开关电源小信号分析方法(完整版)实用资料.doc

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1、开关电源小信号分析方法（完整版）实用资料(可以直接使用，可编辑 完整版实用资料，欢迎下载）Why is it Important to Plot a Power Stage Small-Signal Response?Christophe Basso, ON Semiconductor | Power ElectronicsSep. 16, 2021 Christophe Basso is an Application Engineering Director at ON Semiconductor in Toulouse, France, where he leads an applicat

2、ion team dedicated to developing new offline controllers specifications. He has originated numerous integrated circuits among which the NCP120X series has set new standards for low standby power converters. Read more about Christophe at the end of this article.QUESTION: Why is it Important to Plot a

3、 Power Stage Small-Signal Response?ANSWER: This is the first question you must ask if you are serious about compensating a power supply. Too often, I have seen engineers building a prototype and throwing arbitrarily-selected component values at the error amplifier, hoping it would let the power supp

4、ly at least stabilize after start up. Then, by tweaking compensating components values on-the-fly as the output undergoes a transient step, the power supply is more or less stabilized by taming undershoots and ringing portions. A few prototypes later, the design is validated for pilot run and here w

5、e go for mass production!This is a scenario that I have seen many times while visiting power supply designers as an application engineer for ON Semi. Even if trials and errors must absolutely be banned when it comes down to loop control, I cannot blame these gentlemen for their method. The reason is

6、 simple, 99% percent of an engineers time is spent on safety tests, making sure the converter dies peacefully, without smoke sometimes without noise! when resistance R236 is open or short circuited or when the controller pin 1 is shorted to pin 2 or even worse, to any of the other pins, including hi

7、gh voltage ones! Believe me, testing and solving for safety is an extremely long and tedious exercise, furthermore if extreme cost and time pressure exists. If you overlook important parts of the design (safety limits, stability margins and so on no wonder the telephone rings a few months later, ask

8、ing the design engineer to urgently fly to the remote factory as most of power supplies do not pass the simple start-up sequence: the overshoot trips the Over Voltage Protection (OVP circuit and the converter safely latches off. The money the company believed it has saved by cutting the development

9、time, instantaneously vanishes if a factory enters a line-down situation or worse, if a product re-call is necessary. In short, do NOT neglect stability design by thinking that a simple 0.1-F capacitor across the TL431 will do the job. Spend the necessary time on it, read some of the reference books

10、 and you will quickly realize how new tools can make the stabilization process quite simple at the end.The power stage response is the first thing you need to stabilize your converter. This is how your converter responds to an ac stimulus applied to its control pin while operating in various conditi

11、ons (light load, full load, high or low line and so on. Without it, there is nothing you can do besides trial and errors as already described. I can see several ways to obtain this transfer function:Method 1 - Build A HardwareBuild a hardware, that is to say, assemble your converter using components

12、 that are representative of what will be used in production sites. Your last power supply prototype is obviously a possible solution. Check with your buyer that the components you have soldered are those populating production boards later on. Why? Because some elements, capacitors for instance, hide

13、 stray elements that affect the converter response and having them onboard already will give you a typical response. As an example, Figure 1 shows a (simplified prototype that I purposely built for an active-clamp forward converter; it delivers 5 V/2 A from a 48-V source. I had to stabilize the beas

14、t for an educational purpose. In lack of analytical expression, there is no way I could predict the response. With the help of a network analyzer, I was able to unveil the Bode plot quickly. I could then use that transfer function to select a crossover frequency and implement compensation strategy t

15、o build phase and gain margins. Derive an analytical expression for the power stage. This is probably the most difficult option, in particular if you are not familiar with small-signal modeling. Fortunately, a lot of these equations have already been developed and you should be able to locate your t

16、ransfer function quickly. Lets assume you want to stabilize a current-mode flyback converter. The mathematical expression for this power stage is as follows: In this expression, you can see the position of the various zeros (one of them is in the Right-Half Plane and the effect of the sub-harmonic p

