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All colleagues unite and focus their efforts to write a new chapter of developmentThe Difference Between PFC Power Supplies and Switching Power Supplies
所属分类： Industry News
发布时间：2020-05-14
PFC stands for “Power Factor Correction,” which refers to the correction of the power factor. The power factor is the ratio of active power to apparent power, calculated as active power divided by apparent power. Essentially, it measures how effectively electrical energy is being utilized: a higher power factor indicates greater efficiency in power usage. The power factor serves as an indicator of the efficiency with which electrical equipment consumes power; a low power factor signifies poor energy efficiency. The technology used to improve the power factor of electrical equipment is known as power factor correction.
A computer switching power supply is a capacitor‑input circuit, and the phase difference between current and voltage leads to losses in switching power. To address this, a PFC circuit is required to improve the power factor. Currently, there are two types of PFC: passive PFC (also known as unregulated PFC) and active PFC (also known as regulated PFC).
Passive PFC
Passive PFC is generally categorized into “inductive compensation type” and “valley‑fill circuit type.”
“Inductive compensation” reduces the phase difference between the fundamental‑frequency current and voltage in the AC input, thereby improving the power factor. This approach encompasses both silent and non‑silent variants. However, the power factor achievable with inductive compensation is limited to 0.7–0.8, and it is typically implemented near high‑voltage filter capacitors.
The “valley‑filling circuit” is a novel type of passive power‑factor‑correction topology. Its key feature is the use of a valley‑filling circuit following the rectifier bridge to substantially increase the conduction angle of the rectifier diodes. By flattening the voltage valleys, it transforms the input current from sharp pulses into a waveform that closely approximates a sine wave, raising the power factor to around 0.9 and significantly reducing total harmonic distortion. Compared with conventional inductive passive power‑factor‑correction circuits, this approach offers the advantages of circuit simplicity, robust power‑factor correction, and the elimination of bulky, heavy inductors in the input stage.
Active PFC
In contrast, active PFC consists of inductors, capacitors, and electronic components; it is compact in size and uses a dedicated IC to shape the current waveform, thereby compensating for the phase difference between current and voltage. Active PFC can achieve a high power factor—typically exceeding 98%—but at a relatively higher cost. Moreover, active PFC can also serve as an auxiliary power supply; consequently, circuits employing active PFC often do not require a standby transformer. Additionally, the output DC voltage ripple of active PFC is very low, eliminating the need for large‑value filtering capacitors.
PFC-related technical terms:
1. corrector, power factor (PFC)
Power factor corrector
2. Power-factor corrector (PFC)
Power factor corrector
3. Power factor control (PFC)
Power Factor Control
Anyone who has paid an electricity bill knows that, even when using the same grid‑supplied power, industrial and residential rates are not the same.
Most people might offer this answer: “Industrial electricity is used to create value and generate commercial profits; secondly, industrial equipment tends to cause more environmental pollution; and thirdly, the transmission costs of industrial power are high.” While this response highlights some key points, if you have specialized knowledge or have already learned what power factor is, you could provide a more nuanced explanation: “In industry, most electrical equipment is inductive or capacitive, resulting in a lower power factor compared to residential appliances. This leads to higher reactive power in the grid, requiring utilities to generate additional power to sustain that reactive component—wasting energy in the process. Consequently, industrial electricity users end up paying for this wasted energy.”
So, what methods can we use to reduce reactive power, or in other words, how can we raise the power factor to its optimal value? Brilliant scientists have already devised a solution: power factor correction. The technique of improving the power factor of electrical equipment to bring it closer to 1 is known as power factor correction.
Now let’s take a look at how power supply companies and industrial electricity users are handling this.
Power factor correction methods employed by power supply companies
For the power supplier, the simplest approach is to increase the transmission voltage. Additionally, power factor correction equipment can be installed at various central substations and along the transmission network to improve the overall power factor of the grid, thereby reducing transmission losses, as shown in Figure 2.
Power Factor Correction Method for Industrial Electricity Users
For industrial electricity consumers, power factor correction equipment can be added to circuits with low power factor loads, or high‑power‑factor loads can be used.
Switching power supply It is a type of power supply that utilizes modern power electronics to regulate the duty cycle—the ratio of on‑time to off‑time of switching transistors—in order to maintain a stable output voltage. Switching power supplies typically consist of a pulse‑width modulation (PWM) control IC and MOSFETs. With the continuous advancement and innovation in power electronics, switching‑mode power‑supply technology has also evolved steadily. Today, thanks to their compact size, light weight, and high efficiency, switching power supplies are widely employed in virtually all electronic devices and have become an indispensable power‑supply solution driving the rapid growth of the modern information‑electronics industry.
