Power factor compensation: In the 1950s, an improved method has been proposed for the low power supply efficiency caused by the voltage and current of alternating current appliances with inductive loads (Figure 1) (due to the current lag of inductive loads. 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 that a capacitor 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 voltage on the capacitor is used to compensate the characteristic of the current lagging voltage on the inductor to make the overall characteristic close to the resistance, thereby improving the efficiency is called power factor compensation (AC The power factor can be expressed by the cosine function value cosφ of the phase angle of both the power supply voltage and the load current).


     Since the 1980s, a large number of high-efficiency switching power supplies have been used in electrical appliances. Since the switching power supply uses a large-capacity filter capacitor after rectification, the load characteristics of the electrical appliance are capacitive, which is As a result, when the AC 220V is supplying power to the electric appliance, due to the charging and discharging of the filter capacitor, the DC voltage at both ends of the device appears as a sawtooth ripple. The minimum value of the voltage on the filter capacitor is far from zero, and it is not much different from its maximum value (peak ripple). According to the unidirectional conductivity of the rectifier diode, only when the instantaneous value of the AC line voltage is higher than the voltage on the filter capacitor, the rectifier diode will conduct due to forward bias, and when the instantaneous value of the AC input voltage is lower than the filter capacitor Voltage, the rectifier diode is cut off due to reverse bias. In other words, during each half cycle of the AC line voltage, the diode will only conduct around its peak value. Although the AC input voltage still maintains a substantially sinusoidal waveform, the AC input current shows a high-amplitude spike, as shown in Figure 2. This severely distorted current waveform contains a large amount of harmonic components, causing a serious drop in line power factor.
     In the positive half cycle (1800), the conduction angle of the rectifier diode is much smaller than 1800 or even only 300-700. Due to the requirement to ensure the load power, a very large conduction current will be generated during the extremely narrow conduction angle , So that the power supply current in the power supply circuit is in a pulse state, it not only reduces the efficiency of power supply, but more serious is that it will produce severe AC voltage waveform distortion when the power supply line capacity is insufficient or the circuit load is large (Figure 3 ), and generate multiple harmonics, thereby interfering with the normal operation of other electrical appliances (this is the problem of electromagnetic interference-EMI and electromagnetic compatibility-EMC).

Figure2

      Since electrical appliances have changed from past inductive loads (the early televisions, radios and other power sources used inductive devices of power transformers) to capacitive loads with rectifiers and filter capacitors, the meaning of power factor compensation is not only power supply The problem of different phases of voltage and current is more serious to solve the problems of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) caused by the strong pulse state of the 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 problems of electromagnetic interference (EMl) and electromagnetic compatibility (EMC) caused by the severe distortion of the current waveform caused by the capacitive load. Therefore, the modern PFC technology is completely different from the past power factor compensation technology. It is taken against the distortion of the non-sinusoidal current waveform, 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, so that the system appears Pure resistive technology (line current waveform correction technology), this is PFC (power factor correction).
     Therefore, the modern PFC technology completes the correction of the current waveform and solves the in-phase problem of voltage and current.

Figure 3

      For the above reasons, capacitive electrical appliances that require electrical power greater than 85W (some data show 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.

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

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

      At (00 ~ 1800) t0 ~ t3 time: the voltage is zero at t0 time, the current is zero, the voltage reaches the maximum value at t1 time, the current also reaches the maximum value, the voltage is zero at t3 time, and the current is zero. (Diode turned on 1800)
      During (1800～3600) t3~t4: time: diode reverse bias without voltage and current. (Diode cut off)
      At (3600～5400) t4～t6 time: the voltage is zero at t4 time, the current is zero, the voltage reaches the maximum value at t5 time, the current also reaches the maximum value, the voltage is zero at t6 time, and the current is zero. (Diode turned on 1800)
      Conclusion: In the rectifier circuit without filter capacitor, the voltage and current of the power supply circuit are in phase and the conduction angle of the diode is 1800. For the power supply line, the circuit exhibits a purely resistive load characteristic.

In Figure 5 A, D is the rectifier, R is the load, and C is the filter capacitor. FIG. 5B is a voltage and current waveform diagram of the circuit when the circuit is connected to alternating current.

      At (00～1800) t0～t3 time: the voltage is zero at t1 time, 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, the capacitor C is charged while power is supplied to the load R, Therefore, the amplitude of the current is relatively large. At time t1, due to charging the capacitor C, the voltage Uc on the capacitor reaches the peak value of the input AC. Since the voltage on the capacitor cannot be abrupt, the voltage on the right side of the diode is Uc during t1~t3, and the voltage on the left side gradually increases from the peak value at time t2. The drop is zero, the diode is reverse-biased during t1~t3, and the current is zero during this period. (The positive half of the first alternating current after adding the filter capacitor C, the conduction angle of the diode is 900)
     During (1800～3600) t3～t4: the diode is reverse biased without voltage and current. (Diode cut off)
     At (3600～4100) t4～t5 time: Because 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 magnitude of the decrease is determined by the capacity of C and the resistance of R If the capacity of C is large enough, and the resistance of R is also large enough, its Uc decreases slowly.) Although the voltage on the left side of the diode is gradually rising during t4~t5, the voltage on the right side Uc is still due to the slow discharge of Uc on the right side of the diode Greater than the left, the diode is still reverse biased.
     At (4100～5400) t5～t7 time: the voltage on the left side of the diode rises to exceed the voltage on the right side at time t5. The diode turns on to supply power to the load and charges C. The current flowing through the diode is larger, and the voltage on the left side of the diode gradually decreases at time t6. Since Uc is charged to the maximum value again, the diode enters reverse bias cut-off at time t6~t7.
     Conclusion: In the rectifier circuit with filter capacitor, the voltage and current waveforms of the power supply circuit are completely different. The current waveform is in a strong pulse state in a short time, and the conduction angle of the diode is less than 1800 (according to the load R and the filter capacitor C Depending on the time constant). For the power supply line, this circuit generates a large voltage drop on the line during the extremely short period of strong current pulses (especially for power supply lines with large internal resistance), which causes distortion of the voltage waveform of the power supply line. High-order harmonics produce strong interference to other electrical appliances.
      Power factor correction (PFC)
      The TV we are using now uses an efficient switching power supply, and the internal power input part of the switching power supply uses the full-wave diode rectification and filtering circuit without exception, as shown in Figure 6A, and its voltage and current waveforms are shown in Figure 6B.