17、oles. What is nice with this expression is that you can capture it in a mathematical solver such as Mathcad and plot the magnitude versus frequency response in a snap shot. This is what Figure 2 shows you with a complete Bode plot. What is the difference between this solution and the first one? Usin

18、g the analytical expression, you know exactly what elements create poles and zeros in the converter ac response, or what elements the dc gain H0 is made of. That is to say, once capacitors and inductors have been selected based on ripple requirements, rms currents capability and cost, their respecti

19、ve data-sheets should tell you the spread of stray elements affecting them. If you know the limits within which these poles and zeros will move in production or during the converter lifetime, then you can take provisions during the compensation phase and test your solution efficiency by sweeping val

20、ues in Mathcad while observing phase and gain margins. If your buyer shows up in your office and tells you that a cheaper capacitor has been identified, you will be able to quickly know if your compensation strategy can accept its Equivalent Series Resistance (ESR dispersion.Method 3 - Simulation Mo

21、delin particular ESRs, the simulated response can be very close to reality. A lot of models are available but I derived a set of auto-toggling models based on Vatch Vorprian PWM switch model. These models operate in current or voltage mode and automatically toggle between CCM and DCM. A free-running

22、 version also exists and nicely predicts the response of quasi-square wave resonant converters (so-called QR. These models are available in a lot of different simulator flavors and let you simulate numerous topologies. Figure 3 shows you a current-mode model implemented in a fixed-frequency DCM flyb

23、ack converter. The operating point is set by the Vstim source so that 12 V are delivered to the load. The model inputs are the primary inductance (to compute the operating mode, the switching frequency (65 kHz in this example, the sense resistance and the slope compensation magnitude, if any. The Bo

24、de plot is given in Figure 4 and was immediately obtained owing to the absence of switching events (averaged model. From that response, you can see a first-order behavior in the low-frequency portion while a zero kicks in later on along the x-axis. This zero is created by the output capacitor and it

25、s ESR and may significantly move with temperature and age for instance. Finally, a Right Half-Plane zero starts to manifest itself in the upper frequency portion (gain increases by bringing the phase response further down.Unlike with the analytical solution, SPICE does not give you the transfer func

26、tion telling you how a part variation will affect poles and zeros. However, it is really easy to sweep components values with different distribution profiles and see their effect on the power stage response. Even better, you can compensate the power supply with SPICE and affect tolerances to all you

27、r elements, as in a real production place. Then, by running Monte-Carlo analysis, you will see how the crossover frequency, dc gain, phase/gain margins are affected. It does not mean that this prediction will be verified along the converters lifetime, but if running this type of simulation shows tha

28、t your converter is robust by always keeping adequate margins, chances exist that robustness is confirmed in practice.As a quick summary, I cannot stress enough to be serious about control loop design. Most of the technical questions I have relate to that field and it motivated me to write a book en

29、tirely dedicated to the matter. Actually the subject is wide but I have tried to highlight a few things that I believe a loop designer in the power supply field must understand to make them work efficiently. The method described in the above bullets can also be combined, this is excellent practice:

30、equations tell you what response you should expect from SPICE and where potential offenders hide. SPICE will let you orchestrate the compensation strategy and verify that margins are not violated as component values change. Exploring the response at different operating points is also a child play. F

31、inally, the mandatory hardware testing will tell you if your assumptions and choices were pertinent. Combining these approaches is the recipe to success!Author BioChristophe authored several books in the subject of power electronics: McGraw-Hill published, “Switch-Mode Power Supplies: SPICE Simulati

32、ons and Practical Designs”, in 2021. This work was positively reviewed in several magazines and in a PELS newsletter. Christophe is currently working on a revised second edition due for publication mid-2021. In 2021, he published “Designing Control Loops for Linear and Switching Power Supplies” with

33、 Artech House which is well received by engineers. He holds 24 patents on power conversion and regularly teaches seminars at APEC conferences. He also publishes papers in trade magazines including How 2 Power and PET.Christophe has over 18 years of power supply industry experience. Prior to joining

