also facilitate the realization of multiple outputs of different voltages. , Or multiple outputs of the same voltage.

When the voltage and current ratings of the power switch tubes are the same, the output power of the converter is usually proportional to the number of switch tubes used. Therefore, the more the number of switching tubes, the greater the output power of the DC/DC converter. The output power of the four-tube type is twice that of the two-tube type, and the output power of the single-tube type is only 1/4 of that of the four-tube type.

The combination of a non-isolated converter and an isolated converter can obtain some characteristics that a single converter does not have.

According to the transmission of energy, there are two types of DC/DC converters: one-way transmission and two-way transmission. The DC/DC converter with bidirectional transmission function can transmit power from the power supply side to the load side, and can also transmit power from the load side to the power supply side.

DC/DC converters can also be divided into self-excited and other controlled types. The converter that uses the positive feedback signal of the converter itself to realize the self-sustained periodic switching of the switching tube is called a self-excited converter. For example, the Royer converter is a typical push-pull self-excited converter. The control signal of the switching device in the other-controlled DC/DC converter is generated by an external special control circuit.

According to the switching conditions of the switching tube, the DC/DC converter can be divided into hard switching (Hard Switching)

And soft switch (Soft Switching) two kinds. The switching devices of hard-switching DC/DC converters turn on or off the circuit under the condition of voltage or current flow, so there will be a large overlap loss during the turn-on or turn-off process, the so-called Switching loss. When the working state of the converter is constant, the switching loss is also constant, and the higher the switching frequency, the greater the switching loss. At the same time, the oscillation of the distributed inductance and parasitic capacitance of the circuit will be aroused during the switching process, which will bring additional loss. Therefore, The switching frequency of hard-switching DC/DC converters cannot be too high. The switch tube of the soft-switching DC/DC converter, during the turn-on or turn-off process, or the voltage applied to it is zero, that is, zero-voltage switching (Zero-Voltage-Switching, ZVS), or through the switch tube The current is zero, that is, zero-current switching (Zero-Current·Switching, ZCS). This kind of soft switching method can significantly reduce the switching loss and the oscillation caused during the switching process, so that the switching frequency can be greatly increased, and it creates conditions for the miniaturization and modularization of the converter. Power MOSFET (MOSFET) is a switching device with more applications. It has a higher switching speed, but it also has a larger parasitic capacitance. When it is turned off, its parasitic capacitance is fully charged under the action of external voltage. If this part of the charge is not discharged before it is turned on, it will be consumed inside the device, which is the capacitive turn-on loss. In order to reduce or eliminate this loss, the power FET should adopt the zero voltage turn-on method (ZVS). Insulated Gate Bipolar Transistor (IGBT) is a composite switching device. The tailing of the current during turn-off will lead to a large turn-off loss. If the current flowing through it is reduced to Zero, the switching loss can be significantly reduced, so the IGBT should adopt the zero current (ZCS) turn-off method. When the IGBT is turned off under the zero voltage condition, the turn-off loss can also be reduced, but when the MOSFET is turned on under the zero current condition, the capacitive turn-on loss cannot be reduced. Resonant converter (ResonantConverter, RC), quasi-resonant converter (Qunsi-Tesonant Converter, QRC), multi-resonant converter (Mu1ti-ResonantConverter, MRC), zero voltage switching PWM converter (ZVS PWM Converter), zero current switching PWM Converter (ZCS PWM Converter), zero-voltage conversion (Zero-Vo1tage-Transition, ZVT) PWM converter and zero-current conversion (Zero-Vo1tage-Transition, ZVT) PWM converter, etc., are all soft-switching DC converters. The development of power electronic switching devices and zero-switching converter technology has promoted the development of high-frequency switching power supplies.

basic component

The switching power supply is roughly composed of the main circuit,

It consists of four parts: control circuit, detection circuit, and auxiliary power supply.

1. Main circuit

Impulse current limit: limit the impulse current on the input side when the power is turned on.

Input filter: Its function is to filter the clutter that exists in the power grid and prevent the clutter generated by the machine from being fed back to the power grid.

