Power guru’s understanding of PFC

Power factor compensation: In the 1950s, improved methods have been proposed to address the low power supply efficiency caused by the out-of-phase voltage and current of AC electrical appliances with inductive loads (Figure 1) (due to the current lag of the inductive load). Voltage. Due to the different phases of voltage and current, the burden on the power supply line is increased and the efficiency of the power supply line is reduced. This requires a capacitor to be connected in parallel with the inductive electrical appliance to adjust the voltage and current phase characteristics of the electrical appliance. For example: At that time, it was required that the 40W fluorescent lamp used must be connected in parallel with a 4.75μF capacitor). A capacitor is connected in parallel to an inductive load, and the characteristic of the current leading the voltage on the capacitor is used to compensate for the characteristic of the current lagging voltage on the inductor, so that the overall characteristics are close to resistance, thus improving the low efficiency. This method is called power factor compensation (AC power factor compensation). The power factor can be expressed by the cosine function value cosφ of the phase angle between the supply voltage and the load current).

figure 1

Waveforms of voltage and current in supply lines with inductive loads

Since the 1980s, a large number of electrical appliances have adopted high-efficiency switching power supplies. Since the switching power supplies use a large-capacity filter capacitor after rectification, the load characteristics of the electrical appliances appear capacitive, which results in As a result, when AC 220V supplies power to the electrical appliance, due to the charging and discharging of the filter capacitor, a slightly sawtooth ripple appears on the DC voltage at both ends. The minimum value of the voltage on the filter capacitor is far from zero, and is not much different from its maximum value (ripple peak). According to the unidirectional conductivity of the rectifier diode, the rectifier diode will conduct due to forward bias only when the instantaneous value of the AC line voltage is higher than the voltage on the filter capacitor, and when the instantaneous value of the AC input voltage is lower than the voltage on the filter capacitor When the voltage is , the rectifier diode is cut off due to reverse bias. That is, during each half cycle of the AC line voltage, the diode conducts only near its peak value. Although the AC input voltage still maintains a roughly sinusoidal waveform, the AC input current appears as a high-amplitude spike, as shown in Figure 2. This severely distorted current waveform contains a large number of harmonic components, causing a serious drop in the line power factor.

During the positive half cycle (180°), the conduction angle of the rectifier diode is much less than 180° or even only 30°-70°. Due to the requirement to ensure the load power, a huge amount of energy will be generated during the extremely narrow conduction angle. The conduction current causes the power supply current in the power supply circuit to be in a pulse state. It not only reduces the efficiency of the power supply, but more seriously, it will produce serious AC voltage waveforms when the power supply line capacity is insufficient or the circuit load is large. Distortion (Figure 3), and generate multiple harmonics, thereby interfering with the normal operation of other electrical appliances (this is electromagnetic interference - EMI and electromagnetic compatibility - EMC issues).

figure 2

Since electrical appliances have changed from inductive loads in the past (the power supplies of early televisions, radios, etc. all used inductive devices of power transformers) to capacitive loads with rectifier and filter capacitors, the meaning of power factor compensation is not only the power supply The problem of different phases of voltage and current is more serious, and the more serious problem is to solve the electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems caused by the strong pulse state of the power supply current.

This is a new technology developed at the end of the last century (its background stems from the rapid development and widespread application of switching power supplies). Its main purpose is to solve the electromagnetic interference (EMl) and electromagnetic compatibility (EMC) problems caused by severe distortion of the current waveform caused by capacitive loads. Therefore, modern PFC technology is completely different from the power factor compensation technology of the past. It is adopted for non-sinusoidal current waveform distortion, forcing the AC line current to track the instantaneous change trajectory of the voltage waveform, and keeping the current and voltage in the same phase, making the system appear Purely resistive technology (line current waveform correction technology), this is PFC (power factor correction).

Therefore, modern PFC technology has completed the correction of the current waveform and solved the problem of voltage and current in phase.

image 3

For the above reasons, capacitive load electrical appliances that are required to consume more than 85W (some data show that it is greater than 75W) must add a correction circuit to correct their load characteristics so that their load characteristics are close to resistive (voltage and current waveforms are in phase). and the waveforms are similar). This is the modern power factor correction (PFC) circuit.

Dangers of capacitive loads

Figure 4 below is a half-wave rectifier circuit without filter capacitors, and Figure 5 is a half-wave rectifier circuit with large-capacity filter capacitors. We analyze the waveforms of the currents in the two circuits based on these two circuits.

In Figure 4A, D is the rectifier and R is the load. Figure 4B is a diagram of the voltage and current waveforms in the circuit when the circuit is connected to AC power.

