Pursuing high-quality power supply and demand has always been the goal that countries all over the world want to achieve. However, building a large number of power plants is not the only way to solve the problem. On the one hand, increasing the energy of the power supply, on the other hand, improving the power factor or efficiency of electrical products, in order to effectively solve the problem.
There are many electrical products whose power factor is very low due to the characteristics of their internal impedance. In order to improve the power factor of electrical products, a power factor correction Circuit must be installed at the input end of the power supply. However, the installation of circuits will inevitably increase manufacturing costs, and these costs will be passed on to consumers in the end. Therefore, under the consideration of cost saving, manufacturers usually focus on low prices and are not willing to allow customers to spend more of these environmental protection funds.
Most consumers, because they do not understand the importance of power factor correction circuits, only think that the construction of power plants is the only solution to the problem of insufficient power, which is a major problem in the power supply of most developing countries.
The meaning of power factor
The power (mains) that the power company sends to the user through the power transmission and distribution system is alternating current with a voltage of 100-110V/60Hz or 200-240V/50Hz, and the load impedance of electrical products has three conditions: including resistive, capacitive, and Inductance, etc. Among them, only resistive loads consume power to produce energy conversion such as light or heat, while capacitive or inductive loads only store energy, and will not cause energy consumption. In a purely resistive load condition, the voltage and current are in phase. Under a capacitive load, the phase of the current is leading the voltage, and under an inductive load, the voltage is leading the phase of the current.
This leading or lagging phase angle directly affects the energy consumption and storage conditions of the load, so the real power calculation formula is defined:
P = VICosθ
θ is the angle between V and I, and the value of Cosθ is between 0-1. This value directly affects the actual work of the current on the load, which is called Power Factor (PF).
In order to meet the needs of consumers, the power company must provide S=VI power, and consumers actually only use the power value of P, and some of the energy is used as virtual work and consumed in reactive power. The larger the PF value, the smaller the reactive power consumed, and the smaller the S value that the power company needs to provide, which will reduce the number of power plants to be built.
The structure of the power factor corrector
The main function of the power factor corrector is to make the voltage and current have the same phase and to make the load approximate to resistive. Therefore, there are many ways to design the circuit. Among them, it is classified according to the components used, which can be divided into passive and active power factor modifiers. The PF value of the passive power factor modifier can only reach 70% under the best conditions, which is not applicable under strict power factor requirements. If you want to achieve a PF value of more than 80% in the full voltage range (90V~265Vac) and under light and heavy loads, an active power factor corrector is a necessary choice. The active power factor corrector is mostly a boost circuit structure (Boost Topology).
As shown in Figure 1, Figure 2 is the inductance waveform. The input voltage is required to be 90V~265Vac, and the Vd point is 127V~375V DC voltage. The output voltage Vo is raised to 400V DC by the booster circuit. The working process is as follows :
1. When Q is turned on, the voltage on the Inductor VL=Vd. At this time, Vd, L, and Q form a loop, and Vd charges the inductor L. The loop is shown by the dotted line in Figure 1. At this time, the inductor current ζL follows the same slope Rising, until Q ends, the duty cycle (DT) ends.
2. When Q is turned off, the inductor voltage is reversed and Vd is added to discharge the output terminal through the diode D. At this time, the capacitor C is in a charged state, and RL maintains Vo output, where the value of Vo is the input voltage Vd plus the inductor The value of voltage (-VL) (because the inductor voltage is inverted, -VL is positive instead), the loop is shown as the gray line in Figure 1, until Q turns on again (that is, the (1-D) T period ends) ).
If you want the step-up circuit in Figure 1 to have a power factor correction function, the Q control signal must come from an IC (PFC IC) with a power factor correction function, and the voltage loop and current loop must be used for feedback control. These signals are sent back to the PFC IC to control the on and off of Q, thereby achieving the purpose of current waveform shaping.
There are two types of PFC ICs. One is the Discontinuous Current Mode Power Factor Corrector (DCM PFC), which is suitable for power factor correction with lower power requirements. European energy regulations stipulate that power supplies above 70W must be equipped with PFC circuits. DCM PFC is generally used below 200W. The other is a continuous current mode power factor corrector (CCM PFC), which is generally used above 200W to thousands of W.
Figure 3 DCM PFC in peak current control mode
DCM PFC control method
Regardless of the CCM or DCM PFC, the circuit structure is a boost circuit. The biggest difference is the control mode. DCM PFC generally uses the peak-to-peak current control mode (as shown in Figure 3). This mode is mainly when AC input is bridge-rectified to form an m-shaped voltage waveform, divided by R5 and R6, and then multiplied by an output signal Vc amplified by an error amplifier (Error Amplifer) . This is to give a reference voltage Vm for the peak current flowing through Rs, and this voltage will be adjusted according to the input and output voltages, where the output voltage is divided by resistors R3 and R4, and then passed through the error amplifier. Feedback to the input of the multiplier can keep the output voltage stable when the load changes.
