One Cycle Control of High Power Factor Rectifier simulation
Time:2013-03-21
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With the continuous development of power electronics with nonlinear loads in power electronic devices are widely used in the grid and generate a large number of current harmonics, power system waveform harmonic distortion and the resulting system not only greatly reduce the power factor, but also bring harm to the system.
For more simple control strategy to reduce the PFC costs, reduce the total harmonic content (THD) and EMI, the current three-phase power factor correction to research focused on control strategy and topology structures. Control strategy focused on current-mode control, multi-loop control, single-cycle control, vector control and so on. Single-cycle control of three-phase rectifier control technology to reduce the current distortion, the input current in each switching cycle can be a very good track reference current, so that there is a lot of DC output current harmonics, also in low, the input current distortion, thus achieving high power factor rectifier.
A rectifier topology
Three-phase three-switch PFC circuit shown in Figure 1, two main level and three level 2 structure. Figure 1 (a) for the three-level structure, the two capacitor mid-point of the mid-point potential and grid potential is basically the same, through bi-directional switch Sa, Sb, Sc are the corresponding phase current control. Switch is closed when the corresponding phase of the current amplitude increases, the power is off when the corresponding arm of the diode bridge conduction circuit, the output voltage under Boost inductor current decreases, and thus realize the current control. Figure 1 (b) shows the circuit for the two-level structure, by switching, you can control the phase current. Switch conduction, the inductive energy storage, the inductor current increases; switch is turned off, the power supply to the load and inductor together, current decreases.
Can be seen that these two kinds of topology advantages and disadvantages. Here select the two-level topology, using partition between control, allowed to work in parallel at both ends Boost state, this control method is characterized by only two at any given time work in the high frequency switch mode, so loss smaller. When the circuit in continuous conduction mode, the structure of the circuit the input voltage and input current in phase, to achieve unity power factor and low input current harmonic content.
2 single-cycle control (0CC) technology
0CC technology is developed in the early 90s, a nonlinear large-signal PWM control theory, is an analog PWM control technology. It is the duty cycle by controlling the switch so that each switching cycle of switching the average of the variable is equal to or proportional to the strict control of information quantity. The average input current follow the reference current and is not bound by the load current, even if the load current will not have much input current harmonic distortion. Thus the single-cycle control technique applied to three-phase rectifier can achieve low current distortion and high power factor, the control methods canceled the traditional control method of the multiplier, so that the control circuit reduces the complexity, with fast dynamic response constant switching frequency, robustness, easy implementation, is a good control method.
2.1 OCC basic principles of technology
Figure 2 is a fixed switching frequency of the single-cycle control buck converter schematic.
For the control technology of the single-cycle instructions are single cycle control buck converter with an example. Figure 2, the circuit is, by the opening of the controller to produce a constant frequency pulse opening switches S, diode voltage VD VS after by the integrator output voltage Vinf, when Vinf up to a given voltage Vref comparator output when the flip, the controller turn-off signals sent off switch S; At the same time, the controller issued the reset signal to reset the integrator to zero time, to prepare for the next cycle. You can see, buck converter output voltage is the average diode voltage are defined switching off the diode voltage waveform cycle area under the curve:
If the given signal Vref is a constant, the average diode voltage VS on a constant, so the output voltage to a constant. Integral control of continuous and constant reference volume comparison, if the input voltage becomes higher integral value can quickly reference to control the amount of duty cycle resulting smaller; if the input voltage is lower, then the duty cycle will be larger. If the control reference level is changed, then a switching cycle, the diode voltage is equal to the average change in the amount of control information, control the reference output voltage is equal to the amount. Reference volume control in each step change in its value during the period, the diode voltage integral control reference quantity follow immediately, for the control principle, the duty cycle d by (2) to determine.
Can be seen, the duty cycle d is the input voltage Vg and a given nonlinear function of voltage Vref. So this control method is a nonlinear control. Because of this nonlinear control, making the average voltage VS in each switching cycle and Vref are identical, and independent of the input voltage Vg. Thus, the output voltage V0 is given voltage Vref of a linear function:
2.2 Single-cycle control PFC Rectifier
Single-cycle control is a novel control method does not require multipliers, the whole cycle every 60 ° to be divided, is divided into six intervals. PWM control technique based on the working principle of three-phase rectifier in each 60 ° interval control only two switch-off to achieve unity power factor. Equivalent circuit into two parallel single-phase Boost circuit can be used to control single-phase PFC control circuit technology. Is a fixed switching frequency of the single-phase Boost type PFC, for example, to describe a single cycle of high-power Gun Control in the number of rectifier.
Figure 3 shows the single-cycle control PFC rectifier, eliminating the need for line voltage detector and the multiplier is a simple current control mode, the current detection circuit with the traditional control method used in the multiplier current detection circuit different . Test current of the inductor current Boost power factor correction, the current was measured with a precision non-inductive circuit in series to achieve a sense resistor RS. RS can be located anywhere in the current flowing through the inductor. Typically, the RS set in the DC side, this detection circuit is relatively simple, and does not require isolation.