In (0°~180°) t0~t3 time: the voltage is zero at t0 and the current is zero, the voltage reaches the maximum value at t1 time and the current also reaches the maximum value, and the voltage is zero and the current is zero at t3 time. (diode conduction)

In (180°~360°) t3~t4: time: the diode is reverse biased and has no voltage and current. (Diode cut-off) At (360°~540°) t4~t6 time: the voltage is zero at t4 and the current is zero, the voltage reaches the maximum value at t5 time and the current also reaches the maximum value, and the voltage is zero and the current is zero at t6 time. (Diode conducts 180°)

Conclusion: In a rectifier circuit without filter capacitor, the voltage and current of the power supply circuit are in the same phase, and the diode conduction angle is 180°. For the power supply line, the circuit presents purely resistive load characteristics.

In Figure 5A, D is the rectifier, R is the load, and C is the filter capacitor. Figure 5B is a diagram of the voltage and current waveforms in the circuit when the circuit is connected to AC power.

At (0°~180°) t0~t3 time: the voltage at t1 time is zero and the current is zero. At t1 time, the voltage reaches the maximum value and the current also reaches the maximum value, because at this time, while supplying power to the load R, the capacitor C must also be supplied. Charging, so the amplitude of the current is relatively large. At time t1, due to charging of the capacitor C, the voltage Uc on the capacitor reaches the peak value of the input alternating current. Since the voltage on the capacitor cannot change suddenly, during the period t1~t3, the voltage on the right side of the diode is Uc, while the voltage on the left side gradually changes from the peak value at time t2. drops to zero, the diode is reverse biased and cut off during t1~t3, and the current is zero during this period. (In the first positive half cycle of alternating current after adding filter capacitor C, the conduction angle of the diode is 90°)

During (180°~360°) t3~t4 time: the diode is reverse biased and has no voltage and current. (diode cutoff)

At (360°~410°) t4~t5 time: Since the diode is reverse biased at t3~t4 time, C is not charged, the voltage on C is discharged through the load, and the voltage gradually decreases (the amplitude of the decrease is determined by the capacity of C and the resistance of R Determined by the size, if the capacity of C is large enough and the resistance of R is large enough, its Uc decreases very slowly.) During the period from t4 to t5, although the voltage on the left side of the diode is gradually rising, the voltage on the right side of the diode is slowly discharging due to the slow discharge of Uc on the right side of the diode. Uc is still greater than the left side, and the diode is still reverse biased.

At (410°~540°) t5~t7 time: at time t5, the voltage on the left side of the diode rises to exceed the voltage on the right side. The diode is turned on to supply power to the load and charge C. The current flowing through the diode is larger. At time t6, the voltage on the left side of the diode is again Gradually decrease, because Uc is charged to the maximum value again, the diode enters reverse bias cutoff again at t6~t7.

Conclusion: In a rectifier circuit with a filter capacitor, the voltage and current waveforms of the power supply circuit are completely different. The current waveform shows a strong pulse state in a short time, and the diode conduction angle is less than 180° (according to the load R and filter capacitor Determined by the time constant of C). For the power supply line, this circuit will produce a large voltage drop on the line during the extremely short period of the strong current pulse (especially significant for the power supply line with large internal resistance), which will cause the voltage waveform of the power supply line to be distorted, and the strong pulse will High-order harmonics cause strong interference to other electrical appliances.

How to perform power factor correction:

Power factor correction (PFC)

The TVs we currently use use high-efficiency switching power supplies, and the internal power input part of the switching power supply without exception uses diode full-wave rectification and filtering circuits, as shown in Figure 6A, and its voltage and current waveforms are shown in Figure 6B

A (left) B (right) Figure 6

In order to suppress the distortion of the current waveform and improve the power factor, modern electrical appliances with large power (greater than 85W) and switching power supply (capacitive load) must adopt PFC measures. There are two types of PFC: active PFC and passive PFC. Way.

At present, some manufacturers do not use correction circuits composed of active devices such as transistors. It is generally composed of passive components such as diodes, resistors, capacitors and inductors. For the larger power TVs designed in the past, domestic TV manufacturers add an inductor (appropriate selection) between the rectifier bridge stack and the filter capacitor. Inductance), the characteristic that the current on the inductor cannot mutate suddenly is used to smooth the fluctuation of the strong pulse of capacitor charging, improve the distortion of the current waveform of the power supply line, and the characteristic of the voltage leading the current on the inductor also compensates the characteristic of the filter capacitor current leading the voltage, so that the power Factor, electromagnetic compatibility and electromagnetic interference are improved, as shown in Figure 7.

