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1、OnOn- -Bright confidentialBright confidential1开关电源环路设计与计算开关电源环路设计与计算On-Bright Electronics (Shanghai) Co. LtdOn-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential2内容?开关电源简介?开关电源环路分析(以反激变换器为例)?开关电源的环路补偿(以反激变换器为例)?一个基于OB2263的12W电源环路设计实例?讨论On-Bright Confidential to TenpaoOnOn- -B
2、right confidentialBright confidential3?用途：AC/DC， DC/DC， DC/AC (inverter)?基本拓扑结构：?非隔离型：Buck， Boost， Buck-Boost?隔离型：Forward, Flyback, Half Bridge, Full Bridge, Push-Pull?整流型：全桥型整流全波型整流同步整流(SR)开关电源简介On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential4开关电源简介?调制方式?PWM (pulse wid
3、th modulation)?PFM (pulse frequency modulation)?其它衍生调制方式?控制模式?电压模式?电流模式?开关电源系统可分为两大块?负反馈回路(feedback loop)?保护功能(OVP, OCP, OTP )On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential5开关电源系统基本组成部分 (Voltage Mode PWM System)On-Bright Confidential to TenpaoOnOn- -Bright confidential
4、Bright confidential6开关电源环路分析和设计流程?确定环路基本组成模块， 辨认环路。?EA (error amplifier)?Compensation network?Power Stage?PWM Stage?画出环路的信号流程图, 确定环路的小信号传输函数?确定环路的带宽, 设计补偿网络?根据实际应用，设计和调整外围参数On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential7开关电源环路的小信号传函?EA (例如TL431)及其补偿网络的传函?PWM Stage传函?Po
5、wer Stage 传函 (可参考1)?CCM small signal model?DCM small signal model?考虑斜率补偿后的Power stage 传函On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential8Flyback 系统控制流程图On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential9TL431 及其等效模型On-Bright Confidential to Ten
6、paoOnOn- -Bright confidentialBright confidential10Power Stage小信号模型建立流程1. 建立Power Stage中电感，电容等器件的时域微分方程组2. 对方程组作状态空间平均化(state space averaging)， 也就是对一个开关周期内作平均3. 微扰(perturbation)， 即对方程组作小信号扰动4. 对方程组作线形化(linearization)5. 得到等效的小信号模型On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confide
7、ntial11Flyback PWM Stage小信号模型(CCM)?Power Stage传函:2 2 1 2 22212 1)1 (1)(1 ()()(DnLCsRDnLsDRnDsLCsRnDVsdsvosoom o g+ CRwoz 11=?负载电容ESR 零点:DLDRnwmo rz22)1 ( =?系统右半平面零点:On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential12右半平面零点(RHZ)的直观理解?RHZ在boost, buck-boost, flyback(占空比由输入输出
8、电压和匝比决 定)CCM中都存在,而DCM中没有RHZ。?负载突然增加输出电压下降EA+PWM 反应占空比增大增大(Wrong Way)反激时间减小输出电流减小(通过输出diode)输出电压下降更多 (临时)。此即典型RHZ响应特性。?在DCM中，占空比增大导致输出电流增大，故不存在此RHZOn-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential13PWM Stage 的小信号建模?写出时域方程?微扰和线性化?直流方程组、 交流小信号方程组?PWM Stage小信号模型( )( )( )pkc p
9、kcVtVforVtVtd=0pkc VVD =( )( )pkc Vtvtd)=( )tvVcc)+( )tdD)+( )( ) ( )( )( )( )+=+=+=pkcccccVtvVtdDtdDtdtvVtv)On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential14考虑斜率补偿后的Power Stage 传函?电流模式工作的PWM系统的电压环路内嵌入一个局部的电流环，从而改善系统的瞬态响应?电流模式PWM系统当D50%时，存在Sub-harmonic震荡?斜率补偿能够消除此震荡?电流环
10、小信号模型On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential151)(= ssTs eesTsH?采用电流模式控制，等效在电压环路内引入超前相位补偿，相当于引入微分函数He(s):考虑斜率补偿后的Power Stage传函(CCM)On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential16?CCM模式Flyback的斜率补偿?Fm:LnVmmLnVmLVmkorrog22221=()smTmm
11、F211 +=考虑斜率补偿后的Power Stage传函(CCM)On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential17?存在一个与负载等效电阻有关的滤波极点?存在一个电容ESR引起的零点?存在一个右半平面零点，该零点与输入电压、负载、电感量有 关，很难补偿，设计时应该使环路带宽应该远离这个零点，以 减小其影响( )DsCRDRnDsLCsRRVnVVnRsvsvsGom osensegogoFB+=11)1 (1)(1 ()2()()(0221考虑斜率补偿后的Power Stage传函(C
12、CM)On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential18考虑斜率补偿后的Power Stage传函(CCM)?输入电压越低，右半平面零点频率越低低压输入时有不稳 定倾向！?满载条件下的右半平面零点低于轻载状态满载有不稳定倾向！On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential19DCM模式下Power Stage 小信号传函( )( )sCRSCR VVsvsvooooFBFB2111
13、110+111/12oozoopCRwCRw=?由于Power Stage 无右半平面零点(RHPZ)，所以相对于CCM而言，DCMFlyback系统更容易补偿！On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential20电流模式与电压模式的直观理解?电压模式是占空比直接调制占空比直接调制，变压器电感是开环状态，在外围电压回路 中引入一个DC极点(s=0)?电流模式是占空比间接调制占空比间接调制，变压器电感是闭环状态。?反激变换器类似于buck-boost架构，以buck-boost为例分析。?无
14、论是电压还是电流模式，CCM中RHZ始终存在，且频率相同。()()OOIOILVDVVDVDVV+=1()() ()dVVDvdVVvOIOOIL1+=()DIILO=1()dIDiiLLO1=sLviLL/=On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential21电压模式的信号流程图( )siLoffd()dIDiiLLO1=ddIdsLViLI O=On-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confid
15、ential22电流模式的信号流程图( )siLoffdVi1/sL VoGpwm-1DonDoff()dIDiiLLO1=Variable Gain, is a function of FB and slope compensationFBd()()OIL LLOVVIsLiDii+=1RsOn-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential23负反馈环路稳定性?环路增益loop gain T(s) T(s)包含了系统稳定性信息?幅频-相频曲线，波特图?单位增益带宽(frequency T=
16、0dB)?相位裕量(Phase margin) 45?增益裕量(Gain margin) -12dBOn-Bright Confidential to TenpaoOnOn- -Bright confidentialBright confidential241.左半平面极点: 增益衰减，引起-90度相移 右半平面极点: 不稳定，导致系统震荡 左半平面零点：增益增大，引起90度相移 右半平面零点: 增益增大，引起-90度相移2.负反馈系统DC固有180度相移， 系统的剩下的零极点将对 系统稳定性产生影响。3.系统的环路补偿必须使系统满足稳定性条件，而且要考虑 所有负载和所有电压输入的情形。4.在flyback系统中，为了减小右半平面零点影响，可取单 位增益带宽为1/6-1/10fs(开关频率)零极点对环路稳定性的影响及环
