PFC in the power supply: purpose and principle of operation

Максим Исаев
Reading time: 7 min
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Purpose of the PFC module

Power factor corrector (PFC) or PFC – used in switching power supplies, where the power exceeds 50 W or more. In low-power UPS, as a rule, it is not used.

In a switching power supply, the input circuit is built according to the standard scheme. 

Пассивная схема PFC
Classic AC rectifier circuit

There is a diode bridge at the input, after which a smoothing electrolytic capacitor is installed. A rectifier built according to such a scheme consumes current from the network not according to a sinusoidal law, but in current pulses. In this case, these blocks can consume very large currents from the network.

Диаграмма работы мостового выпрямителя
Bridge rectifier diagram

If we consider the graph of the converter, we can notice the following: when rectifying alternating current with a diode bridge, half-waves of a sinusoidal voltage are obtained. 

A capacitor is installed at the output, which is charged to the maximum amplitude value. When the voltage begins to decrease, the capacitor begins to discharge and, upon reaching a certain value of the next half-wave, begins to charge, consuming current from the network, until the maximum value is reached. 

This process is repeated from half wave to half wave. Thus, the current consumption is concentrated in very short periods of time. The greater the load power, the faster the capacitor will discharge and the longer will be the time during which it will be charged to the amplitude value. This voltage will be approximately 300 – 310 Volts (it all depends on the input voltage of the network).

Since when designing such blocks it is necessary to make the ripple at the output minimal, the capacitance of the capacitor is chosen to be large. This is due to the fact that the capacitor is charged at each half-wave for a short period of time, while the current from the network will be consumed by pulses. When the current charges the capacitor, it determines the angle at which the current flows through the rectifier. 

Потребление тока из сети в классической схеме
Current consumption from the network in the classical scheme

This angle is called the load power factor and depends on the impedance of the power supply, the capacitance of the filter capacitor, and the magnitude of the load. With a small load, the value is small, and with an increase, it increases to 25-30 degrees. It follows from this that the current in the load is not continuous, but has a pulsed value of large amplitude with certain harmonics.

To eliminate the current consumption from the network by impulses, a number of specific devices have been created, which are called power factor correctors. 

Types of power factor corrector

There are the following correction schemes:

  • passive
  • active

For passive power factor correction, circuits with inductance in the input circuit are used. After the diode bridge, a choke is connected, and a capacitor is placed behind it and passive power factor correction is carried out.

Пассивная схема PFC
Passive PFC circuit

If you set a large choke value, then it stores a large amount of energy, which is enough for the entire period of operation, reducing the harmonic oscillations that occur when the current through the rectifier is exceeded. 

Диаграмма работы пассивной схемы PFC
Diagram of passive PFC circuit operation

In practice, the scheme reduces harmonics, improves the power correction factor, but does not completely solve the problem.

With active power factor correction, the load behaves like an active resistance.

The current consumed from the network is not of a pulsed nature, but is close to a sinusoid in shape. The input current must be in the same shape and phase. 

KKM circuitry can be different: step-up and step-down. Most of all, a step-up circuit is used in switching power supplies, as it allows you to get a value close to one COS (F). These converters increase the voltage on the electrolytic capacitor of the rectifier, reducing the current in the high voltage part of the UPS. Most PFC circuits are built according to the scheme of step-up DC-DC converters.

Схема повышающей активной PFC
Boost active PFC circuit

We will consider the operation of this converter using graphic waveforms and a circuit diagram. An oscilloscope must be used to check the incoming pulses at the gate of transistor G.

The principle of operation of the PFC circuit

The power factor input circuit has a diode bridge. It receives a voltage of 220V 50Hz, and at the output of the diode bridge we get a constant voltage with a ripple frequency of 50Hz.

This voltage is no longer applied to the filter capacitor, as in the classical circuit, but to a boost converter made of:

  • throttle L
  • MOSFET transistor
  • PFC-PWM controller
  • diode connected to the output of the filter capacitor (pin +)
Диаграмма работы активной схемы PFC
Diagram of the operation of the active PFC circuit

The main task of this converter is to obtain the shape of the consumption current not by pulses from the network, but the same as the voltage shape, that is, close to sinusoidal.

To obtain a given shape, it is necessary that the pulses are formed on the key gate transistor by some control voltages. 

There is a large amplitude voltage at the output of the diode bridge, and for the formation of pulses of the power transistor switch, 2 conditions must be met:

  1. A rectified pulsating voltage is supplied to the pulse shaper through a divider Rd consisting of resistors R1, R2, R3, reducing the amplitude, and the voltage shape remains the same.
  2. It is necessary to monitor the current consumption from the network. This function is implemented using the resistor R’d in the source circuit of the MOSFET – transistor. Thus, the current will pass through the inductor L and be monitored by the D1 chip on the CS pin.

These conditions are basic for the implementation of the PFC scheme

The pulses at the gate of the transistor must be formed in such a way that when it starts to open (an opening voltage level appears) and current begins to flow through the inductor. 

This current increases linearly and flows through the current sensor (R’d). When the voltage from this current sensor equals the voltage rectified by the rectifier after the divider Rd and R’d, then the transistor should close. When the current flowing through the inductor is zero, the transistor will open again and the current will gradually increase until the next coincidence of the voltage values ​​​​on the current sensor and the rectified voltage from the diode bridge limited by the divider. And this process will be repeated throughout the entire period.

When the device is operating, at the beginning of the sinusoid, the transistor will open for a short time, and when the sinusoid approaches the maximum value, the transistor opens for a longer time.

To stabilize the output voltage, the signal from the capacitor C1 is fed to the pulse shaper through Robr, where an error signal is generated in the D1 chip through the FB pin. This signal affects the duration of the pulse that is generated to drive the transistor from the GO pin of the PFC PWM chip. The duration of the pulses is affected not only by the input, but also by the output voltage.

Depending on the connection load, the output voltage will change and the error signal will change, and the signal will affect the pulse duration. In this case, the input current consumption is reduced to almost a sinusoidal form and the output voltage is stabilized. 

Conclusions:

  • In most electronic devices, these converters are used, which eliminates: phase imbalance, reducing the load on the network, does not distort the shape of the mains voltage.

You can study in detail the operation of the CCM module on the course of electronics.

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