A B Figure 6

     In order to suppress the distortion of the current waveform and improve the power factor, modern appliances with large power (more than 85W) with switching power supply (capacitive load) must adopt PFC measures, PFC is available; active PFC and passive PFC the way.
      At present, some manufacturers do not use correction circuits composed of active devices such as transistors. It is generally composed of passive devices such as diodes, resistors, capacitors and inductors. To the current domestic TV manufacturers, for the TVs with higher power designed in the past, add an inductor between the rectifier bridge stack and the filter capacitor (select appropriately Inductance), the characteristics of the current on the inductor can not be abruptly used to smooth the fluctuation of the strong pulse of the capacitor charging, improve the distortion of the current waveform of the power supply line, and the characteristic of the voltage leading current on the inductor also compensates the characteristic of the filter capacitor current leading voltage, making the power Factor, electromagnetic compatibility and electromagnetic interference are improved, as shown in Figure 7.

Picture 7

     Although this circuit is simple, you can simply add a suitable inductance (appropriately select the value of L and C) to the device without PFC function designed in the early stage, so as to achieve the role of PFC, but this simple, low cost The output ripple of the passive PFC is large, the DC voltage across the filter capacitor is also low, the current distortion correction and power factor compensation capabilities are very poor, and the winding of L and the quality control of the iron core are not good. The serious interference caused by the image and sound can only be a temporary measure for the early PFC-free equipment to enable it to enter the market.
     Active PFC has a good effect. It can basically eliminate the distortion of the current waveform, and the phase of the voltage and current can be controlled to maintain the same. It can basically completely solve the problems of power factor, electromagnetic compatibility, and electromagnetic interference. However, 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 change caused by the charging of the capacitor), and remove the filter capacitor by a "chopping" circuit to The pulsating DC becomes high frequency (about 100K) AC and then after rectification and filtering, its DC voltage is then supplied to the conventional PWM switching 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, whose voltage and current waveforms are in phase and phase. The operation of the chopper circuit is also similar to a switching power supply. Therefore, the active PFC switching power supply is a dual switching power supply switching power supply circuit, which is composed of a chopper (we will call it: "PFC switching power supply") and a regulated switching power supply (we will call it: "PWM" Switching power supply").

Figure 8 chopper part (PFC switching power supply)

      After the rectifier diode is rectified, the filter capacitor is not added, and the unfiltered pulsating positive half-cycle voltage is used as the power supply of the chopper. Due to the chopper's series of "switching" working pulses, the pulsating positive voltage is "chopped" into Figure 9. Current waveform, the characteristics of the waveform are:
     1. The current waveform is intermittent, the envelope and voltage waveform are the same, and the envelope and voltage waveform are in 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. The high frequency "AC" power must be rectified again to be stabilized by the subsequent PWM switching Voltage power supply.
     3. From the external power supply point of view, the power system has achieved the same phase of AC voltage and AC current and the voltage waveform and current waveform 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 to become a DC voltage (power supply) to supply power to a subsequent PWM switching power supply. The DC voltage is called B+PFC in some documents (TPW-4211 is the case). The B+PFC voltage output from the chopper is generally higher than the original +300V after 220 AC rectification and filtering. It chooses high voltage, its inductance has a small wire diameter, a small line voltage drop, a small filter capacitor capacity, and a good filtering effect. It has many advantages such as low requirements for the PWM switch tube of the subsequent stage. Black is the voltage waveform and the red dotted line is the current envelope waveform
      At present, in the PFC switching power supply part, the chopper tube (K) that functions as a switch has two working modes:
      1. Continuous conduction mode (CCM): the working frequency of the switch tube is fixed, and the duty cycle (coefficient) of the conduction changes with the amplitude of the chopped voltage. As shown in Figure 10, the positions of T1 and T2 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). From the figure, it can be seen that all switching cycle times are equal, This shows that at any amplitude of the chopped voltage, the operating frequency of the chopper tube does not change, as can be seen from Figure 10; the duty cycle in each chopping cycle in the high voltage region and the low voltage region is different (T1 The time is the same as T2, and 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 applied at 250W ~2000W devices.
      2. Discontinuous conduction mode (DCM): The operating frequency of the chopper switch varies with the magnitude of the chopped voltage (the "on" and "off" time is equal in each switching cycle. As shown in Figure 11: T1 and T2 have different times It also reflects that the chopping frequency changes correspondingly with the change of voltage amplitude. The chopped voltage is "zero" and the switch stops (oscillation stops), so it is called discontinuous conduction mode (DCM), that is, there is input voltage chopping The tube works, the chopper tube without input voltage does not work. He is generally used in low-power devices below 250W.


     The work is between CCM and DCM, and the work is closer to the DCM mode. After the last turn-on period ends, before the next turn-on period, the inductor current will decay to zero, and the frequency will change as the line voltage and load change.
Advantages: cheap chip, easy design, no conduction loss of switch, choice of boost diode is not decisive;
Disadvantages: Due to the frequency change, there are potential EMI problems, and an accurate input filter is required.