Although this circuit is simple, you can simply add a suitable inductor (appropriate selection of the values of L and C) to the previously designed device without PFC function to achieve the function of PFC, but this simple, low-cost The passive PFC output ripple is large, the DC voltage at both ends of the filter capacitor is also low, the ability to correct current distortion and power factor compensation is very poor, and the quality control of the winding of L and the iron core is not good, which will cause The serious interference caused by images and accompanying sounds can only be a temporary measure to enable PFC-free equipment to enter the market in the early stage.

Principle of active PFC circuit

Active PFC has very good effects. It can basically completely eliminate the distortion of the current waveform, and the phase of the voltage and current can be controlled to be consistent. It can basically completely solve the problems of power factor, electromagnetic compatibility, and electromagnetic interference. , but the circuit is very complicated. The basic idea is to remove the filter capacitor after the 220V rectifier bridge stack (to eliminate the current waveform distortion and phase changes caused by the charging of the capacitor). After removing the filter capacitor, a "chopping" circuit The pulsating DC turns into high-frequency (about 100K) AC and then undergoes rectification and filtering. The DC voltage then supplies power to the conventional PWM switching regulated power supply. The process is; AC→DC→AC→DC.

The basic principle of active PFC is to add a DC-DC chopper circuit between the rectifier circuit of the switching power supply and the filter capacitor (Figure 8 (additional switching power supply)). For the power supply line, the output of the rectifier circuit is not directly connected to the filter capacitor, so For the power supply line, it presents a purely resistive load, and its voltage and current waveforms are in the same phase and phase. A chopper circuit also works like a switching power supply. Therefore, the active PFC switching power supply is a switching power supply circuit with a dual switching power supply. It is composed of a chopper (we will call it "PFC switching power supply" from now on) and a regulated switching power supply (we will call it "PWM from now on"). "switching power supply").

Figure 8

Chopper part (PFC switching power supply)

After rectification by the rectifier diode, 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 Figure 9 Current waveform, its waveform characteristics are:

1. The current waveform is intermittent, its envelope and voltage waveform are the same, and the envelope and voltage waveform are in the same phase.

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 power supply.

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

The high-frequency "alternating current" power is rectified by the rectifier diode and filtered to become a DC voltage (power supply) that supplies power to the PWM switching power supply of the subsequent stage. This DC voltage is called: B+PFC in some materials. The B+PFC voltage output by the chopper is generally higher than the +300V after the original 220 AC rectifier and filter. The reason is that high voltage is used and the inductance is It has many advantages such as small wire diameter, small line voltage drop, small filter capacitor capacity, good filtering effect, low requirements on the post-stage PWM switch tube, and so on. The black line is the voltage waveform and the red dotted line is the current envelope waveform.

Figure 9

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

1. Continuous conduction mode (CCM): The operating frequency of the switching tube is fixed, and the duty cycle (coefficient) of the switch changes with the amplitude of the chopped voltage, as shown in Figure 10. The positions of T1 and T2 in the figure are : T1 is in the low voltage area of the chopped voltage (half cycle), T2 is in the high voltage area of the chopped voltage, T1 (time) = T2 (time), it can be seen from the figure that all switching cycle timesare equal, This shows that the operating frequency of the chopper tube remains unchanged at any amplitude of the chopped voltage, as can be seen from Figure 10; the duty cycle in each chopper cycle in the high voltage area and low voltage area is different (T1 The time is the same as T2, but the width of the rising pulse is different), when the chopped voltage is zero (no voltage), the chopping frequency remains unchanged, so it is called continuous conduction mode (CCM). This mode is generally used in 250W ~2000W equipment.

Figure 10

2. Discontinuous conduction mode (DCM): The operating frequency of the chopper switch changes with the size of the chopped voltage (the "on" and "off" times are equal in each switching cycle. Figure 11: T1 and T2 times are different , also reflects that as the voltage amplitude changes, the chopping frequency also changes accordingly. When the chopped voltage reaches "zero", the switching stops (oscillation stops), so it is called discontinuous conduction mode (DCM), that is, there is input voltage chopping. The tube works, and the chopper tube does not work without input voltage. It is generally used in low-power equipment below 250W.

Figure 11

(3) Critical conduction mode (CRM) or transition mode (TCM): The operation is between CCM and DCM, and the operation is closer to the DCM mode. After the previous conduction period ends and before the next conduction period, the inductor current will decay to zero, and the frequency will change with the line voltage and load.

Advantages: cheap chip, easy to design, no switch conduction loss, the choice of boost diode is not decisive;

Disadvantages: Potential EMI issues due to frequency changes, requiring a precisely designed input filter.

Disclaimer: This article is reproduced from the Internet, and the copyright belongs to the original author. If there are any copyright issues related to the work, please contact us in time, thank you!