We hear this from engineers, PFC, but don’t know what it is, why it matters, and how it works. On this page, I’m going to talk about this special circuit in switching power supply industry with you.
What is PFC circuit?
For understanding what’s FPC circuits, we shall know what is PF(Power Factor) at first.
What is PF(Power Factor)?
Power Factor is defined as a measurement of how effectively you are using electricity.
Generally, there are 2 different power needed when the device working, working power and non-working power. The working and non-working power summed are demanded power, the total demand power we called it apparent power.
- Working power–the “true” or “real” power used in all electrical appliances to perform the work of heating, lighting, motion, etc. the unit is W or kW.
- Non-working power–the “false” or “reactive” power used for some inside components like compressor or ballast to generate and sustain a magnetic field in order to operate. the unit is VAR or kVAR.
- Apparent power–Working power plus non-working power.
The Power Factor is the ratio of Apparent power and Working power. If a circuit/device is 100% efficient, apparent power is equal to working power, the power factor is equal to 1, which is the ideal condition.
What is PFC(Power Factor Correction)?
Definition of Power Factor Correction.
When apparent power(demanded power) is greater than working power, means the power factor is lower than 1, maybe 0.7, 0.75, 0.8……, then we need a technique called POWER FACTOR CORRECTION to improve the power factor and bring the power factor closer to 1, which is also called as Power Factor Controller. This is all you need to know about “what is PFC”if you are not a tech person.
When Power Factor is cosecant value of phase angle θ, P=VIcosθ, when making phase angle θ close to 0°, the power factor is close to 1, like the right waveform.
The waveform of circuits and voltage :
Type of PFC circuits.
Passive PFC
It’s called static PFC also, and uses a harmonic filter at the AC input to correct a poor power factor. Depending on the capacity of the power supply, a large reactor is required. Therefore, passive PFC is commonly used for low-capacity power supplies.
waveform for ideal current waveform of passive PFC.
Active PFC
It’s called Switching PFC also, uses MOSFETs as switching devices, and is commonly used for switched-mode power supplies requiring high efficiency and small size. For power supplies over 100W, the preferable type of PFC is Active Power Factor Correction (Active PFC) since it provides a lighter and more efficient power factor control.
A simple Boost circuit as an example.
The driving method of BOOST is the simplest and can increase the voltage. This circuit is still used in most switching power supplies, and its peak efficiency is 97%. High-power or more demanding application scenarios use other methods.
An inductor is connected after the output of the full-bridge for energy storage:
- When the switch tube is turned on. The AC current only passes through the inductor, and the current in the inductor increases.
- When the switch is turned off, the current stored in the inductor supplies power to the capacitor and the load through the diode.
So whether the Mosfet is on or off, there is always current.
In a words, this inductor and switch tube can ensure that there is always current in the inductor branch. There will be no interruption of the flow of the full-bridge rectifier circuit, which improves the power factor.
Why need PFC circuits for switching power supply?
Working more efficient and Safe
Switching power supply is made up of several circuits that transform an input AC voltage into a stable DC voltage at the output. The most essential of these circuits is the rectifier, which is responsible for transforming the AC voltage into a DC voltage; however, this circuit alone is not enough to ensure adequate operation.
In order for an AC/DC power supply to be efficient and safe, it needs to incorporate isolation, power factor correction (PFC), and voltage reduction. For working stablely, active FPC circuits could make sure the following features:
- The input voltage to 90V~270V.
- Improve power factor to be 0.99.
- No need to use large-capacity filter capacitors, if there is active PFC circuit.
Meeting international regulations
For environment friendly and some other reasons, the European Union (EU) established limits on harmonic currents that can appear on the mains(AC line) of switchmode power supplies at EN61000. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies. Power supplies with PFC circuits that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or better.
