斩控式交流稳压电源电路参数设计.doc

Parameters Design of Main Circuit of AC Constant Voltage Power Supply Based on Dual Directional Reversible PWM ControlAbstract: The constant voltage power supply with dual directional reversible PWM compensation has merits of high precision, high efficiency, small volume, and so on. To ensure the quality of supply, it is important to fix the value of inductance, capacitance in main circuit. From the angle of filtering, and through analysis of the circuit working, detailed formulas of the inductance and capacitance calculation are given and an example of it is given too. In parameters calculation, to reduce the volume, the filter inductance is taken a smaller value but the capacitance is taken a greater value; the DC smoothing capacitance should be chosen according to the amplitude limit of DC voltage rising in inversion, and should be taken as great as possible to maintain voltage stabilizing precision. Keywords: constant voltage; compensation; PWM rectifier; SPWM inverter; filter. INTRODUCTIONFig.1 Struetue diagram of regulate power supplyThere are many kinds of AC constant voltage power supply, of which types of parameter adjustment, self-coupled adjustment, full-power transforming1 have following characters: large volume of electromagnetic components, great harmonics of voltage and current, low power factor, so it has some technical advantages in supply compensatiing within small capacity. The constant voltage power supply with compensation adopts the compensation part being increased to realize the stability of output voltage, it has following characters: simple in circuit, small in capacity, high in precision, quick in response speed. In constant voltage power supply based on dual PWM cotrol, the compensation power can flow in dual direction, is good in comprehensive performances. With the development of micro-computer, speed faster and faster, cost lower and lower, the dual directional reversible PWM constant voltage power supply controled by MCU is becoming the mainstream in AC constant voltage power supply design. MAIN CIRCUIT DESIGN Structure of the constant voltage power supply with compensation of dual directional reversible PWM is shown in Fig.1, Mode of PWM control is adopted in rectifier and inverter.By using PWM rectifier, power factor of AC side can reach to 1; low-order harmonic pollution to line will not be produced; stable DC voltage can be get, and its amplitude is higher than the DC voltage obtained by diode rectifier, which is favorable for the working of inverter. Mode of SPWM control is used in inverter, by which a good sinusoidal wave can be produced. This sinusoidal wave has the same frequency, same or inverting phase with the line voltage, and after series with the line through compensation transformer, a rated output voltage is get finally.A. Structure of Main CircuitMain circuit structure of the constant voltage power supply is shown in Fig.2, rectifier and inverter all adopt single phase full-bridge converter, and switchs adopt IGBT. The primary side of compensation transformer is connected to the inverter output, the second side is in series between the line and the load. In PWM rectifier circuit2, Ls is AC-side inductance, Lf1 and Cf1 constitute secondary filter of DC, Cd is DC smoothing capacitance. In SPWM inverter circuit, Lf2 and Cf2 constitute output filter, by which the compensation voltage is in series with input voltage through compensation transformer T.Fig.2 Diagram of main circult of regulated Power supplyB. Analysis of Power FlowingDotted terminals of compensation transformer T are shown in Fig 3. If , a voltage of opposed-phase to line voltage is need to be a negative compensation. It is shown from the direction of and polarity of that the energy flows from secondary side to primary side of transformer, through inverter and rectifier then feeds back to the line, and PWM rectifier works in inverting state.Similarly, If , a voltage of in-phase to line voltage is need to be a positive compensation, and the energy flows from primary side to secondary side.When the load is a rated one and the system reaches a steady state, power processed by compensation circuit is: (1)C. Parameters Calculation of Main Circuit1) The Compensation TransformerIf the input is , capacity of the compensation transformer is (2）of which, is the rated voltage of secondary of compensation transformer, is voltage of primary, is ratio.Generally, for a certain abundance, DC voltage of inverter should be 20% 30% higher than peak value of the output AC voltage. DC voltage of PWM rectifier is, so （3）Then , and ratio of compensation transformer is （4）finally, is taken to a integer.2)Output Filter of InverterLf2 and Cf2 constitute LC series resonant filter of inverer output3, resonant frequency is .In output voltage of the inverter, harmonic frequency is 4，of which, k is carrier ratio, is carrier frequency, is line frequency, if n = 1, 3, 5, , k = 0, 2, 4, .; if n=2,4,6, k=1,3,5,.In all harmonics, harmonic component of has the highest amplitude and greatest influence.Because the carrier frequency is 100 times greater than the fundamental frequency, so generally 1/10 of the carrier frequency is taken as the resonant frequency of filter. Even so, the cut-off frequency is still far larger than the line frequency.Fig.3 Schematic diagram of power flow as positive compensationAfter defining the resonant frequency, the output current rate of inverter also need to be considered. The ripple value of output current can be expressed as （5）of which, is input voltage of inverter, or namely DC-side voltage of inverter, ; is output voltage of inverter; M is modulation; Tc is switching cycle, or namely the reciprocal of carrier frequency . If the carrier ratio is large enough, then （6）When , the output current ripple is reach to maximum, then （7）The ripple coefficient of inverter output current is ，then （8）So then （9）With the DC-side voltage of inverter, the carrier frequency, the ripple coefficient of current, the rated output current of