34、ON Semiconductor in 1999, Christophe was an application engineer at Motorola Semiconductor in Toulouse. Before 1997, he worked as a power supply designer at the European Synchrotron Radiation Facility in Grenoble, France, for 10 years. He holds an equivalent BSEE from the Montpellier University (Fra

35、nce and a MSEE from the Institut National Polytechnique of Toulouse (France. He is an IEEE Senior member.肯普科技 学习园地 小功率反激式开关电源设计与计算一、原理分析下图为一开关电源原理图220V市电经开关、保险管、热敏电阻、共模抑制电感电容和差模抑制电容，经桥式整流成脉动直流，经电解电容滤波，得到约300V直流电压，通过开关变压器的初级加至开关管漏极（或集电极），这其中在保险管的后面接有压敏电阻，可消除来自电网的超高瞬态尖峰脉冲干扰，如果市电电压异常升高，在一个不太长的毫秒级时间内，压敏

36、电阻阻值迅速降低至欧姆级，大电流熔断保险丝，从而保护了后面的电路。在220V电路中，串有热敏电阻，该电阻在常温下约十几欧姆，开机瞬间利用这一电阻有效减小冲击电流，保护线路、电源开关接点、整流二极管。当电流稳定后，热敏电阻温度升高电阻下降即负温度系数，整机正常工作。在有些高安全设计中，共模抑制电容两端并有一只1W 470K680K的电阻。当电源关断后，电容可能存有电量，人们不小心接触电源插头就可能触电，该 1肯普科技 学习园地电阻就是在关机后迅速放掉电容剩余电压，提高安全性。当接通电源，TOP芯片工作，反馈绕组（实则是供电绕组）直流电压通过光耦加在芯片控制端，该电压5.7V自动重启动进入启动状态

37、，根据交流输入电压的高低与负载的轻重，自动调整脉宽，控制开关管导通时间，即占空比，从而达到取样电压的设计值。开关管在关断的瞬间，变压器由于漏感与分布电容的存在，在感应电压上出现上冲，并以衰减振荡的波形出现，这一上冲将大大超过开关管的BVDS击穿电压，必须要加以限制，保证开关管的安全。在变压器的初级接有削波电路，TOP常选用瞬态抑制二极管和阻塞二极管电路，该电路有效防止过冲损坏开关管，且损耗较之由阻容和二极管组成的吸收电路低一些。某些电路中，在漏极与地之间接有RC串联电路，其功能也是吸收陡峭尖脉冲。在TOP芯片的控制端接有RC串联电路，RC时间常数的大小影响自动重启功能的响应时间。热端与冷端跨接

38、有“安规”电容，将高压侧的高频干扰脉冲入地，该电容的容量大小将影响抗干扰性和漏电流，只好折中选用。TL431是三端精密稳压器，在控制级内部设有一稳定的2.5V参考电压，在主输出电压端接有分压电阻，分压端接在TL431控制级，分压值与2.5V参考值进行比较，得到一个误差值且放大后去推动光电耦合器的发光管使其亮度发生变化，从而使光耦中的光电接收管内阻发生相应的变化，TOP芯片控制电压高低变化，脉宽调整，控制开关管导通时间，自动调整输出电压，控制在一个很小的变化范围内，这是一个闭环控制过程。光电耦合器选用PC817B，一是抗电强度满足要求，二是电流传输比适中，电流传输比大小将影响稳压精度，但不是越高

39、越好，太大的电流传输比将使闭环系统不稳定，甚至引起寄生振荡。TL431的控制极与阳极接有一电容或RC串联电路，其作用类似于负反馈，用于消除自激等不稳定现象。容量大小将影响闭环系统的 2肯普科技 学习园地反应速率，太小克服不稳定作用小，太大将使稳压输出瞬态跟随特性变差，因此也是折中选用。输出电压整流二极管的选用，就目前器件水平，12V以下选用肖特基二极管，其优点突出，超过12V就选用超快恢复二极管，有较高的反向击穿电压，有短的恢复时间，其性能优于快恢复二极管。某些电路在整流管两端并联有电容或RC串联电路，其作用是对5-10MHz电磁干扰有抑制作用,同时可减低反向尖峰电压，保护二极管免于反向击穿。