Main uses
Switching power supply products are widely used in industrial automation control, military‑grade equipment, scientific research instruments, LED lighting, industrial control systems, telecommunications equipment, power‑generation devices, measurement and instrumentation, medical equipment, semiconductor cooling and heating applications, air purifiers, electronic refrigerators, LCD displays, LED luminaires, communication devices, audiovisual products, security surveillance systems, LED light strips, computer cases, digital consumer electronics, and various other fields.
Main types
There are two types of modern switching power supplies: one is a DC switching power supply, and the other is an AC switching power supply.
This section primarily introduces DC switching power supplies, whose function is to convert raw, low‑quality power sources—such as mains electricity or battery supplies—into high‑quality DC voltages that meet the requirements of the connected equipment. The core component of a DC switching power supply is the DC/DC converter. Consequently, the classification of DC switching power supplies follows the classification of DC/DC converters. In other words, the categorization of DC switching power supplies is essentially identical to that of DC/DC converters; the classification of DC/DC converters largely defines the classification of DC switching power supplies.
DC/DC converters can be classified into two categories based on whether there is electrical isolation between the input and output: one type, which provides isolation, is called an isolated DC/DC converter; the other type, which lacks isolation, is called a non-isolated DC/DC converter.
Isolated DC/DC converters can also be classified according to the number of active power switches. Single-switch DC/DC converters include the forward converter and the flyback converter. Dual-switch DC/DC converters comprise four types: the double-transistor forward converter, the double-transistor flyback converter, the push‑pull converter, and the half‑bridge converter. A four-switch DC/DC converter is equivalent to a full‑bridge converter.
Non-isolated DC/DC converters can be classified into three types—single-switch, dual-switch, and four-switch—based on the number of active power devices.
There are six types of single‑switch DC/DC converters: the buck converter, the boost converter, the buck‑boost converter, the Cuk converter, the Zeta converter, and the SEPIC converter. Among these six, the buck and boost converters are fundamental, while the buck‑boost, Cuk, Zeta, and SEPIC converters are derived from them. As for two‑switch DC/DC converters, there is the series‑connected two‑switch buck‑boost converter. Among four‑switch DC/DC converters, the full‑bridge converter is the most commonly used.
When implementing electrical isolation between the output and input, isolated DC/DC converters typically employ a transformer. Because transformers can step up or step down voltage, they help broaden the converter’s range of output applications and facilitate the provision of multiple outputs at different voltages, as well as several outputs at the same voltage.
When the voltage and current ratings of the power switching devices are identical, the converter’s output power is typically proportional to the number of switches employed. Consequently, the greater the number of switches, the higher the DC/DC converter’s output power: a four-switch topology delivers twice the output power of a two-switch configuration, whereas a single-switch topology provides only one-quarter of the output power of a four-switch design.
The combination of non-isolated and isolated converters can achieve certain characteristics that a single converter cannot provide.
Based on the direction of energy flow, DC/DC converters are classified into unidirectional and bidirectional types. A bidirectional DC/DC converter can transfer power both from the source side to the load side and from the load side to the source side.
DC/DC converters can also be classified as self‑oscillating or externally controlled. A converter that relies on the internal positive feedback of the switching device itself to sustain periodic switching is called a self‑oscillating converter; for example, the Royer converter is a typical push‑pull self‑oscillating design. In externally controlled DC/DC converters, the control signals for the switching devices are generated by an external dedicated control circuit.
According to the switching conditions of the power transistor, DC/DC converters can also be classified into hard-switching types.