One more thing to note is that when the error amplifier is used for closed-loop compensation, its gain bandwidth is lower than one-sixth of the mains frequency to avoid interference with the main functions of the PFC circuit, so the values of C1 and C2 Usually not small, about uF-level capacitance. When the multiplier outputs Vm, the voltage waveform at the same moment is still an m-shaped waveform, but it is a reference voltage waveform that has been sorted, and then input to the positive input of the comparator, and to the negative input of the comparator The waveform of the S pole current of Q (that is, the voltage waveform Vs of the voltage drop on Rs) is compared to control the on and off of Q. The waveform is shown in Figure 4.
Figure 4 Action waveforms of each point of DCM PFC
At first, when Q is turned on, the input DC high voltage Vd charges the inductor L, causing the inductor current ζL to rise (see point a to b of the inductor current waveform in Figure 4). At this time, the voltage Vs on Rs also rises until When Vs=Vm (point b), since the voltage at the inverting input terminal of the comparator (Current Comp) is higher than the non-inverting input terminal, the R input terminal of the RS flip-flop (RS Flip-Flop) is at a low potential. At this time, the S terminal is at a high potential, which makes the output of the trigger to be at a high potential, which turns on Qd, while Vg is at a low potential, and Q is in an off state. The voltage VL on the inductor is reversed, and the input voltage Vd makes the diode D conductive. Start to discharge the output RL and C5 (points b to c in the figure). At this time, the load RL is still maintained at a high potential, and the capacitor C5 is charged by the inductor discharge until the inductor discharges to a value of ζL 0 ( c).
When the inductor current ζL is 0, the S terminal of the RS flip-flop inputs a low level, and the R terminal is a high level (because Vm>Vs). At this time, the Q output of the flip-flop is a low level, so that Qd is cut off, and the VGS of Q Is a high potential, so Q is turned on, the inductor voltage VL is positive, the input voltage Vd supplies current flowing through the inductors L and Q, charging the inductor L, so the current flowing through the inductor L continues to rise, until the triangle wave voltage Vs again Until it hits the m-shaped wave Vm (section c to d), and so on, the circuit uses this peak current control mode to obtain the ζL current waveform.
The waveform of ζL is composed of many large and small triangle waves. After all, it is not a sine wave. Therefore, a C3 capacitor must be installed in the circuit to filter out the high frequency components in the inductor current, so that the input sine wave current ζ is a complete basic sine wave Component, its size is the average value of the inductor current ζL. Basically, the peak value of ζL is about twice the peak value of current ζ, which can be used as a reference for selecting Q’s withstand current.
CCM PFC control method
For CCM PFC, the commonly used control mode is the so-called average current control mode, and the control mode circuit is shown in Figure 5.
Figure 5 Boost circuit in average current control mode
In the figure, Vin is DC voltage and Ip is DC current. The voltage and current waveforms at each point are shown in Figure 6.
Figure 6 Waveforms of each point of the boost circuit in average current control mode
The gate of Q is controlled by the comparison result of the Vs voltage and Vc voltage of the PWM comparator: when Vs is greater than Vc, the output of the comparator is low, and when Vs is less than Vc, the output of the comparator is high. Therefore, when the circuit first starts to operate, Vs is less than Vc, at this time the comparator outputs a high potential, and Q is turned on. As shown in Figure 5, Vin charges the inductor L along the dashed path, so the inductor current ζL rises (from a to b). At point b, when Vs is greater than Vc, the output of the comparator changes from a high potential to a low potential, and Q is cut off. , The reverse voltage of the Vin voltage applied to the inductor L charges the capacitor C through the diode D and supplies the voltage to the load (the gray line in the figure). At this time, the inductor L is in a discharged state, so the inductor current ζL decreases (from b to c) ), when the point c is reached, Vs is less than Vc, at this time the comparator outputs a high potential again, making Q turn on again. This is repeated, the current waveform of the current amplifier and the sawtooth wave are compared with each other to generate the driving waveform of Q, so as to achieve the purpose of controlling the load voltage with the average current.
Pay attention to the waveform in Figure 6. In the singular time period such as the ab segment or the cd segment, the waveform of the Vc voltage must have a negative slope before interleaving with Vs. At this time, Vs is a positive slope and must be interleaved, otherwise it cannot be controlled. In even-numbered segments such as bc segment or de segment, both Vc and Vs have positive slopes, but the slope of Vc must be smaller than Vs, otherwise it cannot be interleaved and cannot be controlled. Therefore, when designing the control circuit, you must pay attention to these controls. The key point is to arrange the parameters of the peripheral components, otherwise the circuit will fail to operate, or the circuit will be damaged due to out of control.