40、二、 电路结构1、 脉宽调制芯片UC3842开关管IRF840/2SK1460/2SK2850，工频电压整流可采用桥堆式二极管，电磁兼容必须使用共模抑制电感电容与差模抑制器件，漏极尖峰电压吸收电路采用电阻电容加快恢复二极管。次级主输出电压取样、三端精密稳压器比较输出推动光耦发光管发光，光耦接收管控制芯片调整脉冲占空比。反馈绕组仅给芯片提供工作电源。UC3842工作频率一般取50KHz.2、 TOP系列：脉宽调制-开关管一体化三端器件。漏级尖峰电压吸收采用瞬态抑制二极管与阻塞二极管结构，电路进一步简化，损耗减小，各项保护功能集成于片内，可靠性提高。工作频率为100KHz。3、 近来有一种单片脉宽

41、控制电源芯片，SFQ110/100，很适合做十几瓦以下的开关电源，电路简洁，可靠性高，价格低廉，朋友们不妨一试。三、 可靠性1、 开关管IRF840 BVDS=500V 勉强了一点，可靠性较差，选用2SK1460,2SK2850, BVDS=900V 完全可以胜任。2、 电解电容的质量问题较严重，除了容量指标外，更重要的是耐电压和漏电流0.010.02CF*Uv（A）,特别是脉动直流电压脉动量大的部位，例如紧 3肯普科技 学习园地接整流管的滤波电容要小心选用。3、 脉冲整流二极管选用超快恢复二极管和肖特基二极管，能有效提高整流效率和降低二极管的温升。4、 发热器件例如开关管及大电流整流二极管必

42、须加有效散热器，能提高工作环境温度，提高热可靠性。四、 电路排版1、 6mm，这叫安全距离，安全电压AC3KV，同时要考虑热端与金属外壳的距离也要大于6毫米，确保安全。(如果金属外壳接地，安全距离减半)2、 次级整流二极管紧接的滤波电容尽量减少环路长度，以利于降低纹波系数。采用一点接地原则，特别是高压侧，排版结构可做到：市电-整流-滤波一点接地，开关源极一点接地，有效降低由环路电流引起的有害寄生振荡，在有害的极端情况下甚至使脉冲振荡失控，排版失败。3、 热端与冷端间距五、 开关变压器1、 选磁芯：15W选EE20，1530W选EC2825，3090W选EC35B，所有磁芯的o=2000,因为选

43、用的反激式开关电路，所以磁芯必须磨气隙。从本公司的实际出发，开关电源的功率100W，磁芯不能分档太多，因此只选用三挡，利于生产与管理。2、 确定已知数据：输入电压，负载电流、电压、磁芯有关参数,LAG通过测试求出。3、 将已知数据代入所有计算公式，计算出结果，如果不符合实际，经过多次叠代计算逼近实用。4、有经验表明，绕在线包最外层的次级，在空载时的电压高出满载时的电压4肯普科技 学习园地许多，这是因为漏感太大，波形变坏，在开关脉冲上有高的过冲尖峰波形，经整流将是虚高的直流电压，加负载后电压立即下降。为了减小漏电感的影响，将初级绕组分为两部分，最里半组最外半组，用以提高耦合度，减小漏电感，当然这