There are two main types: hard switching and soft switching. In hard-switching DC/DC converters, the switching devices turn on or off while subjected to voltage or current, resulting in significant overlap losses—known as switching losses—during these transitions. When the converter operates under fixed conditions, switching losses remain constant; moreover, higher switching frequencies lead to greater losses. Additionally, switching events can excite oscillations in the circuit’s distributed inductances and parasitic capacitances, introducing further losses. Consequently, the switching frequency of hard‑switching DC/DC converters cannot be excessively high. By contrast, in soft‑switching DC/DC converters, during turn‑on or turn‑off, either the voltage across the switch is zero—resulting in zero‑voltage switching (ZVS)—or the current through the switch is zero—yielding zero‑current switching (ZCS). This soft‑switching approach significantly reduces switching losses and suppresses oscillations induced during switching, enabling much higher switching frequencies and paving the way for converter miniaturization and modularization. Power MOSFETs are widely used switching devices due to their high switching speeds, yet they also exhibit relatively large parasitic capacitances. During turn‑off, the external voltage charges these parasitic capacitances; if this stored charge is not discharged before turn‑on, it dissipates internally as capacitive turn‑on loss. To mitigate or eliminate such losses, power MOSFETs are ideally operated using zero‑voltage turn‑on (ZVS). Insulated Gate Bipolar Transistors (IGBTs) are composite switching devices. Their turn‑off current tail can cause substantial turn‑off losses; however, reducing the current to zero prior to turn‑off markedly lowers these losses. Therefore, IGBTs are best employed with zero‑current switching (ZCS). While IGBTs can also reduce turn‑off losses when turned off under zero‑voltage conditions, a MOSFET operating under zero‑current conditions cannot eliminate capacitive turn‑on losses. Resonant Converters (RC), Quasi‑Resonant Converters (QRC), Multi‑Resonant Converters (MRC), Zero‑Voltage‑Switching PWM Converters (ZVS PWM), Zero‑Current‑Switching PWM Converters (ZCS PWM), Zero‑Voltage‑Transition PWM Converters (ZVT PWM), and Zero‑Current‑Transition PWM Converters (ZVT PWM) all fall under the category of soft‑switching DC converters. Advances in power electronic switching devices and zero‑switching converter technologies have driven the development of high‑frequency switching power supplies.
Basic components
Switching power supplies generally consist of a main circuit,
It consists of four major components: the control circuit, the detection circuit, and the auxiliary power supply.
1. Main Circuit
Inrush Current Limiting: Restricts the inrush current on the input side at the moment of power-up.
Input filter: Its function is to filter out noise present on the power grid and to prevent noise generated by the device itself from feeding back into the grid.
Rectification and Filtering: The AC power from the grid is directly rectified into a relatively smooth DC voltage.
Inversion: The rectified DC is converted into high-frequency AC, which constitutes the core of a high-frequency switching power supply.
Output Rectification and Filtering: Provide a stable and reliable DC power supply according to the load requirements.
2. Control Circuit
On the one hand, a sample is taken from the output, compared with the setpoint, and used to control the inverter by adjusting its pulse width or pulse frequency, thereby stabilizing the output. On the other hand, based on data provided by the test circuit and evaluated by the protection circuit, the control circuit implements various protective measures for the power supply.
3. Detection Circuit
Provides various operating parameters and instrument data within the protection circuit.
4. Auxiliary Power Supply
Enable software‑based (remote) power‑on, providing power to the protection circuitry and control circuitry (such as PWM ICs).
Main Categories
In the field of switch-mode power supply technology, researchers are simultaneously developing related power electronic devices,
320W single-output switching power supply
320W single-output switching power supply
As switch-mode power‑supply technology continues to evolve, the two approaches mutually reinforce each other, driving annual growth rates exceeding double digits and propelling the industry toward lighter, smaller, thinner designs with lower noise, higher reliability, and enhanced electromagnetic interference immunity. Switching power supplies can be broadly categorized into two main types: AC/DC and DC/DC.
Miniature Low-Power Switching Power Supply
Switching power supplies are becoming increasingly widespread and miniaturized. They will gradually replace transformers in virtually all everyday applications, with low‑power, miniature switching power supplies first finding use in digital displays, smart meters, mobile phone chargers, and similar devices. At present, as the country vigorously promotes the development of smart grids, requirements for electricity meters have risen significantly, and switching power supplies are poised to progressively supplant transformers in meter applications.
Inverted Series-Connected Switching Power Supply
The key difference between an inverting series‑type switching power supply and a conventional series‑type switching power supply is that the former delivers a negative output voltage, opposite in polarity to the positive output of the latter. Moreover, since the energy‑storage inductor L supplies current to the load only when the switch K is off, under identical operating conditions, the inverting series‑type switching power supply produces an output current that is half the magnitude of that of the conventional series‑type supply.
The above information has been compiled from online sources. (We maintain neutrality regarding the statements and opinions presented herein and provide no express or implied warranties as to the accuracy, reliability, or completeness of the content.)