supply, value of Lf2 and Cf2 can be calculated. And in general, is defined 0.2.3) AC-side Inductance of The RectifierTo improve the dynamic characteristic of PWM rectifying system and avoid influence of AC-side parameters, PWM rectifying ofen adopts direct current control5. In the direct current control based on a fixed switching frequency, as the switching frequency is fixed, it is easier to design the transformer and AC inductance, favorable for the limit of switching loss and the reducing of AC-side harmonic current.When PWM rectifier adopts the bipolarity modulation, at the zero-crossing of current, changing rate of current is the most, and now the inductance should be small enough, and the system must satisfy the request of fast current tracking; at the peak of current, the ripple of harmonic current is most serious, now the inductance should be large enough, the system must satisfy the request of current pulsating limit.Fig.5 Diagram of current wave besidepeak The current tracking process of a switching cycle at the zero-crossing is shown in Fig 4, is the PWM rectifying switching cycle.In the stable state, when Fig.4 Diagram of current tracking beside zero-crossingWhen To satisfy the request of fast current tracking, then （10）of which, is current amplitude of instruction, and . From 3 formulas above, then （11）Obviously, the change rate of current is biggest when , the corresponding AC-side inductance should be （12）At the peak of current, the ripple of current is most. The current tracking process of a switching cycle at the peak is shown in Fig 5.In the stable state, when , of which, is the peak of .When , In the stable state, sine current should satisfy in a switching cycle near the peak. In PWM rectifying, , so （13）also, , then （14）Assuming the maximum allowable of harmonic current pulsation is , or , then the inductance should satisfyIn the Bipolar modulation PWM rectifier with direct current tracking, to meet the requirements of fast current tracking and inhibiting current harmonic, the range of inductance is （15）To reduce volume and weight of inductance, generally, AC-side inductance is fixed a small value, and the allowed maximum of current pulsation is taken .4) DC-side Filter of The RectifierFor PWM rectifier of UPF, voltage and current are in-phase in AC side, its input power is （16）Of which, is RMS of input voltage ; is RMS of fundamental current of the rectifier input.DC-side voltage of voltage source PWM rectifier is constant, the instantaneous value is , then the DC-side output current is.As DC-side current is pulsating at 2 times of the line frequency, to get a flat voltage, filtering must be done6. A DC-side filter consists of a second single tuned filter and a smoothing capacitance. And the second single tuned filter meets to （17）The amplitude of AC component in DC-side current is （18）To define voltage coefficient of resonance capacitance to be , then（19） （20）To reduce the capacitance volume, the rated capacitance voltage is chosen a small one, generally fix the voltage coefficient of resonant capacitance .5) Smoothing Capacitance of Rectifier DC-sideThe smoothing capacitance of PWM voltage rectifier DC-side7,8 is using in the cushion of exchange between AC and DC load, in restraining the harmonics, and in stabilizing DC voltage when the inverter feeding back energy, so in the value defining, many kinds of situations should be considered.1) According in restraining the harmonicsIf the ripple coefficient of DC-side voltage is , ripple voltage is . Then without second filtering is （21）Then （22）Normally, taking the ripple coefficient .2) According in the stable DC voltage when the inverter feeding back energyBecause of characteristics of the voltage zero-crossing signal sampling and MCU system, one cycle may differs in state transition between rectifier and inverter. The maximum power feeded back to DC side by the inverter in a line frequency cycle is ， is line frequency, and the feeding back energy will cause the DC voltage to rise. If the voltage change rate of DC-side is ，then （23）So （24）In taking the actual value, a little more than the calculated should be taken. DESIGN PRACTICE A compensating AC constant voltage power supply has following major technical index: total capacity of , input voltage of , output voltage of , efficiency of 95%. 1) Compensation transformerThe rated output current of power supply is , capacity of compensation transformer is , rated voltage of inverter output is , transformer ratio is .2) Inductance of PWM rectifier Compensating current amplitude is , the maximum allowable of current pulsation is , carrier frequency of the rectifier and inverter are all , then switching cycle is , , to reduce the volume of inductance, taking .3) DC-side filterIn this filter, voltage coefficient of the resonant capacitance is taken , the second single tuning capacitance is taken , inductance is taken .4) Smoothing capacitance of DC-sideCompensation transformer ratio is k=5, output voltage precision is 11%. Considering harmonics restraining, voltage ripple coefficient is taken , then ; considering the energy feeded back by inverter, then , . Selecting the max of two, so taking .5) Output filterof inverter In this filter, switching cycle is , , cut-off frequency is ，then ,.6) EfficiencyBy using soft-switching technology of RC parallel buffer, power loss is mainly device losses on-state and compensation transformer losses. If the efficiency of compensation transformer is 95%, the voltage drop on-state of device is 2V, then under rated load, the maximum compensation power, it has,. CONCLUSIONThe constant voltage voltage supply with dual directional reversible PWM compensation has high precision, high efficiency, small volume, and so on. To ensure the quality of supply, it is important to fix the value of inductance, capacitance in main circuit. From the angle of filtering, and through analysis of the circuit working, detailed formulas of the inductance and capacitance are given and an example of it is given too. Through the analysis, f