Rectification and filtering: Directly rectify the AC power of the grid into a smoother DC power.

Inverter: Convert the rectified direct current into high-frequency alternating current, which is the core part of the high-frequency switching power supply.

Output rectification and filtering: According to the needs of the load, provide a stable and reliable DC power supply.

2. Control circuit

On the one hand, samples are taken from the output terminal and compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, according to the data provided by the test circuit, it is identified by the protection circuit to provide The control circuit carries out various protection measures for the power supply.

3. Detection circuit

Provide various parameters and various instrument data in operation in the protection circuit.

4. Auxiliary power supply

Realize the software (remote) start of the power supply, and supply power for the protection circuit and the control circuit (chips such as PWM).

Main classification

People in the field of switching power supply technology are developing related power electronic devices,

320W single group switching power supply

320W single group switching power supply

Miniature low power switching power supply

Switching power supplies are becoming popular and miniaturized. Switching power supplies will gradually replace all applications of transformers in life. The application of low-power micro-switching power supplies must first be reflected in digital display meters, smart meters, mobile phone chargers, and so on. At this stage, the country is vigorously promoting the construction of smart grids, and the requirements for electric energy meters are greatly increased. Switching power supplies will gradually replace the application of transformers in electric energy meters.

Reversing series switching power supply

The difference between the reversing series switching power supply and the general series switching power supply is that the output voltage of this reversing series switching power supply is negative voltage, which is just opposite to the positive voltage output of the general series switching power supply; and because of energy storage The inductor L only outputs current to the load when the switch K is turned off. Therefore, under the same conditions, the current output by the reversing series switching power supply is twice as small as the output current of the series switching power supply.

Among the PFC switching power supplies, the switching regulated power supply is a very important part. The difference between the switching power supply function in PFC and the ordinary switching power supply is not huge, but the power supply is different. Ordinary switching power supply requires 220V rectifier power supply, while PFC power supply is powered by B+PFC.

After rectification, no filter capacitor is added, and the unfiltered pulsating positive half-cycle voltage is used as the power supply of the chopper. Due to the series of "switching" work of the chopper, the pulsating positive voltage is "chopped" into a current waveform. The characteristics are:

1. The current waveform is discontinuous, and its envelope is the same as the voltage waveform, and the phase of the envelope and the voltage waveform is the same.

2. Due to the effect of chopping, the half-wave pulsating DC power becomes high-frequency (determined by the chopping frequency, about 100KHz) "AC" power. This high-frequency "AC" power must be rectified again before it can be stabilized by the subsequent PWM switch. Voltage source is used.

3. From the general perspective of external power supply, the power system achieves that the AC voltage and AC current are in phase and the voltage and current waveforms are in line with the sinusoidal waveform, which not only solves the problem of power factor compensation, but also solves electromagnetic compatibility (EMC) and electromagnetic interference ( EMI) problem.

The high-frequency "AC" power is rectified by a rectifier diode and filtered into a DC voltage (power supply) to supply power to the subsequent PWM switching power supply. This DC voltage is called B+PFC in some materials (TPW-4211 is the case). The B+PFC voltage output by the chopper is generally higher than +300V after the original 220 AC rectification and filtering. The reason It is to choose high voltage, its inductance wire diameter is small, the line voltage drop is small, the filter capacitor capacity is small, and the filtering effect is good, it has many advantages such as low requirements for the downstream PWM switch tube.

At present, in the PFC switching power supply part, the chopper tube (K), which plays the role of a switch, has two working modes:

1. Continuous conduction mode (CCM): The operating frequency of the switching tube is constant, and the conduction duty cycle (coefficient) changes with the amplitude of the chopping voltage.

2. Discontinuous conduction mode (DCM): The operating frequency of the chopper switch tube changes with the size of the voltage being chopped (the "on" and "off" times in each switching cycle are equal).

The PFC switching power supply part and the excitation part of the PWM switching power supply part in the power factor correction switching power supply are all completed by an integrated circuit, and a single IC can complete the design.

This is the end of the related introduction about PFC power supply and switching power supply. Please correct me if there are any shortcomings.

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