44、会给绕制和绝缘处理增加难度，但性能是大有提高，特别是多组输出的变压器更是如此。取样绕组绕满一层或多层，并将其它次级包围，形成一种紧耦合，提高取样绕组的电压调整率。如果要进一步提高其它绕组的电压调整率，可采用多组取样，折中处理。5、 绝缘处理，采用绝缘胶带与挡墙胶带的好处：一是绝缘强度易于达到要求，二是减小绕线包的难度，经过测试合格的变压器再浸渍绝缘处理。6、 线包匝数与线经的选取：在允许范围内作一些调整，考虑因素如下：现有线经优先选定，为减小集肤效应，避免选用大于直径0.41以上的漆包线，载流量不够可采用多股并绕的方法。处理抗电强度，初级是关键,尽量是双层，每层饶满，引出头套套管。六、实验1、

45、高低压实验：设计工作电压范围85V(165V)265V AC，高端升至275V低端降至60V(165V)工作正常。2、极限工作环境温度实验：在恒温箱中，开关电源工作电压分三挡轮流工作：180V/220V/250V，由供电箱每隔15分钟自动转换，温度从常温在8小时内升至110,开关电源保护（无输出），塑料机壳变形，当温度降低，恢复工作。3、抗电强度实验：在常温自然湿度下 ，4000V AC 8mA一分钟。4、短路实验：将5V 1A（取样绕组）及23V 0.35A（非取样绕组）直流电压分 5肯普科技 学习园地别短路实验，电源进入保护状态，去掉短路，电源立即恢复正常工作，未出现元件温度升高。七、纹波

46、测量输出(V)52227TDS-210(mvp-p) 30 40 70 21.6 毫伏表Hz2181(mV) 4.2 8 24八、光纤收发器电源计算TOP221y：保护动作电流IMIT=0.230.28A VSDmax135Vf=100000Hz VDS(ON)=10VU1min=160V P0=5W V0=5V VOR=100V=0.7 Z=0.5 KRP=0.6 VF1=0.4VV1min=*U1min=224VEE20:AL=1.76H/N2 ALG=0.144H/N2 (=0.3)（补充说明：AL及ALG的测量1、2、3、 在骨架中用0.23左右漆包线绕线50匝（N） 将磁芯的端面擦干

47、净，套进线圈，测量电感量L(H) 计算 已知线圈匝数（N），实测电感量L(H)，求有效电感(AL)ALG (H/N2)ALG=L/ N2 已知有效电感ALG，线圈匝数（N），求电感量LL= ALG*N2 已知有效电感ALG，电感量L，求线圈匝数NN=L ALGSJ=0.35cm2 j=4A/mm2 (j=4-10A/mm2 )肯普科技 学习园地DVORmax=VOR+ V-V =0.321minDS(ON)b) IOAVG= P* V=0.032A1minc) IAVGP= I(1-0.5K=0.15ARP)*Dmaxd) NLP= PA =210TLG6e) LPO)+ P=10I2P *

48、KRP (1-0.5KRP)*f *Z(1- =6f) n= VORV=18.5O+VF1g) NNS= Pn=11Ti) BM =IP*LPN100=0.13tP* SJ*j)=40SN2(P11000L-1000A)=0.3mmPLk) 线包每伏匝=11T/5.4V=2.037T/V反馈绕组电压：9.7V反馈绕组=9.7*2.037=20T次级线径：S= 1A4A/mm2=0.25mm2选0.29,S=0.066mm2 4股并绕初级线径：1.13 Ij=0.1mm原理图0.23T2100.29*4T110.23T207选0.23肯普科技 学习园地说明：初级绕组分为两组即1&4。h) 线包的核算与试绕边胶带宽1.5，厚0.15；宽胶带任选，厚0.06层间一层胶带，组间三层胶带，内包一层胶带，外包三层胶带（附：如下图例子根据经验，所使用电源在初级线圈设计上均使用=0.29圈数为50圈，分为两层，初级线圈在设计上为了减小漏电感的影响，将初级绕组分为两部分，最里半组最外半组，用以提高耦合度，减小漏电感，当然这会给绕制和绝缘处理增加难度，但性能是大有提高，特别是针对多组输出的变压器更是如此。反馈绕组采用2股并绕=0.29，共13